Discharge circuits and loads Page: 15 of 62
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DISCHARGE CIRCUITS AND LOADS
during the design of a PFN it is usually more cost-effective to provide the
capacitor supplier with a sketch of the network along with the capacitor
values and inform him of the load peak current, allowing him to design the
capacitors for this service taking into account the current ratios in the
various PFN capacitors.
For very short-duration current pulses in the multikiloamp range, the
PFN capacitor is preferably of the extended-foil type to eliminate internal
arcing and tab interface power losses.2 If the current risetime required
in the load is less than 0.1 z, there is a way of achieving this, which was
started years ago in gas discharge systems. Giver an ideal switch in a
four- to eight-section transmission line, the first four capacitances are
selected in the ratio of 1:2:4:8 and the same impedance per section,
Zo LN. This does not produce a trapezoidal shaped pulse, but it
gives a very fast risetime pulse. In this configuration, water vapor la-
sers were driven with relatively steep rising current pulses. Because they
were direct electron-pumped systems, this increased the electron tempera-
ture significantly and increased their laser efficiency. The risetime is
effectively the discharge time for the very first stage provided the PFN-
load interface inductance L is small enough so that L/RL is less than the
first PFN section discharge time. This is generally a useful technique for
generating a steep wave-front pulse. Time-varying load impedances can be
roughly matched for these lasers b3 setting a ratio for the impedances of Z
to 2:4:8 for the first three sections, creating what is called a tapered
transmission line. This can cause reflection problems, but it is a rather
unique and efficient way of driving such loads as hydrogen fluoride (HF)
lasers, C02 transversely KrF Excimer systems, whose impedance decreases
with time. 'This is a crude but economical approximation to the general
time-varying PFN problem.
For very low ripple on the top of the pulse, there must be many sec-
tions in the PFN. A 20-section PFN can be built to have a peak-to-peak
ripple of onc per cent, which appears to be a practical limit. It is very
difficult to achieve less ripple than this.
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Sarjeant, W.J. Discharge circuits and loads, report, October 15, 1980; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1061299/m1/15/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.