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Magnesium-based photocathodes for triggering back-lighted thyratrons
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Magnesium-based photocathodes for triggering back-lighted thyratrons
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MAGNESIUM-BASED PHOTOCATHODES FOR TRIGGERING BACK-LIGHTED THYRATRONS by Esin B. Sözer ________________________________________________________________________ A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (ELECTRICAL ENGINEERING) May 2012 Copyright 2012 Esin B. Sözer ii Acknowledgments I would like to thank my advisors, Dr. Martin Gundersen and Dr. Chunqi Jiang, for their tireless guidance and support. My experience at USC Pulsed Power laboratory under their supervision taught me not only the technical ingredients of scientific research process but also the requirements to be a good scientist: hard work, perseverance, attention to detail and, of course, clear communication and presentation. I could not come this far without their feedback and encouragement. I would like to acknowledge Lawrence Berkeley National Laboratory, Plasma Applications Group, specifically, Dr. Andre Anders and Dr. Jason Sanders for the deposition of Mg films used for the experiments mentioned in section 4.3.4 and Yung- Hsu Lin for SEM images presented in section 4.3.2. I would like to thank Susan Zarate, Wayne Johnson, Kim Reid, Jaime Zelada for the essential technical and administrative help during my years at the Electrical Engineering-Electrophysics department of USC. I would like to thank all the members of USC Pulsed Power as they have been an important part of my PhD journey; the friendship and support I received during my four years was invaluable. I would especially like to thank Dr. Dan Singleton and Dr. Jason Sanders for being great colleagues, friends and patient listeners. It has been a pleasure to know engineers of your caliber, and a privilege to be able to call you my friends. I would like to thank Dr. Andras Kuthi, Sharon Wu and Yung-Hsu Lin for their ever-positive encouragement and lovely conversations. iii I would like to thank my family members: Drs. M. Tekin Sozer, E. Emel Sozer and Sevin Sozer-Ertoklar. Their tireless encouragement, support and unconditional love have been the single enabling force for everything I achieved. Thank you for teaching me the importance of morality, generosity and humanitarianism, and telling me repeatedly that I will be as valuable regardless of my academic achievements. It has been the greatest gift of confidence to feel valued as a person, especially when everything else felt volatile. I would like to thank my friends who have been my second family during this time where I did not have easy access to my hometown Ankara, Turkey: Dilek Sengil, Beril Inan, Zeynep Tugran, Mustafa Ispir Gurbuz, Ozgun Bursalioglu, Derya Ozkan and all of my Los Angeles family of friends. They listened, valued, provided their best wisdom at times of homesickness and despair that had been the unfortunate constant companions of my PhD years like they had been for many international students and scholars. Finally, I would like to thank Chris, for his patience, support and wisdom and all the proofreading. I surely am lucky to have met you; and I am looking forward to rest of our journey together. iv Table of Contents Acknowledgments.......................................................................................................... ii List of Figures................................................................................................................vi List of Tables ..................................................................................................................x Abstract..........................................................................................................................xi Chapter 1 Pulsed Power Systems and Switching ......................................................1 1.1 Pulsed Power Systems .......................................................................................1 1.2 Pulsed Power Switching ....................................................................................5 1.3 Motivation of Research....................................................................................11 Chapter 2 High Power Gas Discharge Closing Switches ........................................14 2.1 Introduction to gas discharge breakdown ........................................................14 2.2 High Pressure Spark Gaps ...............................................................................19 2.3 Thyratrons........................................................................................................21 2.4 Pseudospark Switches......................................................................................24 2.4.1 Triggering Mechanisms.............................................................................25 2.4.2 Physics of Pseudospark Discharge ............................................................29 Chapter 3 Photocathode candidates and Photoemission..........................................39 3.1 Introduction......................................................................................................39 3.2 Photoemission..................................................................................................42 3.2.1 Spicer’s three step model...........................................................................42 3.2.2 Jensen’s work ............................................................................................43 3.3 Photocathodes ..................................................................................................46 3.3.1 Introduction ...............................................................................................46 3.3.2 Photocathode selection ..............................................................................47 3.4 Photoemission measurements of photocathode candidates for BLT ...............50 3.4.1 Introduction ...............................................................................................50 3.4.2 Experimental Setup....................................................................................50 3.4.3 Results .......................................................................................................52 3.4.4 Discussion..................................................................................................56 3.5 Chapter conclusions.........................................................................................58 v Chapter 4 BLT with Photocathodes.........................................................................60 4.1 Introduction......................................................................................................60 4.2 BLT triggered by unfocused laser beam..........................................................61 4.2.1 Experimental Setup and Procedure............................................................61 4.2.2 Results .......................................................................................................65 4.2.3 Discussion..................................................................................................67 4.3 BLT triggered by partially focused laser beam................................................68 4.3.1 Experimental setup and procedure ............................................................68 4.3.2 Results .......................................................................................................71 4.3.3 Discussion..................................................................................................75 4.3.4 BLT with magnesium thin film .................................................................81 4.4 Chapter conclusions.........................................................................................83 Chapter 5 Summary and Future Work.....................................................................85 5.1 Summary..........................................................................................................85 5.2 Future Work.....................................................................................................88 5.2.1 Pre-switch closure current measurement...................................................88 5.2.2 Plasma simulations ....................................................................................94 Bibliography .................................................................................................................96 vi List of Figures Figure 1 Block Diagram of a pulsed power system.................................................1 Figure 2 Applications of pulsed power at USC (a) Transient plasma ignition electrode(Singleton et al., 2011) (b) Plasma dental probe (Jiang et al., 2009) ...................................................................................................2 Figure 3 The water section of Z was replaced during the Z Refurbishment Project in 2007 (Sandia, 2011)..................................................................4 Figure 4 Temporal development of voltage, current and power loss for plasma closing switch (Bluhm, 2006) ...................................................................6 Figure 5 Semiconductor switches: (a) Thyristor (b) GTO (c) an IGBT with its circuit symbol (Bluhm, 2006) ...................................................................8 Figure 6 Hold-off voltage versus repetition rate characteristics for main high power semiconductor switches (Wang et al., 2007) .................................9 Figure 7 Paschen Curve for variety of gases (Mesyats, 2004) .............................16 Figure 8 The curve separating the regions corresponding to the streamer and the Townsend mechanism of a discharge in air (Mesyats, 2004) ...........17 Figure 9 Cross-sectional view of a trigatron (Hansjoachim, 2006)......................20 Figure 10 Geometry of a hydrogen thyratron (Pirrie and Menown, 2000) ............21 Figure 11 Electric Field Lines on the axis of a typical pseudospark geometry (Riege and Boggasch, 1989) ...................................................................25 Figure 12 Pulsed Low-current Glow Discharge Triggered Pseudospark (Frank and Rath, 1990) .......................................................................................26 Figure 13 Surface Discharge Triggered Pseudospark Geometry (Gortler et al., 1989)........................................................................................................27 Figure 14 Back-lighted Thyratron (Gundersen and Schaefer, 1990) .....................28 Figure 15 Mini BLT and a commercial pseudospark (Jiang et al., 2005a) ............29 Figure 16 Formation of virtual anode (Boeuf and Pitchford, 1991).......................31 vii Figure 17 Dependence of anode-delay time on gas pressure and anode-cathode separation. The delay time decreases with increasing gas pressure and decreases with larger electrode separation. The dashed line is the constant pd below which satisfies Paschen’s criterion at ~10 kV. The das-dot line is f(p,d) above which defines the closure region. The optimum operating points lie between these lines at values of p and d for which the shortest anode-delay times may be obtained (Hoyoung and Kushner, 1989) .................................................................................32 Figure 18 Fundamental equations used in fluid model: (1) is used to get velocities for continuity equations (2) and (3), (4) is the Poisson’s equation used to compute electric field, S is the ionization term computed as a result of Monte Carlo simulation.....................................34 Figure 19 Time to breakdown versus initial charge density as modeled by Pitchford et al. (Pitchford et al., 1995) ...................................................35 Figure 20 Field-enhanced thermionic emission from many small spots (Anders et al., 1994)..............................................................................................36 Figure 21 Time dependence of explosions in vacuum arc cathode spots (Anders et al., 1994)................................................................................38 Figure 22 Potential diagram of a clean metal (Jenkins, 1969) ...............................39 Figure 23 Potential diagram of (a) a clean metal (b) a metal coated with electropositive monolayer or (c) a metal coated with electronegative monolayer (Jenkins, 1969) ......................................................................40 Figure 24 Illustration of Spicer’s three-step model of photoemission on an energy band diagram of a semiconductor (Spicer and Herrera-Gomez, 1993)........................................................................................................42 Figure 25 Collected photoemission current from a porous tungsten matrix containing Sc 2 O 3 , BaO,CaO and Al 2 O 3 dispenser cathode (a) after stopping heating (b) change with temperature (Zhang et al., 2010). (The optical source wavelength is 266 nm.) ...........................................49 Figure 26 Experimental setup for quantum efficiency measurement of photocathode materials............................................................................51 Figure 27 Photo-induced current pulse recorded by a 550 V-biased Faraday cup (a) for a magnesium sample at 3.6×10 -7 Torr; (b) for a molybdenum sample at 6×10 -7 Torr. .............................................................................53 viii Figure 28 Pressure dependence of the quantum efficiency for Mg, Cu and Mo with the collector bias voltage of 550 V. ................................................54 Figure 29 Change in quantum efficiency of magnesium with changing collector bias voltage at a background pressure of 4.8 x 10 -5 Pa (3.6×10 -7 Torr). .55 Figure 30 Relationship between square root of QE and square root of electric field for magnesium at 3.6×10 -7 Torr......................................................57 Figure 31 BLT used in the experiments ..................................................................61 Figure 32 Experimental setup with the cross-sectional view of the switch structure: the inset shows the cathode configuration with molybdenum-cap press-fit.......................................................................62 Figure 33 Experimental Setup ................................................................................63 Figure 34 (a) Definition of delay, arrow shows the time between the laser light hitting the cathode to the switch closure (b) Hold-off voltage vs. helium pressure .......................................................................................64 Figure 35 BLT operating with He plasma ...............................................................65 Figure 36 The switch delays measured at three different helium pressure and switch voltage combinations ...................................................................66 Figure 37 (a) Experimental setup showing the switch cross-section together with electrical circuitry schematics (b) Hollow cathode dimensions and location of the Mg foil indicated with green ....................................68 Figure 38 Typical waveforms of light signal measured by a UV photodiode (shown in arbitrary units (a.u.)), switch voltage, and switch current signals. The dashed arrow indicates the delay, colored block arrows indicate the axes of corresponding colored signals.................................69 Figure 39 Hold-off voltage with and without Mg foil at the switch cathode with changing helium pressure........................................................................71 Figure 40 Switch delay measured with and without Mg foil at the switch cathode at constant switch voltage of 4.1 kV..........................................72 Figure 41 The Mg foil surface (a) before and (b) after 10 6 shots. The inset of (b) shows the bubble-like formations from a side angle. ........................74 Figure 42 Delay of Mg integrated cathodes with different film thicknesses..........81 ix Figure 43 Magnesium film coated cathodes before and after delay measurements (~ 7000 shots). Film deposition was performed at Lawrence Berkeley National Laboratory by plasma applications group........................................................................................................82 Figure 44 Typical waveforms of first and second current peaks after the light signal at low charging voltage.................................................................89 Figure 45 Peak current vs. two charging switch voltage points with plain Cu and Mg integrated Cu cathodes...............................................................89 Figure 46 Full-width at half maximum values of two peaks with changing peak of the current values ................................................................................90 Figure 47 Peak current vs. switch voltage and the full-width at half maximum of the peak currents with changing peak current values .........................91 Figure 48 (a) Current, and voltage, and (b) plasma induced emission waveforms recorded on the axis of hollow cathode between in a pure argon discharge with clean electrodes. The pressure is 0.9 Torr at room temperature. The labels "0," "1,"and "2"refer to different temporal phases of the discharge (Alberta et al., 1994)..........................92 Figure 49 A schematic representation of a two-stage multi-physics simulation for studies of optical triggering of a BLT ...............................................94 Figure 50 UV LED lamp with a wavelength of 255 nm by Sensor Electronic Technology..............................................................................................95 x List of Tables Table 1 Approximate best quantum efficiencies of common photocathodes used for FELs together with measurement conditions and lifetimes (Cultrera et al., 2005; Nation et al., 1999b; Kong et al., 1995; Wang et al., 1996)..............................................................................................48 Table 2 Trigger delay values with increased switch voltages ..............................73 xi Abstract This dissertation presents experimental studies of application of magnesium-based photocathodes to optically triggered pseudospark switches, called back-lighted thyratrons (BLTs). Magnesium was chosen as a low work function metal photocathode for its potential to increase triggering performance of the switch with a higher photoemission performance than traditional BLT cathodes. Improvement in triggering performance of plasma switches is of interest for device development of compact pulsed power systems where the size of switching units can limit the overall size and the mobility of the system. Experiments were conducted on photoemission performance of photocathode candidates under BLT-relevant conditions; and delay and jitter performance of a BLT with photocathode candidates with changing helium pressure and switch voltages. A review of photocathode literature showed that Mg and Cu are the most promising candidates for increasing the photoemission during the triggering of BLTs. As a commonly used BLT cathode in the switch literature, Mo was chosen together with Mg and Cu to be tested under BLT-relevant pressure and field conditions. Quantum efficiency measurements of high-purity foils of Mg, Cu and Mo showed a superior performance of Mg and Cu over Mo. Mg had the highest quantum efficiency of 1.5 x 10 -5 among all three materials. After photoemission measurements in a test bed were concluded, testing of these cathodes for their switching performance was done in two stages. First, an unfocused UV xii laser beam (8.5 ×10 6 W/cm 2 ) with a wavelength of 266 nm was used for delay measurements of a BLT with Mg, Cu and Mo-based cathodes. Mg-based cathodes showed at least a thirty-fold reduction in delay and jitter compared to Cu-based and at least an eighty-fold reduction in delay and jitter compared to Mo-based cathodes at any given helium pressure and switch voltage pair. Subsequently, a partial focusing of the same light source was utilized (7.4 ×10 7 W/cm 2 ) for delay measurements of a BLT with copper electrodes at constant switch voltage and changing helium pressure before and after integration of a Mg foil. These measurements showed an order of magnitude shorter delay and jitter throughout the pressure range when the high-purity Mg-foil was present at the switch cathode. Theoretical estimations of electron emission from the cathode during the triggering suggested that the main mechanism responsible for the observed change in delay and jitter was the increased photoemission due to the lower work function of the Mg cathode and that the effect of temperature on triggering is negligible. SEM images of the high-purity Mg foil integrated at the BLT cathode for 10 6 shots showed signs of melting around the bore hole. No degradation of the switch performance was observed for the duration of 10 6 shots. In conclusion, magnesium-based cathodes for BLTs showed an important potential for small triggering units for optical triggering, especially when the intensity of the optical source is limited. A future work involving plasma simulations is suggested for assessing potential of different cathode/optical source pairs for triggering BLTs. 1 Chapter 1 Pulsed Power Systems and Switching 1.1 Pulsed Power Systems Pulsed power is the science of storing energy over a period of time (usually minutes or seconds) and then discharging it as electrical energy over a shorter period of time (usually microseconds or nanoseconds). A pulsed power system includes an energy storage stage, a load, and a pulse forming stage between these two stages. Figure 1 Block Diagram of a pulsed power system Energy storage is composed of either capacitive or inductive components. The pulse forming stage can be a single high power switch that transfers the stored energy to the load or a more complex system based on a network of high power switches. Pulse forming stage can include opening or closing switches depending on the type energy storage elements: opening switches follow an inductive energy storage unit while closing switches follow a capacitive energy storage unit. In general, the pulse forming stage is responsible for a succession of energy transfer and compression steps. The amount of 2 energy is ideally conserved throughout the stage, thus compression in time corresponds to an increase in the power of the electrical pulse. Switching components that can handle the peak power requirement of the load and have fast turn-on times suitable for energy transfer without modification of the pulse shape are necessary to achieve the needs of any particular load (application). Figure 2 Applications of pulsed power at USC (a) Transient plasma ignition electrode(Singleton et al., 2011) (b) Plasma dental probe (Jiang et al., 2009) Modern pulsed power has originated from work of the John Christopher Martin and his colleagues at the Atomic Weapons Establishment, Aldermaston, U.K., in the1960s. He was a hydrodynamicist who was frustrated by his inability to purchase an adequate X-ray radiography source for imaging, and decided to pursue a new generation of radiography sources that were based on high-power Marx generators, coupled with low-impedance transmission lines, and cold cathode single-stage accelerating gaps (Schamiloglu et al., 2004). The development of pulsed power switching components dates back to WW2 where radar system component requirements lead to milestones of high power switch development. Today, pulsed power technology has applications in 3 wide variety of fields including biomedical engineering, ignition, dentistry, radar systems design, all of which utilize compact system components suitable for portable systems. USC’s pulsed power group is actively involved in some of these applications such that ignition and combustion, and biomedical. Figure 2 shows examples of USC Pulsed Power group’s work. The key enabling technology of compact system design and development for any pulsed power system is the switching technology suitable for system needs. The range of applications not only is varied in field but also in electrical parameter requirements that determine choice of switching components. For example a 1 mJ, 100 ns pulse can be the pulsed power system output for a plasma dental probe, a 70 mJ, or 12 ns pulse for transient plasma ignition, whereas Z machine at Sandia National Laboratories fires only 200 shots every year of 26 MA, 70 ns, 290 TW electrical pulses that are cable of reaching peak X-ray emissions of 350 TW and an X-ray output of 2.7 MJ (Singleton et al., 2011; Jiang et al., 2009; Sandia, 2011). Z machine is quite impressive with the amount of power it can deliver and physical phenomena it allows to study; its home, Sandia National Laboratories is a very important source for pulsed power technologies and historical development of the science and technology related to it. Their 2008 publication on the history of Sandia-based pulsed power is a great historical read of very meticulous detail and a good representation of pulsed power science historical development all over the world (Van Arsdall, 2008). 4 Figure 3 The water section of Z was replaced during the Z Refurbishment Project in 2007 (Sandia, 2011) 5 1.2 Pulsed Power Switching An electrical switch can simply be described as a device with a capability of transitioning from a high impedance state (open) to a low impedance state (close) or vice versa. Closing switches, which are the main area of study for this dissertation, are naturally at a high resistance state. Commutation of a closing switch can be controlled with an application of trigger, i.e. introduction of charged particles to the separating gas medium between the anode and the cathode for a plasma switch or injection of charge carriers into the conduction band for a semiconductor switch. The time lag between the application of a trigger and the switch closure is referred as “trigger delay”. The trigger delay is composed of two components: “statistical” and “formative”. For a plasma switch, the required energy for switch closure must be sufficient for first, charge transport through the gap, and then for sustaining of the ionization and conduction. Since the initial trigger charge density will have a statistical velocity distribution, the trigger delay will “jitter” from shot to shot. Control and repeatability of initial trigger conditions is important for a short “jitter”(Burkes, 1990). 6 Figure 4 Temporal development of voltage, current and power loss for plasma closing switch (Bluhm, 2006) Other critical parameters that determine the choice of the switch to be used for a particular load (application) are mainly hold-off voltage, peak current, rise time (and repetition rate) and lifetime of the switch. Hold-off voltage can be defined as the DC voltage at which device breakdown occurs. Peak current is the maximum current at which the switch can be reliably operated without harming the switch. Repetition rate is the rate at which the switch will be “closed” and/or “opened” without degradation of switch characteristics. Rise time is the transition time it takes for the switch to go from an open to a closed state; it limits the pulse duration and the repetition rate of the system. Lifetime of a switch is usually measured in terms of number of shots the switch can operate 7 without a significant change in its critical parameter ratings (Burkes, 1990). A comparison of hold-off voltage, peak current, rise time, jitter and lifetime parameters of commonly used high power switches with both commercial and literature references can be found in references like Bluhm et al., and Sanders et al.(Sanders, 2011; Bluhm, 2006). High power switches for pulsed power systems can mainly be categorized as solid state and gas-phase (plasma) switches. Solid-state switches are, in general, advantageous with their compact housekeeping requirements; however they are limited in their hold-off voltages, peak currents and rise times. Moreover, occurrence of an arc due to a single time overload of current during their operation can be catastrophic for semiconductor properties whereas it will be only shortening the lifetime of a plasma switch (Schamiloglu et al., 2004). Principal solid-state pulsed power switches include the thyristor, gate-turn-off thyristor (GTO), insulated gate bipolar transistor (IGBT), and metal–oxide– semiconductor field-effect transistor (MOSFET). Figure 5 shows the structure of some of the common semiconductor switches for pulsed power applications. Thyristors are four layer semiconductor devices that can be considered as three p-n junctions or a combination of a n-p-n and a p-n-p transistor sharing a collector junction. GTOs are thyristor switches capable of much higher current rise rate as a result of special gate structures designed for pulsed power applications. Among semiconductor switches, the thyristor has the highest current handling capability however is limited by its rise time. IGBTs combine a bipolar junction transistor with a MOSFET. They are much faster devices compared to thyristors, however have orders of magnitude difference in 8 their hold-off voltage and peak current capabilities compared to plasma switches. Semiconductor switches and their hold-off voltage and repetition rate limitations can be approximately summarized as in Figure 6. These limitations often result in the requirement of several series or parallel semiconductor switches to be used for a single pulsed power system which in turn means a compromise of the compact size of the semiconductor switches (Wang et al., 2007). Figure 5 Semiconductor switches: (a) Thyristor (b) GTO (c) an IGBT with its circuit symbol (Bluhm, 2006) 9 Figure 6 Hold-off voltage versus repetition rate characteristics for main high power semiconductor switches (Wang et al., 2007) Plasma switches including spark gaps, thyratrons and pseudosparks will be explained more in detail in the following chapter. Gas spark gaps have been used under a very wide range of single-pulse (or few-pulse) conditions, and operate with varying lifetimes at very high voltages and currents with very fast switching times. The primary limitation is in lifetime and repeatability; this is because the type of arc formation at high current tends to be erosive, thus altering electrode characteristics. Most large pulsed power machines currently operating under single pulse mode utilize a spark gap switch. Development of a thyratron with high hold-off voltage was an enabling technology for repetitive pulsed power applications, such as radar modulators. The hold-off voltage can be very high, up to 100 kV. Peak currents range to a few kA, much less than what commercial spark gaps are capable of handling; however, pulse-to-pulse repeatability is very good for millions of pulses. Thyratron lifetimes can be a billion shots in applications within specifications. 10 The commercially less widespread pseudospark is similar to the thyratron as it operates at low-pressures (~ 0.1 Torr), unlike spark gaps that operate at atmospheric pressures (760 Torr). They differ in the operation of the cathode. For both the thyratron and the pseudospark, the limitation in current capability is found in the cathode, not in the plasma. They are attractive for applications requiring fast and very high current transitions (Schamiloglu et al., 2004). In addition to two general categories of semi-conductor and plasma closing switches, magnetic switches should be mentioned as a commonly used alternative for today’s pulsed power systems. Magnetic switches use saturable magnetic cores that provide pulse compression as a result of hysteresis loop of ferromagnetic material they are made up of. Saturation of the magnetic core means a reduction in inductance by a factor of the relative permeability of the material and causes the closure of the switch and thus energy transfer (Bluhm, 2006). Today’s pulsed power technologies require use of all of the mentioned switches for different applications. For high power density pulsed power systems with more than tens of kilo amperes of peak currents and few tens of nanoseconds or shorter rise times, plasma switches are the only option available. Pseudosparks, among plasma switches, have attracted attention for a variety of pulsed power applications with their high peak current (up to 100 kA), high current rise rate (~10 12 A/s), high voltage hold-off capability (>30 kV) and homogenous, glow-like discharge operation providing reasonable lifetimes (10 9 shots), and are primary focus of presented work. 11 1.3 Motivation of Research Pseudosparks have attracted attention for a variety of pulsed power applications with their high peak current (up to 100 kA), high current rise rate (~10 12 A/s), high voltage hold-off capability (>30 kV) and homogenous, glow-like discharge operation providing reasonable lifetimes (10 9 shots) (Frank and Christiansen, 1989; Frank et al., 1988; Anders et al., 1994; Bochkov et al., 2001; Jiang et al., 2005b). As pulsed power systems continue to demand higher peak power, faster yet compact switching units, compact pseudosparks remain a promising option while high-power semiconductor switches fail to reach the needs of high peak power (>MW) pulsed power systems. Optically triggered pseudosparks, also called back-lighted thyratrons (BLTs), offer complete electrical isolation of triggering units from the switch allowing the development of an “ultra-compact” BLT with twenty times smaller volume compared to a traditional pseudospark (Jiang et al., 2005b). However, optical triggering with low jitter and delay requires powerful and usually bulky UV lasers. The advantages of pseudosparks over either hydrogen thyratrons or spark gaps in different switching parameters made them subjects of extensive research over a span of two decades (Frank and Christiansen, 1989; Frank et al., 1988; Anders et al., 1994; Bochkov et al., 2001; Jiang et al., 2005b). 12 In the case of BLTs, or optically triggered pseudosparks, the seed electrons are mainly obtained by photoemission from the cathode via a UV light source. Since photoemission is directly proportional to the difference between the photon energy (hν) of the light source and the work function of the cathode material (φ), it is expected that a decrease in cathode work function would result in an increase in photoemission and better switching parameters with less trigger light energy requirement for BLTs. A wide range of optical parameters (usually employing a UV laser) has been used to trigger BLTs in the literature (Braun et al., 1988; Pitchford et al., 1995; Jiang et al., 2005b; Chen et al., 2009). Although some of the important parameters for photoemission such as total light energy, photon energy, optical power density (controlled by focusing the incident light beam), and the electric field (controlled by changing the light spot location on the cathode or the switch voltage) are reported to affect the delay and jitter performance of the switch; investigation of different cathode materials with varying work functions has not been studied extensively. The motivation of this work is to assess the benefits of low work function cathode materials for triggering BLTs. This dissertation is organized as follows: Chapter 2 gives some background information on high power plasma closing switches and their operation principles and physics. Chapter 3 is a look into photocathodes literature and photoemission; in which we introduce magnesium as our subject of study for BLT cathodes and present our experimental work on photoemission measurements of potential photocathodes for BLTs. Chapter 4 has the experimental studies with comparative results and discussion of BLTs 13 with magnesium cathode, and finally Chapter 5 concludes with a summary of our studies and some suggestions for future work. 14 Chapter 2 High Power Gas Discharge Closing Switches 2.1 Introduction to gas discharge breakdown Discharge formation is a result of avalanche multiplication of charged particles to form a conductive path. It is the physical process responsible for switch commutation for plasma closing switches. Two fundamental types of discharge formation can take place depending on nature of the space charge created as a result of them: Townsend and Streamer (Mesyats, 2004). In Townsend discharge space charge of a single avalanche does not distort the electric field in the gap. The dominating factor in the development of the discharge is the secondary electron emission. A description of regions of formation of different discharges can be made by using the parameter x cr , the critical length of an electron avalanche between an anode and cathode at which the field of the ion space charge is equal to the external field. Townsend discharge takes place for x cr >d, where d is the gap distance between the anode and the cathode. Streamer discharge takes place for x cr <d . Streamer discharges also require that a sufficient number of photons or electrons capable of further ionizing the gas molecules near the streamer head to be emitted. A third type of discharge called “multiavalanche discharge” can be defined for x cr <<d, however we will not get into details of it in this section (Mesyats, 2004). . 15 Assuming uniform electric field in a gap, the current I of the electrons arriving at the anode is given as € I = I o e αd 1−γ(e αd −1) (2.1) where d is the gap spacing; I 0 is the electron current from the cathode produced by some external source, γ is the number of secondary electrons per positive ion produced at the cathode and α is the ionization coefficient, defined as the number of ionizing acts of an electron per unit length. The criterion for discharge initiation can be obtained by equating the denominator of this expression to zero. Since γ <<1, the criterion takes the form γe αd ≈1. Using follwong semi-empirical Townsend formula € α = Ape −Bp/E (2.2) where A and B are constants based on experimental data, p is the pressure, and E is the electric field; and assuming γ is a constant, we can get the breakdown voltage in the form of € V breakdown = f pd ( ) : € V breakdown = Bpd ln(Apd) +ln(lnγ) (2.3) This relationship, which had been shown experimentally before Townsend theory, is called Paschen’s law. 16 Figure 7 Paschen Curve for variety of gases (Mesyats, 2004) Paschen’s law is a well known plasma physics law stating that the amount of potential difference required for a gas to breakdown is a function of pressure and gap distance product (pd). It was first stated by F. Paschen in his paper at 1889 (Paschen, 1889). Figure 7 shows a typical Paschen curve for several gases. “Paschen minimum” refers to the pd product where breakdown voltage is minimum: € (pd) min = e A ln 1 γ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ (2.4) High power plasma switches aiming to hold-off very high voltages are developed at both sides of Paschen curve, indicating either high or low-pressure range operation. 17 For the same breakdown voltage, the shape of Paschen curve leads to two different pd product values: one at the right-hand side and one at the left-hand side of the Paschen minimum. Pseudosparks and thyratrons operate at the left-hand side of the Paschen curve, whereas high-pressure spark gaps operate at the right-hand side. Figure 8 The curve separating the regions corresponding to the streamer and the Townsend mechanism of a discharge in air (Mesyats, 2004) Streamer discharge differs from Townsend discharge by the fact that the space charge of an avalanche can transform into a plasma streamer. The electrons of the avalanche ionize and excite gas molecules and atoms leading to photon emission that can eventually lead to photoemission. Photoelectrons together with runaway electrons from the primary avalanche can enhance the electric field inside the avalanche. It has been 18 shown that an overvoltage factor, κ over , can be used to identify regions of Townsend or streamer discharge initiation. A plot of the overvoltage factor versus pd product is shown in Figure 8 (Mesyats, 2004). If the discharge conditions are at the region above this curve, the the streamer mechanism occur; otherwise the Townsend mechanism takes place. 19 2.2 High Pressure Spark Gaps Self-breakdown, high-pressure spark gaps can be described as the simplest closing switches with two electrodes inserted in a high-pressure gaseous medium. Upon application of required overvoltage, transition of the gas from a good insulator to a good conductor (open to close) is achieved (Nunnally and Donaldson, 1990). The operating pressure for high-pressure spark gaps lies at the right-hand side of the Paschen curve. One of the factors that limit the repetitive operation capability of two electrode switches is the discharge channel through which the current flows. This channel has a high inductance that limits the pulse rise times. More importantly, the high current density in the channel leaves a strongly heated metal at the electrodes, which slows down the deionization of the plasma (Mesyats, 2004). Spark gaps can hold-off very high voltages (~100 kV) and currents (10-100 kA); however, arc-type high-density plasma operation not only limits the rise time and the repetition rate as mentioned but also the lifetime of the switch because of its erosive nature on the electrodes. (Schamiloglu et al., 2004). 20 In order to achieve a low delay and jitter, and to be able to synchronize spark gaps in series or in parallel, several trigger mechanisms can be used. Triggered spark gaps include trigatron spark gaps that utilize a trigger electrode coaxially placed at one of the electrodes, as well as electron beam triggered spark gaps and laser triggered gas filled spark gaps (Schaefer et al., 1990). Figure 9 Cross-sectional view of a trigatron (Hansjoachim, 2006) 21 2.3 Thyratrons Although the first idea for thyratrons started as early as 1918 (Langmuir, 1929), hydrogen thyratrons have emerged during World War II as an input switch for power modulators for radars. The hydrogen thyratron operates at the left-hand side of the Paschen curve and was developed by Germeshausen (Kirkman and Gundersen, 1990). Figure 10 Geometry of a hydrogen thyratron (Pirrie and Menown, 2000) 22 A schematic of a typical hydrogen thyratron can be seen in Figure 10. Upon application of a positive trigger pulse to the grid, plasma formation is achieved in the grid-cathode region. This leads to closing of the switch by initiating the breakdown in the grid-anode region and later connecting these two regions of plasmas. Recovery of the switch is usually achieved by starting deionization by keeping the anode voltage slightly negative. Thermionic cathodes which require external heating of the cathode are used to increase the peak current levels (Creedon, 1990). This external heating requirement is absent for similar peak current levels of pseudosparks. Early thyratrons used mercury vapor as filling gas, which was replaced by hydrogen because of two big advantages. First, cathode destruction voltage for hydrogen is much higher which improves hold-off voltage and lifetime parameters. Second, hydrogen has a much lighter ionic mass allowing a smaller recovery time and a higher repetition rate. However, hydrogen has the disadvantage of being a chemically active atom, which causes unwanted impurities in the gas. Hydrogen reservoirs containing metal hydrides that lead to a reversible reaction with hydrogen as a function of temperature are the solution to this problem. These reservoirs utilize a heater to control the mentioned reversible reaction (Pirrie and Menown, 2000). Later, many improvements like ceramic envelope and multi-gap thyratrons lead to very high power ratings with hydrogen thyratrons. One of the most important advantages of hydrogen thyratrons is their long lifetimes. This is partly due to glow discharge operation rather than an arc as with the spark gaps as a result of the fact that they operate at the left-hand side of the Paschen 23 curve. Glow discharge operation causes less electrode erosion because of its diffused nature. Although thyratrons could not achieve the high peak currents of spark gaps, they are still very widely used because of their very high repetition rate capabilities and long lifetimes in many repetitive pulsed power applications. 24 2.4 Pseudospark Switches Pseudosparks are low-pressure plasma switches similar to thyratrons but with a much higher peak current capability and a higher current density operation. Because of special properties of the pseudospark discharge, which can prevent arcing at very high current densities, these switches have lifetimes comparable to the thyratron. Pseudosparks when first invented (Christiansen and Schultheiss, 1979) were regarded as intense electron beam generators. Later they attracted attention as high power switches with short current rise times as well as high current peaks and long lifetimes. Moreover, low pressure (left-hand-side of the Paschen curve) operation provides short recovery times and therefore facilitates high repetition rates (Christiansen and Hartmann, 1990). Pseudosparks have a special axisymmetric hollow cathode-hollow anode structure. At low pressures, the mean free path of electrons – defined as the average distance a particle travels without making any collisions - is very long compared to the distance between the electrodes. Therefore, most of the electrons released at the cathode arrive to the anode without undergoing any ionizing collisions. This is the reason for increased breakdown voltage at low pressures at the left-hand side of Paschen curve. However, hollow geometry of the electrodes provides other possible lengths of discharge paths between anode and cathode thus decreasing breakdown voltage on the axis of 25 switch geometry. (Christiansen and Hartmann, 1990). Figure 11 shows electric field lines at the axis of hollow cathode- hollow anode geometry. Physics of discharge initiation and development will be discussed more in detail in section 2.4.2. Figure 11 Electric Field Lines on the axis of a typical pseudospark geometry (Riege and Boggasch, 1989) 2.4.1 Triggering Mechanisms Triggering is provided by introduction of charge carriers to the cathode hole and the cathode backspace of the pseudospark. Triggering by means of surface discharge, pulsed low-current glow discharge, and optical energy incident on the cathode are the most widely used methods (Frank and Christiansen, 1989). 2.4.1.1 Pulsed Low-Current Glow Discharge Triggering A typical example for this type of triggering is shown in Figure 12. A cylindrical cage forming a hollow cathode separates trigger module and main switch. Pulsed hollow cathode discharge is generated inside the trigger module by applying a negative pulse to 26 the trigger electrode. This low-current discharge initiates the breakdown in the main switch by penetrating through small holes on the cage. A blocking potential is used to accelerate the propagation of the plasma to the main switch when set to zero volts, as well as recombination of the ions and electrons when set to a positive value (Kozlik et al., 1989). Figure 12 Pulsed Low-current Glow Discharge Triggered Pseudospark (Frank and Rath, 1990) 2.4.1.2 Surface Discharge Triggering: Surface discharge triggering is based on electron emission from an insulator surface. Figure 13 shows a typical geometry for the surface discharge triggering. A trigger electrode embedded between two insulator discs is inserted into the hollow cathode region of the main switch. A high voltage pulse is applied to the trigger electrode to obtain emission. Insulators are required to have low breakdown voltages and long lifetimes for good switching parameters (Frank et al., 1988). Surface discharge triggering provides high peak currents and limited repeatability, whereas glow discharge 27 triggering is more repeatable with moderate peak currents (Bochkov et al., 2001). This type triggering is also known for very low jitter and independence of its delay and jitter from trigger pulse character (Frank and Rath, 1990). Figure 13 Surface Discharge Triggered Pseudospark Geometry (Gortler et al., 1989) 2.4.1.3 Optically Triggered Pseudospark (BLT) Back-lighted thyratrons (BLTs) are pseudospark switches with optical triggering (Gundersen and Schaefer, 1990; Kirkman and Gundersen, 1990) and they are triggered by photoemission using a beam of light incident on the back of the cathode in the central region of the switch. A simple geometry of a BLT can be seen in Figure 14. UV light sources like flashlamps or lasers can be used. Optical triggering has advantages of complete electrical isolation of trigger source, simple switch structure and reliable low energy triggering. Moreover, ability to trigger using fiber-optics is useful for perfect synchronization of several switches in parallel or in series (Kirkman and Gundersen, 1990). 28 Figure 14 Back-lighted Thyratron (Gundersen and Schaefer, 1990) As an optically triggered version of pseudospark switches, back-lighted thyratrons (BLTs) offer additional advantages in electrical insulation and flexibility of the triggering system. They are attractive candidates for pulse generators comprised of multiple switches, such as a Marx bank. Moreover, electrical insulation between the switch and the trigger has made it possible to design an ultra-compact switch, the mini-BLT, with 20 times smaller volume than a traditional commercial switch with comparable voltage and current rating. In addition to their high power (40 kV) and high current (4.5 kA) capabilities, mini-BLTs have reasonable lifetime (>10 8 shots) at medium current operation (several kilo amperes) (Jiang et al., 2005b). 29 Figure 15 Mini BLT and a commercial pseudospark (Jiang et al., 2005a) One of the main difficulties of the more widespread use of BLTs is their triggering requirement. Triggering of these switches with commercially acceptable delay and jitter parameters requires UV lasers that can be bulky and costly for compact pulsed power applications. 2.4.2 Physics of Pseudospark Discharge Temporal development of the pseudospark discharge is well established in the literature. It involves two main phases after initiation, hollow cathode phase and high current superemissive cathode phase (Stetter et al., 1995; Hartmann and Lins, 1993; Stark et al., 1995). 30 Hollow cathode phase is responsible for rapid current rise of the discharge due to the hollow cathode effect. During this phase, discharge is homogenous at the axis of the structure with a radial size equal to the bore-hole radius. Couple of tens of nanoseconds later the superemissive cathode phase starts by a change in electron emission mechanism to field enhanced thermionic electron emission. This provides a self-heated cathode process without any external heating. During this phase, discharge expands radially between the cathode-anode gap. High peak currents are due to this phase of pseudospark discharges (Stetter et al., 1995; Stark et al., 1995) 2.4.2.1 Initiation and Hollow Cathode Phase Numerical models developed by Pak and Kushner(Hoyoung and Kushner, 1989, 1990); and Boeuf and Pitchford (Pitchford et al., 1995; Boeuf and Pitchford, 1991) were used to analyze the switch delay parametrically for the initial stages of the switch operation: initiation (also referred as Townsend discharge phase) and the hollow cathode phase. Pak and Kushner investigated a numerical BLT model by looking at the dependence of switch delay on pressure, geometry, and initial photon density. They developed a numerical model based on finite differences to solve the continuity equations for electron and ions. The source term for the ion and electron continuity equations initially comes from photo-emitted electrons generated at the cathode (assuming a constant quantum efficiency of 2×10 -7 ), later the source term is calculated via combined 31 photoemission due to electron impact excitation of the gas, and the secondary emission due to ion flux on the cathode. The time-dependent Poisson’s equations are used to update the electric field distribution at each time step and the resulting field is used to obtain transport coefficients (drift velocity, ionization coefficient, and diffusion coefficient) as a function of E/N (electric field/ gas number density). Figure 16 Formation of virtual anode (Boeuf and Pitchford, 1991) Simulation results showed shorter switch delays with increasing optical fluence, pressure and anode-cathode space, and identified the virtual anode formation as a cathode anode 32 necessary condition for triggering. Although these trends well-agree with experimental findings, this model did not take into account the high energy tail of electron energy distribution that effects the ionization events in nonequilibrium conditions like hollow cathode phase and thus has an important role in the commutation of the switch (Hoyoung and Kushner, 1989). They later integrated a multi-beam model for electrons to represent nonequilibrium effects and observed the hollow cathode initiation through this model. However parametrical analysis of implications of this model on the triggering was not studied in detail (Hoyoung and Kushner, 1990). Figure 17 Dependence of anode-delay time on gas pressure and anode-cathode separation. The delay time decreases with increasing gas pressure and decreases with larger electrode separation. The dashed line is the constant pd below which satisfies Paschen’s criterion at ~10 kV. The das-dot line is f(p,d) above which defines the closure region. The optimum operating points lie between these lines at values of p and d for which the shortest anode-delay times may be obtained (Hoyoung and Kushner, 1989) 33 Boeuf and Pitchford used a hybrid fluid-particle model first to identify different stages of switch operation (Boeuf and Pitchford, 1991), and then to parametrically study switch delay (“time to breakdown”) (Pitchford et al., 1995). The model used time and space dependent charged particle densities determined by fluid equations; and then the space charge densities and the electric field distribution computed by Poisson’s equation to generate the particle current leaving the cathode. The particle current and the electric field distribution are used to find the ionization source term via a Monte Carlo simulation. Monte Carlo simulation is helpful to model the electron energy distribution more realistically by randomly choosing the locations where electrons emitted from cathode, according to the spatial distribution of the flux from the cathode deduced from the fluid model. Fluid model represents the electron population by mean values of velocity, energy and density, this assumption is sufficient to compute electric field distribution; however high energy tail of the electron population that can not be represented by mean values are not accounted for, which makes an important difference in ionization source term. Monte Carlo simulation does not make any assumptions on form of energy distribution function. This, in turn, allows a more realistic representation of nonequilibrium effects such as hollow cathode effect that occur during pseudospark discharge. This model does not take into account any photoionization -by plasma emitted photons- (previously shown to be negligible by Pak et al.) or thermionic effects, thus only used to model the initial two stages of pseudospark discharge where commutation occurs. 34 Figure 18 Fundamental equations used in fluid model: (1) is used to get velocities for continuity equations (2) and (3), (4) is the Poisson’s equation used to compute electric field, S is the ionization term computed as a result of Monte Carlo simulation Identified stages of pseudospark discharge by Boeuf and Pitchford are (1) Townsend Discharge, (2) Plasma Formation, (3) Onset of Hollow Cathode (4) Plasma Expansion and Current Runaway, (5) Enhanced Electron Emission from the Cathode. These stages agree well with experimental work on temporal development published later (Stark et al., 1995). 35 Figure 19 Time to breakdown versus initial charge density as modeled by Pitchford et al. (Pitchford et al., 1995) The trends of dependence of time to breakdown on gas pressure, applied voltage, switch geometry and initial charge density are in good agreement with experimental results. This model also showed that there is a critical charge density to initiate the switch commutation that is also a function of the location the initial charge density was introduced. Below this critical value if plasma forms, it never grows to reach the cathode and form virtual anode, thus the field distortion is insufficient to increase the ionization rate above the electron loss rate. Also, they observed a minimum or saturated value of time to breakdown, which is limited by the number of electrons that can be extracted from the hollow cathode during an ion transit time in the main gap. Figure 16 shows the 36 formation of the virtual anode, which is a critical stage for switch commutation to occur. Figure 19 shows change in delay around the critical charge density (5 × 10 9 cm -3 for this model). 2.4.2.2 Superemissive cathode phase Homogenous nature of pseudospark discharge while reaching very high current densities comparable to arc discharges (up to 10 4 A/cm 2 ) has the very important advantage of less erosion of switch electrodes thus increasing the lifetime of the switch. However the physics of the mechanisms that leads to these high current densities with a glow type discharge is not trivial for an originally cold cathode like in pseudospark switches. Hartmann and Gundersen has analyzed electron emission mechanisms that can actually lead to such a high current density discharge with a cold cathode and concluded some type of thermionic emission has to take place and this is possible at the very thin surface layers of the electrodes by ion bombardment (Hartmann and Gundersen, 1988). Figure 20 Field-enhanced thermionic emission from many small spots (Anders et al., 1994) 37 Work by Anders et al. suggested that vacuum arc-like cathode spots spread across the surface of the cathode are more likely to explain the high current densities observed (Anders et al., 1994). They showed that the power density of homogeneous ion bombardment heating in the hollow-cathode phase is too small to create observed plasma density. Their model suggested “with increasing bulk plasma density, the thickness of the cathode sheath decreases until the homogeneous electric surface field ignites simultaneously many cathode spots, leading to the transition to the high-current phase”. Other experimental evidence on erosion rates of the pseudospark electrodes also supports this vacuum-arc-like behavior model. In pseudosparks these explosive processes are believed to take place over a large area where the high electric field is sustained by virtual anode (plasma sheath) at the proximity of the cathode. Thus, the appearance of the plasma is homogenous due to macroscopically larger area of the many cathode protrusions. Figure 21 shows delay time for cathodic microprotrusions measured with a tungsten cathode (Anders et al., 1994). Anders et al. also suggested a correction to terminology of high-current density phase of pseudospark, calling it “superemissive cathode” instead of “superemissive glow” as was originally called by Hartmann et al. (Hartmann and Gundersen, 1988). In their subsequent paper they further added a self-sustained sputtering mechanism of the cathode to the picture based on the new experimental data (Anders et al., 1995). 38 Figure 21 Time dependence of explosions in vacuum arc cathode spots (Anders et al., 1994) 39 Chapter 3 Photocathode candidates and Photoemission 3.1 Introduction Figure 22 Potential diagram of a clean metal (Jenkins, 1969) At the surface of a solid, there is a potential barrier that prevents the escape of electrons unless they can be given enough energy. The simplest case is a clean metal whose potential diagram is shown in Figure 22. The distribution of electron energies in 40 the metal obeys the Fermi-Dirac distribution. According to Fermi-Dirac distribution, at absolute zero temperature, there are no electrons with energies above the Fermi level. The height of the potential barrier above the Fermi level at the surface is called the work function ϕ (usually given in electron volts, contracted to eV). When the metal is heated the Fermi energy distribution is modified (Figure 22), so that some of the electrons have enough energy to escape over the potential barrier into the vacuum. This form of electron emission is called thermionic emission (Jenkins, 1969). Figure 23 Potential diagram of (a) a clean metal (b) a metal coated with electropositive monolayer or (c) a metal coated with electronegative monolayer (Jenkins, 1969) Electrons can also escape by other means like absorption of optical energy from photons (photoemission) or quantum mechanical tunneling as a result of bending of the potential barrier due to electric field (field emission). Bending of the potential barrier can also be due to thin surface coatings, facilitating the emission. Thermionic cathodes take advantage of this mechanism to reach very low work functions. For example, a clean tungsten surface has a work function of 4.54 eV while cesium coated tungsten’s work 41 function can be as low as 1.5 eV. This effect can be at the opposite direction (poisoning) depending on the type of the surface layer. Figure 23 shows potential diagrams of a clean metal surface with electropositive and electronegative surface monolayers: in other words, poisoned or lower work function coated surfaces respectively. Suitability of any given cathode material for certain type of emission is not only a function of its work function, but also operating temperature, field and other material properties that affect the electron emission and lifetime of the cathode. As BLTs operate mainly by emission due to optical energy from their cathodes, we will discuss some of the well-known cathodes and their photoemission performances after an introduction to photoemission theory. 42 3.2 Photoemission 3.2.1 Spicer’s three step model Figure 24 Illustration of Spicer’s three-step model of photoemission on an energy band diagram of a semiconductor (Spicer and Herrera-Gomez, 1993) Spicer’s three-step model defines photoemission as a process that can be represented by product of factors governing absorption of light energy on the solid surface, transport of the excited electrons and the escape of electrons from the surface with sufficient energy. This three-step process creates an easy-to-follow physical picture of photoemission based on bulk absorption coefficient, reflectivity, scattering length of the target material. This model has been widely used to compare photoemission processes 43 of semiconductors versus metals including the response times. However, it does not account for thermal or quantum mechanical effects. A derivation of quantum efficiency (QE, ratio of number of emitted electrons to absorbed photons) using Spicer’s model (Spicer and Herrera-Gomez, 1993) results in following expression: € QE = P E (hv) (a PE / a) (1+l a /L) (3.1) where α PE /α is the fraction of electrons excited above vacuum level (step1, α PE represents the part of the absorption where the electrons are excited above the vacuum level and have a possibility to escape), l α (hv ) = -1/α(hv) is the absorption length, l α /L is the ratio of absorption length to scattering escape length (step 2), P E (hv) is the probability of escape of electrons reaching surface with sufficient energy to escape (step 3, accounting for backscattering effects). Figure 24 shows the three steps for a semiconductor together with representative drawings of distribution of electrons at each step. 3.2.2 Jensen’s work Motivated to model photoemission from coated thermal dispenser cathodes, Jensen et al. developed a model to represent electron emission as a result of combined photoemission and thermal emission due to heating of the electron gas (Jensen et al., 2003). 44 This photoemission model closely follows modified Fowler-Dubridge (MFD) model. MFD model, like Spicer’s model, uses an approach to define total emission as a product of factors. However, it differs from Spicer’s three-step model is in its usage of different scattering factors, emission, probability estimates, and an explicit focus on temperature-related modifications. Using Richardson approximation for transmission probability, an expression for current density due to photoemission can be obtained: € J λ =q(1−R) I λ ω × U[β T (ω −φ)] U[β T µ] ⎧ ⎨ ⎩ ⎫ ⎬ ⎭ (3.2) where , J λ is the electron current density, I λ is the laser intensity, R is the reflectivity, q is the electron charge, β T =1/k B T with k B Boltzmann’s constant, and T is the temperature, µ is the chemical potential or Fermi level, ϕ is the effective work function defined as € φ =Φ− 4QF with Q=q 2 /16πε 0, ε 0 is the permittivity of free space and F is the product of electric field and electron charge, ħω is the photon energy, U(x) is the Fowler- Dubridge function and the factor containing U functions is an estimate of emission probability. The quantum efficiency at its asymptotic limits, where € β T (ω −φ) is large, thus for low temperature and short wavelength optical source, approaches to: € QE =(1−R) ω −φ µ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 2 (3.3) 45 This form of QE corresponds to widely used quantum efficiency formulation in metal photocathode literature (Srinivasan-Rao et al., 1995; Nakajyo et al., 2003). Jensen et al.’s following work proposed modifications to MFD-based approach that accounts for quantum mechanical reflection and tunneling and involves a more realistic evaluation of scattering factors. As a result, a moments-based approach was proposed as a more accurate theoretical model of photoemission, especially for cathodes with partially coated surfaces instead of MFD based approach (Jensen et al., 2007). As we will only work on uncoated metal cathodes without any heating, details of this more complex model will not be given here. 46 3.3 Photocathodes 3.3.1 Introduction Photocathodes have been mostly studied for short duration electron beam generation for free electron lasers (FELs) and other particle accelerators. The control of the electron beam duration by nano, pico or femtosecond laser pulses is advantageous for these applications where the beam width is critical. Although thermionic cathodes have been used in the past for such applications the controllability of the pulse width of the electron beam makes photocathodes only viable option today. Free electron lasers require long-lived, reliable, capable of producing nano-Coulomb electron bunches in picosecond timescales (Jensen et al., 2003). Some of these requirements are common with what is needed to trigger a BLT type switch. BLT requires a high enough charge density (~nC) in nanosecond timescales (not limited by picosecond response times like FELs) and of course, cathodes that can live through reasonable lifetimes (~10 7 shots). However, the operating environment of BLTs has two major differences from FELs, (1) the gas and pressure environment: BLTs operate in 100s of mTorr helium or hydrogen whereas FELs operate around 10 -7 Torr vacuum, (2) the accelerating fields: for FELs can be quite large (~10-100 MV/m) compared to fields at the cathode of a BLT (~10 kV/m). 47 We used the published photocathode literature as a guide to determine most promising candidates for cathodes for BLT and then tested their photoemission efficiency under BLT-relevant operating gas and pressure environments. 3.3.2 Photocathode selection Common high vacuum photocathode materials can generally be divided into two categories: semiconductor and metal. Recent reviews of photocathodes (Kong et al., 1995; Nation et al., 1999) have shown semiconductor cathodes have higher QE and lower work function, however they suffer from short lifetime (up to hundreds of hours) and high sensitivity to contamination. K 2 CsSb, Cs 3 Sb, and cesiated GaAs require ultra-high vacuum conditions and even then have short lifetimes of tens of hours. K 2 CsSb and Cs 3 Sb photocathodes are reported to require to be renewed almost on a daily basis. Cs 2 Te cathodes look advantageous compared to other semiconductor cathodes with a longer lifetime of hundreds of hours. However they require UV irradiation, and are still prone to contamination, especially if exposed to air. These qualities make semiconductor cathodes too difficult to maintain for a plasma environment at few hundreds of Torr pressure range as with BLTs. Metal photocathodes, such as magnesium and copper, are attractive because of their fast response time, ease of preparation, relative insensitivity to contamination, and the long lifetime (orders of years). However, their high work function usually requires intense UV irradiation of the cathodes to obtain reasonable electron yield. 48 Table 1 Approximate best quantum efficiencies of common photocathodes used for FELs together with measurement conditions and lifetimes (Cultrera et al., 2005; Nation et al., 1999; Kong et al., 1995; Wang et al., 1996) Material Wavelength (nm) QE Pressure (Torr) E field (MV/m) Lifetime Cs 2 Te 263 13% 10 -9 -10 -10 >20 T 1/2 >100 h CsI 209 2% 10 -9 -10 -10 T 1/2 >150 h K 2 CsSb 527 8% 10 -9 -10 -10 >20 T 1/2 <4 h LaB 6 355 0.1% <10 -7 ~1 day Cs 3 Sb 527 4% 10 -9 -10 -10 >20 T 1/2 <4 h Mg 266 0.2% <10 -7 90 > year Cu 266 0.05% <10 -7 >100 > year Some thermionic cathodes including BaO, Sc 2 O 3 ,CaO, Al 2 O 3 based thermionic dispenser cathodes have been tested for their photoemission performance because of their lower work function (Zhang et al., 2010; Leblond, 1992). The highest quantum efficiencies obtained with these cathodes are in the order of 10 -4 , which is in the order of that of Mg and Cu. However, these dispenser cathodes require continuous (or some type of) heating for the reported QE performance to be reliably sustained, which makes them unattractive compared to metal cathodes. 49 Figure 25 Collected photoemission current from a porous tungsten matrix containing Sc 2 O 3 , BaO,CaO and Al 2 O 3 dispenser cathode (a) after stopping heating (b) change with temperature (Zhang et al., 2010). (The optical source wavelength is 266 nm.) In the case of BLTs, long lifetime is critical and required light energy can be as low as 10 µJ with reasonable delay and jitter times (Braun et al., 1988). As a result, a high-yield metal photocathode can improve switch’s triggering parameters while maintaining its performance for years. Table 1 lists the approximate best quantum efficiencies reported for various photocathodes. 50 3.4 Photoemission measurements of photocathode candidates for BLT 3.4.1 Introduction Photoemission measurements under BLT-relevant pressure and field conditions are an important first step to estimate the potential of any photocathode for BLT. Magnesium, and copper as the most commonly utilized metal photocathodes; and molybdenum as the most common BLT cathode were chosen to be studied under mentioned conditions. Quantum efficiency measurements of high-purity thin foils made up of Mg, Cu and Mo were conducted under controlled helium pressure and varying DC electric field. 3.4.2 Experimental Setup A schematic of the experimental setup together with the measurement circuit is shown in Figure 26. It consists of a vacuum chamber and a vacuum system including an ion pump and backed up with turbo and rotary vane pumps. Inside the chamber, photocathode samples are placed on a 45 degree sample holder facing the incident laser beam. The sample, fixed on the top of the sample holder, is at ground potential. A 9.5 mm diameter, 12.7 mm long cylindrical Faraday cup, was placed ~10 mm above the 51 sample. Current measurements are done using a Tektronix DPO4104 oscilloscope by measuring the voltage across a 50 Ω resistor. The measurement circuit is a high-pass filter configuration, which is designed to pass the fast photoemission electrons generated by 5 ns full-width at half maximum (FWHM) laser pulses. The bias voltage can be varied between 0-550V DC. A frequency quadrupled Nd: YAG laser (λ = 266 nm) at a repetition rate of 10 Hz and a pulse width of 5 ns (FWHM) was used as the light source. The average incident energy per pulse on the cathode is 54.9 µJ. The laser beam was directed into the chamber without any focusing. The spot diameter on the sample surface was measured to be 3 mm. Figure 26 Experimental setup for quantum efficiency measurement of photocathode materials. Photocathode samples of 0.25 mm thickness are used for Mg (99.9% purity, Alfa Aeser), Cu (%99.9985 purity, Alfa Aeser) and Mo (99.95% purity, Alfa Aeser). Each sample was polished with 1200-grit SiC sand paper, and ultrasonically cleaned for 30 minutes in hexane bath. Samples were unavoidably exposed to air for several minutes 52 during the transfer to the vacuum chamber. After the transfer, they are baked inside the vacuum chamber at 120 °C at a base pressure of 3 x 10 -7 Torr for 2 hours. Samples were then irradiated with 54.9 µJ energy laser pulses at 266 nm for an hour before conducting photoemission measurements. 3.4.3 Results Figure 27 (a) and (b) show typical current waveforms for magnesium at 3.6×10 -7 Torr and for molybdenum at 6×10 -7 Torr when the collector is biased at 550 V, respectively. The fall time of both of the negative current pulses (rise time of the electron emission) are 5 ns, which corresponds to the laser’s pulse width. Quantum efficiency is calculated by integrating the current pulse over time up to the point it rises back to zero level to obtain the total electron charge emitted, and then taking the ratio of number of electrons generated to number of photons according to equation (3.4), where I is the current pulse, e is the electron charge, E opt is the average optical energy incident on the sample, h is the Planck’s constant and ν is the light source frequency. The average laser energy incident on the sample was measured using a Si photodiode based laser powermeter to be 54.9 µJ, this value is used for all QE calculations. € QE = I dt /e ∫ E opt /hv (3.4) 53 Figure 27 Photo-induced current pulse recorded by a 550 V-biased Faraday cup (a) for a magnesium sample at 3.6×10 -7 Torr; (b) for a molybdenum sample at 6×10 -7 Torr. The change in quantum efficiencies of Mg, Cu and Mo samples with changing pressure were measured by increasing the background He pressure in the range of 3×10 -7 to 0.1 Torr. The upper limit of the pressure range of our quantum efficiency measurement experiments at 0.1 Torr was chosen as a convenient vacuum level for BLT 54 operation and is orders of magnitude higher than vacuum conditions for the reported QE values for these materials in the literature as reported in Table 1. The results with the collector biased at 550 V, are shown in Figure 28. Data points are the average of QE values calculated according to equation (3.4) using five electron emission measurements at each pressure point. Error bars show the range of these measurements. Figure 28 Pressure dependence of the quantum efficiency for Mg, Cu and Mo with the collector bias voltage of 550 V. Magnesium has the highest quantum efficiency among all three samples, up to 1.5×10 -5 at 0.1 Torr. Its QE increases from 1.34×10 -5 to 1.45×10 -5 with increasing pressure for the range of 3×10 -7 to 1×10 -3 Torr. For pressures higher than 1×10 -3 Torr, we have measured the QEs of 1.4×10 -5 and 1.5×10 -5 at 1×10 -2 Torr and 0.1 Torr, respectively. The quantum efficiency of Cu shows similar pressure dependence to that of 55 Mg with the highest QE value of 1.4×10 -5 at 0.1 Torr. The QE of molybdenum increases by 9% with the pressure increasing from 3×10 -7 to 1×10 -4 Torr. At 1×10 -4 Torr it reaches a maximum value of 1.2×10 -5 . For pressures higher than 1×10 -4 Torr, the QE decreases with increasing pressure. A quantum efficiency of 1×10 -5 at 0.1 Torr was measured. Additionally, we are in the process of conducting more QE measurements of these three metal photocathodes for the pressures higher than 1×10 -3 Torr to further analyze the pressure dependency of the quantum efficiency. Figure 29 shows quantum efficiency of magnesium as the bias voltage at the collector changes from 50 to 550 V at 3.6×10 -7 Torr. Since the distance between the biased collector and the sample surface is 1 cm, we can approximate the electric field on the sample surface to be changing from 50 V/cm to 550 V/cm using parallel plate approximation. Quantum efficiency increases from 1.5×10 -6 to 1.3×10 -5 in this electric field range. Figure 29 Change in quantum efficiency of magnesium with changing collector bias voltage at a background pressure of 4.8 x 10 -5 Pa (3.6×10 -7 Torr). 56 3.4.4 Discussion Implementing Mg or Cu photocathodes into BLTs has a good potential of improving the performance of BLTs. At 0.1 Torr, Mg and Cu photocathode samples show a QE of 50% more than that of molybdenum which is the most common BLT cathode material. Over the entire pressure range, Mg showed the best QE performance, most likely due to its relatively low work function. The increase in QE values with the increasing pressure for Mg and Cu may be due to laser induced low-pressure glow discharge current generated near the sample surface. As the gas pressure increases, so does the laser- induced discharge current. For Mo photocathode, we propose that the same laser energy is not able to initiate a gas discharge at the surface of the sample. This may be due to the relatively high work function combined with the effect of thicker oxide layer on the Mo surface. Due to its hardness, the Mo sample was not as easily polished as the Cu and Mg samples. This relative increase in surface roughness likely resulted in a relatively thicker oxide layer (consistent with (Roy et al., 1999)). Additionally, removal of oxide layer with laser cleaning for Mo is expected to be more difficult due to higher bond disassociation energy of Mo-O compared to Cu-O and Mg-O (Haynes and Lide, 2011). Moreover, our results indicate that quantum efficiency of the photocathodes can be easily increased using higher electric fields regions within the BLT geometry. The electric field dependence of QE, as described in Nakajyo (Nakajyo et al., 2003), is expected to be proportional to square of the difference between incoming photon energy (hv) and the cathode effective work function (ϕ) as in Equation 1: 57 € QE α (hν −φ) 2 (3.5) where the effective work function is modified by the applied electric field E according to the Schottky effect, € φ =φ 0 −b βE (3.6) The constant € b = e 4πε 0 , and b is the local field enhancement factor due to micro-geometry on the cathode surface. Thus, the QE is most sensitive to variations in the electric field on the cathode surface when the photon energy is very near the cathode work function. The linearity between square root of QE and square root of electric field for magnesium at 3.6×10 -7 Torr is shown in Figure 30. Parallel plate approximation is used in this figure to calculate the E field values (50 V to 550 V at 1 cm distance). Figure 30 Relationship between square root of QE and square root of electric field for magnesium at 3.6×10 -7 Torr 58 3.5 Chapter conclusions A potential diagram (energy band diagram) of a clean metal was used to introduce the concept of work function of a solid surface. Work function is defined as the potential barrier between the Fermi level of a solid to the vacuum level and can be modified due to surface impurities. Photoemission models by Spicer et al. and Jensen et al. are introduced (Jensen et al., 2003; Spicer and Herrera-Gomez, 1993). Jensen et al. showed that an asymptotic approximation of quantum efficiency for short wavelengths and low temperature for uncoated metals was shown to be: where R is the reflectivity, ħω is the photon energy, the chemical potential or Fermi level, ϕ is the effective work function defined as € φ =Φ− 4QF with Q=q 2 /16πε 0, ε 0 is the permittivity of free space and F is the product of electric field and electron charge. Photocathode literature, mostly motivated to be utilized for FELs or other accelerators, was briefly reviewed. The operation conditions of cathodes for FEL-type applications have two main differences than of a BLT cathode: (1) The pressure range of BLT operation is orders of higher gas pressure, thus involve ! QE =(1"R) !# "$ µ % & ' ( ) * 2 59 interaction with plasma (2) Electric fields are significantly smaller for BLT cathodes. The most promising cathodes for an environment like BLTs were chosen to be metal photocathodes Mg and Cu from this review. Mg, Cu and Mo high-purity foils are tested for their photoemission performance in a pressure range of 10 -6 to 0.1 Torr He at 550V/cm field. Mg showed the highest quantum efficiency in the order of 10 -5 at 0.1 Torr. 60 Chapter 4 BLT with Photocathodes 4.1 Introduction Literature on photocathodes and the results of quantum efficiency measurements of photocathode candidates for BLTs have shown magnesium’s potential as a promising cathode candidate for BLT with its ease of maintenance and relatively low work function and higher quantum efficiency among metal photocathodes. In this chapter, delay and jitter measurements of a BLT with Mg, Cu and Mo-based cathodes are presented with a low optical intensity condition, where the triggering light source was utilized in its unfocused form. Subsequently, delay and jitter performance of a BLT with copper electrodes is compared before and after integration of a high-purity Mg foil at its cathode using an order of magnitude increase in optical intensity of the light source by partially focusing the laser beam on the cathode surface. Partial focusing of the light source allowed the investigations to be performed under a wider range of switch voltage and gas pressure conditions. 61 4.2 BLT triggered by unfocused laser beam 4.2.1 Experimental Setup and Procedure The experimental setup with a cross-sectional view of the switch and the schematics of electrical circuitry are shown in the Figure 32. The outer and inner diameters of the electrodes are 19 mm and 17.4 mm respectively. Diameters of the central bore holes on the planar surfaces of both the anode and cathode, the thickness of the planar surfaces around the bore holes, and the anode- cathode separation are all 3 mm each. A Pyrex glass envelope is used for insulation and the vacuum sealing of the structure. Figure 31 BLT used in the experiments 62 A compact frequency-quadrupled Nd:YAG laser (wavelength of 266 nm), with 3 mJ energy per pulse and 3 mm diameter spot size measured at the switch cathode is used as optical source for triggering. The pulse width of the laser pulse is 5 ns (FWHM). The optical setup includes a mirror with high reflectivity at 266 nm wavelength to direct the laser light to the switch cathode through a sapphire window. The timing of the laser light is measured using a UV sensitive photodiode. Figure 32 Experimental setup with the cross-sectional view of the switch structure: the inset shows the cathode configuration with molybdenum-cap press-fit. Three different cathode configurations are used for the delay measurements. First one is a molybdenum-based cathode replicating the most commonly used configuration for BLT cathodes in the literature. The inset of Figure 32 shows the Mo-cap press-fitting onto the copper electrode. Second one is the bare copper cathode configuration where 63 the cathode is identical to the anode, machined from oxygen-free solid copper. The third kind of cathode configuration is similar to the bare copper cathode however utilizing a high purity Mg foil of 0.25 mm thickness, shaped to fit at the back of the cathode with full contact to the cathode back surface, as shown in Figure 32. The Mg foil (%99.9 purity, Alfa Aeser) is polished with 1200-grit SiC sand paper and ultrasonically cleaned in hexane for 30 minutes before being placed in the switch. Figure 33 Experimental Setup 64 Helium is used as the operating gas. The pressure range for the reported data is from 0.6 Torr to 1 Torr. Switch voltage is adjusted to the 65% of the hold-off (self- breakdown) voltage at each pressure as a reliable operating point below self-breakdown. Figure 34 (b) shows the hold-off voltage values for the pressure range of the delay measurements. The delay is defined as the time between the rising edge of the photodiode signal to the voltage rise across the switch indicating the switch closure. Figure 34 (a) shows the typical photodiode and switch voltage signals and the dashed arrow indicates the delay. Figure 34 (a) Definition of delay, arrow shows the time between the laser light hitting the cathode to the switch closure (b) Hold-off voltage vs. helium pressure 65 Figure 35 BLT operating with He plasma 4.2.2 Results Figure 36 shows the delay and jitter for three cathode configurations measured at three pressures with corresponding switch voltages of 65% of hold-off voltage at each pressure. The pressure and switch voltages are listed together in the figure. The delay values show average of 15 measurements. 66 Figure 36 The switch delays measured at three different helium pressure and switch voltage combinations The shortest delays for each cathode configuration, all at 0.8 Torr operating pressure and 8.5 kV switch voltage, are measured to be 140 ns, 4.2 µs and 11.2 µs for Mg foil, bare Cu and Mo-cap cathodes respectively. The jitter for Mo and Cu electrodes is around 1 µs and for Mg cathode around 10 ns throughout the pressure range. The cathode configuration with magnesium foil has more than an order of magnitude shorter delay and jitter compared to both bare copper and molybdenum-cap cathodes. The delay with Mo-cap cathode is the highest among the three configurations. 67 4.2.3 Discussion The delay is observed to increase in the following order of cathodes: Mg foil, bare Cu and Mo-cap cathode. This is in agreement with the quantum efficiency measurements of the same materials under BLT-relevant conditions, reported earlier (Sozer et al., 2009). Reliable and consistent triggering of the switch was not achieved at low pressures (<0.6 Torr) or at switch voltages below 45% of the hold-off voltage at a given pressure, except for the cathode configuration with Mg foil where triggering was achieved down to 0.5 Torr. Partial focusing of the laser light, thus increasing the optical power density approximately ten times (from 8.5×10 6 to 7.4×10 7 W/cm 2 ), has changed this behavior allowing triggering the switch at lower pressures (<0.4 Torr) and at switch voltages down to 15% of the hold-off voltage. This effect can be explained by a combination of increased charge density due to smaller spot size with the same total optical energy, and a possible small contribution of thermionic emission due to laser heating. However, even when the light is partially focused the effect of emission due to laser heating is expected be at least an order of magnitude smaller than the photoemission. These observations show clear benefit of utilizing Mg foil at the switch cathode not only for shorter delay but also for flexibility in operating parameters like gas pressure and switch voltage with lower optical power density. 68 4.3 BLT triggered by partially focused laser beam 4.3.1 Experimental setup and procedure The switch cross-section together with electrical circuitry schematics and optical triggering setup is shown in Figure 37. The BLT used for these experiments has a symmetrical, hollow anode-cathode structure. Each hollow electrode, made up of oxygen-free copper, is cylindrical with a planar face that has a central bore hole. The outer diameter of the electrodes is 19 mm, and the inner diameter is 17.4 mm. The bore hole diameter, the thickness of the planar surface of the electrode around the bore hole, as well as the anode-cathode gap are 3 mm each. A Pyrex glass envelope is used for insulation and vacuum sealing of the structure. Figure 37 (a) Experimental setup showing the switch cross-section together with electrical circuitry schematics (b) Hollow cathode dimensions and location of the Mg foil indicated with green 69 The switch is in parallel with a 16 nF capacitor bank and a 3 Ω damping resistor arranged in a circular low-inductance configuration. The charging of the capacitors is provided by a DC negative high voltage supply through a high-impedance charging resistor of 5 MΩ. The anode is grounded. Two electrical signals measured by Tektronix P6015A high voltage probes (electrical connections shown in figure) are the voltage across the switch and the current through the switch. The optical triggering setup used for triggering is composed of a frequency- quadrupled Nd:YAG laser (λ=266 nm) directed to the switch via a mirror with high reflectivity at 266 nm wavelength and a UV lens that partially focuses the laser beam onto the cathode. The optical energy and the beam diameter at the switch cathode are 3 mJ per pulse and 1 mm, respectively. The timing of the laser pulse is measured using a UV sensitive photodiode. Figure 38 Typical waveforms of light signal measured by a UV photodiode (shown in arbitrary units (a.u.)), switch voltage, and switch current signals. The dashed arrow indicates the delay, colored block arrows indicate the axes of corresponding colored signals 70 A high purity (99.9%, Alfa Aeser) Mg foil of 0.25 mm thickness that was shaped to fit exactly behind the cathode back surface was used. The foil was polished with 1200- grit SiC paper and ultrasonically cleaned in hexane bath for thirty minutes before being placed in the cathode. The location of the foil is indicated in the Figure 37. Hold-off voltage and peak current measurements were conducted in self-ignited breakdown mode of the switch. This mode of operation, where the charging voltage is increased until the switch breakdown occurs, does not involve optical triggering. The trigger delay of the optically triggered switch is measured from the rising edge of the photodiode signal to the voltage rise across the switch indicating the switch closure. Figure 38 shows the typical photodiode, switch voltage, and switch current signals with the dashed arrow indicating the delay. 71 4.3.2 Results 4.3.2.1 Self-Ignited Breakdown Figure 39 Hold-off voltage with and without Mg foil at the switch cathode with changing helium pressure Figure 39 shows the hold-off voltage of a self-ignited BLT filled with helium from 0.4 Torr to 1 Torr, with or without Mg foil inserted at the back surface of the cathode. Peak current values follow the same trend as the hold-off voltage, decreasing with increasing pressure, and range from 6 kV (at 0.4 Torr) to 1.5 kV (at 1 Torr). Hold- off voltages above 25 kV are measured at 0.4 Torr. Higher hold-off voltage values are 72 possible to sustain at lower pressures. No significant difference in the hold-off voltage or the peak current values was observed with the insertion of the Mg foil in the switch cathode. 4.3.2.2 Trigger delay and jitter The trigger delay and jitter measured at a constant switch voltage of 4.1 kV for a pressure range of 0.4 Torr to 1 Torr are shown in Figure 40. An amplitude of 4.1 kV for the switch voltage is chosen as a reliable operating point at 1 Torr (corresponding to 65% of the hold-off voltage at this pressure). This allows us to maintain a constant switch voltage for the trigger delay and jitter measurements for the whole pressure range. Figure 40 Switch delay measured with and without Mg foil at the switch cathode at constant switch voltage of 4.1 kV 73 The delay decreases with increasing pressure and is an order of magnitude shorter throughout the pressure range when there is Mg foil at the switch cathode. The shortest delays at this switch voltage are 2.42 µs with 160 ns jitter and 200 ns with 20 ns jitter without and with the application of Mg foil, respectively. Effect of the switch voltage Trigger delays were also measured by increasing the switch voltage to 65% of the hold-off voltages at each pressure. The pressure, corresponding switch voltage and the measured delay and jitter under these conditions are listed in Table 2. The trigger delay shortens significantly with increasing switch voltage for both “with” and “without Mg foil” integrated cathodes compared to the delay values at same pressures shown in Figure 40 at a switch voltage of 4.1 kV. Table 2. Trigger delay values with increased switch voltages with Mg foil without Mg foil Pressure (Torr) Switch Voltage (kV) Delay (ns) Jitter (ns) Delay (ns) Jitter (ns) 0.4 17.8 280 40 842 319 0.6 14.3 171 9 1130 187 0.8 8.6 146 17 645 38 1 4.1 199 20 2485 160 74 The shortest delay was measured to be 146 ns with 17 ns jitter at 0.8 Torr when Mg foil was applied at the cathode. The relative reduction in delay due to increased field is significantly more when there is no Mg foil at the switch cathode. Despite this effect, the delay still is at least three times shorter throughout the pressure range when there is Mg foil at the switch cathode while the jitter is shortened at least by a factor of two. 4.3.2.3 Lifetime of the Mg foil Figure 41 The Mg foil surface (a) before and (b) after 10 6 shots. The inset of (b) shows the bubble-like formations from a side angle. 75 Lifetime of the switch with Mg foil was investigated at moderate current levels (~2 kA) up to 10 6 shots. After 10 6 shots, no significant degradation of the switch performance was observed. However, a difference in surface characteristics around the bore hole was visible in SEM pictures taken after 10 6 shots. Figure 41 shows the foil surface at 1.7 mm away from the center of the bore hole before and after the shots. The vertical lines on the surface before the switching are due to polishing with sand paper. The bubble-like formations observed after 10 6 shots are signs of melting and fast freezing afterwards, which can be attributed to the high density plasma (also called superemissive discharge) that forms at the later stages of the switch operation, and is capable of reaching high temperatures (Anders et al., 1994). 4.3.3 Discussion Results presented here show an impact of the integration of Mg foil to the copper cathode on the trigger delay. Additionally, the magnitude of this impact changes with the electric field. At 0.4 Torr, when there is no Mg foil at the cathode, the reduction in delay due to the increase in field induced by changing the switch voltage from 4.1 kV to 17.8 kV is more than an order of magnitude (from 14.84 µs to 0.84 µs) whereas for the same change in field the reduction in delay is only less than a factor of two (from 0.43 µs to 0.28 µs) when there is Mg foil at the cathode. In order to understand the underlying physics of these results, it is crucial to look into electron emission mechanisms involved in optical triggering process. 76 Estimation of the cathode surface temperature is important for the following two reasons: (1) photoemission is a function of temperature (Jensen et al., 2003); and (2) the local heating may result in thermionic emission, laser ablation and laser-induced plasma in addition to photoemission. The maximum expected temperatures on the surface of the cathode as a result of laser heating under our experimental conditions could be estimated using the time- dependent heat-conduction equation: € T(t) = 2I m (1−R) K κt π +T 0 (4.1) where I m is the laser intensity (W/m 2 ), R is the reflectivity of the surface (reflectivity values at 266 nm wavelength of 0.85 (Gesell et al., 1973) for Mg surface and 0.33 for Cu surface (Haynes and Lide, 2011) are used), K is the thermal conductivity (W/Km), T 0 is the initial surface temperature (293 K), and κ is given by K/ρc, where ρ is the mass density (kg/m 3 ), c is the specific heat (J/kgK) (Bechtel, 1975). A more complete calculation should take into account both spatial and temporal nonuniformity of the laser illumination, and the fraction of photons leading to direct photoemission instead of heating for our laser wavelength. Since both of these factors would lead to a lower local heating, using this equation for a worst-case scenario is adequate. The estimates of the surface temperatures calculated using (1) show that the surface temperature can rise up to 850 K on Mg cathode and 1350 K on copper cathode. 77 At these temperatures the dependence of photoemission is dominated by its asymptotic limits where € (hν−φ)/kT is large (hν is the photon energy, φ is the effective work function, T is the temperature and k is the Boltzmann’s constant) and can be represented by (2) for short wavelengths (hν > φ) (Jensen et al., 2003). Consequently, the temperature dependence of photoemission disappears at these temperature and wavelength conditions. Furthermore, calculated surface temperatures are not sufficient to result in significant laser ablation or laser-induced plasmas seen at higher power densities of lasers where evaporation of the target material is possible. (1380 K for Mg, 2840 K for Cu) (Bogaerts et al., 2003; Wang et al., 1992). The thermionic emission given by field modified Richardson-Laue-Dushman equation given in (Anders et al., 1994) is negligible compared to photoemission at the calculated surface temperatures. Even compared to photoemission with a modest value of 10 -6 for quantum efficiency (ratio of number of emitted electrons to number of incident photons), contribution of the thermionic emission to the total emission is less than 10% at temperatures below 2000 K. Photoemission can be represented by quantum efficiency (QE) indicated in (2) where α is a material dependent constant. The QE is proportional to the square of the difference between the photon energy (hν, in this case 4.67 eV) and the work function of the cathode at zero electric field (φ 0 ), plus a field related term ( € b βE ) where b is a constant given by € e 4πε 0 (e is the electron charge and ε 0 is the permittivity of free space), β is the field enhancement factor and E is the electric field. € QE =α hν −φ 0 +b βE ( ) 2 (4.2) 78 An approximation of parameter α can be calculated using the asymptotic approximation of QE at short wavelengths (hν > φ) given in (Jensen et al., 2003) by substituting generic parameters for reflectivity and chemical potential of Cu and Mg. Under these assumptions, α is 0.003 eV -2 for Mg and 0.013 eV -2 for Cu. Since Mg has a lower work function (3.66 eV) than copper (4.5 eV), the overall photoemission according to (2) is approximately 8 times higher for Mg than it is for Cu at zero electric field. Exact theoretical evaluation of QE requires information on microscopic field enhancement at the emission sites, exact reflectivity and work function of the emission surface and any residual coating of the surfaces together with the exact temperature distribution which are either too complicated to measure or not measurable. The results also show that the impact of electric field on the trigger delay is more significant for the bare Cu cathode compared to the Mg foil inserted cathode. Using (2), assuming no macroscopic surface roughness (β=1), and a rough estimate of 500 V/cm for the field based on the electrostatic simulation presented in (Sozer et al., 2009) the fraction of the field enhancement is calculated to be 2% of the total photoemission for a Mg cathode, whereas it is 10% for a Cu cathode. This calculation is in agreement with the trend observed in our experimental results. 79 We conclude that an increase in photoemission due to decrease in work function of the cathode is the main reason for the observed reduction in delay when Mg foil is present. This also explains why no significant difference is observed between “with” and “without Mg foil” integrated cathodes in hold-off voltage and peak current measurements during self-ignited breakdown mode of the switch where the electron injection with photoemission is absent. A direct comparison of the measured trigger delays to previous BLT delay results is not trivial since the measurement conditions varied in more than one parameters affecting the photoemission (i.e. in optical source wavelength, energy, intensity, operating gas and switch voltage) or not all of the mentioned parameters were reported. The UV laser used in our experiments is one of the most compact in its category and has lower light energy output than most reported light sources at similar wavelengths reported in the literature for triggering BLTs (Jiang et al., 2005b; Braun et al., 1988; Chen et al., 2009; Pitchford et al., 1995). The only work where triggering with lower optical energy than used here with similar order of magnitude delays reported is by Braun et al. (Braun et al., 1988) with significantly higher photon energy (5.58 eV) and in hydrogen, where the critical number of electrons for triggering is expected to be less than it is for helium due to the smaller electron-impact cross section of helium. In addition, cathodes with Mg foil did not show significant change in delay or jitter up to 10 6 shots. Although signs of melting observed in SEM pictures show a potential of degradation in switching parameters at higher number of shots due to change 80 of the cathode surface conditions, the previously reported limiting factor on lifetime for molybdenum based cathodes (Braun et al., 1988), which was the metallization of the glass envelope after 10 5 shots, was not observed during our experiments for either with or without Mg foil integrated cathode. Furthermore, Mg foil has shown to increase the range of pressure and switch voltages for reliable operation even at low intensity (8.5×10 6 W/cm 2 ) illumination compared to copper and molybdenum based cathodes (Sozer et al., 2011 ). As a result, Mg-based cathodes showed a potential to be advantageous for systems where the minimum wavelength of the optical source is limited and lower optical energy and intensity sources are preferred. This is promising for the development of compact triggering units for BLTs. 81 4.3.4 BLT with magnesium thin film Figure 42 Delay of Mg integrated cathodes with different film thicknesses Triggering of BLT with Mg film deposited cathodes showed similar delays as Mg foil. The increased delay at high pressure is due to not only lower electric field but also erosion of the film from the cathode surface after several thousand shots at optical intensity of 8×10 7 W/cm 2 . The surface of the films where the laser spot hits are visible as seen in Figure 43. The damage threshold measurements for Mg films deposited on Cu substrates were reported. A 1 µm depth damage was observed at an energy density of 600 µm/mm 2 . This energy density is 6 times less than the energy density used in the delay measurement shots of Figure 43. As we did not observe any improvement in delay and jitter as a result of using thin films, we can conclude that either thicker layers of 82 magnesium or lower energy density of optical source are preferable for Mg film applications to BLT. (Srinivasan-Rao et al., 1998) Figure 43 Magnesium film coated cathodes before and after delay measurements (~ 7000 shots). Film deposition was performed at Lawrence Berkeley National Laboratory by plasma applications group. 83 4.4 Chapter conclusions Delay measurements of BLTs with Mg, Cu and Mo-based cathodes were conducted using optical triggering source of 8.5×10 6 W/cm 2 intensity. The delay got shorter with the following order of material-based cathodes: Mo, Cu and Mg. Mg-based cathode showed a thirty-fold reduction in delay compared to Cu-based cathode and eighty-fold reduction in delay compared to Mo-based cathode. Light intensity affects the operable range of switch voltage and gas pressure. An order of increase in light intensity by partial focusing of the laser beam allowed the delay measurements to be performed with changing only one of these parameters instead of both simultaneously. Delay measurements of a BLT with copper electrodes using 7.4×10 7 W/cm 2 optical intensity and constant switch voltage of 4.1 kV showed that the delay and jitter decrease with increasing gas pressure. Same measurements before and after integration of a Mg foil at the switch cathode showed an order of magnitude shorter delay as a result of the presence of the high-purity Mg-foil. Theoretical estimations of electron emission from the cathode during the triggering suggested that the main mechanism responsible for the observed change in delay and jitter was the increased photoemission due to the lower work function of the Mg cathode. 84 SEM images of the high-purity Mg foil integrated at a BLT cathode for 10 6 shots showed signs of melting around the bore hole. No degradation of the switch performance was observed during the lifetime test of 10 6 shots. Mg thin-film coated copper cathodes (230 nm and 920 nm in thickness) for BLTs were tested. No difference in the delay and jitter values were observed with changing Mg thicknesses even when compared to Mg-foil (250 µm in thickness) integrated switch. Moreover, erosion of the Mg was more severe for thinner layers, thus did not offer any advantage compared to thicker Mg cathodes. 85 Chapter 5 Summary and Future Work 5.1 Summary In this dissertation, the effect of magnesium-based photocathodes on optical triggering of a pseudospark switch, called back-lighted thyratron (BLT), was studied. A theoretical look into triggering mechanism based on photoemission suggests an increase in total electron emission due to a lower work function cathode. Among well-studied photocathodes in the literature, magnesium and copper were chosen as the most appropriate candidates for BLT. Two main criteria in this selection were long lifetime and ease of maintenance of the cathode while providing lower work function than traditionally utilized BLT cathodes. Molybdenum-based cathodes were included in parts of these studies to provide a comparison to a traditionally preferred cathode material for BLTs. The studies involved three main stages: (1) Quantum efficiency measurements of high-purity magnesium, copper and molybdenum foils in a test bed; (2) Switching delay and jitter measurements of magnesium, copper and molybdenum-based cathodes with low-intensity (8.5×10 6 W/cm 2 ) optical condition; (3) Measurement and comparison of switching delay and jitter of a BLT with plain copper electrodes before and after integration of a high-purity Mg foil at its cathode. For this last part of the study a higher 86 optical intensity (8.5×10 7 W/cm 2 ) was used to allow the measurements for a wider range of switch voltage and gas pressure conditions. Summary of the results of each stage is as follows: (1) The quantum efficiencies (QE) of Mg, Cu and Mo were investigated in a pressure range of 10 -7 to 0.1 Torr as photocathode candidates for back-lighted thyratrons. A quantum efficiency of 10 -5 for Mg and Cu photocathodes at a He pressure of 0.1 Torr promised a potential improvement in switch performance (e.g., shorter switch trigger delay and jitter) when implemented within a BLT. Strong dependence of the QE of photocathodes on the electric field at the sample surface due to Schottky effect was observed. (2) The delays of BLTs with Mg, Cu, and Mo-based cathodes with varying switch voltage and helium pressure were investigated. The Mg-based cathode configuration shows a thirty- shorter delay and jitter compared to Cu and eighty-fold shorter delay and jitter compared to Mo-based cathode configurations. Moreover, presence of Mg at the switch cathode is observed to increase the pressure and switch voltage range that reliable triggering is achieved under low optical power density (8.5 x 10 6 W/cm 2 ) conditions. (3) Delay measurements of a BLT made up of copper electrodes with and without Mg foil integrated at its cathode were conducted. It was shown that insertion of the Mg foil shortened the delay and jitter of the switch by an order of magnitude at a constant switch voltage of 4.1 kV for the same triggering light intensity. The presence of the Mg foil decreased the trigger delay and jitter by at least three times when the switch voltage was increased to 65% of the hold-off voltage throughout the pressure range of 0.4 Torr to 87 0.8 Torr helium. The estimations of the highest surface temperature together with the photoemission of the cathodes have showed that the main reason for the observed results is an increase in photoemission due to lower work function of the magnesium cathode is. The shortest delay of 146 ns with 17 ns jitter at 0.8 Torr is measured when switch voltage is at 8.6 kV using 3 mJ optical energy at 266 nm wavelength for triggering. The delay and jitter was maintained up to 10 6 shots. Metallization of the glass envelope was not observed as a limiting factor on lifetime, however signs of melting seen on the Mg surface after 10 6 shots may be an indication of eventual degradation of lifetime. Mg thin film deposition tested under these conditions showed that the damage threshold is not high enough for film thickness below 1 µm to survive. Mg-based cathodes are advantageous for optical triggering of pseudosparks where the optical source energy and intensity are limited. As a result, we have shown a benefit of magnesium-based cathodes compared to traditional BLT cathodes for the first time. The improvement in delay and jitter was even more pronounced for low intensity light sources, where future of compact triggering units may lie. 88 5.2 Future Work 5.2.1 Pre-switch closure current measurement In order to measure the photoelectrons emitted from the cathode of a BLT, we also performed current measurements of the switch before the full switch closure. These preliminary measurements show a potential of the method to provide information on discharge development, however data acquisition must be improved to be able to obtain a sufficient amount of data for conclusive results. The experimental setup was the same as the setup for delay measurements shown in Figure 37 with two differences (1) the switch current was measured with higher sensitivity probe (×1) across the 3 Ω resistor, and (2) the switch charging voltage was below 700 V, which is less than 10% of the hold-off voltage of the switch at 800 mTorr where the preliminary measurements were performed. The limit for charging voltage was the damage limit of the oscilloscope where the measurements through a 50 Ω terminating resistor were done. Initial goal of these experiments were to be able to see the direct photoemission current. Although, this goal was not achieved, several microseconds after the light pulse (measured with fast photodiode) a current signal with two distinct peaks was observed: an initial peak of few nanoseconds and followed by a peak of 1-2 microseconds width. Typical waveforms of the both peaks are shown in Figure 44. 89 Figure 44 Typical waveforms of first and second current peaks after the light signal at low charging voltage This current characteristic was consistent after every light pulse and the values of these peaks increased with increasing switch voltage. First measurements were performed at two values of switch voltage with different cathodes. Figure 45 shows the peak current amplitudes of both peaks. The data points are the average of five measurements each. Figure 45 Peak current vs. two charging switch voltage points with plain Cu and Mg integrated Cu cathodes 90 Another important characteristic of these current peaks are their pulse widths. A look at their full-width at half maximum (FWHM) values versus the peak current amplitudes show that the width of the first peak does not change with changing peak current amplitude, whereas the width of the second peak linearly decreases with increasing peak current amplitude. Figure 47 shows the FWHM values of each current measurement. For the case with Mg foil integrated cathode two sets of five current measurements can be distinguished corresponding to two different values of the switch voltage. Figure 46 Full-width at half maximum values of two peaks with changing peak of the current values Similar behavior was observed when a higher sensitivity voltage source was used as seen in Figure 47. Detection of this current signal was not achieved for the case without the Mg foil. 91 Figure 47 Peak current vs. switch voltage and the full-width at half maximum of the peak currents with changing peak current values The physical process responsible for these peaks is the hollow cathode discharge formation and was analyzed in detail with a transient argon hollow cathode discharge in (Alberta et al., 1994). Experimental results and their comparison to simulation results were reported to show that the first peak corresponds to a high brightness spark behind the cathode and a peak in charge multiplication in the simulation model. This current is not as a result of charge transportation but it is a displacement current resulting from the rapid expansion of the plasma into the hollow cathode backspace (a rapid change in electric field around the cathode). This also explains its short duration since the duration corresponds to the time needed for charge multiplication and is expected to be much less than the time needed for charge transportation. The second peak in the current signal is due to actual charge transportation, thus is a conduction current of the hollow cathode discharge (Alberta et al., 1994). The width of this current is a function of the electric field in the gap determining the acceleration of charged particles. 92 Figure 48 (a) Current, and voltage, and (b) plasma induced emission waveforms recorded on the axis of hollow cathode between in a pure argon discharge with clean electrodes. The pressure is 0.9 Torr at room temperature. The labels "0," "1,"and "2"refer to different temporal phases of the discharge (Alberta et al., 1994) As we observed in the preliminary results the first peak can be cathode material dependent since the higher number of initial electrons would result a higher rate of change of the electric field. Thus, it can be used to evaluate the initial electron emission strength by means of following charge multiplication peak during the hollow cathode discharge phase. This method has the advantage of eliminating the emission processes associated with the superemissive cathode phase where explosive vacuum-arc and field- enhanced thermionic emission dominate. Consequently, this measurement allows analysis of a simpler picture of discharge formation at hollow cathode phase where the initial charge introduction and the secondary electron emission are the main contributing mechanisms. 93 The successful experimental design should include a higher sensitivity current measurement, while keeping a nanosecond rise time. This requires a careful design of competing requirements of the signal amplification and fast rise time. First step in amplification can be the increasing the damping resistor value from 3 Ω to a higher value. An amplifier circuit with low input capacitance also can be used for amplification with fast response times. 94 5.2.2 Plasma simulations Figure 49 A schematic representation of a two-stage multi-physics simulation for studies of optical triggering of a BLT Optimization of triggering of a BLT requires an understanding of the physics of a two stage process (1) introduction of initial charge density to the switch gap by photoemission from the switch cathode, (2) formation of the conducting path between switch anode and cathode following the introduction of the initial charge density. While photoemission is a function of optical energy, cathode material (work function and Fermi energy), cathode surface conditions (i.e. coating and scattering considerations), electric field and temperature; plasma formation is a function of charged particle densities, 95 background gas type (ionization cross-sections of the gas species), electric field distribution throughout the switch geometry and temperature of the plasma, all of which change temporally and spatially during the discharge formation. An ideal effort for optimization of delay and jitter of a BLT switch requires control of all of these parameters accurately. As this effort may not be practical experimentally, a simulation work where a mathematical representation of the critical parameters can be controlled easily would benefit cathode studies for BLTs. Photoemission from experimental measurements can be used as an input parameter for a hybrid fluid-particle plasma model like Pitchford and Boeuf had used to estimate following discharge formation and switch closure (Boeuf and Pitchford, 1991; Pitchford et al., 1995). This type of modeling and simulation would allow assessing the potential of different types of cathode material/ optical triggering source pairs and making an accurate estimation of potential of utilizing low intensity sources like UV LEDs for triggering BLTs. Figure 50 UV LED lamp with a wavelength of 255 nm by Sensor Electronic Technology 96 Bibliography Alberta, M. P., Derouard, J., Pitchford, L. C., Ouadoudi, N. and Boeuf, J. P., 1994. Space and time dependence of the electric field and plasma induced emission in transient and steady-state hollow cathode discharges. Physical Review E, 50, pp.2239-52 Anders, A., Anders, S. and Gundersen M. A., 1994. Electron emission from pseudospark cathodes. 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Asset Metadata
Creator
Sözer, Esin B.
(author)
Core Title
Magnesium-based photocathodes for triggering back-lighted thyratrons
School
Graduate School
Degree
Doctor of Philosophy
Degree Program
Electrical Engineering-Systems
Degree Conferral Date
2012-05
Tag
OAI-PMH Harvest
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-oUC11291409
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UC11291409
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etd-SzerEsinB-467-0
Document Type
Dissertation