KEK-79-4 April 1979 P PERFORMANCE TESTS OF CROWBAR CIRCUIT FOR KLYSTRON PROTECTION Hisashi KOBAYAKAWA, Koji TAKATA and Isao TOYAMA NATIONAL LABORATORY FOR HIGH ENERGY PHYSICS OHO-MACHI, TSUKUBA-GUN IBARAKI, JAPAN KEK Reports are available from Technical Information Office National Laboratory for High Energy Physics Oho-machi, Tsukuba-gun Ibarakl-ken, 300-32 JAPAN Phone: 02986-4-1171 Telex: 3652-534 (Domestic) (0)3652-534 (International) Cable: KEKOHO PERFORMANCE TESTS OF CROWBAR CIRCUIT FOR KLYSTRON PROTECTION Hisashi KOBAYAKAWA Depertment of Physics, Nagoya University, Nagoya, 464, Japan Koji TAKATA National Laboratory for High Energy Physics Oho-machi, Tsukuba-gun, Ibaraki-ken, 300-32, Japan Isao TOYAMA Nichicon Capacitor Ltd. Kusatsu, Shiga-ken, 525, Japan Abstract A crowbar circuit to protect high power klystron for the rf accele­ ration of the PF storage ring has been constructed for the experimental purposes. Brief description of the test circuit and results of the performance tests are presented. The circuit is designed for 50 kV operation and works sufficiently well for our purpose: electrical energy of 9 kJ switched within 5 ps and thus only 3 J fed to the load. 1. Introduction The PF storage ring (2.5 GeV) requires more than 500 kW of continuously rated rf power at 500 MHz. The rf generators consist of A klystrons each having an output power of 180 kW in the continuous mode. The klystron will be fed by a high voltagd dc power supply (50 kV, 10 A). The klystron beam voltage is produced by a conventional 12 phase YA bridge rectifier followed by a single stage LC filter, the design of which achieves a reasonably low level of ripple content of the beam voltage (0.2 %pp) . 2 3) The output of the power supply shall have a high-speed crowbar ' for the protection of the klystron load. The crowbar shall operate automatically, in a few psec, to protect the klystron from excessive beam and/or body current. We have constructed a crowbar circuit for experimental purposes and for the help of actual design of the HV power supply. The circuit contains three ignitrons (Mitsubishi MI-3200E) in series connection for the crowbar switch, and also a 7 uF charging capacitor which will be used as a filter in the actual HV supply and would be the biggest energy source to damage the klystron. The primary of the HV power supply, 3 (j) 6.6 kV ac, shall be cut off with crowbar operation within 100 msec, and the crowbar operation will be held on until the primary is de-energized. The test circuit has an additional capacitor (0.7 yF) as an equivalent of the primary ciucuit in the actual HV supply. Performance tests have been done and the circuit works expectedly well, suppling only 3 J to the thin copper load at 50 kV charging voltage. We describe briefly the structure of the test circuit, contents of the performance test and its results. - 1 - 2. Brief description of the test circuit The schematic diagram of the crowbar test circuit is shown in Fig. 1. The specifications are as follows: (1) 60 kV high voltage generator; Output voltage (max. 60 kV) is stepped up from 100 V ac by a variable transformer and the Cockcroft-Walton rectifier. (2) Charging capacitor (C_) and a resistor (R_) regarded as a filter module of the actual power supply; C = 7 yF, Rf = 20 JJ. (3) Capacitor (C ) regarded as an additional current source with a resistor (R ) when the crowbar is operated in the actual HV supply; C = 0.7 yF, R =1 W2. s * s (4) Crowbar switch; (4-a) three ignitrons MI-3200E (Mitsubishi). (4-b) trigger module, containing three high voltage pulse transformers (see Fig. 2 for schematic diagram), (5) Sensing circuits; (5-a) 0.1 R Manganin (Mn) resistor: (i) to detect abnormal current and send a signal to fire the crowbar, (ii) to measure the waveform of the output current with an oscilloscope. (5-b) 0.1 0. Mn resistor for waveform measurement of the crowbar current. (5-c) RC divider to measure the crowbar voltage (1/1000). (6) Resistor (R) for the shunt in the output; R = 4.5 Q. (7) The high voltage switch and short-circuiting materials (copper wires or thin metal foils). - 2 - 3. Crowbar characteristics We give here briefly the crowbar characteristics for the following parameters C£ = 7 uF, R = 20 $2, C = 0.7 yF, and E = 4.5 fi. The stored energy in the charging capacitor (Cf = 7 uF) at 50 kV is approximately 9 fcJ. To see how the crowbar works, we used either thin copper wires 0.18 mmtp, 200 mm long whose critical energy for melting is about 20 J, or aluminum/copper foils with an iron needle for the sparking point. The calculated energy getting into, for example, the thin wire is approximately 55 J without crowbar switch that is enough to melt it away. When crowbar operates this energy is cut down to ~3 J. Expected current waveforms in the load are shown in Fig. 3 when the crowbar is on and off, respectively. Values indicated in the figure are estimated by using the circuit inductance and resistance actually measured. 4. Performance results Correction of the meterings and RC divider for the voltage measurement is carefully done. Two shunt circuits (crowbar current, load current) are also carefully calibrated (time constant is about 1 usee) with both dc and pulse currents. Three different materials are used as the test load; (1) closing with thick copper wire, (2) melting tests with thin copper wires (0.18mm<j>, 200 mm long), (3) punching holes on the thin foils (aluminum/copper) by spark currents from a needle point. We have studied varying the charging voltage from 10 to 50 kV with two values of resistors Rf = 20 and 45S2, and with dividing capacitors C. = 0.2 UF and 2000 pF (see Fig. 2 for the definition of C. ). The ig ig typical waveforms are shown in Fig. 4 for the cases with and without - 3 - crowbarring. Fig. 4(a) is a picture of the load current, crowbar current and crowbar voltage at a charging voltage of 45 kV with Rf = 20 fi and C. = 2000 pF. Fig. 4(b) is the same as (a) except that the load and C is connected with a 9 m, 50 JJ coaxial cable in order to simulate the actual cabling. Rise time (x..) in picture (a) is roughly 0.5 psec, and 3 usee after the high voltage switch is closed, the crowbar ignitrons start firing. Then about 2 sec (x„) later, the load current is almost cut out. So the current flows into the load during only first 4 psec and the succeeding current is switched into the crowbar by-pass. Without crowbarring the decay time (x ) is 190 ysec. The thin copper wires 0.18 mm<j>, 200 mm long, are used for the melting study"by which one could imagine the damages on the klystron electrodes. Without crowbarring, one can see a small spike in the wavefcrm of the load current (Fig. 5(a)). This is supposed to be a sign of melt. After this the current still falls with the same time constant. It may suggest that a conductive region around the vapourized wire sustains the current. The energy fed before melt, estimated from the waveform, is about 16 J which is roughly equal to the calculated energy (20 J) to vapourize the wire. With crowbarring, the main energy in the capacitor is well by-passed, so the load wire suffers no damages up to 50 kV (Fig. 5(b)). The holes on the aluminum and copper foils due to the localized spark current from the needle point are useful to imagine another type of damage on the klystron electrodes. Sometimes abnormal discharge on the electrode is localized in a very small area. The electrical energy and therefore damages also concentrate into this small point and develop at its vicinity. We can not exactly estimate the relation between the size of the hole and energy (or voltage). We might say rather qualita- - 4 - tively that the size (d), diameter of the hole, is approximately linear to the charging voltage (V), as the energy to melt foil is proportional 2 to td with t the foil thickness and the stored energy in condensor is 2 to V , The measured results given in Figs. 6 and 7 indicate a rough relation between d and V. The diameter ratio obtained from the aluminum 1/2 foils of 20 ym and 55 ym is approximately (55/20) , although the extraporated V-d lines seem not to cross the zero point. We cannot, however, explain the difference between the aluminum and copper foils only by taking into account the densities and specific-heats of them. When the crowbar works, we have still small holes on the foil as given on the right side of the Fig. 7. A small hole, for example, with 1.5 mm on 20 ym foil at 50 kV corresponds to the energy dissipation of 2 ~3 J, roughly. 5. Timings and Waveforms measured (1) Measurement of breakdown time and jitter. The breakdown time (T, ) and its jitter (T.) are defined as the average time and its maximum deviation, taken between the trigger pulse start and the breakdown of the ignitrons. Fig. 8 is a super­ imposed picture of the waveforms of five discharges, showing T, = 3 ys and T = 0.5 ys at charging voltage of 20 kV (C. = 2000 pF).