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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