Evaluation of a High-Pressure, Coaxial Spark Gap for Pulsed Ring

Home , Spark gap, Trigatron

EVALUATION OF A HIGH-PRESSURE, COAXIAL SPARK GAP FOR

PULSED RING-DOWN APPLICATIONS

COLT JAMES

A MASTER THESIS

ELECTRICAL ENGINEERING

Submitted to the Graduate Faculty

of Texas Tech University in

Partial Ful llment of

the Requirements for

the Degree of

MASTER OF SCIENCE

Approved

Dr. James Dickens Committee Chairman

Dr. John Mankowski

Fred Hartmeister Dean of the Graduate School

December, 2007 Copyright c 2007, Colt James Texas Tech University, Colt James, December 2007

ACKNOWLEDGMENTS

I would like to thank my adviser, Dr. Dickens, for guidance and help throughout this project. I would also like to thank the other member of my committee, Dr.

Mankowski, for his support and knowledge. Special thanks go out to the Dr. John

Krile and Shad Holt, who provided a great deal of advice and knowledge. I would also like to thank the sta of the Center for Pulsed Power and Power Electronics.

This project could not have been accomplished without the help of Danny Garcia,

Dino Castro, Elmer Thornton, and Shannon Gray. I would also like to thank all of my colleagues in the lab, speci cally my oce mates Greg Edmiston, Thomas

Holt, Andrew Young, and Mohamed Elsayed, for all their advice and help. Finally,

I would like to thank my friends and family for their invaluable support.

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CONTENTS

ACKNOWLEDGMENTS ...... ii

ABSTRACT ...... iv

LIST OF TABLES ...... v

LIST OF FIGURES ...... vi

CHAPTER

I INTRODUCTION ...... 1

II BACKGROUND THEORY ...... 4

2.1 Closing Switches ...... 4

2.1.1 Breakdown Mechanisms ...... 5

2.1.1.1 Townsend Discharge ...... 5

2.1.1.2 Streamer Breakdown ...... 6

2.1.2 Erosion Mechanisms ...... 10

2.2 Previous Work ...... 13

III EXPERIMENTAL SETUP ...... 15

3.1 Spark Gap ...... 15

3.1.1 Electrostatic Modeling ...... 17

3.2 Test Bed ...... 21

3.2.1 Trigger System ...... 22

3.3 Diagnostics ...... 23

IV EXPERIMENTAL RESULTS ...... 25

4.1 Preliminary Results ...... 26

4.2 Final Results ...... 30

V CONCLUSIONS ...... 39

BIBLIOGRAPHY ...... 41

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ABSTRACT

The design and jitter performance of a high pressure, coaxial spark gap for use in pulse ring-down applications is presented. Additional comparisons with trigatron style triggering are also presented. The spark gap is triggered by eld distortion of a center plane electrode. The switch was tested up to 75 pulses per second (pps) with a maximum switching voltage of 50 kV in nitrogen. Analysis will focus on jitter measurements taken over the full lifetime of the switch. This paper presents the results of this analysis along with comparisons from the literature. Speci cally, switch jitter and lifetime will be evaluated as a function of switch geometry as a whole and as a function of trigger electrode geometry.

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LIST OF TABLES

2.1 Summary of jitter experiments...... 13

4.1 Summary of electrode erosion data for preliminary tests. Negative

values indicate a loss of mass, while positive numbers indicate an

increase in mass...... 27

4.2 Summary of switch jitter for the preliminary tests...... 28

4.3 Summary of electrode erosion data for nal tests. Negative values

indicate a loss of mass, while positive numbers indicate an increase

in mass...... 30

4.4 Summary of electrode erosion data for preliminary tests. Negative

values indicate a loss of mass, while positive numbers indicate an

increase in mass...... 32

4.5 G{10 mechanical and electrical properties...... 37

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LIST OF FIGURES

1.1 Diagram of a phased array antenna system. The variable delay is

made up of all the electrical connections necessary for array operation. 1

1.2 Example of pulse ring-down. Sample waveform used for phased array

testing...... 2

2.1 Diagram of a three-electrode triggered spark gap.[1] ...... 4

2.2 Diagram of a trigatron spark gap.[1] ...... 5

2.3 Diagram depiction of the streamer breakdown process, taken from

Nasser[2]. (a) Formation of primary electron avalanche. (b) Photons

are emitted from excited gas. (c) Secondary avalanches are formed

from photo-electrons emitted near the cathode. (d) Positive space

charge begins to build up at the anode. (e) Streamer propagating due

to successive avalanches. (f) Some branches die out while avalanches

continue to feed others. (g) Completed streamer channel...... 8

2.4 Potential distribution in a three-electrode spark gap.[1] Top - Main

electrode at high, positive voltage with the trigger electrode oating.

Middle - Incoming trigger pulse causes an increase in potential dif-

ference between main electrode and trigger electrode. Bottom - Es-

tablishment of arc between main and trigger electrodes brings them

to the same potential and increases the potential di erence between

trigger electrode and ground electrode...... 10

3.1 Spark gap cross section ...... 16

3.2 Machined Bruce pro le electrodes...... 17

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3.3 Comparison of electrode pro les. Left - Standard Bruce pro le, Right

- Modi ed Bruce pro le ...... 18

3.4 Electrostatic eld simulation of Bruce pro le electrodes...... 19

3.5 Electrostatic eld simulation...... 19

3.6 Potential distribution throughout the gap for each stage of the break-

down process. Top - Main electrode at high, positive voltage with the

trigger electrode oating. Middle - Incoming trigger pulse causes an

increase in potential di erence between main electrode and trigger

electrode. Bottom - Establishment of arc between main and trig-

ger electrodes brings them to the same potential and increases the

potential di erence between trigger electrode and ground electrode. 20

3.7 Test bed equivalent circuit ...... 21

3.8 Test Bed Load ...... 22

3.9 PFL Trigger System ...... 23

4.1 Top - Annular midplane, Left - Point midplane, Right - Radius mid-

plane ...... 26

4.2 Overlay of ve voltage waveforms. The magnitude of the trigger

voltage was scaled down to allow the waveform to t in the graph. . 27

4.3 Image of the anode and cathode taken after 10 million shots. The

baes can be seen surrounding the electrodes...... 28

4.4 Standard deviation jitter recorded over 10 million shots...... 29

4.5 Statistical delay times over the recorded lifetime of the electrodes. . 29

4.6 Comparison photo of the second tested anode after 10 million shots. 31

4.7 Neutral biased jitter vs. cathode biased jitter of the annular midplane

over tested lifetime of the switch ...... 33

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4.8 Comparison of midplane geometries. All measurements taken with

the midplane in the cathode biased position...... 34

4.9 Comparison of midplane geometries. All measurements taken with

the midplane in the neutral biased position...... 34

4.10 Close-up image of the e ects of breakdown inside the G{10 housing. 38

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CHAPTER 1

INTRODUCTION

The goal of this project is to evaluate a long-lifetime, low-jitter spark gap for pulsed ring-down phased array applications. Phased array antenna systems provide a means of removing the mechanical problems involved with large high gain antennas by connecting many smaller antenna elements together. The trade o with array antennas is that, while mechanically simpler, electrically they are much more complex and the connections between them become very important. This is why precise switching components are necessary. An array usually consists of a source, a number of antenna elements, and the electrical connections between them,

Fig.1.1. If the source is connected to all the elements at the same time a beam will be produced with a gain proportional to the number of elements and phase normal to the antenna elements. The radiated electric eld pattern can be changed by varying the time delay in which the antenna elements are switched. By switching the elements in di erent orders (Fig.1.1), the directed phase angle can be changed.

This also means that an inability or variation in ring the array elements can cause an unwanted phase angle shift and loss of power on target.

Figure 1.1: Diagram of a phased array antenna system. The variable delay is made up of all the electrical connections necessary for array operation.

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Array systems can either be continuous or pulsed. In this project a modi ed pulsed system will be built to transmit a ringing pulse. The pulse forming network that would be used to produce a single square pulse to propagate on a matched antenna load in a typical pulsed array system will be removed. This will result in a reduction of size and weight. Instead, a high-voltage switch will be placed within each antenna element. The antenna elements will be charged to a high potential, and when the switch is triggered a fast rising pulse will be produced that will ring down after several cycles, Fig. 1.2.

Figure 1.2: Example of pulse ring-down. Sample waveform used for phased array testing.

For a system like this to be possible, switching components with certain characteristics are needed. They need to be able to switch high voltages and the required energy per pulse; spark gaps are the best answer. Timing jitter will be a key factor for transmission of power; simulations have shown a jitter of < 10ns will be required[3]. Finally, for this system to be able to operate for long periods without maintenance a long lifetime switch will be required.

The processes that govern spark gap jitter and lifetime will be explored and previous research done in these areas will be detailed. An experimental setup

2 Texas Tech University, Colt James, December 2007 designed to test these parameters and the e ects of switch geometry on jitter will be discussed. Finally, the results of detailed lifetime and jitter experiments will be presented along with conclusions obtained from the data.

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CHAPTER 2

BACKGROUND THEORY

2.1 Closing Switches

The two mainstays for high voltage and high current switching are the eld distortion spark gap and trigatron spark gap. A diagram of the eld distortion spark gap appears in Fig. 2.1 and the trigatron spark gap in Fig. 2.2, both from Bluhm[1].

An understanding of spark gap operation in general, and speci cally jitter, begins with understanding the breakdown process and electrode erosion mechanisms.

Figure 2.1: Diagram of a three-electrode triggered spark gap.[1]

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Figure 2.2: Diagram of a trigatron spark gap.[1]

2.1.1 Breakdown Mechanisms

The eld distortion and trigatron, when operated in its normal mode, behave quite di erently. As explained in the following sections, the eld distortion gap follows the Townsend breakdown process where an electron avalanche is initiated at the cathode and moves toward the anode until gap closure is achieved. The trigatron gap follows a special case of this process where successive avalanches build up from the anode causing positive ion streamers to eventually close the gap.

2.1.1.1 Townsend Discharge

Consider two parallel plate electrodes; one, the cathode, at x = 0 and the anode at x = d. When an electric potential is applied to the anode a uniform electric eld is created between the two plates. Assuming an electron then gets emitted from the cathode, it will move through the gap toward the anode and cause an electron avalanche, with the number of electrons,

n = eαd (2.1)

5 Texas Tech University, Colt James, December 2007 where α is Townsend's rst ionization coecient. Once the electron avalanche reaches the anode the electrons will be absorbed and the positive ion tail of the avalanche will move back toward the cathode where each ion has a probability, γi, of producing another electron that will start its own electron avalanche on the way to the anode. This electron multiplication process will continue and if the number of electrons from secondary emission becomes greater than the initial number of electrons, n0, from external radiation, the current from electron motion becomes self-sustaining and the gap is said to have reached breakdown. From Nasser[2], if the current in the gap can be written as

i eαd = (2.2) αd i0 1 − γi(e − 1) then the condition for self-sustained current is reached when when the denominator is equal to zero or,

αd γi(e − 1) = 1 (2.3)

This transition to self-sustained current constitutes a spark discharge.

2.1.1.2 Streamer Breakdown

Various sources[4][5] have demonstrated that, when operated in common mode

(meaning over-voltaging of the trigger pin launches a streamer closing the gap), a trigatron does not follow the standard Townsend discharge but instead breaks down via a mechanism known as streamer breakdown. A streamer breakdown begins, as the Townsend mechanism does, with the ionization process initiated by an electron avalanche. If the parallel plate model from before is revisited, once an electron is emitted an electron avalanche begins to form on its way to the anode. As the

6 Texas Tech University, Colt James, December 2007 avalanche moves through the gap the electrons will collisionally excite or ionize the gas with the ratio of excited molecules, θ, to ionized molecules de ned as

θ f = (2.4) α with f being a factor of 10 or more in some gases. A photon is produced by each excited molecule as it returns to a lower energy state. These photons each have a chance of impacting the cathode and, if the work function of the cathode is lower than the energy of the photon, producing a secondary photo-electron. Each photo-electron in turn creates another avalanche. When the avalanche reaches the anode the electrons are absorbed, leaving a cloud of positive ions and as subsequent avalanches reach the anode, a positive ion channel begins to form as each of the electron fronts are absorbed. As photons are continually emitted, photoelectrons from the cathode continue the electron avalanche mechanism in the gap until the ion channel grows long enough to close the gap resulting in self-sustained current and breakdown. This process is outlined in Fig. 2.3.

The transition between these two mechanisms occurs around the breakdown condition for the Townsend discharge. If γ is low enough such that

αd γi(e − 1) < 1 (2.5) a Townsend discharge cannot be achieved, it may still be high enough that successive electron avalanche generations will be created and an ion channel will begin to form. While the transit time of photons in the gap is several orders of magnitude faster than either that of electrons or positive ions, since a streamer breakdown is characterized by several avalanche generations transitioning into a streamer statistical delay times are similar for both mechanisms.

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Figure 2.3: Diagram depiction of the streamer breakdown process, taken from Nasser[2]. (a) Formation of primary electron avalanche. (b) Photons are emitted from excited gas. (c) Secondary avalanches are formed from photo-electrons emitted near the cathode. (d) Positive space charge begins to build up at the anode. (e) Streamer propagating due to successive avalanches. (f) Some branches die out while avalanches continue to feed others. (g) Completed streamer channel.

Breakdown in a triggered spark gap is just an extension of these basic breakdown processes. A standard three-electrode spark gap is con gured such that the cathode is grounded and the anode is at high potential. The anode is usually charged to approximately 80-90% of the self-break voltage, where self-break is the voltage required for critical electron avalanche multiplication, or streamers, and self-sustained current. A negative voltage pulse on the order of the charging voltage is applied to the trigger electrode which initiates the breakdown process. The potential distribution through the gap can be seen in Fig.2.4, taken from Bluhm[1]. In the rst

8 Texas Tech University, Colt James, December 2007 picture the potential is evenly distributed through the gap. The electric eld is high enough for individual electron avalanches to move toward the anode but too low for the electron multiplication necessary for breakdown. In the second picture the trigger pulse is applied. A negative voltage pulse will e ectively double the potential di erence between the anode and the trigger electrode. This will enhance the electric eld past the point necessary for a critical electron avalanche. Breakdown will occur between the anode and trigger electrode and both will then be at the same potential. The last picture shows this and the increased potential between the trigger and cathode. Another arc will form between the trigger electrode and cathode closing the gap. Due to the overvoltaging from the trigger pulse and UV radiation created by the rst arc this breakdown process happens very quickly.

Of interest is another mechanism possible in the three-electrode spark gap. If the operating voltage of the main gap is much lower than the self-break voltage then the trigger pulse is not enough to completely over-voltage the gap. The only mechanism available to fully break down the gap comes from the UV radiation produced by the rst arc. The UV radiation will slowly reduce the hold-o voltage of the gap through photo-ionization until the gap fully breaks down. This continuous process happens much more slowly than the previous case and leads to statistical delay times that are much longer and, therefore, higher jitter.

The breakdown process for the trigatron is similar to the three-electrode gap.

Streamers are initiated from the trigger pin when the trigger pulse is applied. These streamers grow outward toward the anode and, at the same time, toward the cathode. The end result is a streamer arc connecting the cathode to trigger electrode and trigger electrode to anode. Due to the fast nature of the streamer breakdown and the concurrently forming arcs this breakdown process happens very fast.

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Figure 2.4: Potential distribution in a three-electrode spark gap.[1] Top - Main electrode at high, positive voltage with the trigger electrode oating. Middle - Incoming trigger pulse causes an increase in potential di erence between main electrode and trigger electrode. Bottom - Establishment of arc between main and trigger electrodes brings them to the same potential and increases the potential di erence between trigger electrode and ground electrode.

2.1.2 Erosion Mechanisms

Electrode and insulator erosion are some of the most important factors limiting the lifetime of high voltage and high current spark gaps. Erosion mechanisms have been studied in depth by Soviet scientists during the Cold War Era. A great deal of the work done by American scientists was done by Donaldson[6] in the 80's.

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Many di erent techniques for modeling these processes have been reported. To understand how switch characteristics, speci cally jitter, will be a ected over the lifetime of operation, a basic knowledge of some of these mechanisms is needed.

Assuming a uniform electric eld between two metal electrodes, such as the example from the previous section or the eld between two uniform eld electrodes

(Bruce, Rogowski, etc), then as a high enough potential is applied breakdown will occur, whether Townsend or streamer, and a stationary arc will connect both electrodes. The resistance of the arc will be e ectively zero and current ow between the electrodes will only be limited by the external circuit and can be any where from kA to MA in magnitude. Energy is transferred to the electrodes through conduction, radiation, and convection from the arc. This energy heats the electrodes and they begin to melt before the material is nally removed by a combination of ablation

(explosive removal) or vaporization. Both electrodes will have material removed at approximately the same rate, however, if they are symmetrical the cathode will also gain material from the anode and so appear to not lose much material. These erosion mechanisms can be explained by solving the heat conduction equation near the electrodes surface

δT(z, t) δ2T(z, t) J2(z, t) ρc = k + e (2.6) 2 δt δz σe where ρ is the material density, k is the thermal conductivity, c is the speci c heat,

σe is the electrical conductivity of the electrode material, T is the temperature, Je is the electrode current density, and z is the direction perpendicular to the electrode surface. Belkin[7] solved this equation for a case valid over a wide range of pulse times. From his solutions the normalized amount of material eroded is

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£ mm (2.7) mm = Qh/cTeff and vaporized material

£ mv (2.8) mv = Qh/cTeff where

tp Qh = q(t)Ae(t)dt (2.9) Z0 and

Teff = Tmp − T0 (2.10)

is the e ective temperature rise. Tmp is the melting point of the material, T0 is the initial temperature, and tp is the pulse width. This material erosion causes craters and protrusions on the electrode surface.

After the arc dissipates, the electrode material will begin to cool and the whole process will start over again with the next pulse. This temperature cycling can lead to microscopic cracks in the electrode material. All this erosion damage can e ect voltage hold o and stability and can eventually lead to the electrodes frac- turing. Since the electric eld is no longer uniform throughout the gap, voltage instability can mean a decreased probability of forming a streamer channel or electron avalanche. This decreased probability can lead to increased statistical delay times and an increase in the standard deviation jitter.

Insulator erosion is governed by these same mechanisms and while it is unlikely these mechanisms would lead to fracture and absolute failure, even over very long

12 Texas Tech University, Colt James, December 2007 lifetimes, interactions between the insulator material and switch arc can lead to de- composition of the insulator into conductive compounds which can severely reduce the hold o voltage of the switch. This can be of even greater importance than electrode erosion in the design of high voltage spark gaps. Ways of reducing the e ects of insulator erosion due to the switch arc include removing the insulator far enough away such that heat transfer is only via conduction or including a shield or bae inside the switch to protect the insulator material.

2.2 Previous Work

Extensive research has been done in the area of spark gap design and construc- tion, especially in the area of increasing the pulse repetition rate and life time.

Switch jitter often seems to have been of secondary concern except where speci c applications have concentrated on it. Table 2.1 summarizes some of the literature on low jitter switching[8, 9, 10, 11, 12].

Table 2.1: Summary of jitter experiments. Author Switch Type PRR Voltage Jitter Grothaus, et al Trigatron 1 kHz - Burst 500 kV 2 ns Deutsch Laser Triggered Single Shot 120 kV 1 ns Winands, et al Field Distortion 100 Hz 34 kV 23 µs McPhee, et al Trigatron 100 Hz 500 kV 1 ns McPhee Trigatron { 500 kV <1 ns Mayes, et al Field Distortion Single Shot 30 kV <1 ns

The trigatron has been given a great deal of attention due to simplicity of its design and very low reported jitter due to the speed at which the streamer breakdown process occurs. The inevitable drawback to this switch design is the dramatically increased erosion rate of the trigger pin. McPhee et al[8] reported an erosion rate on one of their prototype spark gap designs of 25 grams/Coulomb which led to an estimated lifetime of only 104 shots. The design of a new trigatron for ex-

13 Texas Tech University, Colt James, December 2007 tended lifetime use only allowed them to extend the usable life to 106 shots with comparable jitter. However, at 100 pps operation this is only a usable lifetime of approximately 3 hours. While the rep-rated eld distortion spark gap in the literature reported drastically higher jitter, 23 µs, they also demonstrated much longer lifetimes. Winands et al[12] were not able to report actual erosion rate numbers, but were able to estimate an erosion rate of < 8 ¢ 10−6 cm3/Coulomb; leading to an estimated lifetime of 1010 shots. While the trigatron and eld distortion spark gaps have shown very good results, a spark gap with both very low (<5 ns) jitter and very long (107 − 108) lifetime has yet to be demonstrated.

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CHAPTER 3

EXPERIMENTAL SETUP

A coaxial spark gap was constructed for testing. In order to carry out the necessary tests a high voltage test bed consisting of a pulse forming line and resistive load were also built. The experimental setup consists of the spark gap under study, the high voltage test bed, and all associated diagnostics necessary for the tests.

3.1 Spark Gap

The spark gap is based on an original design, by Jane Lehr[13], for a high pressure trigatron spark gap to be used in high repetition rate applications. The trigatron design was converted over to a three-electrode eld distortion spark gap by Shad Holt for use in electrode erosion studies. The present design incorporates a trigger system, detailed later in the chapter, to allow jitter measurements to be taken. A cross-sectional view of the spark gap design can be seen in Figure 3.1.

The outer housing is made of G-10. The G-10 provides a rigid structure necessary to keep the internal switch components in place during pressurization. The insulating material inside the housing is made of KEL-F (PCTFE), a high density plastic that is relatively impermeable to lighter gases such as H2 and o ers excel- lent chemical resistivity. Chemical resistivity could help play a role in reducing the e ects of insulator erosion. The KEL-F insulators allowed a wide variety of gases to be used as the insulating medium in the spark gap in a number of di erent experiments. In this experiment only high pressure (> 100 psig) N2 was used. The inner walls of the insulators are corrugated. The corrugations act as a reservoir for impurities in the switch arc that could limit repetition rate and otherwise build up on the switch walls providing a conductive path. The gap spacing is set using alu-

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Figure 3.1: Spark gap cross section minum baes and copper shim stock. The variability of the baes and shim stock allow for di erent midplane geometries to be tested. The midplane can e ectively be biased toward the anode or cathode by changing the amount of shim stock used to set the gap spacing of each electrode without needing to rearrange the con guration of the actual midplane. Di erent midplane con gurations (ie, pin, etc) can be tested by substituting the annular midplane with the new con guration. The baes also act as a two-fold shield. They reduce the build up of contaminants on the insulator walls and help shield the insulator material from the switch arc, this reduced the e ects of insulator erosion. The combination of the liner corrugations and baes are intended to eliminate the issue of surface tracking inside the switch.

This would allow the switch to be used for extended periods without maintenance.

The spark gap electrodes are made of Cu25W75 and machined to an approximate

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Bruce pro le. The Cu25W75 was chosen because in previous erosion experiments it

8 has shown to have attainable lifetimes of 10 shots or more??. Cu25W75 also has superior corrosion resistance while maintaining machinability compared to other allows of W. The Bruce pro le electrodes are designed to produce a uniform electric eld between the anode and the cathode. A uniform electric eld is important for erosion because any eld enhancements on the electrode surface would cause the arc between anode and cathode to concentrate on that point, causing more material erosion in that area. Uniform eld electrodes allow for a high enough electric

eld throughout the entire gap increasing the probability of developing an electron avalanche.

Figure 3.2: Machined Bruce pro le electrodes.

3.1.1 Electrostatic Modeling

Electrostatic testing was done to determine what e ects, if any, the baes and midplane would have on the electric eld between the anode and cathode and also to determine if the baes a ect the potential distribution throughout the

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Figure 3.3: Comparison of electrode pro les. Left - Standard Bruce pro le, Right - Modi ed Bruce pro le gap as compared to the theoretical model discussed in the rst chapter. The rst simulation shows that the electric eld between the modi ed Bruce pro le electrodes is indeed uniform. A Bruce pro le electrode, as de ned in the literature[14], consists of a at plane with a sine curve transition down to a circular back edge. Due to limitations in the machining process the circular back edge had to be removed and a simple at edge was substituted. The area of the electrode that was modi ed is shielded by the aluminum baes and, therefore, the modi cation should have no impact on the electric eld. A comparison of the two electrode pro les can be seen in Figure 3.3.

The electrostatic model was carried out using Ansoft Maxwell 2D. The simulation was run with the anode biased to 50 kV, the cathode grounded, and the midplane, when present, oating between the two voltages. The results of the simulation can be seen in Figure 3.4 and Figure 3.5. As shown, the baes and midplane produce a slight distortion of the electric eld between the anode and cathode.

However, since both the bae and midplane are radially symmetric this distortion should have little a ect on the erosion of the electrodes, and, as can be seen in the electrode erosion section, a uniform erosion pattern was achieved.

Fig. 3.6 shows the evolution of the potential throughout the gap as the breakdown process proceeds. Comparison with the theoretical model show roughly equiv-

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Figure 3.4: Electrostatic eld simulation of Bruce pro le electrodes.

Figure 3.5: Electrostatic eld simulation. alent potential distribution. There is a change in the distribution near the tips of the electrode baes which would indicate an enhancement of the electric eld at these points. The electric eld is uniformly higher between the two electrodes so this enhancement should have little e ect.

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Figure 3.6: Potential distribution throughout the gap for each stage of the breakdown process. Top - Main electrode at high, positive voltage with the trigger electrode oating. Middle - Incoming trigger20 pulse causes an increase in potential di erence between main electrode and trigger electrode. Bottom - Establishment of arc between main and trigger electrodes brings them to the same potential and increases the potential di erence between trigger electrode and ground electrode. Texas Tech University, Colt James, December 2007

3.2 Test Bed

The experimental test bed is made up of the pulse forming line (PFL), resistive load, and triggering system. The PFL consists of two, 8.5 m RG-220 coaxial transmission lines and is designed to hold o a minimum of 60 kVDC. The test bed power supply can only provide 20 mA of current and so the test bed is limited to 100 Hz repetition rate because the capacitance of the PFL is ∼2.5 nF, which produces a 10 mA current draw at 50 kV. The spark gap has been operated at higher repetition rates in previous experiments, however, due to this limitation jitter measurements cannot be recorded at repetition rates above 100 Hz.

Figure 3.7: Test bed equivalent circuit

The test bed produces an 85 ns pulse transferring 2 J (<1 mC) per pulse into a 25 Ω load. The load is made up of; four 6 Ω ceramic washer resistors supported by two 24" aluminum anges that also serve as the ground return. 18" annular copper heat sinks located between each pair of resistors provide adequate thermal dissipation for extended runs of the experiment. An exploded view of the load base and ground return path can be seen in Figure 3.8.

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Figure 3.8: Test Bed Load

3.2.1 Trigger System

Two separate triggering systems were used in this project. Preliminary tests were carried out using a high voltage pulser. The pulser produced a -100 kV, 15 ns pulse with a rise time <10 ns. The pulser was an acceptable trigger source, however, it was limited to 50 Hz burst operation and was being used by other experiments and so a PFL trigger was built. The PFL trigger equivalent circuit can be seen in Figure 3.9. A 1 m section of RG-220 coaxial cable is charged negatively

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up to 100 kV and then a N2 pressurized spark gap is allowed to self-break sending a 10 ns pulse to the midplane of the main spark gap. The trigger pulse has a rise time of approximately 3 ns which should provide acceptable jitter measurements based on the dependence of switch jitter on fast rising trigger pulses (10kV/ns)[9].

The power supply for the PFL trigger can only supply 2 mA of current maximum, but due to the short length of coaxial cable used this is adequate to reach 100 Hz repetition rate. Initial jitter tests using the PFL trigger were performed both with and without a 50 Ω load on the PFL. In some switch con gurations the unloaded PFL actually produced dramatically lower jitter than the same con guration with the load attached. However, leaving the cathode of the trigger gap oating produced very unreliable triggering and also required much higher charging voltages and so it was decided all jitter tests would be performed with the 50 Ω load in place and comparisons could be made from there.

Figure 3.9: PFL Trigger System

3.3 Diagnostics

Jitter measurement requires accurate measurement of the leading edge of both the trigger and the main gap pulse. The main gap pulse was recorded using a

NorthStar PVM-2 high voltage probe. Preliminary tests done using the SOS Pulser used a capacitive voltage divider built into the pulser itself for trigger measurement.

A coaxial voltage probe was built when the PFL trigger was implemented. The

23 Texas Tech University, Colt James, December 2007 coaxial probe is a capacitive voltage divider with a fast response (∼1 ns) to record the short pulse used to trigger the spark gap. Erosion measurements were performed using a GK2202 electronic scale. The scale has an accuracy of +/- 1 mg.

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CHAPTER 4

EXPERIMENTAL RESULTS

Switch jitter measurements were taken in nitrogen over the lifetime of the electrodes. Mass measurements of the electrodes and baes were recorded using a scale with one milligram accuracy. The switch was triggered at approximately 75 pps and jitter measurements were then taken by recording random samples of the trigger and switch voltage waveform. Jitter measurements were taken with three di erent midplanes; an annular midplane, a point midplane, and a radiused midplane. The three midplane geometries have an equivalent 7/8" aperture and can be seen in Fig. 4.1. Each midplane was also tested in three positions; equi-distant between anode and cathode, biased toward the cathode, and biased toward the anode. Once jitter had been recorded for all midplanes and positions the switch was run for approximately half a million shots at self-break to erode the electrodes further. The switch was run at self-break to decrease the amount of maintenance needed on the triggering gap. Mass and jitter measurements were then repeated and the whole cycle was continued until a total of 107 shots had been reached. Referring to Fig. 4.2, statistical delay time in this experiment is calculated as the time at which the rise time of the main gap pulse reaches 90% of its nal value minus the 10% point of the trigger pulse fall time. Jitter is calculated as the standard deviation

v u u u 1 N σ = t (x − x)2 (4.1) N i i=1 X of all the shots taken for one dataset.

25 Texas Tech University, Colt James, December 2007

Figure 4.1: Top - Annular midplane, Left - Point midplane, Right - Radius midplane

4.1 Preliminary Results

Preliminary tests were done to determine the lifetime of the switch based on jitter. An annular midplane was triggered in a neutral position between the anode and cathode. Jitter data was recorded every 5 million shots because of the expected long lifetime of the electrode material (108 shots). A summary of erosion measurements is shown in Table 4.1. The erosion measurements clearly show the erosion mechanisms at work. While both electrodes are heated and eroded by the switch arc, the anode shows most of the erosion while the cathode shows very little. Fig. 4.3 show the anode and cathode after 10 million shots. The formation of pits on the electrode surface can clearly be seen along with the build up of copper nitride (Cu3N). The copper nitride is a greenish ceramic that, while conductive, could a ect the eld shape.

The jitter results for the preliminary tests are summarized in Table 4.2. A large increase in jitter can be seen from the beginning of the tests to the end. Fig. 4.5

26 Texas Tech University, Colt James, December 2007

Figure 4.2: Overlay of ve voltage waveforms. The magnitude of the trigger voltage was scaled down to allow the waveform to t in the graph. shows what appears to be a threshold in the lifetime for the jitter some where between 5{10 million shots. It can be assumed this apparent threshold is caused by the inevitable breakdown voltage instabilities that come about from electrode erosion. Fig. 4.5 shows the change in statistical delay times in the spark gap over

Table 4.1: Summary of electrode erosion data for preliminary tests. Negative values indicate a loss of mass, while positive numbers indicate an increase in mass. Mavg Mnew(g) Mfinal(g) ∆M Shot (g) Anode 73.844 73.598 -0.33% -2.66E-08 Anode Baﬄe 40.299 40.325 0.06% 2.81E-09 Cathode 71.871 71.828 -0.06% -4.65E-09 Cathode Baﬄe 40.367 40.399 0.08% 3.46E-09

27 Texas Tech University, Colt James, December 2007

Figure 4.3: Image of the anode and cathode taken after 10 million shots. The baes can be seen surrounding the electrodes.

Table 4.2: Summary of switch jitter for the preliminary tests. New 10¢106 Shots Change 50 Hz Jitter 2.61 ns 26.28 ns 1006% 1 Hz Jitter 400 ps 2.22 ns 555% time. Their increase over time lends evidence to the erosion mechanisms causing an e ective end of lifetime for this application around 10 million shots. In the next section it will show that upon further testing this does not seem to be the case.

Even with an end of life of approximately 10 million shots this demonstrates a eld distortion spark gap with very low (<5 ns) jitter for up to and over 1 million shots. After 10 million shots at 50kV no change in hold o voltage nor physical degra- dation was noticed in the KEL{F insulators. Visual inspection at each weighing showed the buildup of contaminants inside the switch to mostly be upon the electrode baes and only lightly on the insulators. One problem that was encountered approaching the 10 million shot mark was surface tracking inside the G{10 housing.

Removing approximately 10 mil (0.01 in) of material from the inside of the housing proved to be an e ective way to alleviate this problem.

28 Texas Tech University, Colt James, December 2007

Figure 4.4: Standard deviation jitter recorded over 10 million shots.

Figure 4.5: Statistical delay times over the recorded lifetime of the electrodes.

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Table 4.3: Summary of electrode erosion data for nal tests. Negative values indicate a loss of mass, while positive numbers indicate an increase in mass. Mavg Mnew(g) Mfinal(g) ∆M Shot (g) Anode 75.575 75.287 -0.288% -3.20E-08 Anode Baﬄe 39.672 39.708 0.036% 4.00E-09 Cathode 71.751 71.685 -0.066% -7.33E-09 Cathode Baﬄe 39.496 39.514 0.018% 2.00E-09

4.2 Final Results

After the preliminary tests were completed the interior of the housing and insulators were thoroughly cleaned and new baes, electrodes, and midplane were machined. This series of tests was conducted to determine where and why the apparent jitter increase appears and also to test the di erent midplane geometries discussed in the introduction of this chapter.

Once again the erosion measurements are summarized in Table 4.3. As expected, these measurements match almost exactly the preliminary experiment. A comparison photo taken of the anode after 10 million shots shows a slightly increased build up of Cu3N. Uniform erosion can clearly be seen across the face of the electrode. Comparisons between all nine midplane tests are summarized in Table 4.4. The anode biased position was determined to be unusable. Moving the midplane past neutral toward the anode resulted in signi cantly decreased hold o voltage, as expected, and the spark gap could not be triggered in this position. Since no data could initially be gathered for this position it was removed from the tests. This left the neutral and cathode biased positions to test with the three midplanes. Of the two positions, the cathode biased one showed the best performance. The cathode biased position consistently triggered more reliably and often demonstrated single nanosecond, and even sub-nanosecond, jitter. The rst thing that can be noticed from Fig. 4.3 is that, regardless of midplane geometry, moving the midplane closer

30 Texas Tech University, Colt James, December 2007 to the cathode results in lower jitter. In the annular and point midplane cases, the jitter improvement is approximately two orders of magnitude and even in the worst case, the radiused midplane, jitter improvement is still a factor of fteen over the neutral position case. A comparison of the jitter over the course of 10 million shots shows that moving the midplane toward the cathode caused a much lower, consistent jitter. While very low jitter (sub-ten nanosecond jitter was recorded) is attainable with the midplane in the neutral position, it uctuates greatly over time. When the midplane is moved toward the cathode it is repeatably low over the course of the test.

Figure 4.6: Comparison photo of the second tested anode after 10 million shots.

Using the annular midplane as an example and referring to Fig. 4.7, the jitter performance between the neutral bias case and cathode biased can be seen. Both positions exhibit sub-ten nanosecond jitter; however, for the neutral position this is only a rare occurrence. The jitter, and in turn statistical delay times, uctuate wildly, sometimes as much as hundreds of nanoseconds. This appears similar to the lengthened breakdown process that occurs when the gap is severely under-voltaged; however, charging voltages were kept constant through out the experiments. The

31 Texas Tech University, Colt James, December 2007

Table 4.4: Summary of electrode erosion data for preliminary tests. Negative values indicate a loss of mass, while positive numbers indicate an increase in mass. Annular Midplane Point Midplane Radius Midplane (ns) (ns) (ns) Neutral Position 236.34 2204.69 2468.17 Cathode Bias 2.52 11.39 161.83 Anode Bias N/A N/A N/A lower jitter for the cathode biased position is due to the almost constant statistical delay time of formation of the pulse. The reason for this can be found by taking the trigatron as an example. As explained, the trigatron breakdown process involves streamers growing out concurrently from the trigger electrode to the anode and cathode. The electrostatic simulations shown in Chapter 3 show only a slight eld enhancement between trigger electrode and cathode when the trigger pulse arrives.

By moving the midplane closer to the cathode the electric eld at these points is greatly enhanced. If the eld enhancement is great enough it is possible that the breakdown process is happening concurrently between the trigger electrode, cathode, and anode. This concurrent process would greatly reduce the statistical delay times and, in turn, reduce jitter. This data also explains the results from the preliminary tests. The preliminary tests were done with the midplane in the neutral bias position and these uctuations would explain the steep rise in jitter that appeared to signal the end of lifetime for the electrodes. More data points would have showed the jitter uctuating even higher and from time to time return to low values like it does here. Fig. 4.7 shows a comparison of the three di erent midplane geometries in the cathode biased position. Fig. 4.9 shows all the geometries in the neutrally biased position.

The performance of all three midplane geometries appears in Table 4.4. The radiused midplane performed considerably worse than the other two geometries.

The jitter using this geometry was a factor of 10 higher than the point midplane

32 Texas Tech University, Colt James, December 2007

Figure 4.7: Neutral biased jitter vs. cathode biased jitter of the annular midplane over tested lifetime of the switch and almost 100 times higher than the annular midplane. The radiused midplane also required approximately 25% higher voltage in order to trigger reliably, except in some cases in the cathode biased position where triggering was not possible with any reasonable voltage. This increased trigger voltage requirement could be due to a decreased surface area compared to the annular midplane or decreased eld enhancement compared to the point midplane. Since breakdown is a statistical process the electric eld needs to be high enough every where in the gap to increase the probability of getting out an initiatory electron. This is why uniform eld electrodes are so often used. If the surface area of the trigger electrode exposed to

33 Texas Tech University, Colt James, December 2007

Figure 4.8: Comparison of midplane geometries. All measurements taken with the midplane in the cathode biased position.

Figure 4.9: Comparison of midplane geometries. All measurements taken with the midplane in the neutral biased position. 34 Texas Tech University, Colt James, December 2007 the gap is reduced then the probability of initial avalanches being in the vicinity of the high eld region created by the incoming trigger pulse is reduced. This decreased probability would lead to increased statistical delay times. This problem might be resolved by decreasing the e ective aperture of the midplane which would move more of the radiused spoke into the gap. Alternatively, radiused spokes could be added to the midplane to increase the surface area around the gap. The annular and point midplane both performed drastically better than the radiused midplane and are much easier to fabricate. Add to this the fact that any modi cations to the radiused midplane would e ecively mimick the geometry of either of the other two and the radiused midplane is shown to simply be an inadequate subset of the rst two geometries. Unless speci c tests or applications call for such a geometry the annular and point midplane perform markedly better and are easier to construct.

Geometry tests were discontinued after 10 million shots as the e ects of both midplane position and geometry were considered fully characterized. Lifetime tests were continued with the annular midplane in the cathode biased position.

This con guration showed both the lowest and most reliable jitter measurements and so should be the best indicator of the end of lifetime. Antenna simulations conducted[3] have shown a jitter of <10 ns is required for acceptable operation of the phased array. This will set the upper bound for lifetime. When the spark gap approaches a jitter of 10 ns that will indicate the maximum usable life.

After 10 million shots on the new electrodes there was no recorded change in jitter. This further disproves any conclusions made from the preliminary tests.

However, after 5{6 millions shots on the new electrodes, surface tracking began again on the interior of the G{10 housing. The switch was dismantled and machined again, but the problem was not resolved. The housing was then lled with dielectric grease in an attempt to alleviate the problem. The grease has a higher voltage hold-

35 Texas Tech University, Colt James, December 2007 o than air and should work similarly to pressurizing the housing. The grease lled switch was reassembled and tested and the a ect was complete breakdown between the high voltage connection and ground through the grease. The e ects of this breakdown are shown in Fig 4.10. The cause of this short is most likely localized enhancement of the electric eld in the grease due to air bubbles or other voids in the grease. The dielectric material needs to be uniformly distributed through out the housing. If the dielectric is properly applied it will be able to hold o a higher voltage than that of air. If the dielectric is not uniformly distributed, which is most likely the case here, the dielectric will be able to hold o the increased electric

eld but breakdown will occur in any air bubbles or spaces the dielectric does not

ll. This breakdown could lead, as it did in this case, to complete breakdown from the high voltage node to ground. The e ects of the breakdown had to be machined o the interior of the housing and all the switch components had to be thoroughly cleaned to return the switch to operational status. At this point it seems quite clear that the G{10 housing is the limiting factor for lifetime of the spark gap. The electrical and mechanical characteristics of G{10 are listed in Table 4.5. The mechanical properties of the G{10 are necessary to pressurize the switch safely and therefore, another material is unlikely to serve as a proper substitute. Comparable polymers (500 { 1000 V/mil), such as nylon or delrin, have signi cantly lower mechanical strengths. Delrin has a tensile strength of only

75 MPa and nylon has a maximum tensile strength of 100 MPa, so there are no other clear alternatives. Other possible ways of increasing the hold-o voltage of the housing include machining grooves in the interior similar to those on the exterior or applying a dielectric coating to the interior of the housing. The coating was the least invasive and potentially damaging so a 4 mil coating of Kapton (dielectric strength of 3.3 and voltage hold-o of 7 kV/mil) was applied. The Kapton performed well

36 Texas Tech University, Colt James, December 2007

Table 4.5: G{10 mechanical and electrical properties. Mechanical Properties Electrical Properties Tensile Strength 262 MPa Dielectric Constant 5 Flexural Strength 517 MPa Dielectric Strength 800 V/mil Compressive Strength 448 MPa compared to the complexity of installation showing that a dielectric coating is capable of successfully increasing the hold-o voltage of G{10. A spray coating of PTFE (1.5 kV/mil hold-o ) could be applied when the housing is fabricated to provide adequate increase to the hold-o voltage then the switch could be assembled for use without worrying about eventual surface tracking. Lifetime tests were performed to 25 million shots with no change in jitter. Erosion measurements predict an erosion rate of approximately 2.56¢10−4gram/C (1.79¢10−5cm3/C) for the anode. The cathode erosion rate should be similar, but since material ejected from the anode builds up on the cathode this erosion rate is dicult to quantify. If a layer approximately 1 mm thick is allowed to erode from the electrode surface before either the jitter increases due to voltage instability or over all hold-o voltage is changed due to increased gap length than it can be calculated that about 0.113 cm3 of material is allowed to erode. This leads to an estimated lifetime of 50 million shots and a total charge transfer of 6500 C. Recall that the erosion measurements were taken under self-break conditions. Based on the work done by Dickens[15], triggered spark gap erosion for low coulomb transfer applications is 50% less than operation at self-break. This mean if the spark gap is used in only triggered operations then the lifetime estimate is closer to 100 million shots.

37 Texas Tech University, Colt James, December 2007

Figure 4.10: Close-up image of the e ects of breakdown inside the G{10 housing.

38 Texas Tech University, Colt James, December 2007

CHAPTER 5 CONCLUSIONS

The purpose of this experiment was to evaluate a spark gap design for pulsed ring-down and phased array applications. Pulsed ring down applications presently being designed require both a long lifetime switch and a switch that has very low jitter (<10 ns). Many e orts have been made to increase the lifetime of spark gaps, however, most have overlooked switching jitter as an important aspect of spark gap operation. The test setup built for this project allows jitter tests to be conducted over the lifetime of the test spark gap. The e ects of switch geometry (midplane position and geometry) can also be tested with minimal down time. A high pressure coaxial spark gap has been designed and evaluated. Sub-nanosecond jitter has been reported in some long-life trigatrons[8]. The main drawback to the trigatron design however, is an increased erosion of the trigger pin leading to a decrease in switch lifetime. Using the materials described a midplane triggered spark gap has been built and tested that exhibits low jitter with less than 1 mg/C of material eroded. The projected lifetime of the electrode materials used is on the order of 108 shots[6] with little variation in jitter demonstrated up to and over 107 shots. Thus far the limiting factor has proven not to be the electrodes or insulator material but the G-10 used as housing material. Possible ways of increasing the lifetime of the housing include increasing the surface area of the interior of the housing by machining grooves in the surface like those on the exterior or lining the interior with a material or a coating with higher dielectric constant and voltage hold o characteristics. The e ects of midplane geometry on switching jitter have also been categorized. It has been shown that by moving the midplane closer to the cathode, e ectively mimicking an inverted trigatron spark gap with the cathode and midplane in the same plane, very low jitter (∼2 ns) can be obtained. These jitter measurements rival many of the trigatrons detailed in the literature while maintaining a lifetime of upwards of 20 million shots. Based on these results a phased array antenna system could be designed and imple-

39 Texas Tech University, Colt James, December 2007 mented to run at 100 Hz without maintenance or intervention for close to 13 days (300 hours) of continuous operation. This system would maintain voltage hold-o and beam directivity, based on jitter, over the full course of this operation.

40 Texas Tech University, Colt James, December 2007

BIBLIOGRAPHY

[1] Hansjoachim Bluhm. Pulsed Power Systems: Principles and Applications, chapter 4. Springer, 2006.

[2] Essam Nasser. Fundamental of Gaseous Ionization and Plasma Electronics. Wiley- Interscience, 1971.

[3] John Walter, James C. Dickens, John J. Mankowski, and M. Kristiansen. Theoretical Pulsed Ring down Antenna Array Performance. In 16th Int. Pulsed Power Conf., 2007. Albuquerque, NM USA.

[4] A. J. McPhee, S. J. MacGregor, and S. M. Turnbull. An investigation of trigatron breakdown by two di erent mechanisms. In 10th Int. Pulsed Power Conf., pages 775{780, 1995.

[5] P. F. Williams and F. E. Peterkin. Triggering in trigatron spark gaps: A fundamental study. J. Appl. Phys., 66:4163{4175, 1989.

[6] Anthony L. Donaldson. Electrode Erosion in High Current, High Energy Transient Arcs. PhD thesis, Texas Tech University, Lubbock, TX, December 1990.

[7] G. S. Belkin. Dependence of electrode erosion on heat ux and duration of current ow. Sov. Phys. Tech. Phys., 15(7):1167{1170, 1971.

[8] A. J. McPhee, I. C. Somerville, and S. J. MacGregor. The Design and Electrostatic Modelling of a High Voltage, Low Jitter Trigatron for Repetitive Operation. In 10th Int. Pulsed Power Conf., 1995. Albuquerque, NM USA.

[9] J. R. Mayes, W. J. Carey, W. C. Nunnally, and L. Altgilbers. Sub-nanosecond Jitter Operation of Marx Generators. Pulsed Power Plasma Science, 2001. Digest of Technical Papers, 1:471{474, 2001.

[10] F. Deutsch. Triggering of a pressurized spark gap by a laser beam. Brit. J. Appl. Phys., 1:1711{1719, 1968.

41 Texas Tech University, Colt James, December 2007

[11] M. G. Grothaus, S. L. Moran, and L. W. Hardesty. Recovery characteristics of hydrogen spark gaps. In 9th Int. Pulsed Power Conf., 1993.

[12] G. J. J. Winands, Z. Liu, A. J. M. Pemen, E. J. M. van Heesch, and K. Yan. Long lifetime, triggered, spark-gap switch for repetitive pulsed power applications. Rev. Sci. Instrum., 76:085107{1{085107{6, 2005.

[13] Jane M. Lehr, Michael D. Abdalla, Frederick R. Gruner, Brett C. Cockreham, Michael C. Skipper, Sean M. Ahern, and William D. Prather. Development of a Her- metically Sealed, High-Energy Trigatron Switch for High Repetition Rate Applications. IEEE Trans. Plasma Sci., 28(3):1469{1475, Oct 2000.

[14] F. M. Bruce. Calibration of uniform eld spark gaps for high voltage measurements at power frequencies. In Proc. IEE, volume II, page 138, 1947.

[15] J. C. Dickens, T. G. Engel, and M. Kristiansen. Electrode performance of a three electrode triggered high energy spark gap switch. In 9th Int. Pulsed Power Conf., pages 471{474, 1993.

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