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Design of 5.5MJ Charge Dump Power Supply for the PPPL FLARE Experiment P. A. Melnik, A. H. Bushnell, P. E. Sieck, J. E. Stuber, S. Woodruff F. Hoffmann, J. JaraAlmonte, H. Ji, M. Kalish, A. Zhao Woodruff Scientific Inc, 4000 Aurora Ave N, Suite 6, Seattle, WA 98103 USA [email protected] Princeton Plasma Physics Laboratory, PO Box 451 Princeton NJ 08543 USA [email protected] T. E. Ziemba, K. E. Miller Eagle Harbor Technologies, Inc 169 Western Ave, Suite 263 Seattle, WA 98119 USA [email protected] Abstract—The Facility for Laboratory Reconnection Experiments (FLARE) is an intermediate laboratory experiment currently under construction at Princeton University by a consortium of five universities and two Department of Energy (DoE) national laboratories, located at the Princeton Plasma Physics Laboratory (PPPL). The goal of FLARE is to provide experimental accesses to new regimes of the magnetic reconnection process and related phenomena directly relevant to heliophysics, astrophysics, and fusion plasmas. The device comprises a vacuum chamber and 9 coils sets that are independently programmable to provide the poloidal and toroidal magnetic fields required to form plasma and study the effects of magnetic reconnection. Each of these 9 coil sets requires a separate pulsed power system, it is the design of the power systems that is reported here. The 9 separate pulsed power systems combine to produce over 5.5MJ of energy to the experiment and each presented their own unique challenges. The most energetic power system is a 3.4MJ, 19.2mF capacitor bank charged to 20kV that provides the guide field, with a rise time of approximately 12ms it delivers an average peak current of 40kA over 5.3ms to 12 coils wired in series. The poloidal field coils consist of two separate coilsets each requiring 540kA peak current which is produced by two 20kV, 2.64mF capacitor banks. The design of the two driver coilsets each charged to 60kV will also be presented. has a maximum voltage and current specification, as well as a desired rise-time, and crowbar time. The scope of the design is to provide concepts and engineering for the circuits that will energize these coils to meet the specifications as provided by PPPL. The final result of this design consists of PSpice models of each circuit and models of coupled circuits to analyze how they interact, detailed wiring schematics of each circuit, box diagrams showing all systems involved in operating each circuit, full engineering drawings of each custom components and engineering layouts of each capacitor bank, and a complete bill of materials for each bank. Keywords—Pulsed power, Plasma physics, High energy RLC circuits, modular capacitor bank I. INTRODUCTION The Princeton Plasma Physics Laboratory (PPPL) Facility for Laboratory Reconnection Experiments (FLARE) device depicted in Fig. 1 consists of 9 separate coil systems, as outlined in Table 1 [1, 2]. Each coil system comprises multiple coils, so for example, there are 4 coils in the Poloidal Field A (PF-A) system that are connected in parallel. Each coil system Fig. 1 The FLARE device under construction at PPPL. TABLE 1. FLARE EXPERIMENT POWER REQUIREMENTS Coil System OH EF GF PF-A PF-B TF-A TF-B DC-I DC-O #Subcoils 2 2 12 4 4 4 4 2 2 Voltage (kV) 20 1.4 20 20 20 20 20 60 60 Current (kA) 180 26 40 540 540 250 250 50 50 Energy (kJ) 545 389 3,395 418 418 123 123 4.4 10.5 Day 1 (%) 0 33.3 20 33.3 33.3 33.3 33.3 0 0 Risetime (ms) <0.45 >30 >12 <0.11 <0.11 <0.08 <0.08 <0.01 <0.03 Crowbar Peak Peak Peak Peak-50% Peak-50% Peak70% Peak70% Peak Peak II. COMMON CIRCUIT DESIGN POINT A. Circuit Description banks the crowbar timing is variable so ignitron switches are used. Following a shot, the dump switch is closed to ensure the capacitors are fully discharged. Capacitor fuses are thin wires designed to blow during an over-current event. They will be used on each capacitor and will connect the capacitor to the busbar. B. Circuit Analysis Fig. 2 Generalized circuit schematic A generalized bank schematic is shown in Fig. 2. Starting on the left side of the figure, the module is charged by connecting both positive and negative supply lines. A power supply protection circuit is shown (although the components are distributed between the charge supply rack and the capacitor modules). A bleed resistor is connected in parallel with each capacitor and is used to slowly drain bank charge in case of failure of all other dumps (i.e. if left alone overnight the bank will passively discharge below the NFPA 70E safe approach threshold of 50 V). Before each shot, the dump load relays are opened. Then the charging relays are closed and the charging supply charges the caps. When the set-point is reached, the charging switches are opened. The capacitors are then discharged through the inductive and resistive load of the coil, and crowbarred with a delay corresponding to peak current. For some banks the first swing current will always be greater than zero so a uni-polar crow bar switch (i.e., diode commutation) may be used. For the poloidal and toroidal field An extensive PSpice analysis was completed for all capacitor banks. The inductance, capacitance, and resistance of all buswork, harnessing, and connections was calculated and included in the model with a safety factor. The vendor supplied specifications were used for all components. Additionally, the mutual inductance between all coupled magnet coils was included and complete models of all toroidal and poloidal field capacitor banks were developed. A PSpice model of all poloidal field capacitor banks encompassing all mutual inductance between the coils is shown in Fig. 3. The resulting transient analysis using likely bank timings is shown in Fig. 4. Detailed circuit modelling and simulations are a valuable tool for outlining acceptable performance ranges, validating safety features, and cost optimization. As the most energetic of the nine capacitors banks, the guide field (GF) is also the most expensive, more than twice the cost of any other bank. Multiple iterations in the design of the guide field were completed in an effort to reduce cost. These included simulating the wiring of the 12 magnet coils that make up the guide field in different configurations. Every possible parallel wiring configuration was explored and priced but a series configuration was found to be the cheapest and least complex. The guide field capacitor bank is meant to produce a steady magnetic field during the plasma discharge which requires a certain current waveform. Different combinations of bank voltage and capacitance were simulated and priced until the least expensive combination was found. Fig. 3 PSpice model of all poloidal field capacitor banks III. ENGINEERING DESIGN 600KA A full engineering design was completed for all capacitor banks. The banks were designed to minimize the inductance and resistance of all buswork and harnessing. The copper buswork connecting all capacitors for a given bank will be water-cut from one large sheet. This eliminates any contact resistance and additional inductance between busbars. The buswork is also designed such that the positive and negative busses can be cut from one standard ¼” sheet. The busses are insulated from each other with 8 mylar sheets creating a spacing of a ¼”. The insulating mylar is also designed with an approximately 1.5” overhang so the tracking distance in air is well over 1” per 10kV. Poloidal Field Bank Currents with Coupling PF-A 400KA PF-B OH 200KA DC-I EF 0A DC-O -200KA A. Full Assembly -400KA 27.6ms 27.8ms 28.0ms 28.2ms 28.4ms Time Fig. 4 Transient analysis of poloidal field capacitor discharges 28.6ms A completed render of the poloidal field-A capacitor bank is shown in Fig. 5. Assembly renders and full engineering drawings of all custom fabrication parts have been completed for all nine capacitor banks required for operating the FLARE experiment. The layout for each capacitor bank is similar to the layout of the PF-A bank. The high energy density capacitors are placed on steel pallets with spacer blocks between them and on the perimeters to avoid translation. Custom copper wire fuses are used to connect each capacitor to the buswork. Fig. 5 Full render of the PF-A capacitor bank The size of the fuses is calculated according to the exploding wire phenomena and are designed to handle the maximum capacitor current but will blow during an overcurrent event [3]. Current limiting resistors at the forward switch and in the crowbar line are made of stainless steel and are designed to minimize damage in the event of a catastrophic ground fault. The Richardson Electronics NL8900 ignitrons are mounted in custom coaxial cages to create a magnetic field free environment for each device. 500 W infrared heaters are used to maintain a thermal gradient so the anode is 10°C warmer than the cathode. Each ignitron is water cooled with ¼” NPT hose connected to specified water chillers and pumps. G10 fiberglass rods and channel beams are used to mount all buswork to the pallets. Custom clamps of fiberglass plate and threaded rod have been designed to mount the busbars to the capacitors. Large custom clamps are also used on the interconnecting buswork for the capacitors banks with current and time requirements that create displacement of the opposite polarity layered buswork due magnetic pressure between them. All nine capacitor banks are charged with individual bipolar power supplies. Two TDK-Lambda charging supplies are wired in series to create the bipolar output. The two output lines each include a series resistor to improve regulation and limit output current under fault conditions. The lines are then split to run a cable to each capacitor module. At the capacitor module, the charge cable is connected across a diode that protects the charge supply in the event of a bank pre-fire (when the charge supply is connected to the bank). The diode prevents any bank reversal during pre-fire from imposing a reverse voltage at the charge supply. After the diode is a resistor that limits the charge current for the bank (and limits the protection diode current under reversal). The final connection to the capacitor buswork is made via a normally-open DPST Ross relay (E40-2PNO) which is only connected for charging, and is disconnected immediately prior to programmed discharge. Each capacitor bank includes a resistor sized to dissipate the full-charge energy of the bank. The resistor material is an aqueous solution of copper sulfate with brass electrodes in a polycarbonate reservoir. The electrolyte concentration is tuned to discharge the bank to below 50 volts (the NFPA 70E safety threshold) within 30 seconds. This dump rate was chosen based upon a conservative estimate of the time required from removal of the Kirk key at the operator station to entry of the bank enclosure. The dump sizing is also designed to allow several sequential full-energy dumps at 3 minute intervals, but in this operation mode the resistor temperature shall be monitored remotely by the operator to ensure the temperature does not exceed 60°C (as per ASTM C1055) and that the water level is maintained. The normallyclosed dump relay on each module (Ross E40-NC) will be energized only during charge, and will be de-energized to engage the dump upon removal of the Kirk key or at any emergency stop or interlock break. A temperature sensor is located on the outside lower section of each dump. The data acquisition and control (DAQ) wiring diagram shows all of the connections that need to be made to the bank modules, power and control systems. From the left of Fig. 6, 208 and 110 power is fed to the bank enclosures via a Kirk key controlled isolation switch. This same switch can be energized by an Emergency Stop (E-stop) button located in the control room. This E-stop is digitized by both the FLARE control DAQ and the capacitor charge/dump power supply (CCDPS) control DAQ. If energized, the switch will drop all power to the enclosure, thereby killing power to the HV dump (normally closed) and charge (normally open) relays, and dumping bank energy into the cap dumps. The 110 and 208 power is delivered to the charging supply rack (located on its own separate pallet), and 110 is also delivered to an isolation transformer mounted on the bank module pallet. Connections to the inductive load are made by multiple triax cables in parallel, the number of which is dependent upon the current waveform and subsequent ohmic heating. Water is connected to the ignitron switches along 1/4" tubes from a shared chiller unit. The charge, dump ground relays are controlled by individual fiber-optic-enabled switches, with pulse signals sent from the CCDPS DAQ rack. Temperature sensor data are transmitted by fiber-optics from the pallet to the DAQ after conversion of voltage to frequency, then reconverting at the DAQ. A BNC connection is made from the current sensor integrator to the DAQ fast data acquisition (sampling at 1 MHz or faster). Timing synchronization is provided by the FLARE control DAQ. Fig. 6 Data acquisition and control wiring diagram Switch firing is controlled here by the FLARE control computer and DAQ, by transmission of fire signal by fiberoptic connection. CCDPS DAQ requires 110V as input. CCDPS control computer requires 110V as input. Phenomena" under subcontract from PPPL under PO94088 REFERENCES [1] [2] ACKNOWLEDGMENT *Work supported by NSF Grant Number: PHY-1337831 Title: "MRI Consortium: Development of a Large Plasma Device for Studies of Magnetic Reconnection and Related [3] H. Ji et al, “Status and Plans for the Upcoming FLARE”, BAPS 2015 Z. Gao, “Statement of Work for Design of Capacitor Charge/Discharge Power Supply (CCDPS) for FLARE”, FLARE-CCDPS-150828, Revision 0, Sept. 9th 2015 V. Babrauskus, I.S. Wichman, “Fusing of Wires by Electrical Current”, Fire and Materials Conf. Proc. 2011