ft ‘AD—A039 460 ARMY ELECTRONICS COMMAND FORT MONMOUTH N .~J F/S ~/5MOCILATOR — REPETITIVELY PULSED FIELD EMISSION ELECTRON BEAM SU—ETC(U)APR 77 5 .~J DEZEP4fl.. W WRIGHT • S SCHNEIDER

LMCLASSIPIED ECOM—1469 Pit. a

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Resea rch and Development Technical Repo rt

~~ ECOM - 4489

MODULATOR-REPETITIVELY PULSED FIELD EMISS IONELECTRON BEAM GUN INTERFACE

George J0 DezenbergU. S. Army Missile CommandW 0 Wrig htS. Schne iderElectronics Technology & Devices Laboratory

April 1977D D C

F { ~~~~ ECOM

~~ A R M Y E L E C T R O N I C S CO M M A N D F O R T M O NM OU TH , NEW J E R S E Y 0 7 7 0 3

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N O T I C E S

: ~DiscIaimers

The findings in this report are not to bo construed as anofficial Department of the Army position, unless so desig-nated by other authorized documents.

The citation of trade names and names of manufacturers inthis report is not to he construed as official Governmentindorsement or approval of commercial products or servicesre ferenced herein.

Disposition

Destroy this report when it is no longer needed. Do notreturn it to the originator.

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~~~~~ BEFORE COMPLETING FORM1. REPORT NUMBER 2. GOVT ACCESSION NO. 3. RECIPi ENT’ S CATALOG NUMBER

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-~~-~----——-——--——— —-——-— ———---——- ~----—-— 1 R~ ~~~~ “ ~. ab~~ ~~~~~~~~~~ Modulator — Repetitively Pulsed Field Emission

1 p~~~nica1~~ep’~~t.Electron Beam Gun Interface.

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7. A UTHOR(s ) 8. CONTRACT OR GRANT NUMBER(a)

George J. Dezenberg. US Army Missile Cmd ,Redstone Arsenal, AL andW. Wri~ht , and S. Schneider. USAETDL. Ft Monmouth __________________________

9. PERFORMING ORGANIZATION NAM E AND ADDRESS 10. PROGRAM ELEMENT. PROJECT . TA SKAREA & WORK UNI BER S

U~~’Army Electronics Command —

(ATTN: DRSEL—TL-BG) .

Fort Monmouth , NJ 07703 lL~~~

27~Ø5~j

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II. CONTROLLING OFFICE NAME AND ADDRESS . PORT

Beam, Plasma and Display Technical Area ____________________________US Army Electronics Te chnology and Devices Lab

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SCHEDULE

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Approved for public relea ution unlimited. I

• 17. 015 RIBUTION STATEMENT (~f lb. abstract ait.t.d In Block 20. It diIf.rsnt from Report)

18. SUPPLEMENTARY NOTES

19. KEY WORDS (Continue on rev•rse aid. if n.c.asa,y aid identify by block numb.r)

Electron Beam GunBlumlein ModulatorPulse PowerField Emission

• Hig h Voltage20. AUSTRACT rCait~~u. ~~~~~~ .1* ii ~~~ I d .nf lf y by block ,um~b.c)

A field emission electron beam gun is repetitively pulsed with a modulatorin a Blumlein arrangement. The modulator is operated in an unmatched conditionwith the output connected directly to the gun. The gun is a time varying mono—tonically decreasing impedance load while the modulator impedance is constant .The modulator—gun configuration produces an initial voltage peak which ap—proaches twice the value of the charge voltage to promote gun emission. Afterthe Initial peak, the load voltage assumes a value determined by the gun

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iuçedance. Peak voltages in excess of 350 kV and peak currents up to 8 kA

have been delivered in 5 jis pulses by the modulator to the gun. The modu-

lator routinely operates at 50 Hz repetition rate, 135 kV recharge voltageand about 3 amps of average current.

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MODULATOR - REPETITIVELY PULSED FIELD EMISSIONELECTRON BEAN GUN INTERFAC ’

by

George J. Dez~tibergUS Army Missile Command

Redstone Arsenal , Alabama 35809

William Wright and col SchneiderUS Army Electronics Command

Fort Monmouth , New Jersey 07703

ABSTRACT

A field emission electron beam gun is repetitivelypulsed with a modulator in a Blumlein arrange’nent . Themodulator is operated in an unmatched condition with theoutput connected directly to the gun. The gun is a timevarying monotonically decreasing impedance,load while themodulator impedance. is constant . The modulator—gun config-uration produces an initial voltage peak which approachestwice the value of the charge voltage to promote gun emission.

- After the initial peak, the load voltage assumes a valuedetermined by the gun impedance. Peak voltages in excessof 350 kV and peak currents up to 8 k.A have been deliveredin 5 ~s pulses by the modulator to the gun . The modulatorroutinely operates at 50 Hz repetition rate, 135 kV rechargevoltage and about 3 amps of average current .~ , -

IntroductionThe modulator ançl electron beam gun described in this paper are a

part of a repetitively pulsed CO2 laser system called CCEBL (ali acronymfor Cold Cathode Electron Beam Laser) operating at the US Army MissileCoimsand. The CCEBL uses an unmodulated sustainer field as shown inFigure 1. The voltage on the sustainer capacitor is at a value belowlaser gas breakdown. Conduction occurs when electrons from the electronbeam gun are injected into the sustainer field region. Conduction ceaseswhen the electron beam gun is turned off. Since the electron beam guncontrols sustainer conduction and hence, laser action, the modulator-guninterface is the determining factor in the operation of the CCEBL.

Cold cathode, or field emission, electron beam guns similar to tl1eone described in this work have been used in GO2, CO, excimer, :ndelectrically initiated chemical laser systems . Most of these laserapplications have been in Marx—generator driven single pulse sy ’tems.A GIN—400 generator was used to provide up to 300 kV, 500 A pul~ c.r ofabout 1—us duration at a 5 Hz rate to a cold—cathode electron beam gunused in a CO2 laser.’ A 15—cm by 50—cm cold-cathode electron beam gunhas been operated at a 50—Hz rate using a lumped element line driver,pulsed—transformer combination.2 The open core pulse transformer had asecondary to primary turns ratio of 17.5 to 1 and provided 250 kV,

4 750 A pulses of 3 us duration to the gun. This paper describes thecharacteristics of the first Bluinlein modulator driven repetitivelypulsed cold—cathode electron beam gun.

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r _Gun Characteristicsthe d iode gun used in this work, designed with the aid of John 4

Rink 1 , is shown schematically in Figure 2. A stainless steel gun vacuumchamber housed the aluminum cathode structure, consisting of a 15—cmwide by 1.3—cm thick plate surrounded by a 5—cm diameter field gradingring. A single 2—meter long, 25—nm thick tantalum strip emitter ismounted at the center of the cathode normal to the plate. The emitteredge projects 1.5—cm from the front surface of the cathode. The cathodeis mechanically held in the rear by two lucite high voltage bushingsseparated by 1.22 m. Beldon YR—13181 coaxial cable is used to makeelectrical connection to the cathode through the oil—filled bushings.The gun anode consists of the flat stainless steel plate and beryllium—copper foil holder shown to the left in Figure 2. The foil holder wasslotted to provide about 70% open area. The electrons accelerated intothe slotted areas transmit into the laser gas region (sustainer region)through a 25—urn thick, 15—cm by 200—cm wide aluminum foil. In additionto serving as a transmissive element for the electrons, the aluminumfoil window is a vacuum seal between the atmospheric pressure laser gasmixture and the 4 X 10 6 torr gun pressure.

An excellent physical explanation of the operation of the cold—cathode electron beam gun is given in reference 4. The main gun featuresgleaned from this reference are: electron emission is from a plasmainitiated at the emitter strip, gun current reaches the space chargelimited value near the beginning of the pulse (in about 2 ~ls

k), and forconstant gun voltage temporal current rise occurs. This latter featureresults in a time varying, monotonically decreasing gun impedance.

Typical cold—cathode electron beam gun voltage and current temporalbehavior obtained with Marx—generator excitation is illustrated inFigure 3. These data were obtained using a two stage Marx—generatorwith 0.925 uP per stage and charged to 100 kV. Gun anode—cathodespacing dak, the distance bqtween the tantalum strip emitter edge andthe inner surface of the foil holder, was 15—cm in Figure 3. Thetemporal behavior of gun impedance for the experimental conditions ofFigure 3 is illustrated in Figure 4. Impedance is calculated by dividingthe voltage by the current in Figure 3 on a point—by—point basis. Zerotime in Figure 4 is taken to correspond to Marx—generator erection time.During the initial 2 us of the pulse triggering~noise obscured the voltageand current traces in Figure 3. Hence, the data were averaged in thisregion. This is indicated by the broken line in Figure 4. The timevarying, monotonically decreasing behavior of cold—cathode electron—beamguA impedance is clearly illustrated in Figure 4.

Modulator Requirements and DesignIn addition to matching to a time varying impedance similar to

tha t illustrated in Figure 4, the modulator should satisfy the followingrequirements for efficient gun operation:

(1) High Initial voltage is required . The electric field onthe tantalum strip emitter must be greater than l0~ V/cm to produceinitiation.”

* (2) Initial voltage risetime should be short. A high dV/dtis required to produce a multitude of initiation sites on the tantalumstrip. ’ A dV/dt of at least 200 kV in less than 100 nsec would bedesirable in the CCEBL system. -

(3) High voltage throughout the pulse is required . Thiswould insure minimum aluminum foil window heating and adequate electron

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range to uniformly ionize the laser gas. About 200 kv would be desirablein the CCEBL system.

(4) A very fast voltage fal . at LIIC ~ iid ~t the pt.lse isrequired . This is needed to reduce aluminum toil window heating. At100 kV more than 30% of the electron energy is deposited in the aluminumfoil. This percentage increases witi, decreasing .cLta~a.

The initial modulator designed to satisfy the above requirementsused PFN’s in a Blumlein arrangement and a d amper circuit to compensate[or the time varying gun impedance. 5 Aichough the original design con-cept is valid, diode reliability problems were enc~It!ntered d~~ r~high voltage and current operating conditions of the d a m per . This ledto the modulator redesign shown schematically in Figure 1. There is adistinct design philosophy difference between the initial modulatordesign and the present design. Initially, with the use of a d ampercircuit, the voltage was constant a~ross the gun . The present modulator—gun configuration produces an initial voltage peak which approachestwice the value of the charge voltage to promote gun emission. Afterthe initial peak, the load voltage plateaus at a value determined bythe gun impedance.

The gun modulator , as shown in Figure 1, uses two ten stage PFN ’sin a Blumlein arrangement as before. 5 However, the only diode used inthe present modulator is the front—end clipper diode. The charging diodewas replaced wi th a 260—ohm charging resistor. The end of line clipperconsisting of a resistor and diode in series’ was replaced with an endof line termination consisting of a resistor and capacitor in serb s.The resistor value is equal to the PFU impedance value of 17—ohms andthe capacitor value is 0.02 pF. This value was chosen to be close tothe 0.016 uF value of PFN capacitance per stage. Both the resistor inthe end—of—line termination and the charging resistor are used to dissi-pate the reflected energy resulting from either non—initiati-’~ thegun or gun arcs. Two parallel 15—rn lengths of 60 ~ Beldon YR—l3l8lcoaxial cable are used to interconnect the modulator and gun. T-~i3minimizes loop inductance and also offers an approximate impedance matchbetween the 34 Q modulator impedance and t ime gun impedance.

Modulator—Gun PerformanceSingle shot gun voltage — cur ren t behavior is shown in Figure 5

for an anode—cathode spacing of 17.5 cm and charging voltages of 125 kVand 175 kV. Peak voltages are 210 kV and 260 kV or 1.7 and 1.5 timesthe charge voltages of 125 kV and 175 kV , respect ively. The lower rela—tive peak value at 175 kV charge results because the PFN voltage reachesthe gun initiation voltage value of about 150 kV sooner at 175 kV thanat 125 kV charge. Gun conduction loads the FFNpreventlng it from

• . reaching the open circuit value of twice the c iarge voltage. Gun impe-dance for the above conditions is shown in Figure 6. Note that the gunimpedanc e at 175 kV charge is always less than the impedance at 125 kVcharge. This voltage dependence of impedance is due to the space chargelimited conduction characteristics of the gun. That is, the initialcurrent is proportional to voltage to the three—halves power. Theimpedance levels in Figure ó show that thd modulator operates with apositive mismatch during most or the pulse and a negative mismatch atthe end of the pulse.

• The voltage risetime of the modulator is about 1 us , as shown inFIgure 5. The loop inductance of both PFN ’s in series accounts for0.8 us of the risetime . The remainder is due to the interconnections.

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The 1 ps modulator risetime limits the lifetime of 25 urn thick tantalumstrip emitters to about l0~ pulses. Thinner emitters have much shorterlifetimes. The number of initial emission sites on the strip emitteris determined by the initial voltage risetime .” Fewer sites are producedat slower voltage risetimes. Hence, more emission per site and morelocal emitter errosion occur .

The modulator output waveforms for single shot and repetitiveoperation at 170 kV charging voltage are compared in Figure 7. Thehigher amp li tude traces for both gun vol tage and current are singlesho t data; the lower ampli tude , wider traces are 50 superimposed pulsesat a 50—Hz rate. The ampli tud e reduct ion of about 20% is caused by thehigh power supply impedance resulting in recharge time exceeding theinterpulse interval. The 1~ % amplitude jitter , estimated from thewidth of the trace, is due to the SCR controller in the Megavolt, Inc.250 kV , 2A power supp ly. The recharge cycle consists of a t rain ofcurrent pulses at a 720—Hz rate which produces a staircase PFN chargevoltage. When the firing time is unsynchronized to the charging wave-form, the PFN voltage can be at a different step for each pulse. Thisamplitude jitter can be greatly reduced by operating the modulator ata repetition rate exactly a submultiple of 720 Hz and synchronizingf i r ing t ime to line frequency. The time j i t t e r was in the order of200 nsec. At a power supply setting of 170 kV the PFN recharge voltageat 50 Hz was about 135 kV and the average current was 3A.

ConclusionsThe modulator described in this paper successfully drives a cold—

cathode electron beam gun at 50 Hz. Comparable single shot laserbehavior has been obtained with both the Blumlein modulator and a twostage Marx—generator operating at the same 4.6 kJ energy storage. Workis being performed to optimize laser output by tailoring the impedancecharacteristics of some of the network sections to improve gun perform-ance. The technique has been established in a low voltage simulationcircuit using an electron beam bombarded semiconductor as a time varyingload. The simulated load does not precisely match gun characteristics;hence, actual modulator changes must be varied emperically. C- nsidera-tion is also being given to adding a peaking circuit consisting of alow inductance capacitor and spark gap at the gun connection to increasethe initial rate of application of voltage to the gun and hence, increaseemitter l i fe t ime. -

References1. I. K. Babaev, G. G. Dolgov—Savel’ev, L. L. Kozorovitskii,

I. D. Kon’kov, I. A. Leont ’ev, V. C. Lyakishev, V. K. Orlov,V. F. Razumtsev, S. N. Telepin, D. D. Khondkevich, and N. V. Cheburkin,Soy. J. Quant. Electron. 4, 777 (1974).

2. C. Loda, System, Science and Software Report No. SSS—75—264 (1975) .3. J. P. Rink, Tech. Digest 1973 International Electron Devices

Meeting, 448 (1973).4. S. Singer, J. S. Ladish, and M. J. Nutter , Los Alamos

Scientific Laboratory Report No. LA—TJR—75—1585 (1975).* 5. W. H. Wright, S. Schneider, and A. J. Buffa, IEEE Conference

Record of 1976 Twelfth Modulator Symposium, 163 (1976).

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