A 10 GW PULSED POWER SUPPLY FOR HPM SOURCES ∗
1
B.M. Novac, 1M. Istenič, 1J. Luo, 1I.R. Smithξ, 1J. Brown,
2
M. Hubbard, 3P. Appelgren, 3M. Elfsberg, 3T. Hurtig,
3
C. Nylander, 3A. Larsson and 3S.E. Nyholm
1
Department of Electronic and Electrical Engineering,
Loughborough University, Loughborough, Leicestershire LE11 3TU, UK
2
Dstl, Fort Halstead, Sevenoaks, Kent TN14 7BP, UK
3
Swedish Defence Research Agency(FOI), Grindsjön Research Centre,
SE-147 25, Tumba, Sweden
Abstract
I. INTRODUCTION
A research activity involving the detailed
consideration of novel high voltage transformers
(HVTs) for pulsed-power applications has recently
begun at Loughborough University (LU). Although the
main goal is the demonstration of a compact and
lightweight unit employing magnetic self insulation
under vacuum conditions, the initial stage of the work
is directed towards the development of a conventional
air-cored HVT as a main component in a compact
power supply for HPM sources. In cooperation with the
Swedish Defence Research Agency (FOI), the power
supply has been tested with a HPM source of the
vircator type.
The power source for the system uses a 70 kJ/25 kV
capacitor bank and an exploding wire array to generate
a 150 kV voltage pulse in the primary circuit of the
HVT. A pressurised SF6 spark gap in the secondary
circuit sharpens the high-voltage output, so that pulses
approaching 500 kV and with a rise time below 100 ns
are generated on a 20 Ω high-power resistor. The peak
power produced by the power supply is in excess of
10 GW. Measurements provided by various diagnostic
techniques are analysed with the aid of a detailed
numerical code.
Experimental results are presented from final testing of
the system, where a reflex triode vircator replaces the
20 Ω resistor. Measurements made of the microwave
emission using free-field sensors are presented for
various electrode configurations. Comments are made
with the microwave emission from the same vircator
powered by a Marx generator at FOI.
∗
ξ
Modern applications of HPM generators require the
development of compact pulsed power supplies capable
of generating a voltage pulse of up to 1 MV and of
sustaining the voltage for up to 1 µs when a
considerable microwave output is required. No simple
solution is available for this type of power source, and
although Marx generators represent the most obvious
solution, they are complex and have a high
volume/weight ratio. An attractive alternative is
however based on explosive magnetic flux
compression generators (FCGs) particularly when only
a single-shot and possibly expendable system is
required. To generate the necessary high-voltage pulse,
it is necessary to attach a conditioning unit
incorporating an opening switch (OS) to the FCG
output. One of the simplest high-voltage OS is the
exploding wire array (EWA), in which a number of
long thin metallic wires are transformed into plasma
columns in a high-pressure N2 or SF6 environment. A
number of such systems have been produced at
Loughborough, and based on the experience gained it
was decided that, before a 1 MV EWA was attempted,
a much reduced system would be produced. This would
be coupled to a HPM vircator provided by FOI and
would be tested as part of ongoing considerations into
the use of vircators in compact HPM power sources.
The paper describes a 10 GW generator based on a
relatively low voltage EWA and coupled to the vircator
through a high-voltage transformer (HVT). The main
results obtained during the many stages of the
development programme are presented, and followed
by results from the overall system.
Work supported in part by EPSRC (UK) through Grant GR/T01044/01 and the Swedish Armed Forces
email: I.R.Smith@lboro.ac.uk
II. SYSTEM DESCRIPTION
A. Pulsed power supply
An equivalent electrical scheme of the overall system
is given in Figure 1. The power source is a low
inductance 70 kJ/25 kV capacitor bank [1] feeding an
EWA through a flat, low inductance transmission line
and two 1.2 MA parallel connected closing switches
[2]. The EWA consists of 37 parallel connected OFHC
99.99% purity copper wires, 250 µm diameter and
370 mm long and mounted in a flat, plane geometry
(Figure 2). Insulation between the plane of the wires
and the copper strip part of the current return path is
provided by a 25 mm thick polyethylene plate with an
expendable 5 mm polyethylene layer that is replaced
after each firing. Although the use of cylindrical
geometry and thinner wires in a pressurised gas would
provide a more efficient EWA, the simpler, cost
effective, reliable and easy to replace solution was
preferred in this case.
A spiral-strip geometry HVT was used with the
following
electromagnetic
characteristics:
primary/secondary self inductance 255 nH/5.75 µH and
mutual inductance 1.1 µH. The output is coaxial with
the HV centre directly connected to a SF6 pressurised
spark gap (SG) mounted inside the HVT. The SG
design used ANSYS® software to ensure a high field
enhancement factor (FEF) of 1.31, while avoiding
surface breakdown inside the switch by maintaining a
very low electric field strength close to the plastic
cylindrical container. During preliminary testing, a
CuSO4 doped water resistor formed a 23 Ω high power
load (HPL). The HVT, SG and HPL, together with
their corresponding connections and diagnostics are all
mounted inside a plastic transformer-oil filled tank
(Figure 3). A metallic surface covers the bottom of the
tank and is connected to ground through a low
inductance circuit. The transformer oil is permanently
filtered and a sealed cover protects the oil from copper
debris resulting from the EWA explosion.
B. Vircator
The vircator at FOI is usually powered by a Marx
generator [3, 4] delivering approximately 200 kV to
250 kV over the diode with a peak power of 2 GW.
The reflex triode type vircator is built into an eight
inch, four way, stainless steel, vacuum cross. One of
the ports of the vacuum cross is bolted on to the oil
tank with the connections for the high voltage and the
ground submerged in oil. Two other ports are
connected to a vacuum pump and to the adjustable
cathode holder respectively, while the fourth faces a
semi-anechoic chamber. The 120 mm diameter anode
(A) consists of a woven copper mesh of 71 %
transparency or a stainless steel mesh of 55 %
transparency fitted inside a brass circular holder. The
cathode (K) is a circular brass disk of diameter 90 mm,
on which an emitting surface of velvet cloth is glued.
The A-K distance can be adjusted by screwing the
cathode brass stock inwards or outwards. In the
experiments presented the velvet cloth had a diameter
of 62 mm and the distance between the anode and the
cathode was 10, 12 and 14 mm. A vacuum level of 10-5
torr was obtained in the vircator housing.
C. Diagnostic equipment
The current in the primary circuit (Figure 1) is
monitored by a pair of in situ calibrated pick-up probes
positioned in a tunnel in the flat transmission line and
that in the secondary circuit (Figure 1) by a 40 MHz
commercial current transformer. Current through the
vircator is monitored using a pair of 20 MHz current
transformers. A fast LU built, 500 kV capacitive
divider [5] measures the secondary voltage of the HVT
which is used to detect accurately the SG breakdown
voltage. A commercial 40 MHz, 1 MV capacitive
divider records the load voltage and the light emitted
by the EWA explosion is detected by optical fibres,
each coupled to an opto-electronic converter. The
electromagnetically decoupled output signals are used
to trigger the five 500 MHz battery-powered
oscilloscopes that provide recordings of all the above
measurements.
Radiation from the vircator is measured using several
B-dot probes and two 20 GS/s, 4 GHz and 5 GHz
oscilloscopes. To facilitate the radiation diagnostics,
the vircator is installed in a semi-anechoic chamber.
The system overall arrangement is shown in Figure 4.
Figure 1 Equivalent electrical scheme of the system
III. PRELIMINARY EXPERIMENTS
The development of the overall system involved six
different stages, but space restrictions allow only the
most significant experimental results to be presented.
Figure 5 shows the primary current, its time rate-ofchange and the load voltage measured with the SG
short-circuited. Load powers in excess of 10 GW were
generated in some experiments.
Figure 2 EWA before a shot
Figure 3 Arrangement inside the oil tank during
preliminary testing
IV. NUMERICAL MODELLING
Accurate modelling of the system is needed to both
ensure a successful design and to interpret correctly
experimental data should a failure occur. The present
system design was based on the well proven 2D
filamentary technique [6], and involved modelling of
nonlinear elements of both the EWA and the vircator.
A simple phenomenological model using voltage and
current measurements established for the EWA both
the normalised dynamic resistance REWA(t)/REWA(0) and
the specific Joule energy w/m deposited in the wire,
where
w(t ) = ∫ REWA (t ) I 2 (t )dt and m is the EWA mass.
The corresponding model for the vircator used a
dynamic impedance obtained from Marx driven
experiments performed at approximately the same
voltage level.
V. FINAL SYSTEM TESTING
Due to space restriction, only the most relevant
results obtained during final testing of the complete
system are presented. A more detailed analysis will be
published elsewhere.
In the most successful experiments a stainless steel
mesh was used with the A-K distance set to 10 mm
while the power from the pulsed power supply was
varied. Figure 6 shows the results from an experiment
where the peak voltage applied to the vircator was
250 kV In this particular experiment the corresponding
pulse length was limited to 170 ns due to a breakdown
across the HVT primary. Microwaves, measured
0.95 m in front of the output window, were emitted for
140 ns coinciding with the length of the voltage pulse
although the strongest field was emitted during only
some 40 ns.
Figure 4 Overall view of the complete system
The peak magnetic field of 50 A/m corresponds to
approximately 1 MW/m2 assuming far-field conditions,
which is less than what obtained in previous FOI Marx
generator experiments. In the frequency spectrum two
peaks can be observed: one at the frequency 5.1 GHz
occurring at about 8.76 µs and one of 3.3 GHz
occurring at about 8.78 µs. The frequency shift is
explained by the vircator voltage falling from a higher
to a lower value. Noting that the voltage is
approximately 250 kV and 180 kV at the times for the
two frequencies the oscillation frequency f can be
calculated using [7]:
f =
5
6π
eV
, where V is the
md 2
voltage applied across the A-K gap, d is the A-K
distance and m and e are the mass and electron charge
respectively. This expression gives a frequency of
5.6 GHz and 4.7 GHz respectively, agreeing not very
well with the measurement. However this difference
between theory and measurements is also observed in
other experiments when the vircator is powered by a
Marx generator [4].
100
0
-100
H-field [A/m]
Voltage [kV]
Voltage over Vircator
300
200
8.66
8.68
8.7
8.72
8.66
8.68
8.7
8.72
8.74
8.76
Time [µs]
H-field
8.78
8.8
8.82
8.74
8.76
8.78
8.8
Time [µs]
Normalized Power Spectrum of H-Field signal
8.82
50
0
-50
1
0.5
0
0
1
2
3
4
5
6
Frequency [GHz]
7
8
9
10
Figure 6 Results from final system testing: (top)
voltage pulse across the vircator, (middle) microwave
pulse and (bottom) FFT spectrum
VII. REFERENCES
Figure 5 Results during preliminary testing at full
power (SG shorted): (from top to bottom) dI/dt and
integrated primary current (dots are theoretical
predictions) and load voltage. The load peak power is
in excess of 10 GW.
VI. CONCLUSIONS
A 10 GW pulsed power generator has been designed,
constructed and successfully tested at LU coupled to a
HPM vircator load provided by FOI. The common LUFOI experiments, with a vircator not in any way
optimized for the pulsed power supply, show that it can
be powered with this type of system. All shots
produced microwave radiation with frequencies
between 1 GHz to 5 GHz depending on the vircator
parameters and the level of voltage. A complete
analysis of the results will be published elsewhere.
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D.F.Rankin
and
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