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Shiva Star - Marauder Compact Torus System

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SHIVA STAR - MARAUDER COMPACT TORUS SYSTEM
Abstract
The MARAUDER (Magnetically Accelerated Rings to Achieve
Ultrahigh Directed Energy and Radiation) System is composed of
multiple banks currently being used to form and accelerate a compact
torus. The system consists of four banks: gas valve bank; slow magnetic
field bank; toms formation gun bank; and accelerator bank. The accelera-
tor bank is the SHIVA Star 9.5 MJ capacitor bank modified to operate with
a reduced number of modules, a precautionary requirement for the initial
testing of the acceleration phase of a compact torus. The normal 36
modules and six arms of SHIVA Star have been reduced to operation of
three arms and a choice of either one or two modules per arm as
acceleration banks. The four forward modules of the three arms can be
used as a system crowbar on the accelerator bank with the addition of a
buss shorting clamp on each module. Operation of the SHIVA Star
capacitor bank in this modified mode allows for lower energy operation
(550 kJ and 100 kV) with the flexibility of adding additional modules as
experiment energy requirements increase. This approach permits rela-
tively non-destructive testing of the load at suitable energies. Initial test
results are discussed, relative primarily to bank performance during
MARAUDER acceleration tests with SHIVA Star in the reduced energy
mode.
Introduction
The 9.5 MJ SHIVA Star capacitor bank (Figure 1) has in the past
been used to study soft radiation production by the implosion of thin foils
using inductive store pulsed compression [1] and plasma flow switch
techniques [2]. Present work at the Phillips Laboratory has been the
formation and acceleration of a large compact torus (CT) with a future
goal of very high energy acceleration using the SHIVA Star capacitor
bank for acceleration.
The design and system integration of the CT experiment has been
an on-going project since early in 1988. At that time, the first of the design
concepts and assemblies were under consideration. Phase IA was the
formation of a large CT (105 cm OD and 90 cm ID). Requirements for this
experiment were a gas injection system, magnetic field bank, and
formation gun bank. Phase 1A was set up in an area away from the existing
SHIVA Star assembly where toms formation could be studied and the
phase 1A system could be tested before it was connected to SHIVA Star.
Figure 1. 9.5 MJ SHIVA Star Capacitor Bank.
J.D. Graham, D. Gale, W. Sommars, M. Scott,
Albuquerque Division
Maxwell Laboratories, Inc
Albuquerque NM 87119
Y. G. Chen
Balboa Division
Maxwell Laboratories, Inc
San Diego CA 92123 Phase IB of the SHIVA experiment required relocation of the
formation experiment and integration into the SHIVA Star capacitor bank
to allow for experimental testing of both the formation and acceleration
of a large CT. The goal of initial experiments was to successfully trigger
all of the banks in the correct firing order and to achieve torus acceleration.
To allow for load testing at higher voltages and lower energies, these first
studies were done in a reduced energy mode on SHIVA Star, allowing for
operation of three modules of the 36 module system. Phase IB effort
began in May 1990, and toms formation shots started in October 1990.
First acceleration shots were done in January 1991 after modifications to
the SHIVA Star capacitor bank. These modifications to the SHIVA Star
bank will be the main topic of discussion with descriptions of the Phase
IB integration of the formation system of Phase IA. The purpose is to
show the flexibility of the modular approach available with the SHIVA-
style modules.
System Design And Performance
SHIVA Module
The SH1VA-type module (Figure 2) is constructed with twenty-
four 6.1 ktF capacitors in a differentially charged configuration and is
switched with four rail gap switches. This standard module style is used
in the acceleration and formation banks of the SHIVA Star CT experi-
ment. Each module may be charged over a range from 30 kV to 120 kV;
nominal module energy is 250 kJ. This modular approach has been used
on the SHIVA-type systems for 18 years and has produced the very high
energies of the 9.5 MJ SHWA Star capacitor bank [ 1].
CRDVYAR
V
ACCELERATOR I~
Iill ~'.,/"
.
"t~'~l
MODULE
<
I SHORTING
CLAMP
[ OUTER '
~li~~ SHORTING CLAMP
Figure 2. SHIVA-Type Module with Added Shorting Clamp.
SHIVA Star--~Acceleration Bank
Initial acceleration testing for the MARAUDER experiment
required that accelerator energies be reduced to permit study of the early
behavior of the torus, to minimize the possibility of catastrophic failure,
and to aid in the design of the next phase of higher energy CT acceleration.
990
The present assembly allows variations in the operating configuration of
the acceleration bank. The four forward modules may be used as
crowbars, and one or two of the outside modules on each of three arms
may act as the acceleration modules. (Figure 3.)
Accelerator
Formation
Bank
Figure 3. SHIVA Star Accelerator/Crowbar Configurations.
The Phase IA and
IB
hardware was not optimized for accelerating the
torus at the full SHIVA Star energy of approximately 9.5 MJ. The plan for
the accelerator bank operation is to use three of the 36 modules from the
SHIVA bank's six ann configuration to provide for the addition of
unused modules as the load parameters allow. In the present design, only
three of the six arms are used for the accelerator bank, and three of the 36
modules of SHIVA Star are to be used as acceleration modules. This
arrangement provides a capacitance of 109.8 gF driving 15 nH to the
main accelerator insulator and 28 nH to the initial torus attachment to the
acceleration region shown in Figure 4.
ACCELERATION
REGION
FORMATION
REGION
ACCELERATOR TRANSMISSION UNE 1"~ / ]
g2 dll / ..... '
II
T /II
FORMATION
r~..o,o,u.o
II
!
1
x tK;
Figure 4. SHIVA Load Assembly Side View.
Additional accelerator modules can be added by installation of
similar hardware to the other three arms. This would provide a total of 12
acceleration modules and 24 crowbar switches in the present mode of
operation. The possible modes of operation and initial parameters are
shown in Table 1. Table 1.
Accelerator Configurations
C m L m R rn L total T/4
Modules (ktF) (nil) (mr2) (nil) ktsec)
3 109.8 4.9 0.787 28 2.75
6 219.6 2.45 0.393 25.45 3.7
12 439.2 1.22 0.197 20.72 4.74
36 313 0.40 0.066 16.0 7.2
The crowbar modification employs switches on the four for-
ward modules as crowbar switches, with other changes including
addition of module shorting clamps, series resistors of 10 mr2 in the
transmission lines, trigger peaking gap switches and 50-50 gap
spacing of the rail gap switches. The shorting clamps include a copper
compression gasket to short the front-to-rear plates of the module,
removing the capacitance and using the buss plates as the trans-
mission line for those rail gap switches serving as crowbars. Series
resistance in the transmission lines will reduce module reversal
from 90% to <60% when the crowbar circuit is fired. The peaking gap
uses a trigatron style irradiation pin which will provide a stronger
trigger pulse for triggering the crowbars at lower voltages. Resistor
connections to the rail gap knife edge are made with resistive spark
plug wire to lower resistor cost and provide a low inductance resistor
for the 50-50 gap geometry. The crowbars will allow the shunting of
energy from the load and insulator surfaces to the crowbars to
mitigate system contamination after the acceleration of the torus has
been completed.
Formation Gun Bank
The formation gun banks, located at the ends of three of
the SHIVA arms, consist of three SHIVA style modules, each rated
for 100 kV operation. These modules operate in an inverted mode
with the ground on the top transmission line and use a sloped parallel
plate transmission line feed under the SHIVA Star transmission lines.
The system capacitance is 109.8 gF and inductance to the insulator
is 27 nil, with an additional 10 nH to the gas injection level. Short
circuit shot waveforms are shown in Figure 5 for 40 kV charge.
i
Figure 5.
991
Formation Bank 40 kV Short Circuit Shot 1.8
MA Peak at 5~ts/Div.
The inductance is 42.2 nH with the short installed at the beveled
flange level above the gas injection site (Figure 4). Line termination
resistors were installed in the transmission line to reduce line ring-up and
aid in switch channel formation in the event of system prefires (Figure 4).
The system circuit (Figure 6)differs from the short shot circuit by
the addition of series resistors which increase the inductance by 16.7 nH
and resistance by 4.1 m.Q. (The series resistor placement is shown in
Figure 4.) The load inductance has been reduced to the site of the gas
injection ports in Figure 6.
FORMATION TRANSMISSION
BANKS LINE
8mf~ 5nil 19.7nH 1.2mf~
20txH 30nil
09.8~F
.033mf~ 160mf~
PARALLE L TE R M I NAT ION
-- INDUCTORS RESISTORS
SERIES
RESISTORS
16.7nH 4.1 mfl
12.5nH
Figure 6. Formation Circuit.
L
O
A
D
Magn¢fi~ Field Bank
The magnetic field bank is a 10 kV-750 kJ slow discharge
capacitor bank used to provide the radial magnetic field for CT formation.
The current waveform for this system, shown in Figure 7, is for a 2.5 kV
charge and 11 ms rise to peak current with diode crowbar system acting
at Ipeak to stop bank reversal.
Figure 8. Gas Valve Bank Current 116 Output 2.74 kA20~/Div.
3.5 kV Charge.
The valves are connected with 60 RG-214 cables attached to
the output buss of the Magneform. This configuration allows 50 valves
to fire even with the failure of one series leg of 10 valves. The use of
the electro-mechanical valves for this application has proven to be an
economical means of fielding a 60 valve system and can be improved
upon in many ways (i.e., faster opening and closing times). The present
valve opening and closure times were measured with a laser reflec-
tometer for full movement of 0.020 inch at 400 ~ts and 1000 Ixs closure
times (12.5 psia.).
System Operation in Phase IB
Operation of the MARAUDER experiment requires indepen-
dent triggering of each separate bank (magnetic field, gas puff,
formation acceleration and accelerator crowbar) at a prescribed time
(Figure 9). The initial testing started with an isolation test on each of
the different trigger systems. Isolation is accomplished by monitoring
the triggers during test firing the trigger systems and using standard
isolation techniques (fiber optic trigger isolation in addition to EMI
boxes and inductive isolation for cables) to shield against pretriggering
of the systems. Testing progressed to triggers and bank charging to
isolate additional current loops and sources of pretriggering.
Figure 7. 2.5 kV Charge, I Peak 3.8 kA, 2 ms/Div.
Presently inner and outer magnetic field coils are used to
provide a field of approximately 0.1 to 0.5 Tesla in the formation
region.
Gas Valve Bank
The gas valve bank is constructed from a Maxwell Magne-
form® bank that was modified to hold eight 10 kV 240 l.tF capacitors.
The present gas valve system uses only two of the capacitors to drive
60 electro-mechanical gas valves in a series andparallel arrangement.
Each valve is a Honeywell Skinner normally--closed valve (type B2
DA! 175)[3]. Every valve coil has been replaced with a pulsed drive
coil of seven turns of AWG 12 wire, and the internal spring has been
changed from a 7 to a 50 lb/in spring. A mild steel cover for shielding
from external magnetic fields also has been fitted over each coil. The
valves have been successfully operated with argon over a wide range
from 3 to 100 psia. The electrical circuit drives six parallel sets of 10
valves in series. Each leg operates at a peak current of 2.74 kA and a
total current of 16.4 kA with a time of 54 Its to peak. The current
waveform is shown in Figure 8.
I '-b "sd- 6- is ,s)]
r !- 2mS ,.[. ARGON "T"
To ' GAS FORMATION ACCELERATION
VALVE GUN BANK
MAGNETIC BANK BANK
FIELD BANK
Figure 9. MARAUDER Tmae Line (Argon 12.4psiaTest).
Initial formation bank short shots compared favorably with
simulations. Early testing showed that reversals were about 80% on the
formation gun banks. This was subsequently lowered to 46% after the
first peak by the use of series resistive elements in the transmission
lines. The resistors reduced the coulomb transfer in the switches and
peak current by 16%, with reduced formation bank prefires and switch
maintenance but higher operating voltages to correct for the 16% loss
in energy. The formation banks have been operated at 80 kV in the
formation mode and in a combination of formation and acceleration
modes. The peak current delivered by the formation bank to the load
at 80 kV was 2.6 MA with the peak at 3.0 lxs and the formation insulator
flashing at 6.0 to 6.5 IXs after current rise. A typical current waveform
is shown in Figure 10.
The acceleration banks operating at 40 kV delivered 1.5 to 1.8
MA to the toms and showed early insulator flashover at higher voltages
992
Figure 10. Formation Current 3.0 MA Peak and 5ItS/Div.
and energies. MACH2 simulations of the early acceleration testing
showed that the toms can be pushed off the accelerator wall in the
straight feed [4], an event that will lead to early insulator flashover. The
flashover is indicated in the current waveform (Figure 11) at one Its
after current rise; this shows the slower rise time and the subsequent
changes to the shorter ringing frequency which indicate a flashover
near the vacuum insulator at approximately 15 nil.
Figure 12. 80 kV, Current Peak 5.6 MA, 2kts/Div.
Condusion
Independent trigger control of the MARAUDER experiment
has been demonstrated concurrent with operation of the SHIVA Star
bank in a reduced energy mode. The formation and acceleration
banks have been operated at 80 kV with satisfactory trigger system
isolation. Reduced energy operation has permitted the exploration
of load parameters at energies not catastrophic to the MARAUDER
load. Future testing will include the addition of a conical accelera-
tion section to replace the straight section of phase IB, additional
accelerator modules, and testing of the crowbar circuits.
SHIVA.Star CT formation and acceleration experiments arc
continuing with eight to 10 shots per day in each operational series.
Acknowledgements
This work is on-going at the USAF's Phillips Laboratory,
Kirtland AFB, New Mexico under contract F29601-90-C-0019.
The authors would like to thank Dr. J. H. Degnan for his suggestions
and support in the conversion Of SHIVA to operation in a reduced
energy mode. Also appreciated are the efforts of our Maxwell
Albuquerque Division technical crew to implement this change
Figure 11. Accelerator 40 kV, 3.7 MA Peak, 2its/Div.
[1]
The acceleration bank operating at 80 kV showed insulator
flashover at 0.2 Ixs after current rise and inductance reduced to 15 nH
from the 28 nH to the expansion region. The waveform in Figure 12
shows bank current and early insulator flashover with ringing at the
lower inductance.
[2]
The bank crowbar system has not yet been fired, but trigger
circuits have successfully been tested. Before the crowbar system is
activated, series resistance will be installed in the SHIVA Star
transmission lines to reduce current reversal of the accelerator modules
to the crowbars. Without the series resistance the reversal would be
near 90% and would greatly reduce module life.
[3]
[4]
993
References
R.E. Reinovsky, W.L. Baker, Y.G. Chen, J. Holmes, and
E.A. Lopez, "SHIVA Star Inductive Pulse Compression Sys-
tem," Transactions of the 4th I.E.E.E. International Pulsed
Power Conference, 1983, pp. 196-201.
J.H. Degnan, W.L. Baker, K.E. Hackett, D.J. Hall, J.L.
Holmes, J.B. Kriebel, D.W. Price, R.E. Reinovsky, J.D.
Graham, E.A Lopez, M.L. Alme, G. Bird, C.B. Boyer, S.K.
Coffey, D. Conte, J.F. Davis, S. Seiler, and P.J. Turchi,
"Experimental Results from SHIVA Star Vacuum Inductive
Store/Plasma Flow Switch Driven Implosions," in Transac-
tions of the 6th I.E.E.E. Pulsed Power Conference, 1987, pp.
332-335.
Honeywell Skinner Valve Division, catalog V86, page 2.3.
C. R. Sovinec, private communication.
Chapter
Compression and acceleration of magnetized plasmas is relevant to fusion for two reasons. Certain types of magnetized plasmas can be compressed and accelerated without fluid instability growth. These are magnetized plasmas rings or compact toroids1,2. Because of their stability, they can be compressed and accelerated over meters of distance and several microseconds of time, enabling economic scaling to much higher energy operation. Other types of implosions and compressions, e.g., Z-pinches, are limited by instability growth3 to much shorter acceleration distances (a few cm) and times (less than 100 nanoseconds), making it very expensive to scale their operating energies to the fusion regime. An important second advantage of magnetized plasmas is that discussed by Lindemuth and Kirkpatrick in their magnetized target fusion (MTF) concept4. Reduced electron thermal conduction losses and increased alpha energy deposition result in reduced requirements of fuel density-radius product for achieving fusion ignition. In this paper, we discuss two experimental efforts at the Phillips Laboratory relevant to this topic. These are our Compact Toroid2,5,6,7 and Solid Liner/Working Fluid8,910,11 efforts. Though these efforts have potential fusion application, their present support is for the applications of intense X-ray generation and achieving high density and pressure in the laboratory, respectively.
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