AAWSAP DIRD, Inertial Electrostatic Confinement Fusion, March 2010

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bank discharges. The pulse shaper is a tapped inductive line with each tap terminated
by a pulse-forming network (PFN). Each tap performs a filtering function for a discrete
Fourier function or band of frequencies, which results in a smoother, and more uniform
pulse rise and fall, and also provides more constant load impedance matching, as the
IEC tube has high input impedance. This aids in the forward coupling of energy into the
IEC by minimizing backward or reflected standing waves generated by the dynamic
impedance discontinuity presented by the spark gap and the cathode to anode circuit
path. The PFN, in effect, serves as a pulsed coupled transformer with a low input
impedance and a high output impedance to drive the IEC tube. Components for the PFN
are off-the-shelf and commercially available. The spark gap trigger is activated by an
IGBT (isolated fate bipolar transistor) driven by the power control circuits.
A stable multivibrator timer circuit is reset each time the spark gap is fired by a fast
phototransistor circuit that detects spark-generated photons. This is coupled by a light
wave-guide or optical fiber to provide high voltage isolation between the solid-state
control circuits and the high voltage sections. This optically isolated signal is input to
signal processing to give time of flight measurements. A digital or manual
potentiometer range selector controls the time delay by increasing or decreasing the
resistance in an RC network. When the capacitor in this network charges to a nominal
voltage threshold, the timer circuit changes its logic state and drives the gate of the
spark gap trigger IGBT on, and this initiates the spark gap arc. This RC time period
ramp is an analog of the energy storage capacitor charging ramp, which also begins
after the main arc occurs and drains the energy store. The longer the charge time, the
higher the charge in the energy storage capacitor bank and the higher the voltage is
when the next pulse is initiated. The voltage applied to the IEC tube is thus varied over
a nominal 40 percent range. The peak output voltage can therefore be varied over a
range of 50 kV to 120kV. There is a reciprocal relationship between the pulse repetition
frequency and the output pulse peak power.
Design of the Total Integrated Interrogation System
The integrated system is illustrated conceptually in Figure 5.2.
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