919 IEEE TRANSACTIONS ON MAGNETICS, VOL. 29, NO. 1, JANUARY 1993 A Pulse Forming Network and Test Fixture for Screening Electrothermal Chemical Candidate Propellants Duk I. Chang Electric Armament Division U. S. Army Research, Development and Engineering Center Picatinny Arsenal, NJ 07806-5000 Brian Long Parker Kinetics Design, P.O.Box 596, Wharton, NJ 07885-0596 Donald Chiu, Rod King Energetics and Warheads Division U. S. Army Research, Development and Engineering Center Picatinny Arsenal, NJ 07806-5000 Joseph Hershkowitz Geo-Centers, Lake Hopatcong, NJ Abstract-A pulse power supply has been designed to support the development and evaluation of candidate propellants for an Electrothermal Chemical (ETC) gun program a t ARDEC. The flexibility needed for this evaluation is achieved through the modular design of the test fixture and power supply. The test fixture is comprised of a plasma chamber and instrumented combustion chamber interconnected by a convergent-divergent nozzle. The combustion chamber is terminated by an exhaust nozzle and overall thrust is measured. The volume of both c h a m b e r s a n d nozzle configurations can be varied to optimize performance. A 10 mm gun tube is an option. The power supply is readily reconfigured to provide either an RLC or PFN discharge. Either configuration can store up to 34 kJ and provide a pulse between 0.5 to 2 ms. Load behavior and the effects of parasitic energy losses on circuit performance are presented. The results will be compared with PSpice c i r c u i t models. A presentation of the experimental pulse power system and resulting data will be provided. controlled electrical discharge into the combustion chamber. This paper will discuss mainly the design process and test results of the elctrical energy source and only touch upon the test fixture. 11. ETCTEST FIXTURE The ETC modular test fixture, shown in Fig. 1, is akin to a 10 mm gun, properly scaled, and is adaptable for firing in a gun configuration. The test fixture is a thick-wall vessel consisting of a "plasma" chamber feeding a "combustion" chamber thru a nozzle with the second chamber exhausting through a nozzle to the exterior. The nozzles are designed to produce choked flow. The internal diameter and the length of the chambers can be modified by inserting different elements into the body. The power supply connects to the anode through windows in the pedestal, which is mounted on a thrust gauge, with the cathode connection made through the body. Two pressure gauges measure the pressure in the combustion chamber. 111. PULSE POWER SUPPLY I. INTRODUCTION A. Design process ARDEC is participating in a DOD effort to provide a major increase in the effectiveness of gun propellants. The ETC program seeks to extend projectile velocity, leading to greater penetration and range, by going beyond the limitations of solely chemical sources. The principal intent of the test fixture is to provide a testbed for evaluating the candidate gun propellants/working fluid from both industry and ARDEC with respect to performance potential. The evaluation will be done on a small scale here at ARDEC by providing a Manuscript received by March 2, 1992. Parker Kinetics Design is sponsored by ARDEC under Contract DAAA-21-90-C-0050. Geo-Centers is sponsored by ARDEC under Contract DAAA-21-89-D-0030 The design parameters for this power supply required the delivery of 3 to 12 kJ to the load in pulses between 0.5 and 2 ms. Load values predicted by the Loeb-Kaplan model [l] ranged between 50 mi2 and 150 mi2 for this particular test fixture. A square wave was the preferred pulse shape, however an RLC discharge was not excluded. It was clear the power supply would require a great deal of flexability to meet the design parameters. To provide the flexability necessary to vary the pulse width and to match the load values it was decided to build a five section pulse forming network, PFN, (Fig. 2 and 3) utilizing 0018-9464/93$03.00 0 1993 IEEE I 920 In the design process, safety for an operator always took precedence. To insure safety, all the accessible doors leading into the power supply room and the test fixture room have interlocks incorporated into the control system to safe the power supply if a door was opened. Also, there are plexiglass doors in front of the power supply rack itself to protect the operator. A safety stick is available to drain/short the capacitors in the event of soft-dump system failure. Any residual voltage retained by the capacitors is monitored at the control room located 130 feet away from the power supply room. A dc voltmeter is also mounted at the power supply room so that the operator can verify the voltage is zero before proceeding into the room. The interlock conditions are monitored in the control room. The power supply will not operate unless the interlock system is satisfied. 25 30 20 s v 25 m 15 A PSpice* model was used to evaluate the power supply performance under different operating conditions, including a nonlinear load based on the Loeb-Kaplan equations for confined high pressure discharges. These equations describe the plasma parameters as functions of the discharge parameters, such as ablation rate, plasma temperature, resistivity, density, current and plasma chamber dimensions. For a given geometry the plasma resistivity falls off exponentially with increasing current. This load behavior was also verified by BRL [4]. This equation along with the analog behavioral modeling feature of PSpice was used to control a variable resistor to simulate the load. The model indicated that the plasma quickly moves into the milliohm range and remains fairly constant over the duration of the pulse. Current and Energy simulations for the nonlinear Loeb-Kaplan model are shown in figure 4. Power supply configurations for simulations LK1, LK2, and LK3 correspond to test #1, 2, and 3 mentioned in the 'Power Supply Tests and Results' section. B . Power Supply Tests and Results Initial testing of the power supply was performed by discharging energy into a configurable test load. The test load shown in Fig. 5 can be configured from 25 mQ to 400 mQ. * PSpice is a circuit analysis program from MicroSim Corporation, 20 Fairbanks, Imine, CA gl 15 2 0 10 Y 10 5 -e 5 0 0 1 0.0 In addition to personnel protection, hardware protection was also incorporated. When the test is conducted on the plasma load, it is possible that the load may become an open circuit during the discharge cycle and give rise to a fast change of current vs time [3]. This would create an unwanted high voltage in the inductor since the voltage of an inductor is governed by Ldi/dt. To protect the inductor from over voltage, a calibrated spark gap with a resistor in series is placed in parallel with the plasma load as depicted in Fig. 2 (SW3 and Rs). The gap will break down at a set voltage and provide an alternate energy dissipation path in the event of an open circuit failure at the load. 20 0.5 1.0 1.5 2.0 3 .Ox10.- Time(=) Fig. 4. Current and energy obtained by PSpice model based on Loeb and Kaplan equations. 1 4 5 8 Fig. 5 Test Load Four tests were conducted. The first three tests were performed in PFN configuration. Test #1 used only one 500 pF capacitor, test #2 used two 500 pF capacitors and test #3 used all three capacitors in each section of the five section PFN. These configurations at appropriate charge voltages store 8.5 kJ, 17 kJ and 34 kJ of energy, respectively. The last test involved an RLC configuration discharge at full energy. The maximum energy transfer occurs when the load impedance matches to the characteristic impedance of the power supply. Using (1) the matching impedance was calculated for different power supply configurations. PFN Z = (L/C)''2 RLC Z = 2(L/C)'j2 Though the test load is reconfigurable, it was found that exact load matching could not be obtained. Therefore the closest test load configuration was used. For all three PFN discharges a 100 mQ load was used and for the RLC discharge a 50 mQ load was used. Table I shows the test setup and the numerical results. Fig. 6, 7 and 8 show the results of the simulation and the experimental PFN tests. Current and voltage were measured at the load by using current transformers which provide inductive isolation from the experiment. Actual load values calculated from the voltage 92 1 r-r-i Maximum energy delivered at the narrowest pulse width determined the 3 kV operating voltage for the capacitors. Due to limited power supply room space, it was decided to use high energy density metalized capacitors with an individual clamping diode across each section of the PFN to provide the necessary protection against voltage reversal on the capacitor. Therefore, the power supply (Fig. 3) consists of fifteen 3 kV, 500 pF high energy density capacitors, five 10 p H inductors, five clamping diodes, and an ignitron (sw2 in Fig. 2)used as a closing switch Cl=C?=C3=C4=C5= 1500 pF Ll=U=L3=L4=L5= 10 pH DIODES: CE A780.4200 V Q = 100 Q, 150 watts Rload = Plasma Load R, = 4- 100 mR. Solid Disc type SW1: Soft Dump Ross Relay SW2: Size D Ignitron SW3: Calibrated Spark Gap Fig. 2. The 34-kT Power Supply Schamatic 3AP 120- THRUSTGAUGE Fig. 1. ETC Test Fixture multiple capacitors in each section. Adding and deleting these capacitors provide flexibility in choosing the desired power supply configuration. This multiple component method could also have been applied to the inductors but limited space and the relative cost of inductors to capacitors discouraged this option. Also, the modularity was achieved by mounting the components on fiber-glass unislruts (Fig. 3) where they can easily be reconfigured. The fiber-glass unistrut rack measures 83 in x 52 in x 16 in. The capacitors rest on angles which allow them to be easily unbolted from the bus work and slid back to remove them from the circuit. The inductors and bus work are mounted to vertical unistruts. This provides for easy alignment during the construction via sliding channel nuts as well as providing insulation between components. The upper shelf houses the charging power supply, soft dump and side D ignitron. By matching the PFN impedance to the predicted load values over the range of pulse width requirements it was determined that three 500 pF capacitor in parallel feeding a 10 pH inductor in each section of the PFN would provide a reasonable range of values within the design parameters [21. Fig. 3. Photo of 34-kJ Power Supply 922 and current data were different from what was expected. The calculated load values from the data were used for the PSpice model simulation. The measured currents in Table I are an average reading of each current profile. C.Discussion The test load dictates that Test #1 is underdamped, test #2 is slightly overdamped, and test #3 is overdamped ciruit. This can be seen in Fig. 6, 7, and 8 current profiles, respectively. Also from the trailing LR decay in Fig. 6, we can verify the clamping diode has operated as desinged under the reverse voltage condition. The test results followed extremely close with the PSpice model calculations. This close agreement indicates the effects of parasitic losses can be neglected. It should be noted that these three PFN tests were conducted within a half hour period. This demonstrates the ease with which this power supply can be reconfigured. Time (sec) Fig. 7. Current and energy profile of Test #2 where two capacitors were used in the each section of five section PFN. TABLE I m AND THE RESULTS Tw S 20 35 30 15 Power Supply Config. PFN 2500 I C 0 CLOLH) 50 Charge volt (kV) Matched R1-d (ma) Actual R1-d (ma) Expected I,, (kA) &A) Measured,,I Stored Energy (kJ) Delivered Energy(ktJ) Pulse Width (msec) 2.6 140 110 9.6 10.5 8.5 8.48 1 PFN 5000 50 2.6 100 107 13 12.5 17 16.7 1.25 PFN 7500 20 RLC 7500 10 3 73 50.6 38 33 34 33 1.5 50 3 81.6 107 17 16 34 32 1.5 1 25 p 10 15 U " 10 5 M 0;l Y E w 5 0 0 0.0 0.5 1.0 1.5 2.0 3 .Ox 16 ' Time(=) Fig. 8. Current and energy profile of Test #3 where two capacitors were used in the each section of five section PFN. 10000 , Iv. 6000 -E Y SUMhfARY AND CONCLUSION The design and performance of the power supply meets requirements operating into fixed resistive loads. In the Spring of 1992 the power supply will be used to create plasma and a later paper will give details of the plasma test and working fluid performance. REFERENCES - 2000 0.0 0.5 Time (sec) Fig. 6. Current and energy profiie of Test #1 where only one capacitor was used in the each section of five section PFN. A. Loeb and Z. Kaplan, "A Theoretical model for the physical processes in the confined high pressure discharges of Electrothermal Launchers," IEEE Transactions on Magnetics, vol. 25, NO. 1 , pp342-346, January 1989 121 C. N. Glasoe and J. V. Lebacqz, Pulse Generator. New York: McGraw-Hill, Inc.. 1948 [3] Conversation with Dr. Bob Greig at GT-Devices. [4] G. L. Katulka, H. Burden, A. Zielinski, K. White, "Electrical energy shaping for ballistic application in electrothermal guns," December 1991, BRL Technical Report# BRL-TR-3304, U.S. Army Ballistic Research Laboratory, Aberdeen Proving Ground, MD 21005-5066 [l]