Page 74 - Tunable Lasers Handbook

3 Tunable Excimer Lasers 55
As just mentioned, Burnham and Djeu separated the preionization from the
main discharge. This originally required separate capacitors and switches for the
two circuits and also imposed timing considerations between the discharges.
Present-day commercial systems have very cleverly combined the two by forcing
the peaking capacitors to be charged through small gaps via an arc that provides
for the preionization. The diagrams in Fig. 18(a) show one of a row of such a
peaking capacitor array. An alternate, efficient technique [Fig. 18(b)] is that of
corona preionization using the voltage rise time of the system to induce a voltage
on the surface of a dielectric by generating displacement currents in the dielectric.
Commercial lasers using the preceding techniques usually provide laser energies
as high as 1 J/pulse with an operating pulse width between 20 to 30 ns.
A major advance in discharge laser technology is attributed to Lin and Lev-
atter [72,73]. They studied the details of streamer formation and postulated that
there is a region in discharge parameter space where long stable discharges are
possible, This is accomplished by very uniform preionization and very fast volt-
age rise times. They developed a laser with X-ray preionization and a series rail-
gap switch to accomplish the very fast voltage rise time. Such a system, shown
in Fig. 19, indeed showed greatly improved laser performance. However, the
stringent requirements make commercialization of the technique difficult.
Attempts to satisfy the Lin-Levatter criteria led to the study of magnetic
pulse compression techniques to transform a slow rising pulse to a very fast one.
The technique has the added benefit of substantially lowering the current and the
rate of current rise through the switch. This will greatly improve the switch life-
time. However, due to the hysteresis loss in the magnetic material, oil cooling is
generally necessary and results in substantial complications for a commercial
system. Lambda Physik has incorporated the technique into some of their prod-
uct lines for the purpose of preserving switch lifetime. Figure 20 gives a
schematic of the pulse power setup. Due to the development of hollow anode
thyratrons by English Electric Valve, Ltd., which allow 50% inverse current tran-
sients through the switch, switch lifetime considerations are no longer as severe
a problem as previously the case. The use of pulse compression to shorten
greatly the voltage rise time was first successfully implemented by Laudenslager

H.V.

FIGURE 1 9 Circuit for very fast voltage rise time incorporating a series rsl-gap switch.