SIMPLE CIRCUITS TO

PULSE-FIRE IGNITRONS FOR EP-RR 16 DISCHARGE H DEC 1067 /G&l

C. F. VANCE

March, 1967

Department of Engineering Physics

Research School of Physical Sciences

THE AUSTRALIAN NATIONAL UNIVERSITY

ra, A.C.T., Australia.

HANCOCK TJ163.A87 EP-RR16. f T.J 1 63* 1924138 . A87 F. P - R R 1 6 A.N.U. LIBRARY This book was published by ANU Press between 1965–1991. This republication is part of the digitisation project being carried out by Scholarly Information Services/Library and ANU Press. This project aims to make past scholarly works published by The Australian National University available to a global audience under its open-access policy. SIMPLE THYRISTOR CIRCUITS TO PULSE-FIRE

IGNITRONS FOR CAPACITOR DISCHARGE H DEC 1967

by

C. F. VANCE

M arch. 1967

Publication EP-RR 16

Department of Engineering Physics Research School of Physical Sciences THE AUSTRALIAN NATIONAL UNIVERSITY C anberra, A.C.T. A ustralia S UMMARY

Simple thyristor circuits have been used for firing capacitor-discharge ignitrons in a physics * experiment. Experience of over 18, 000 discharges shows that the problem of semiconductor failure in an environ­ ment of high voltage transients can be overcome by atten­ tion to screening and filtering. Speed limitations of present-day are discussed and found acceptable for medium-speed plasma experiments.

*(By October 1967, 48, 000 trouble-free discharges have been obtained).

ii 1 . INTRODUCTION

The use of thyristors in place of has been advertised by a number of manufacturers for some time, including their use for firing ignitrons. However, de­ tailed information on this application is elusive and it has been necessary to assemble fragments of information from many sources and from local experiments to arrive at the designs described herein.

No comprehensive discussion of design principles is attempted; it is not sug­ gested even that the designs are optimum, but they have survived exposure to an environ­ ment of high voltage, high current transients which plagued some earlier workers in the field.

The first units developed were General Purpose Firing Modules (section 2). These fire ignitrons, the of which may be as much as 10 kV from earth, through insulated pulse . The primary circuits are ’’earthy” and the inputs are deliberately attenuated. Thus large input pulses of 80 volts or more can be used in long trigger pulse cables, and false firing due to interference eliminated.

To drive these firing modules, trigger pulse (section 3) are used, these boost 6 volt step outputs from a digital tim er3 * to 90V pulses in 70 ohm cable.

Finally, for the special application of firing a clamp ignitron without bringing voltage measuring leads outside the capacitor room, a self-contained Clamp Trigger Unit (section 4) was installed. It is believed that double screening of this unit has been an essential aspect in the survival of its semiconductors.

2. GENERAL PURPOSE FIRING MODULES

When ignitrons are used for capacitor discharge,1 it often proves inconvenient to earth their cathodes. This has in the past led to two alternative solutions: firing units direct coupled to the ignitron, 2 having high-voltage insulated mains supply and input trigger pulse connection; or earthy firing circuits with high-voltage insulation on output pulse transformers. The former is intrinsically capable of faster rising igni­ tor current and may be considered essential if the main ignitron current rise time is very short. The latter method gives an ignitor current whose initial rate of rise is limited by the pulse transformer leakage inductance which, for single layer windings, is roughly proportional to the insulation thickness. Where the slower firing pulse is acceptable, the pulse transformer option brings two main advantages: the earthed primary circuits can share a charging circuit and the transformer ratio is adaptable to suit the optimum working voltage of the chosen form of primary switching. 3 In the installation used on the toroidal plasma physics experiment at the Australian National University, it was decided to try thyristors instead of thyratrons for discharging the capacitor into the pulse transformer (Figure 1). After hearing of thyristor breakdown due to failure to observe limiting rates of current turn-on, it was encouraging to find one thyristor type designed for pulse duty. 5 (In two years no other GENERAL PURPOSE FIRING MODULE 2

UR 70 560 K

Y 40216 On back piugqmg panels : Shunt Monitor, 6Vln70n/ fo r 600 A

Fig.I I gnitron Firing M odule GENERAL PURPOSE FIRING MODULES 3

supplier has quoted us an equivalent.) By plotting the turn-on current-time curve on logarithmic scales and doing the same (on tracing paper) with typical waveforms, such as a half sine-wave and a critically damped waveform, it is very easy to find the short­ est permissible time to a given peak current. For the type 40216 thyristor working at 600A peak, the minimum rise time is 8 y sec for a half sine waveform and 10 y sec for a critically damped waveform. The latter was selected because of the 5V reverse igni­ tor voltage limit quoted for most ignitrons.

As a compromise between rated firing conditions for BK24, BK178 and GL7703, secondary output for the units was chosen as 1. 8kV open circuit, 200A short circuit. All three types have been operated at capacitor bank voltages up to 9 kV, without a known failure to fire.

The (Figure 1) do not conduct during normal operation, as the circuit is somewhat over-damped. Their function is to protect the thyristors against reverse breakdown in case a unit is triggered with its secondary open-circuited. The are an extended foil type sold for photo-flash duty. It was found that ordinary rectangu­ lar cased paper capacitors gave only half output, which implies an internal inductance about equal to the transformer leakage inductance (3 u H, referred to primary). The transformer construction is described in the Appendix.

The input attenuation shown in Figure 1 was chosen to ensure firing of thy­ ristors of the manufacturer's minimum sensitivity.'^ Since the current batch proved to be about three times as sensitive as this, the 5 ft between gate and has been further shunted so that the average unit requires about 60V to fire. A high voltage level in the trigger cables is necessary to swamp interference.

Although the rise time of ignitor current delivered by these units is 10 y sec, tests have shown that the time delay from the application of an 80V trigger pulse to the start of main current in a GL7703, discharging capacitors from 9kV, is only 2 y sec.

3. TRIGGER PULSE AMPLIFIERS (Figure 2)

These are necessary because the basic timing signals are positive-going steps of only 6V, derived from a digital timer. ^ The poor signal to noise ratio, in a labora­ tory with currents (in a discharge plasma) rising at a few times 10^ amp/sec, necessi­ tates shunt capacitors on the input (470 pF) and output (0.033 y F). In addition the 0. 47 y F capacitors, which are discharged to produce the output, are not charged until after interference from operation of A. C. contactors is over. Finally, the lieft pre­ set compensate for varying sensitivity of the 2SF105 thyristors. These are of a type normally used for "mains duty", but all but one out of seven proved accep­ table for fast pulse duty. This one (since replaced) turned out to require a lower voltage, but longer duration, input to fire. A typical 2SF105 in this circuit has a delay of less than a microsecond followed by a current rise time (10% to 90%) of about one microsecond. Output pulses from some of these units are used to trigger the firing modules (above), and other output pulses are used to trigger oscilloscopes. TRIGGER PULSE AMPLIFIERS 4

470 K

2 S F 10^

Output 90V peak info 70 n malched cable. 2 2 K

470 K ■AAAA

Fig.2 Trigger Pulse 4. CLAMP TRIGGER UNIT 5

The economical use of energy storage capacitors calls for the use of a clamp to limit the voltage reversal. In the present experiment the clamp switch is a BK178 ignitron, with a "holding” anode. Rather than take a capacitor voltage signal out to trigger the "earthy" input of a general purpose firing module, it was considered worth while making a self-contained unit to mount directly below the ignitron and use its holding anode supply.

There is a dearth of information about time delays of ignitrons at low ignitor voltages so tests were made which showed:

a. On first applying ignitor voltage, a current flows which is substantially what one would calculate from the "cold" ignitor resistance (a few tens of ohms) and the applied voltage.

b. This continues almost constant for a time which depends strongly on applied voltage, after which the ignitor current rapidly rises, at a rate determined by the external circuit, and the holding anode current begins within a fraction of a microsecond.

In one BK178, 200V gave a "current pause" lasting about 500 y sec, 300V gave 50 y sec and at 400V this delay was only 2 y sec. In the present application the voltage on the main capacitor bank goes through zero at the rate of only 5 volt/ y sec, so that 400 V supply for ignitor firing is acceptable. In the clamp trigger unit (Figure 3), this is obtained from the same insulated mains transformer as the holding anode supply, using a doubler circuit to charge a 20 y F electrolytic capacitor. This is discharged at a peak current of 120 A, by a 40216 thyristor inside a screened box connected to the ignitron ca­ thode.

In an inner screened box is the voltage discriminator which fires the thyristor. This is easily adjusted by means of the 500 ft preset so that the 2SH15 unijunc­ tion just fires with + 90V D. C. on the ignitron anode. Tests with a 5V/ y sec ramp input voltage then show that the holding anode current begins before the main anode reaches 100V. Considering the 100 M ft input resistor, this means an input current of less than 1 y A is ultimately firing a clamp ignitron which will take over 10 kA. This being a (pulsed) current gain of 1010, the doubled screening is probably essential.

In the present experiment, the ignitron anode lead goes through a 60 m ft re­ sistor to the negative (earthy) side of the main capacitor bank and the cathode lead has up to +10 kV suddenly switched on to it at the start of a discharge. The single IN459A prevents excessive reverse base voltage on the input transistor (2N3565). However the 10D6 diodes occasionally failed (they are outside the screens). The 0. 022 y F capacitor appears to have cured this.

The small capacitor shunting the 100 Mft input resistor has to withstand lOkV. It consists simply of a short copper tube surrounding a perspex tube, the other being a short metal cylinder soldered to the input end of the 100 M ft resistor, housed in­ side the perspex tube. Since this capacitor is not easily adjusted, the other capacitor of the voltage divider was adjusted for optimum transient response, resulting in the 3900pF shown. LM TIGR UNIT TRIGGER CLAMP

oA PF M O CM

Fig.3 Clamp Trigger Unit. 6 5. CONCLUSION 7

Some aspects of the design of thyristor circuits, for pulse-firing of ignitrons have been discussed and present-day speed limitations indicated. In the case of a med­ ium speed plasma physics experiment, experience of over 18, 00(? discharges has shown that thyristor circuits can be made reliable, even in an environment of high voltage tran­ sients, by proper attention to screening and filtering.

6. ACKNOWLEDGEMENTS

The author wishes to thank Mr. R. W. Parkes for assistance with the construc­ tion and testing of these firing units.

* (By October 1967, 48, 000 trouble-free discharges have been obtained). 8

APPENDIX

Since pulse transformers for the firing module could not be bought ready made, these were made on the premises. Details are appended for those in similar difficulties.

Core: 2 loops of C-core HWR50/18/4. Form er: 50/8988 Case assembly: Z371068 P rim ary: 20 turns size 0.10 in. x 0. 020 in. flat copper strip (BS1844 enamelled), end turns edge bent and sleeved with PTFE, re­ tained with type 59 "Scotch" polyester tape.

Inter-winding 0. 003 in. thick mylar cut 1/8 in. wider than bobbin; eight insulation: turns with 0. 0005 in. copper screening foil trapped between first and second turn.

Secondary: 64 turns 20 B and S , with PTFE sleeving on leads and end turns, retained with type 59 tape. The winding is over­ wrapped with this tape and baked to thermo-set it for maxi­ mum oil resistance.

Term inals: Primary, type TLS1-G; Secondary, type TLS1-J, rated at 20. lkV flashover in air. The case is vacuum filled with outgassed transformer oil and sealed with an O-ring com­ pressed between a 0-BA screw and a hank bush soldered into the can. This obviates air bubbles and the problem of soldering an oily can (with which some established manu­ facturers have difficulties!)

Preliminary tests of breakdown under oil suggest an internal flashover voltage from secondary to the foil screen (connected to the earthed can) would be about 30 kV. Thus, external flashover of the terminals is favoured. The secondary terminals connect to output sockets through 2^ in, of 7/0.0048 in. wire, one of which fused when plasma in­ stabilities produced voltage ’’spikes" of 40 or 50kV. This unit was otherwise intact and has operated satisfactorily since the "spikes" were suppressed. 9

REFERENCES

1 SMART, D. L. : "Some Switching Problems in Thermonuclear Research," P roc. I.E.E., Paper No. 2932 (106A, supp. 2, p. 107), April, 1959.

2. KNIGHT, H. de B. , HERBERT, L. , and MADDISON, R.C.: "The Ignitron as a Switch in High-Voltage Heavy-Current Pulsing Circuits," Proc. I. E.E., Paper No. 2947 (106A, supp. 2, p, 131), April, 1959.

3. LILEY, B.S., SMITH, J. M. , and VANCE, C. F. : "The Control System and Logic of the A. N. U. Toroidal Plasma Machine, " To be present­ ed at I. R.E.E. Convention, Sydney, 1967.

4. General Electric Company (New York;: SCR Manual, 3rd Ed. , p. 32, 1964.

5. Radio Corporation of America: "RCA 40216 Silicon Controlled Rectifier— Design Considerations and Device Date for Use in High-Current Pulse Application" (Application Note SMA-29).

6. British Insulated Callender’s Cables, Ltd. : "Energy Storage Capacitors" (Publication No. TD CA 1). RI

Publications by Department of Engineering Physics

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EP-RR 17 Bydder, E. L. On the Evaluation of Elastic Sept. ,1967 and Inelastic Collision Fre­ quencies for Hydrogenic-Like P la sm a s

EP-RR 18 Stebbens, A. The Design of Brushes for M ar. ,1964 Sept., 1967 W ard, H. the Homopolar Generator at The Australian National U niversity Copies of this and other Publications (see list inside) of the Department of Engineering Physics may be obtained from: The Australian National University Press, P.O. Box 4, Canberra, A.C.T., 2600. Australia. Price: SA1.00 Copyright Note: Reproduction of this publication in whole or in part is not allowed without prior permission. It may however be quoted as a reference.