A HIGH VOLTAGE AND HIGH REPETITION RATE PULSER FOR SPARK CHAMBERS* Stanford Linear Accelerator Center Stanford University, Stanford, California 94305

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1 TN-7 l-20 July 1971 A HIGH VOLTAGE AND HIGH REPETITION RATE PULSER FOR SPARK CHAMBERS* M. Gan, F. Bulos and H. L. Lynch Stanford Linear Accelerator Center Stanford University, Stanford, California * Work supported by the U. S. Atomic Energy Commission.

2 INTRODUCTION We describe the high voltage pulsing system used to drive a set of wire spark chambers for an experiment studying n-p interactions. The pulsing system consisted of charging cables, a series hydrogen thyratron, a trigger amplifier, and a 15 kv de power supply. The system produced rectangular pulses up to 7,5 kv, 220 nsec long into a 6 Q load at rates as large as 180/set. The rise time was about 20 nsec and the propagation delay was about 90 nsec, of which 60 nsec was due to the series thyratron. The experiment used seven such pulsing systems, each of which was pulsed about 2 X 10 7 times without failure. I. CHARGING SYSTEMS Each thyratron drove one spark chamber, which consisted of two gaps e Each gap was supplied by four 50 D cables connected in parallel. Resistive termination for the cables was provided on each gap. The high voltage pulse was generated by resistively charging a set of pulse forming cables (RG 213) 85 feet long. Charging these to 15 kv produced an output pulse of 7.5 kv and a duration of 220 nsec. In the experiment eight such charging cables in parallel were used per thyratron; however, provision was made for the use of a smaller number of cables if desired. It is necessary to adjust the charging resistance to match the number of cables in order to preserve the maximum repetition rate of 180/set; on one hand the thyratron must be allowed to deionize (since no quenching circuit was used), and on the other hand the cables must be completely recharged for the next pulse. It was found that a time constant of about 700 psec was optimum 0 Figure 1 shows the method of adjusting the charging resistance. -l-

3 II. THE THYRATRON ASSEMBLY The ceramic thyratron (ITT F 103) was mounted in a forced air cooled cylindrical cage whose geometry was chosen so that the characteristic impedance was approximately 6 Q2. The thyratron is used as a series switch connecting the set of charging cables and a matching set of output cables. The charging cables are connected in parallel at one end of the cage (plate end) and the output cables at the other (cathode end). For this particular tube we were able to reduce the residual inductance by clamping the output connection to the metal base of the tube instead of the cathode pin provided. This was possible because in this tube the cathode is internally brazed to the metal base. The thyratron circuit is shown schematically in Fig. 2. The trigger amplifier delivers a pulse of about 800 volts at grid G2 of the thyratron. This causes the thyratron to fire and it acts as the series switch between the charged input cables and the output cables. The principle of operation is identical to that of the well-known mercury switch pulser. The output is then l/2 the charging voltage. Since the heater and cathode of the tube are electrically connected internal to the tube it is necessary to place a high frequency choke and filter between the filament transformer and the heater input leads to prevent any damage to the transformer. A similar arrangement is also used in the reservoir monitor output 0 The tube is designed for operation with zero volts control grid bias and has a keep-alive electrode (auxiliary grid), and a hydrogen reservoir. The hydrogen pressure is controlled by a second heater. It was found that the minimum delay of 60 nsec was obtained with a keep- alive current of 15 ma and a reservoir voltage of 4.3 volts. Increasing the keep- alive current beyond 15 ma did not decrease the delay. In any case provision -2-

4 was made for the application of an external supply to vary the keep-alive current by means of a jack on the back of the unit. The limit on the reservoir voltage is partly set by the pulse repetition rate of 180/set which causes heating of the tube; using a higher voltage may prevent the tube from extinguishing. Although the rated peak anode current is 350 amps, driving a 6 fi load with a 6 kv pulse means a peak current of 1000 amps. Such operation had no adverse effect on the thyration; seven units were pulsed in excess of 2 x lo7 times without any sign of failure,, The output pulse is shown in Fig. 3 for four different loads and charging cables of 50 feet long. Note that the rise time degrades somewhat as the load impedance decreases. This is partly due to residual inductance of order of 0.05 ph, and at high currents there may be a saturation of the tube s current capacity. III. TRIGGER AMPLIFIEL-R The main thyratron requires a large pulse at its grid to fire. The trigger amplifier converts a NIM standard logic signal (750 mv into 50 Q) into a l-10 kv signal suitable for the main thyratron and for spark gaps, Each unit contains two complete, identical amplifiers sharing only the power supplies. Each half is capable of driving two main thyratrons, although we only used one per amplifier. The amplifier was designed to operate at rates up to 200/set and has a propagation time of about 30 nsec. This minimum delay, however, could only be obtained by a careful selection of the EFP 60; only about one in three were acceptable. This tube could be replaced by an avalanche transistor Marx generator 0-3-

5 Circuit diagrams are shown in Figs. 4 and 5. To prevent interaction of the input and output, all low level signals are shielded and located on the upper side of the chassis, while the output circuitry (DP 30) is placed on the underneath. Figures 6-8 show a photograph of the layout of the trigger amplifier; Figs. 9, 10 show the thyratron unit, and Fig. 11 shows the complete two-thyratron assembly. IV, POWER SUPPLY The high voltage supply, built by Universal Voltronics, is specified to provide a regulated output of 1 to 15 kv at currents up to 65 ma. The regulation is important because the trigger rate in the experiment fluctuates. One power supply is used per thyratron so that each chamber s high voltage can be individually adjusted. The current capacity is required to fully charge the cables at the maximum repetition rate of 180 pps. The power supplies have been modified to signal an overload condition, due to thyratron misfire; an audible signal is emitted and a NIM logic level signal is sent to an inhibit input in the trigger logic. V. CLEARING FIELD The clearing field potential was a 50 V dc level upon which was superimposed a V pulse of adjustable length; the pulse was applied after every accelerator pulse, whether or not the chambers had been triggered. A length of 3 msec was used in the experiment. It was applied to the kround wire plane of the spark chamber; this way the keep-alive bias of the thyratron and the clearing field do not interact. The ground plane is isolated from the high voltage ground, i.e., the aluminum frame as shown in Fig. 12. The 0.2 pf capacitor at the chamber bypasses the HV pulse, and the RC filter between the chamber and the clearing field supply prevents interaction between sparks and the clearing field supply. -4-

6 Care must be exercised in applying so large a clearing field voltage. It was found that it was possible for the clearing field to reignite old sparks and damage the chamber if the rate of rise was too large. For a 300 V pulse,a time constant significantly less than 100 psec is potentially dangerous. One clearing field supply powered two chambers. Each supply consisted of a series SCR to charge the chamber capacitors and a shunt SCR to discharge them. The four SCR units shared a common V dc power supply. Figure 13 shows a schematic diagram of one SCR unit. An additional SCR unit with a dummy load was used as a standard reference for a set of comparators which continuously monitored the clearing field. If any chamber voltage sagged an alarm was sounded. Figure 12 shows a block diagram of this complete scheme. Figure 14 shows the schematic of the clearing field voltage comparator and warning alarm 0 ACKNOWLEDGEMENTS The invaluable assistance of M. Lateur and R. Pickup with the mechanical fabrication of the system is gratefully acknowledged. -5-

7 FIGURE CAPTIONS Schematic of charging resistance box. Schematic of main thyratron. 3. Output pulse shape for various loads. 4. Schematic of trigger amplifier. 5. Power supplies for trigger amplifier. 6. Top view of trigger amplifier. 7. Bottom view of trigger amplifier. 8. Front view of trigger amplifier. 9. Top view of thyratron with cage removed. 10. Front view of thyratron. 11. Chamber pulsing assembly, consisting of two thyratrons, one dual trigger amplifier, and charging cables. 12. Block diagram of clearing field. 13. Schematic of pulsed clearing field supply. 14. Schematic of clearing field monitor.

8 TO DC HIGH VOLTAGE SUPPLY C ot5 50kfi D (1 NUMBER OF CONNECT CONNECT CHARGING CABLES JUMPER CABLES lo BETWEEN I 2 I TI, T2 IBANDT5 1 I I 4 TI,T2 T3.T4 I BAND T5. T6 AND T8 TO CHARGING VOLTAGE MONITOR *AN ADDITIONAL 30 kil SERIES RESISTANCE IS PROVIDED IN THE DC HlGH VOLTAGE POWER SUPPLY. 1672Al Fig. I

9 TO TRIGGER AMPL 4.7 kq 25W 100~ w TO PULSE SHAPING CABLES HV INPUT (O-15kV) 8 SPECIAL HV CONNECTORS IN PARALLEL 800V A ii- G2 --- G, --- THYRATRON ITT F103 R EXTERNAL POWER TO G, (TEST) HV INTERLOCK POWER SUPPLY Fig. 2

10 LOAD = 25Cl LOAD = 12s1 LOAD = 852 LOAD = 6sZ THYRATRON OUTPUT HOR SENS = 5Onsechm VERT SENS = 4kWcm Fig A3

11 -15v t15v IOOOV 385V f I 1 t 4 kv NOMINAL 0-IOkV 2000 pf I 20kV.Ol.r l-69 56kQ 1 uv3u t X I I8kfl IN SERIES OP30 MONITOR 56OQ STANDBY OPERATE Fig. 4

12 II B I.A TYPE IOB UNIVERSAL VOLT. BPE 2-5-S 4 3 O-2kV I 5mA = v O-200pA UNIVERSAL VOLT. Q IA TYPE 108 BPE IO-IO-S = IOkV I IOmA IOkD loomi r- I A SET TO IkV 0-IOkV OUT - ~ ooa +O-,OOpA 90-lOOpA F16X Fl6X, -15ov IOOmA 3 WB I,.\, 303 v IYUV l8kn B POWER/MATE RAl ov -50v -15v -7v I IOk&! 5w 5 =.I/.LF- *ONE FOR EACH CHANNEL I = Fig. 5

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19 t w z!i 9 3 f 4. -PHI, r r

20 CLEARING FIELD TO CHAMBER o-5oov DC 0 IN $A Fig. 13

21 DIFF MONITORS MPL. SCH. TRIG. +6V +6V s 1 CA3000 > I I I J--K 9 I ( I 1 CA3001 2Nl7ll SC628 ALARM GE 756 I (4) 1 I N = -4.7ki-l.lOki2 t -ii RESET (PUSH BUTTON 1 P DIFF. AMPL. 4.7k4-6V SCHMITT TRIGGER Fig. 14