Spark Chamber Pulsing SystemLouis Lavoie, Sherwood Parker, Charles Rey, and Daniel M. Schwartz Citation: Review of Scientific Instruments 35, 1567 (1964); doi: 10.1063/1.1719210 View online: http://dx.doi.org/10.1063/1.1719210 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/35/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Rapid Charging System for Spark Chamber Pulsers Rev. Sci. Instrum. 41, 687 (1970); 10.1063/1.1684619 Spark Chambers: A Simplified System for the Observation of Particle Trajectories in Two Types ofChambers Am. J. Phys. 35, 582 (1967); 10.1119/1.1974193 Spark Chamber Performance in High Intensity Pulsed Magnetic Fields Rev. Sci. Instrum. 34, 33 (1963); 10.1063/1.1718116 Spark Chamber Track Measuring System Rev. Sci. Instrum. 33, 859 (1962); 10.1063/1.1717993 Observations on Pulsed Spark Chambers Rev. Sci. Instrum. 32, 499 (1961); 10.1063/1.1717422

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FATIGUE MACHI:-.IE 1567

brated against a Pratt and Whitney supermicrometer. From these calibration data the following reference curves were plotted: (a) pounds vs millivolt output for the load cell, (b) inches vs millivolt output for the extensometer. Although these curves are not reproduced here, they show a nearly linear relationship between load or displacement and output.

Figure 5 .shows the stress-strain hysteresis loops re­corded from a test sample of pure copper under controlled constant total strain amplitude. Figure 6 illustrates the work-hardening behavior of an aluminum alloy undergoing increasing strain amplitudes.

The experiments reported above represent some typical tests and applications of the fatigue machine described in this paper. No attempt is made to interpret the experi­mental data. It was intended to show that precise cyclic

THE REVIEW OF SCIENTIFIC INSTRUMENTS

stress-strain curves can be recorded under a variety of controlled test conditions.

More complete information concerning the design, con­struction, and performance of the fatigue machine is available from the authors.l

ACKNOWLEDGMENTS

The authors wish to express their gratitude to W. W. Woods, who designed and supervised the assembly and calibration of the electronic systems, to R. E. Wilbert, who designed the mechanical components, including the extensometer, and to C. Peecher and F. Bohn, of the B.S.R.L. machine laboratory, who carried out the precise machine work.

1 R. M. N. Pelloux and S. D. Brooks, Boeing Scientific Research Laboratories, Document DI-82-0268 (July 1963).

VOLUME 35. NUMBER 11 NOVEMBER 1964

Spark Chamber Pulsing System*

LOUIS LAVOIE, SHERWOOD PARKER, CHARLES REY, AND DANIEL M. SCHWARTZ

The Enrico Fermi Institute for Nuclear Studies, The University of Chicago, Chicago, Illinois

(Received 4 May 1964; and in final form, 10 July 1964)

A spark chamber pulser is described in which several avalanche transistors and a step-up transformer drive di­rectly an air spark gap whose trigger electrode is surrounded by barium titanate dielectric. Output pulses of from 1 to 25 k V with a risetime of 1 nsec and an output impedance of less than 1 U can be obtained. The total delay from the I-V input pulse to the high voltage output pulse ranges from 17 to 65 nsec depending on the desired voltage and the mode of operation of the spark gap. Jitter times are less than 3 nsec. When Elkonite-tipped electrodes are used, the gap life is in excess of 5 000 000 pulses. Methods are given for reducing the delay to less than 25 nsec at 20 k V and the recovery time to less than 200 Ilsec at 10 k V.

I. INTRODUCTION

T HIS article describes a pulser system of relatively simple and compact construction which attempts to

minimize delay, jitter, rise, and recovery times, and gives reliable operation for a large number of pulses. The basic components used are several avalanche transistors, a step­up transformer, and a spark gap having a trigger electrode surrounded by barium titanate.

II. AVALANCHE TRANSISTOR TRIGGER CIRCUIT

Thy physics of avalanche breakdown and the charac­teristics of transistors operated in the avalanche mode have been known for some time,1,2 but only recently have such

* Research supported by the U. S. Office of Naval Research. 1 S. L. Miller and J. J. Ebers, Bell System Tech. J. 34, 883 (1955);

S. L. Miller, Phys. Rev. 99, 1234 (1955); A. G. Chynoweth, Phys. Rev. 109 1537 (1958); W. Shockley and J. Gibbons, Proc. IRE 46, 1947 (1958); R. C. Macario, Electron. Eng. 31, 262 (1959); D. J. Hamilton, J. F. Gibbons, and W. Shockley, Proc. IRE 47,1102 (1959);

devices been seriously considered as useful circuit ele­ments.2- 4 Their use in the first stage of spark chamber pulsers is described in Refs. 3 and 4.

Because of the low trigger threshold of the spark gap, the avalanche circuit needs only four transistors and a step-up transformer. The circuit is shown in Fig. 1. Under load it delivers a 5oo-V pulse to the input of the transformer with a delay of 5 nsec and a rise time of 5 nsec.

Potentiometers can be used to set the voltage across each transistor close to its individual breakdown voltage, rather than the usually lower voltage that would result from the product of the series I cO current and the collector-

R. Fullwood, Rev. Sci. lnstr. 31, 1186 (1960); R. H. Shaver, Semicond. Prod. 5, 29 (1962).

2 W. M. Henebry, Rev. Sci. lnstr. 32, 1198 (1961); M. Bramson IRE Trans. Nuclear Science NS-9, No.4, 35 (1962). '

3 J. Fischer and G. T. Zorn, IRE Trans. Nuclear Science NS-9, No.3, 261 (1962).

• Q. Kerns, University of California Radiation Laboratory Rept. 10887 (1963).

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1568 L. LAVOIE et al.

I +Y (-600Y) I lOOK

:FIG. 1. Avalanche transistor trigger circuit.

base resistance. However, this was observed to increase the maximum output voltage by only about 15%.

Because of the low source impedance of the avalanche transistors (",20 Q), it is possible to step up their output voltage pulses with a transformer and still be able to drive a reasonable load. A Ferroxcube 208F12S-102 toroid wound with a 4 to 10 turns ratio produces a 1000-V pulse with an additional delay of 5 nsec and a risetime of 10 nsec. As the step-up ratio is increased beyond this point, the output impedance becomes too large compared with the load im­pedance and the risetime and peak voltage consequently drop.

Although no systematic search for high voltage ava­lanche transistors was made, we have found four types particularly suitable in terms of avalanche voltage and cost: Philco or National Semiconductor types 2N2087 and 2N2478, National Semiconductor NSll10, and the Motorola 2N2218. The average avalanche breakdown volt­age was about 100 V for the 2N2218 and about 160 V for the 2N2087 and 2N2478. 5

III. THE SPARK GAP

A. General

Many spark gap designs employing various trigger techniques have appeared in the literature. A rather com­plete discussion of these is given in Craggs et al. 6 and Williams. 7 In general, they require large trigger pulses or accurate adjustments.

The gap described here fires reliably with relatively small trigger pulses (500 V) and requires only normal ma­chining tolerances (0.05 mm). It takes advantage of a well­known effect: the reduction of electric field in an insulator due to its dielectric constant and the consequent over­volting of an adjacent air gap. 6, 7 If a cylinder of dielectric constant k, inner radius r2, and outer radius r3 is inserted

• The 2N2087, 2N2478, and 2N2218 are not made specifically for avalanche service and so should be tested for ability to avalanche.

6 J. Craggs, M. Haine, and J. Meck, J. lust. Elect. Eugrs., Pt. lIlA 93, 963 (1946); J. Craggs and J. Meek, High Voltage Laboratory Technique (Butterworths Scientific Publications Ltd., London, 1954).

7 T. J. Williams, "The Theory and Design of the Triggered Spark Gap," Sandia Corporation Report SCTM 186-59(14) (May 1959, unpublished).

in a cylindrical gap of inner radius r1 and outer radius r 4, the field at either electrode is intensified by the ratio

Despite the end effects present in actual spark gap trigger electrodes, this equation gives a reasonably good estimate of this ratio. The first test was made using linen Bakelite (phenolic) as the dielectric. For the specific dimensions of the gap used, the equations predicted a ratio of 3: 1. It was observed that the use of the dielectric reduced the pulse height necessary for breakdown from 5.5 to 2 kV.

____ High ________

¥ Yoltage ---... _ Electrode _

tl·t::~:;:1f~'O'OI" I" ~GrOund~ BaTi0 3 ~ Electrode / BoTI03

Trigger E'ectrode~ Open Trigger Covered Trigger

FIG. 2. Spark gaps with dielectric triggers.

The dielectric adopted for actual use is barium titanate (BaTiOa), the dielectric used in most high voltage ceramic capacitors. It has a high dielectric constant (over 10(0) and, being a ceramic, is strongly resistant to spark heating. While Bakelite burns away from 5000 to 100000, 0.1 J, 9-kV pulses, barium titanate lasts for over 5000 000 pulses.

B. Modes of Triggering

Two geometric arrangements are used, one in which the metal trigger electrode top is exposed and one in which it is covered (see Fig. 2). In both types the barium titanate is bound tightly to the trigger electrode.8 Since the dielec­tric constant of the ceramic is very high, nearly all of the voltage appears across the air gap. For reliable triggering it is only necessary that the air gap at some point be less than about 0.008 mm/100 V of trigger pulse height. To minimize the effects of erosion on the gap life, it is best to have the trigger electrode centered so that an air space of less than this value completely surrounds it. Typical trigger gap input capacities are 4 to 5 pF.

The are three main differences between the perform­ances of the covered and open trigger electrodes: (1) trig­gering mode, (2) the delay time from the trigger-input to the high voltage output, and (3) feedback of the output pulse at the input. A threshold exists for trigger pulse voltages, above which a spark will occur between an open trigger electrode and the ground electrode, with no voltage on the high voltage electrode. When covered trigger

8 These dielectric cylinders were ultrasonically' cut from barium titanate pieces removed from Sprague 20-kV, 500-pF, capacitors.

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PULSING SYSTEM 1569

electrodes are pulsed and the high voltage electrode is groUnded, no such spark occurs. In operation, with voltage on the high voltage electrode, the main gap fires on trigger pulses that can be as much as two to three times smaller than the trigger spark threshold. The actual value of trigger voltage at which this occurs will depend on the trigger gap spacing. The delays fall in the band labeled "nonsparking mode" in Fig. 3. They are measured by extrapolating the linear rises of the input and output pulses back to zero and taking the difference. The varia­tions in delay for various gaps are comparable to the half­width of the band, and the jitter times are typically 0.5 to 3 nsec. , As the trigger voltage is increased, the delays for open trigger gaps drop continuously. When the voltage becomes great enough to break down the trigger gap, they lie

, in the band marked "sparking mode." There is no signifi­cant drop in delay time for further increases. No such drop is seen for covered triggers, whose times are always in the nonsparking mode band. The shortening of the gap delay in sparking mode operation may be due to photons from the small trigger spark traveling ahead of the main elec­tron avalanche, ionizing the air, and starting secondary avalanches.

Holdoff Voltage (kVl Z 4 6 B 10 12

0.15" Electrode Sp<Jcing

14

Non Sparking Mode

Sparking Mode

0.20"

FIG. 3. Spark gap delay vs main electrode spacing and holdoff voltage for flattened electrodes. Trigger voltage for non-sparking m~de-SOO to 1000 V, for sparking mod~-greater than 1300 V. Open tngger electrode must be used for sparkmg mode operation.

Since no visible spark ever goes to the covered trigger electrode, a much smaller signal is fed back to the input. The feedback signal seen at the input to the avalanche transistor circuit is between 70 and 100 V for open triggers and less than 5 V for covered ones.

The trigger pulse height required for reliable sparking mode operation (normally 1500-1800 V with our gaps) can be reduced by applying a static bias of the same polar­ity as the pulse to the trigger electrode. Since the static breakdown voltage of a gap is always less than the pulsed breakdown voltage, the bias must be kept to

500-600 V. This bias reduces the necessary trigger voltage by 400-500 V.

It is to be noted that pulses of either polarity will trigger the gap, but the high voltage electrode should be positive.

C. Pressurized Gaps

In some critical applications it may be necessary to obtain gap delays of the order of 5 nsec. Two methods of obtaining these fast times were tested. The first consisted of pressurizing the gap.6,8 Since a pressure of n atmospheres of air increases the breakdown field by a factor on n, the gap spacing can be decreased by the same factor for any given voltage. The avalanche drift velocity remains un­changed since the higher field and air density contribute opposing factors of n. The shorter gap spacing then should cause a reduction in delay time by a factor of n. This was observed to occur in practice. The trigger voltage threshold also increases by a factor of n.

D. Cascaded Gaps

The second method of obtaining very short delay times involves the use of the cascaded spark gap. Such gaps are described in Ref. 3. From data shown in Fig. 3, it is seen that low voltage gaps operating in the sparking mode have the shortest delay times. Thus, when a cascaded gap is triggered by an open trigger electrode operating in this mode, the first intermediate gap (which is biased at 2 to 3 k V) breaks down very rapidly. The first intermediate electrode is then pulled down to ground potential and over-voltages the second gap. The overvoltage, plus the copious supply of photons and electrons, causes it to break down even more rapidly. The delays are shown in Fig. 4. The upper line shows the total gap delay, a considerable portion of which is due to the 6-nsec rise of the trigger pulse. The lower line shows the delay from the start of the voltage rise on the first intermediate electrode to the start of the output pulse. Of this, 1.5 nsec was in the delay be­tween the two intermediate electrodes.

40 '0 '" g 30

'" o ~ 20 a. o

<.:> 10

6

Stotic Holdoff Voltage' kV)

FIf? 4. Ga~ delay curves for a three-stage cascaded gap with the first mtermedlate electrode at 2.7 kV (0.5-mm spacing from ground elec~rode) and the second at 7;5 k V. (0.89-mm spacing from first inter­mediate electrode). Open pomts: delay from the first intermediate electrode to the final output. Trigger voltage 3 kV.

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1570 L. LAVOIE et at.

From + HV

r-_--r ____ ~r__---_._--::-~_']'_----_lN-U-I-31---_4 O.l~F. 20 r

lOOK

FIG. 5. Spark gap recovery circuit. All resistors 10%, 1 W unless otherwise specified.

-330Y _-----.L--...l---L---l.--L...L----V\/\r--h.v\,---i

4

6.3Y

The operation of the cascaded gaps was found to be very stable. Once the ratios of the intermediate voltages were made consistent with the spacings chosen (within 10%), the main high voltage could be lowered 40-50% with the breakdown remaining stable, although with a somewhat longer delay time. The operation was also found to be independent of the stored electrostatic energy of the intermediate gaps. Capacities of 20 pF to 0.05 /IF in parallel did not affect its operation.

I t was interesting to note that if a single intermediate electrode with no hole was used, the main gap would not trigger at all. This indicates that large pulsed overvoltages make only a minor contribution and that the main gap is triggered primarily by the photons and/or electrons of the triggering and intermediate sparks.

In principle, one could pressurize the cascaded gap and further reduce the delay time, but there is little point in going to these lengths unless the extra 5 nsec is truly crucial or unless one wishes to operate the gap in the 100-kV range at very high speed .

o 5 , I

em

Output Coble Guide--u-/J

HV(DC)in (Modified BNC),,\

~'----i'I==

..--3 of 7 Output Connections Shown (Modified BNC)

~Alumjnum

~ Electrodes

~ Lucite

_ Barium Titanate

Output Connection Plate

Coupling Capacitor (30kY 2000 pf Shown)

_"""':1----+ HV Electrode ~,;",.,c;;;;_-Trigger Electrode

LUCile Lock Nut 1t~'1I~~~LGroUnd Electrode

Avalanche CirCUit ----+l---~ .. ~ Output to Trigger Electrode (Sliding Spring Contact)

Avo lanche Transistor Circuit in this space

Avolonche Collector Supply (DC) in(UG931 /ul

\Electrode Adjustment (Ground -HV Electrode Spacing)

FIG. 6. Sectional view of the assembled pulser.

E. Recovery Time

When the spark gap voltage recovery time is reduced below 5 msec to permit operation in excess of 200 pulses per second, untriggered refiring occurs for two reasons:

(1) If the voltage recovers sufficiently while too many ions or metastable states capable of giving free electrons still remain, another avalanche may form. This source can be removed by directing a stream of air past the elec­trodes. 6 •9 For 12.7-mm-diam electrodes and a 100-/lsec RC recovery time constant, an air velocity of about 100 m/sec is adequate. Such speeds are easily obtained from a normal laboratory compressed air outlet.

(2) As the charging resistor is lowered to reduce the RC recovery time, the current through the gap with C discharged (= V supply/ Rcharging) becomes large enough to sustain a continuous arc. This effect is eliminated with the circuit shown in Fig. 5. The left half of the 12AU7 and the

FIG. 7. Exploded view of the pulser.

9 K. Wilkinson, J. Inst. Elect. Engrs., Pt. lIlA 93, 1090 (1946).

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PULSING SYSTEM 1571

NU13po are nonnally off, the 6BQ6 is on at low plate current, and the right half of the 12AU7 is on hard. A fraction of the negative signal from the spark gap output pulse is used to trigger the multivibrator. The 200-V posi­tive signal from the right plate of the 12AU7 drives the grid of the 6BQ6 cathode follower, which in turn drives the grid of the NU131 positive 100 V. The resulting current surge of ,....,0.2 A passing through the 70-kQ load resistor keeps the plate within 200 V of ground for about 100 J,lsec for supply voltages of up to 15 kV. The current through the spark gap, 200 V /10 KQ= 20 rnA, is then low enough so that the spark is extinguished.

Using two NU131's in parallel with a 25-kQ load resistor and a 2000-pF charging capacitor, we have operated a lO-kV air spark gap with pulse separations of 200 J,lsec.

F. Life Tests

In a life test of electrode materials, copper, Elkonite,ll and stainless steel electrodes operated for more than 15000000 pulses with a 2000-pF capacitor charged to 9 kV and discharged into a load of 500 pF in parallel with 20 Q. The gaps were fired by open trigger electrodes operat­ing in the sparking mode with 3-kV trigger pulses. Brass and aluminum electrodes lasted for 5 000000 to 10 000 000 pulses.

Gaps using copper and stainless steel electrodes were found to fail after about 500000 pulses when fired by covered triggers driven by a 700-V pulse from two ava­lanche transistors. Failure was apparently due to a shorting of the dielectric-ground air gap by electrode metal de­posits. However, the use of Elkonite-tipped electrodes extended the life to over 5 000 000 pulses. Copper is used for the threaded part of the electrode to prevent a binding to the aluminum ground plate which we found occurs with some other materials after several million pulses.

IV. THE COMPLETE PULSER

Two types of pulsers have been constructed using the methods described above. One is shown in Figs. 6 and 7. It normally uses a covered trigger electrode with an internal capacitor of 500 to 2000 pF, and can trigger a set of slave gaps or drive spark chambers of up to 2000 pF. With the

10 An 18-kV triode made by the National Union Electric Corpora­tion, Bloomington, Illinois.

11 A sintered copper-tungsten electrode material made by P. R. Mallory and Company.

FIG. 8. Block diagram of complete system.

output plate shorted to the high voltage electrode, and attached by coaxial cables to capacitors located at the load, larger capacities can be driven. As many as seven cables can be driven in parallel. Specially modified BNC fittings are used for. the high voltage connections. The housing has provisions for taking either a moving stream of air or pressurization in the region of the spark gap. A block diagram of the complete system is shown in Fig. 8.

The second kind of pulser has been made by modifying the spark gap of Rey.12 This used two adjustable electrodes with a trigger wire midway between them. The trigger wire was removed and the ground electrode was replaced with the open trigger-ground electrode combination shown in Fig. 2. The triggers of a number of these gaps are driven by the unit described above. Open triggers are used for speed since output feedback at the input is no problem here. These gaps can be used with larger capacities, can be placed close to their loads, and have small area circuit paths to minimize inductance.

ACKNOWLEDGMENTS

R. Gabriel and M. Neumann provided many interesting and useful discussions, and R. Sumner assisted in construct­ing the test setups. We are especially pleased to acknow­ledge the contribution of J. Lovda whose enthusiasm and careful and imaginative fabrication of much of the spark gap assembly significantly added to the final success of this work.

12 C. Rey and S. Parker, Nucl. Instr. Methods 20,173 (1963).

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