Circuit for the preionization and main discharge of a pulsed gas laser

A circuit for the preionization and main discharge of a pulsed gas laser provides that the sparks intended for the preionization of the working gas have an independent switch function. Capacitive energy storage means 7, 8, 9, 10 are connected low-inductively to the spark electrodes 3, 4, 5, 6. The current flowing in the spark discharge is used selectively for the preionization by means of the spark or for generating a compressed high-voltage pulse. The compressed high-voltage pulse is applied as prepulse to the main electrodes to momentarily greatly increase the voltage obtaining between the main electrodes and thus initiate and supply a homogeneous main discharge.

The invention relates to a circuit for the preionization and main discharge 
of a pulsed gas laser. 
Pulsed gas laser, for example excimer lasers, N.sub.2 lasers and CO.sub.2 
lasers, are often stimulated transversely by a plasma discharge (TE gas 
laser). The plasma discharge (also called main discharge) takes place 
perpendicularly to the optical axis of the laser. The energy necessary for 
the plasma discharge is stored in a capacitor and transferred to the 
plasma on discharge thereof. The plasma discharge takes place between main 
electrodes disposed in the laser chamber parallel to the optical axis. 
The power and also other qualities of the laser depend inter alia 
substantially on the homogeneity of the plasma discharge. To obtain the 
necesary homogeneity of the plasma discharge at different pressures of the 
working gas depending on the gas mixture a so-called preionization is 
necessary prior to the plasma discharge (main discharge). Details on the 
preionization will be found for example in: A. J. Palmer: "A physical 
model on the initiation of atmospheric pressure glow", Appl. Phys. Lett. 
25 (1974), 136; J. I. Levatter and S. C. Lin: "Necessary conditions for 
the homogeneous formation of pulsed avalanche discharges at high gas 
pressure", J. Appl. Phys. 51 (1980), 210; and G. Herziger et al.: "On the 
homogenisation of transverse gas discharges by preionization", Appl. Phys. 
24 (1981), 267. 
The preionization of the working gas is carried out inter alia with sparks. 
Sparks are sources of intense radiation which is adequately short-waved to 
photoionize in a substantially one-stage process an atom or molecular type 
present in the working gas and thus to generate free electrons in adequate 
concentration in the space between the main electrodes. The UV light 
irradiation by the sparks must therefore reach the space between the main 
electrodes so that on adequate concentration of the free preionization 
electrons the plasma discharge starts homogeneously. The sparks are 
generated between the spark electrodes which are provided additionally to 
the main electrodes. The spark electrodes are generally disposed in the 
vicinity of the main electrodes (K. Miyazaki et al.: "Efficient and 
compact discharge XeCl laser with automatic UV preionisation", Rev. Sci. 
Instru. 52 (1985), 201). It is also possible to make the main electrodes 
of optically substantially permeable metallic screens or sieves so that 
the spark electrodes can be disposed behind the main electrodes (C. R. 
Tallman: "A study of excimer laser preionization techniques", Topical 
Meeting on Excimer Lasers 1979, Paper WB4-1; R. S. Taylor et al.: 
"Time-dependent gain and absorption in a 5 J UV-preionized XeCl laser", 
IEEE QE 19 (1983), 416; Levatter: EP No. 0033424; A. J. Kearsley et al.: 
"A novel pre-ionisation technique for discharge excited rare gas halide 
lasers", Opt. Comm. 31 (1979), 181; S. Watanabe and A. Endoh: "Wide 
aperture self-sustained discharge KrF and XeCl lasers", Appl. Phys. Lett. 
41 (1982), 799). An arrangement of the spark electrodes laterally adjacent 
the two main electrodes is apparent from the works by C. R. Tallmann: "A 
study of excimer laser preionization techniques", Topical Meeting on 
Excimer Lazers 1979, Paper WB4-1; C. E. Webb: "Quantum Electronics and 
Electro-Optics", Ed. by P. L. Knight, 1983, John Wiley & Sons Ltd., p. 3; 
and A. J. Kearsley et al.: "A novel pre-ionisation technique for discharge 
excited rare gas halide lasers", Opt. Comm. 31 (1979), 181. 
With regard to the electrical supply a distinction may be made between two 
different groups of known preionization systems by means of sparks: On the 
one hand in the so-called autonomous preionization circuits the current 
supply of the spark electrodes takes place independently of the supply of 
the plasma discharge (R. S. Taylor et al.: "Glow discharge characteristics 
of a 0.8 Joule multi-atmosphere rare gas halide laser", Opt. Comm, 25 
(1978), 231 or R. S. Taylor et al.: "Time-dependent gain and absorption in 
a 5 J UV-preionized XeCl laser", IEEE QE 19 (1983), 416) whereas in the 
so-called automatic ("integrated") spark preionization the electrical 
supply of the spark electrodes is integrated into the plasma discharge (K. 
Miyazaki et al.: "Efficient and compact discharge XeCl laser with 
automatic UV preionization", Rev. Sci. Instr. 52 (1985), 201; J. I. 
Levatter: EP No. 0033424; A. J. Kearsley et al.: "A novel pre-ionisation 
technique for discharge excited rare gas halide lasers", Opt. Comm. 31 
(1979 ), 181). 
On the other hand in one type of spark preionization a plurality of sparks 
were fed in series (travelling wave) from a current path (R. S. Taylor et 
al.: "Time-dependent gain and absorption in a 5 J UV-preionized XeCl 
laser", IEEE QE 19 (1983), 416; S. Watanabe and A. Endoh: "Wide aperture 
self-sustained discharge KrF and XeCl lasers", Appl. Phys. Lett. 41 
(1982), 799) whereas in another type of spark preionization the sparks are 
fed independently of each other (parallel). In the latter preionization in 
the supplying of a predetermined spark discharge between associated spark 
electrodes it is not necessary to supply the adjacent spark discharges as 
well (A. J. Kearsley et al.: "A novel pre-ionisation technique for 
discharge excited rare gas halide lasers", Opt. Comm. 31 (1979), 181; C. 
E. Webb: "Quantum Electronics and Electro-Optics", Ed. by P. L. 
Knight--1983, John Wiles & Sons, Ltd., p. 3; Miyazaki et al.: "Efficient 
and compact discharge XeCl laser with automatic UV preionisation", Rev. 
Sci. Instr. 52 (1985), 201). 
In all the circuits cited so far for spark preionization the sparks are 
supplied from highly inductive power circuits. This high inductance of the 
power circuits supplying the spark discharge is due to the fact that the 
spark current during the preionization flows in series with a punctual 
switch such as a spark gap or thyratron (cf. e.g. Optics Communications, 
31, (1979), p. 181, FIG. 1). The maximum spark current is limited by the 
power capacity of the switch. In addition the impedance of the spark 
discharge is very small and thus the adaptation of the highly inductive 
spark supply circuit to the spark discharges very poor. The inductance of 
the spark supply circuits known from the cited publications rapidly 
reaches values above 100 nH so that within one oscillation period only a 
very small part of the stored energy is converted to the spark formation 
and thus the preionization. 
European patent application No. 33,414 discloses a circuit of the 
"automatic" type mentioned above for spark generation in which the switch 
itself is a low-inductive linear spark gap (rail gap). This circuit 
comprises a low-inductive power circuit for supplying the sparks. The 
sparks are fed independently (parallel). 
In all the known spark preionization systems a switch is used which lies in 
series with the spark discharges and the spark discharges take place only 
"at the command" of said switch. In the known preionization systems the 
preionization spark discharge thus does not perform and independent 
switching function. 
The already mentioned high inductance of the power circuit feeding the 
spark discharge, i.e. the poor adaptation of the power circuit to the 
low-inductive spark discharge, is not detrimental in the known circuits of 
the aforementioned "automatic" type because the capacitively stored energy 
is to be transferred for the greater part to the plasma discharge (main 
discharge). In the aforementioned autonomous systems in which the sparks 
are fed from a circuit separate from the main discharge circuit 
considerable disadvantages result: Firstly, the efficienty of the 
conversion of the capacitively stored energy to the preionization is low. 
Also, the radiation emission of the sparks takes place over a relatively 
long period and a consequence of this with electronegative gases such as 
HCl or F.sub.2 is that the free electrons initially generated by 
photoionization are again captured (electron attachment). Thus, due to the 
long period of the spark preionization the concentration of the free 
electrodes in the space between the main electrodes is undesirably 
lowered. Also, an unnecessarily large amount of electrical charge is 
conducted by the spark discharges and this can impair the life and 
functionability of the spark electrodes and be contrary to the requirement 
of the long life of the laser of at least some 100,000,000 shots. In 
addition, the large charge transport can lead to gas contamination. 
It is also to be noted that the ionization effectiveness of the sparks is a 
very rapidly increasing function of the spark current. It is thus 
desirable to transfer the energy used for autonomous preionization in an 
as intense and short as possible a current possible to the spark 
discharge. 
The problem underlying the invention is to remedy all the aforementioned 
disadvantages. In particular, an autonomous circuit for the electrical 
supply of a pulsed TE gas laser is to be provided which permits 
selectively an excellent preionization and/or the generation of a 
compressed high-voltage pulse. If the circuit is used for preionization 
the latter should be as intensive as possible, convert the capacitively 
stored energy with high efficiency, be less than 200 ns, subject the 
switch (e.g. a thyratron) generally provided in the pulse high-voltage 
source to little load and ensure a long life of the preionization system, 
in particular of the spark electrodes. Moreover, a limited charge transfer 
by the sparks is desirable for avoiding gas soiling and contamination. 
According to the invention this problem is solved in a circuit having the 
features set forth in the preamble of claim 1 in that the capacitive 
energy storage means (capacitor) is connected low-inductively to the spark 
discharge taking place between the spark electrodes, the capacitive energy 
storage means is charged by the pulse high-voltage source up to the static 
breakdown voltage, and that the current flowing in the spark discharge is 
used selectively for the preionization by means of the spark and/or for 
generating a compressed high-voltage pulse which is entered as working, 
switching or control pulse into a circuit of the laser other than the 
voltage supply of the spark electrodes, such as for example the supply 
circuit of the plasma discharge. 
Thus, according to the invention the spark discharge between the spark 
electrodes is used either for the preionization or for the pulse 
compression or for both. Pulse compression means that energy capacitively 
stored over a relatively long period of about 100 nsec to several hundred 
nsec is converted without substantial losses to a high-pressure pulse 
compressed, i.e. shortened, in time by for example a factor of 10. Said 
compressed high-voltage pulse can be supplied as working, switching or 
control pulse for other purposes than the spark preionization to other 
switching elements of the laser, for example to the primary side of a 
pulse transformer or the supply circuit of the plasma discharge. 
Thus, according to the invention each individual spark between the spark 
electrodes itself acts like a switch which conducts the energy stored 
capacitively prior to the spark breakdown with very low inductance via the 
spark discharge. 
If a plurality of spark electrodes are arranged adjacent each other in the 
laser chamber the spark discharge takes place partically simultaneously 
for all spark electrodes. For once a single spark is the first to 
flashover the spark discharge generates with a negligibly small delay in 
the adjacent spaces between the spark electrodes enough charge carriers to 
immediately cause the spark discharge there as well. 
In a preferred embodiment of the invention the capacitive energy storage 
means, for example a capacitor, is connected in parallel to the spark 
discharge gap between the spark electrodes. 
To allow the lowest possible inductive feeding of the spark current in a 
preferred embodiment of the invention the current path, measured along the 
shortest travel, on which at least 50% of the spark discharge current 
flows in the time in which said spark current serves for preionization is 
not longer than 100 cm for all sparks. 
It has surprisingly been found that in contrast to the prior art, where 
pointed spark electrodes are preferred, particularly good results can be 
achieved, in particular a long life of the spark electrodes and very 
stable and reliable operating conditions, if the spark electrodes are made 
blunt on their opposing sides. Particularly suitable are convexly curved 
or frusto-conical spark electrodes. 
It has also surprisingly been found that the spark length and thus the 
spark electrode spacing should be relatively great, preferably in the 
region of 25 to 30 mm. 
In a further embodiment of the invention the compressed high-voltage pulse 
generated as described above is applied as so-called prepulse directly to 
the main electrodes so that at the main electrodes a voltage increase 
takes place which together with the preionization already previously 
initiated leads to breakdown and initiation of the main discharge. 
For the quality of the main discharge, in particular its homogeneity and 
time behaviour, it is necessary for the preionization also to take place 
as homogeneously as possible, i.e. uniformly in space. 
The task of the preionization and of said prepulse is to increase the 
initially very small concentration of free electrons in the working gas 
(presumably less than 1 electron per cm.sup.3) in as homogeneous as 
possible a manner up to values of 10.sup.13 to 10.sup.15 per cm.sup.3. 
This increase in the concentration of free electrons by 13 to 15 powers of 
ten is achieved in two stages. The increase by about the first 7 powers of 
ten of the concentration of free electrons is effected by the 
preionization whereas the following powers of ten are obtained by the 
so-called avalanche process (field multiplication). The preionization 
operation has substantial influence on the homogeneity of the entire 
electron multiplication process and thus also on the homogeneity of the 
main discharge on which the power and other qualities of the laser mainly 
depend. If during the preionization phase between the main electrodes of 
the laser chamber an adequately high voltage already obtains free 
electrons are also generated by the avalanche process. If however this 
avalanche process already starts before the concentration of free 
electrons has reached values of about 10.sup.6 to 10.sup.7 per cm.sup.3 
the desired homogeneous generation of free electrons may be considerably 
impaired. In other words: The initial concentration of about 10.sup.7 free 
electrons per cm.sup.3 must be generated substantially by the 
preionization itself (and not by an avalanche process). The number of 
electrons generated by avalanche processes is a sensitive function of the 
voltage between the main electrodes in the laser chamber. To prevent 
avalanche processes form disturbing the concentration of free electrons of 
about 10.sup.7 per cm.sup.3 generated by means of preionization (that is 
for example by ionization of UV radiation) the voltage between the main 
electrodes must be kept correspondingly small in this phase of the 
preionization. The exact value of a voltage still permissible between the 
main electrodes (i.e. a voltage at which the avalance processes do not 
disturb the homogeneous electron generation) depends on the composition of 
the working gas and the intensity of the preionization. Generally 
speaking, with increasing intensity of the preionization the admissible 
voltage between the main electrodes may also increase. A certain 
simulation of these processes shows that with all conventional 
preionization methods lasting in each case up to a few hundred ns to 
generate about 10.sup.7 free electrons per cm.sup.3 the voltage between 
the main electrodes must not be appreciably greater than twice the 
breakdown voltage (the voltage which leads to breakdown at the 
aforementioned higher electron concentrations). 
The exact time control of the voltage between the main electrodes and its 
time adaptation to the preionization is made difficult above all because 
the generation of free electrons during the preionization depends 
substantially on the time integral of the preionization intensity whereas 
the field multiplication (avalanche processes) depends exponentially on 
the time. 
The compressed high-voltage pulse generated according to the invention and 
applied as prepulse directly to the main electrodes can, with the 
inductances and capacitances provided, be adapted in time so that the time 
variation of the voltage between the main electrodes automatically meets 
the requirements described above without other control and monitoring 
means. If the inductance L1 of the circuit in which the high-voltage pulse 
serving as prepulse is adequately small (e.g. less than 5 nH, this 
depending however on the geometrical arrangement of the circuit elements), 
the prepulse can be given an ideal time profile with which it firstly 
considerably reduces the voltage between the main electrodes (so that in 
this phase of the preionization no troublesome avalanche processes occur) 
whilst in a subsequent phase the prepulse substantially increases the 
voltage between the main electrodes until the breakdown voltage is 
reached. The prepulse thus changes its sign with time. Its polarity at the 
beginning is chosen such that the voltage between the main electrodes is 
substantially reduced (e.g. to 0). This period with reduced voltage 
between the main electrodes lasts for example 15 ns. In this time the 
working gas between the main electrodes is preionized by the preionization 
sparks homogeneously to for example 10.sup.7 free electrons per cm.sup.3. 
Only thereafter does the time variations of the prepulse cause the voltage 
between the main electrodes to increase very rapidly until due to the then 
starting field multiplication of the free electrons the plasma discharge 
takes place in the working gas (the concentration of the free electrons is 
increased here to values of about 10.sup.13 to 10.sup.15 electrons per 
cm.sup.3). It is obvious that the polarity of the prepulse in the field 
multiplication phase is the same as the voltage applied by capacitive 
energy storage means to the main electrodes so that in this phase the two 
voltages add up whereas previously the voltage made available by the 
capacitive energy storage means was reduced by the prepulse. 
According to a preferred embodiment of the invention it is therefore 
provided that the prepulse is applied directly to the main electrodes in 
the laser chamber in such a manner that prior to the increase in the 
voltage a momentary reduction of the voltage occurs. 
Such a variation of the prepulse can be achieved with the low-inductive 
connecting of the capacitive energy storage means for the spark electrodes 
according to the invention to the spark discharge taking place between the 
spark electrodes in that negative and positive interferences as known in 
high-power pulse electronics are utilized.

FIG. 1 shows the basic circuit diagram of a circuit for preionization of 
the working gas of a TE gas laser and/or for generating a compressed 
high-voltage pulse. Two spark electrodes 1, 1' are made substantially 
hemispherical and face each other a distance of 25 to 30 mm apart. It is 
also possible to arrange two blunt electrodes or a plurality of blunt 
electrodes of a common plate-shaped counter electrode opposite each other. 
A pulse high-voltage source 2 of conventional construction, i.e. for 
example comprising parallel-connected capacitors and a thyratron as 
switch, feeds the capacitor C.sub.1 serving as capacitive energy storage 
means successively up to the static breakdown voltage. The voltage of the 
capacitor C.sub.1 is applied to the spark electrodes 1, 1'. A spark 
discharge then takes place between the spark electrodes 1, 1' and effects 
the preionization of the working gas of the laser. Since the capacitor 
C.sub.1 is connevcted low-inductively to the spark discharge gap good 
adaptation of the spark suply circuit to the inductance of the spark is 
achieved. The electrical energy stored in the capacitor C.sub.1 (from the 
pulse high-voltage source 2) is therefore converted with good efficiency 
to the spark formation so that the preionization also takes place with 
correspondingly good efficiency. The preionization lasts only some tens of 
nanoseconds. 
As apparent from the Figures the supply circuit for the spark discharge 
does not require its own switch. The switch in the pulse high-voltage 
source 2 (e.g. a thyratron) is not subjected to any particular load. The 
spark thus acts as "switch". 
Charging of the capacitor C.sub.1 lasts typically about 50 to a few hundred 
ns. 
FIG. 4a shows the variation of the charge current of the capacitor C.sub.1 
with time. The charge quantity stored in the capacitor C.sub.1 is denoted 
by Q.sub.1 and corresponds to the area beneath the curve according to FIG. 
4a. 
FIG. 4b shows the variation of the spark current with time, the two time 
scales of FIGS. 4a and 4b being identical. 
At the instant T.sub.1 the spark discharge is initiated between the spark 
electrodes 1, 1'. The energy stored in the capacitor C.sub.1 is 
transferred mainly in a current pulse to the spark discharge. The charge 
Q.sub.2 flowing in the spark current is substantially equal to the charge 
Q.sub.1 of the capacitor C.sub.1. The start instant T.sub.1 of the spark 
current corresponds substantially to the instant T.sub.1 (e.g. 400 ns) of 
the charging of the capacitor C.sub.1 according to FIG. 4a. 
It is also apparent from FIG. 4b that the high-voltage pulse formed by the 
spark discharge is considerably compressed in time compared with the 
charging time period, typically by a factor of 10, i.e. the period T.sub.1 
-T.sub.2 according to FIG. 4b is about ten times shorter than the period 
0-T.sub.1 according to FIG. 4a. 
The good adaptation of the supply circuit to the spark impedance also 
manifests itself in the rapid decay of the oscillations of the spark 
current shown in FIG. 4b (the negative swing of the spark current shown in 
FIG. 4b is a so-called "overshoot"). 
In the Figures L.sub.1 denotes the current path along which the current 
flows in the preionization of the working gas of the laser. L.sub.2 
denotes the current path in which the current flows in the generation of a 
compressed high-voltage pulse. 
C.sub.1 denotes the capacitive energy storage means (capacitor) whose 
energy is used for generation of the spark discahge while C.sub.2 denotes 
the capacitive energy storage means (capacitor) whose energy is used for 
generation of a compressed high-voltage pulse. 
FIG. 2a shows a circuit with which a preionization and/or the generation of 
a compressed high-voltage pulse can be carried out. If only a compressed 
high-voltage pulse is to be generated and is to be introduced into another 
element of the laser the spark current flowing between the spark 
electrodes 1, 1' serve solely to generate the short high-voltage pulse 
according to FIG. 4b. Simultaneously, however, the spark discharge between 
the spark electrodes 1, 1' according to FIG. 2a can also be used 
optionally for the preionization of the working gas of the laser. 
FIG. 2b shows a variant of the circuit according to FIG. 2a which requires 
no further explanation. 
FIGS. 3a and 3b show further circuits in which it is immediately clear that 
simultaneously both the preionization by means of the capacitor C.sub.1 
via the current path L.sub.1 and the generation of a compressed 
high-voltage pulse by means of the energy stored in the capacitor C.sub.2 
via the current path L.sub.2 are possible. The two capacitors C.sub.1 and 
C.sub.2 are simultaneously charged by the pulse high-voltage source 2. 
The term "another component of the laser" in the drawings can for example 
be the primary winding of a pulse transformer or the supply circuit of the 
plasma discharge (main discharge). 
The configuration of the spark electrodes specified and the spacing apart 
specified effect that the spark discharges with a plurality of spark 
electrode pairs disposed in the laser chamber take place practically 
simultaneously. By the adaptation of the low-inductive spark supply 
circuit to the impedance of the spark the spark discharge is completed 
after some tens of nanoseconds. 
Since the spark circuit is low-inductive (its inductance is typically more 
than 30 times smaller than the inductance of the pulse high-voltage source 
2) the peak spark current is considerably higher than the peak current of 
the pulse high-voltage source 2. Since the brightness of the spark 
increases disproportionately greatly with the spark current an intensive 
preionization takes place. 
Consequently, the energy required for the preionization is not greater than 
a few Joule. The efficiency in the conversion of the energy stored in the 
capacitor C.sub.1 to UV light is greater than in the known preionization 
systems. 
The life of the preionization system is substantially greater than that of 
conventional systems in which in the spark discharge charge quantities up 
to about 10 times greater are transported. 
The generation of the compressed high-voltage pulse does not subject the 
spark electrodes to much load either. Since moreover the energies of the 
compressed high-voltage pulses are relatively small and make up only a 
small part of the energy of the plasma discharge and this energy is 
transferred through a plurality of sparks (typically more than 20), the 
load on the spark electrodes remains small so that the erosion processes 
typical of spark gaps do not occur. Since the sparks are disposed together 
with the other components of the laser receiving the power in the laser 
chamber the inductance in the consumer circuit ("other component of the 
laser") remains comparable to the inductance of the spark discharge 
circuit for the preionization. Although the compressed high-voltage pulse 
contains only a small amount of energy compared with the energy of the 
plasma discharge its power nevertheless remains high so that it cannot be 
switched by so-called "punctual switches", such as a thyratron. The use of 
the spark as switch makes a punctual switch in the laser chamber 
superfluous. 
In all possible circuits it must be ensured that the capacitive energy 
storage means C.sub.1 and C.sub.2 can be charged from the pulse voltage 
source before the initiation of the spark discharge. This must be 
permitted by the design of the component referred to in the drawings, 2, 
3, 3a as "another component of the laser". 
FIGS. 5 and 6 show a use according to the invention of the compressed 
high-voltage pulse generated as described above as prepulse for 
controlling the plasma discharge (main discharge). 
The homogeneous main discharge for stimulating the working gas of the laser 
burns between the main electrodes 1 and 2. The arrangement of the main 
electrodes 1, 2 and of the spark electrodes 3, 4, 5 and 6 is known per se 
and need not be discussed in detail here. 
From pulse voltage sources (not shown) known per se the capacitors 7, 8, 9 
and 10 are charged via the connections 12 and 13. The capacitors 7 and 9 
as well as 8 and 10 store the electrical energy from which a spark 
discharge between the spark electrodes 3 and 4 as well as 5 and 6 
respectively is fed. This energy also serves to generate a 
time-compressed, i.e. short high-voltage pulse, as described above. 
The charging of the capacitors 7, 8, 9 and 10 last typically a few hundred 
ns. To enable the switching element of the pulse voltage source (not 
shown), for example a thyratron, to operate under relaxed conditions, all 
the spark discharges between the spark electrodes 3 and 4, 5 and 6 (still 
more spark electrodes can be provided) are intiated synchronously. As soon 
as the spark discharge starts the sparks generate short-wave radiation by 
which the working gas in the laser chamber and in particular between the 
main electrodes 1 and 2 is preionized, i.e. by the short-wave radiation 
free electrons are generated in the working gas. 
The capacitive energy storage means 7, 8, 9 and 10 are charged by the pulse 
voltage source (not shown) in such a manner that a voltage is achieved 
between the spark electrodes 3 and 4, 5 and 6 of several times the static 
breakdown voltage. Thereafter the energy stored in the capacitive energy 
storage means (capacitors) is switched by the sparks themselves, i.e. the 
spark executes an independent switch function. Since the capacitors 7 and 
9, 8 and 10 are connected in each case low-inductively to the spark 
discharge gaps between the spark electrodes 3 and 4, 5 and 6 respectively, 
the spark discharge not only effects the preionization but at the same 
time also generates a compressed high-voltage pulse. Said high-voltage 
pulse is applied as so-called prepulse by means of the capacitors 9 and 10 
to the main electrode 1. As a result between the main electrodes 1 and 2 a 
voltage variation occurs which is influenced by the prepulse. Apart from 
the prepulse transmitted via the capacitors 9 and 10 at the main electrode 
1 there is also the charge voltage of the capacitive energy storage means 
(capacitors) 11 and 11' which have been charged previously via the supply 
line 14 from a voltage source (not shown) known per se, such as a pulse 
voltage source. To generate long pulses of a few hundred nd duration 
typically the capacitors 7, 9, 8 and 10 are charged via the supply lines 
12 and 13 to voltages above 20 kV whilst via the supply lines 14 the 
capacitors 11, 11' are charged to below 10 kV. These values are only by 
way of example. 
Typical values for the capacitances of the capacitors 11 and 11' are 1.3 
.mu.F per meter discharge length. The capacitors 7, 8, 9 and 10 each have 
a typical capacitance of 30 nF per meter discharge length and the sum of 
the capacitances of the capacitors 9 and 10 corresponds substantially to 
the sum of the capacitances of the capacitors 7 and 8. 
At the instant of the initiation of the spark discharges between the spark 
electrodes 3 and 4, 5 and 6 the charging of all the capacitors is 
completed. 
In FIG. 5 two inductances essential to the circuit are shown schematically. 
L1 is the inductance of the circuit in which the prepulse in the 
initiation and on strikeover of the main discharge closes its current path 
whilst L2 is the inductance of the circuit in which the discharge current 
of the main discharge flows. The latter inductance is measured on the 
shortest travel along which the current path closes on the two terminal 
sides of the capacitive energy storage means 11, 11'. 
The ratio of the two said inductances (i.e. their magnitude) plays an 
important part in the generation of the prepulse. The capactive energy 
storage means 11, 11' for the main discharge usually has a capacitance 
which is greater than the sum of all the other capacitances so that all 
rapid voltage changes which are transmitted via the capacitors 9 and 10 
from the spark electrodes 3 and 5 to the main electrode 1 are 
short-circuited by the capacitive energy storage means 11, 11' to ground 
(FIG. 5). 
Since the voltage changes in question here are very rapid (far below 100 
ns) all the voltage changes at the main electrode are subjected to the 
laws of the inductive voltage divider. To obtain as high and intensive a 
prepulse as possible it is desirable to make the quotients of the 
inductances L2/L1 as large as possible. On the other hand, the rapid 
energy depositing in the main discharge between the main electrodes 1 and 
2 requires an inductance L2 which is as small as possible. In the example 
of embodiment described with a ratio of the inductances L2/L1 of about 4 
good results are obtained. The very low inductance L1, whose value depends 
on the selected geometrical arrangement of the conductors and components, 
has in the example of embodiment a value of about 5 nH so that the 
inductance L2 can be kept in the range of about 20 nH which still permits 
a favourable transfer of the energy stored in the capacitive energy 
storage means 11, 11' to the main discharge. 
The inductance L2 consists substantially of the natural inductance of the 
electrical lead-throughs and lines. It is also possible to attach at the 
point 19 (FIG. 5), i.e. in the circuit between the main electrode 1 and 
the capacitive energy storage means 11, 11' associated therewith, in 
addition a magnetically saturable inductance. Such magnetically saturable 
inductances are known per se, for example this may be a flat band with 
core (see the publications (3), (4), (5) and (6) cited at the beginning). 
For the period of the prepulse this saturable inductance at 19 increases 
the inductance L2 so that the inequality L2 &gt;&gt;L1 is satisfied. For the 
period after the prepulse this inductance at 19 becomes saturated so that 
in this phase the requirement of as small as possible an inductance L2 is 
met. The use of a saturable inductance is one variant of the example of 
embodiment shown. 
A comparison of the circuit arrangement described with the aid of FIG. 5 
shows that it is possible here to apply the prepulse directly and with 
high efficiency to the main electrode 1 without an additional separation 
being necessary between the main electrode 1 and the capacitive energy 
storage means 11, 11' associated therewith. 
To obtain the desired time variation of the prepulse the voltages which in 
turn are applied via the supply line 14 to the capacitive energy storage 
means 11 of the main electrodes 1 and 2 and the voltages which are applied 
via the terminals 12 and 13 to the capacitors 7, 8, 9 and 10 must have the 
same sign (measured with respect to the common ground potential, FIG. 5). 
FIG. 6 shows the time variation of the voltage at the main electrode 1 
influenced by the prepulse. At the instant of the strikeover of the spark 
discharges between the spark electrodes 3 and 4 and 5 and 6 respectively 
the charge voltage of the capacitors 11, 11' is present between the main 
electrodes 1 and 2. The initial polarity of the prepulse is selected such 
that firstly the prepulse transmitted via the capacitors 9 and 10 to the 
main electrode 1 greatly reduces the voltage thereof, for example to a 
value close to 0. This reduced voltage between the main electrodes 1 and 2 
is maintained for a short time of for example 15 ns. In this time the 
working gas between the main electrodes is preionized by the short-wave 
radiation from the spark discharge. Here for example the 10.sup.7 free 
electrons per cm.sup.3 mentioned at the beginning are generated. Only 
thereafter does the voltage between the main electrodes 1 and 2 increase 
rapidly until due to the filled multiplication of the free electrons 
(avalanche processes) then starting the discharge occurs in the working 
gas, the concentration of the free electrons being about 10.sup.13 to 
10.sup.15 per cm.sup.3. During this phase of the field multiplication the 
polarity of the prepulse is the same as the charge voltage of the 
capacitors 11, 11' present at the main electrodes so that the two voltages 
add up. This time sequence of the prepulse is possible because the 
inductance L1 is made adequately small. 
Compared with the prior art set forth in the introduction to the 
description the circuit according to the invention of FIG. 5 also has the 
advantage that with the initiation of the energy transfer from the pulse 
voltage source (not shown) via the terminals 12 and 13 to the capacitors 
7, 8, 9 and 10 the entire laser system with all its processes takes place 
completely automatically, i.e. the preionization, the generation of the 
prepulse and the main discharge. Consequently, no special time 
synchronization and controls are necessary for these three processes.