Inductively stabilized, long pulse duration transverse discharge apparatus

An inductively stabilized, long pulse duration transverse discharge apparatus. The use of a segmented electrode where each segment is attached to an inductive element permits high energy, high efficiency, long-pulsed laser outputs to be obtained. The present apparatus has been demonstrated with rare-gas halide lasing media. Orders of magnitude increase in pulse repetition frequency are obtained in lasing devices that do not utilize gas flow.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the production of stable 
electric avalanche discharges and more particularly to an inductively 
stabilized, long pulse duration transverse discharge apparatus. This 
invention is the result of a contract with the Department of Energy 
(Contract No. W-7405-ENG-36). 
High pressure, transverse discharges are inherently unstable. In rare-gas 
halide gas mixtures the degree of stability as measured by the energy 
loading and stable discharge time depends on the gaseous components of a 
particular laser medium. As one increases the energy deposition in the 
gaseous medium, streamer arcs are observed throughout the discharge. This 
results in a limited lasing time since the useful energy deposition time 
is limited by the time it takes for the streamer arcs to propagate across 
the electrode spacing. Studies of the details of streamer arc formation 
have shown that stringent requirements exist for the preionization 
electron density and uniformity as well as for the voltage rise time in 
order to insure sustained stable discharge operation. See, e.g., S. Lin 
and J. J. Levatter, Appl. Phys. Lett. 34, 505 (1979), and J. J. Levatter 
and S. Lin, J. Appl. Phys. 51, 210 (1980). The inherent drawback of 
systems built incorporating these results is a need for a very fast 
voltage rise time which necessitates the development of a very low 
inductance switch or some pulse sharpening scheme which precludes the use 
of conventional thyratron switches. A solution to this problem is the 
passive stabilization of the electrodes, and specifically, inductively 
stabilized electrodes. 
Resistive stabilization of high pressure gas discharges is known in the 
art. Inductively stabilized discharges are a substantial improvement over 
resistively stabilized discharges since there are no ohmic losses which 
cause electrode heating (in addition to that from the cathode fall) and 
ultimately limit pulse repetition rate, and since there is no energy loss 
which limits energy deposition to the discharge. 
Resistive stabilization of transverse discharges has been applied to 
several rare-gas halide lasing systems in order to circumvent the problems 
in scaling such lasers. The effect of resistive ballasting on the 
discharge stability of a uv-preionized discharge-excited, XeCl* laser is 
described in "Resistive Stabilization of a Discharge-Excited XeCl* Laser," 
by D. C. Hogan, A. J. Kearsley, and C. E. Webb, J. Phys. D: Appl. Phys. 
13, L225 (1980). The authors suggest therein that distributive resistive 
ballasting of the transverse discharge electrodes must be provided so that 
if a localized region of high current density were to develop, the voltage 
in that region of electrode would decrease and further growth of the 
incipient arc would be suppressed. Arc-free discharges are reported by the 
authors to be maintained for relatively long time periods thereby allowing 
relatively long output pulses to be obtained. No mention, however, is made 
of alternative measures to distributed resistive ballasting of a single 
electrode. As mentioned hereinabove, resistively stabilized discharges 
result in ohmic losses which cause electrode heating which ultimately 
limits the pulse repetition rate. The subject invention substantially 
eliminates this limitation. 
SUMMARY OF THE INVENTION 
An object of the instant invention is to provide an apparatus for producing 
stable transverse discharges in high pressure gaseous media. 
Another object of the subject invention is to provide an apparatus for 
producing stable discharge rare-gas halide laser action. 
Yet another object for my invention is to provide an apparatus for 
producing long-pulsed operation stable discharge rare-gas halide laser 
action. 
Additional objects, advantages and novel features of the invention will be 
set forth in part in the description which follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by practice of the invention. The objects and 
advantages of the invention may be realized and attained by means of the 
instrumentalities and combinations particularly pointed out in the 
appended claims. 
To achieve the foregoing and other objects, and in accordance with the 
purposes of the present invention, as embodied and broadly described 
herein, the apparatus of this invention may include a segmented planar 
first electrode, each segment having one terminal of an inductive element 
attached for the purpose of limiting the rate at which an electric current 
can pass through the individual segments, a second terminal of each 
inductive element being tied together forming a common bus, a second 
planar electrode parallel to and spaced apart from the first electrode and 
being approximately coextensive therewith, a gas impermeable enclosure 
surrounding the first and second electrodes, means for establishing a 
potential difference between the bus and the second electrode suitable for 
producing a first, controlled puled electric avalanche discharge in the 
volume formed by the two electrodes, and means for initiating the electric 
discharge. Preferably, the electric discharge initiating means includes a 
third, substantially planar electrode spaced apart from and parallel to 
the second electrode on the side of the second electrode away from the 
first electrode. Means are provided for causing a second, pulsed electric 
discharge in the volume formed between the second and third electrode. 
Preferably, also, the second electrode allows ultraviolet light generated 
by the second electric discharge to enter the volume formed by the first 
and second electrode thereby preionizing the gaseous medium located in 
this first volume and uniformly initiating the first pulsed electric 
avalanche discharge in that region. It is preferred that the third 
electrode includes an electrical conductor insulated by a nonconducting 
medium which limits the current flow in the second pulsed electric 
discharge between the third electrode and the second electrode to 
essentially displacement current. 
The apparatus of the present invention then provides a system for 
depositing high energy per unit time into a gaseous medium over long time 
periods. The long lasing pulses resulting therefrom lead to much improved 
beam quality. The ability to deposit energy in a discharge over long time 
periods eliminates the need for a fast voltage rise time thereby allowing 
the use of conventional thyratron switches. Moreover, inductively 
stabilized discharges do not have the ohmic losses which occur when the 
discharge is stabilized by resistive loading and which results in 
electrode heating and the ultimate limitation of the pulse repetition 
rate. There is, further, no energy mechanism which limits energy 
deposition to the discharge. These principles have been applied to the 
development of an inductively stabilized rare-gas halide miniature laser.

DETAILED DESCRIPTION OF THE INVENTION 
Reference will now be made in detail to the present preferred embodiment of 
the invention, an example of which is illustrated in the accompanying 
drawings. Turning now to FIG. 1a, a segmented cathode 13 was constructed 
such that each segment is associated with an inductor 15, and all of the 
inductors are tied to a common bus 10 which is attached to a pulsed high 
voltage source 17. Parallel to and spaced apart from the cathode is a wire 
screen anode 11 which is held at ground potential. A third, composite 
electrode is spaced apart from and parallel to the wire screen electrode 
on the other side thereof from the segmented cathode. An electrically 
conducting element 12 is surrounded by an insulating material 9 which 
electrically isolates it from a direct discharge to the wire screen anode 
11. Preferably, this insulating material is a quartz tube. The conducting 
element is attached to a trigger device 16 which provides a low energy, 
high voltage pulse to it timed to fire before the pulsed source 17 is 
fired. The three electrodes are surrounded by a gas impermeable envelope 
14 which is provided with inlet and outlet ports 18, 19 for the purpose of 
filling and removing the gaseous lasing medium from the enclosure. The 
ports 18, 19 are designed to permit a static fill operation of the 
discharge apparatus. Windows 20, 21 are fitted with highly transmitting 
material when the apparatus is used as a laser. In a preferred embodiment 
of my invention, 87 separate segments with 0.79 mm separation between 
adjacent segments comprise a 27.6 cm total discharge length having a 20.8 
cm active discharge length. The anode-to-cathode electrode separation is 
about 2.5 mm and the discharge width is approximately 4 mm. The 
configuration shown in FIG. 1a has also been constructed using 87, 4.7 
.OMEGA., 1/4 W carbon resistors in parallel giving a net resistance in 
series with the discharge of 54 m.OMEGA.. The 4.7 .OMEGA. array was the 
lowest value used for comparison experiments using resistors since the 
discharge operation shows it to be only marginally stable with arcs being 
observed in the discharge for a substantial number of gas mixtures and 
pressures. This result has been published in "Inductively Stabilized 
Rare-Gas Halide Mini-Laser for Long-Pulsed Operation," by Robert C. Sze, 
J. Appl. Phys 54, 1224 (1983). Previous experience in developing miniature 
excimer laser has shown that for an approximately 4 mm gap spacing, the 
stable discharge time for an unstabilized laser is only 10 ns. (See, e.g., 
R. C. Sze and E. Seegmiller, IEEE J. Quant. Elec. QE-17, 81 (1981)). 
Therefore, ultra fast pulsed charging circuits were required to deposit 
all of the usable energy within the stable discharge time. However, using 
the inductively stabilized discharge electrodes of the present invention, 
ultra fast circuits are no longer necessary. No arcing has been observed 
for most gas mixtures investigated in the research which formed the basis 
of the subject invention at electrode spacings up to 1 cm and for total 
energy deposition times greater than 200 ns. Prevention of arcs is 
accomplished by the nearest local controlling inductor in the following 
manner. As the discharge region begins to become unstable, there is a 
rapid increase in current. However, the increase in current causes a 
voltage drop across the inductor (v=Ldi/dt) which translates into a 
decrease in voltage across the gap. The drop in voltage across the gas 
immediately quenches the arc formation. 
FIG. 1b shows a second embodiment of my invention identical in all respects 
to that shown in FIG. 1a except that the cathode 50 is a solid elongated 
electrode having inductive elements 15 spaced apart along its longest 
dimension. All of the inductors are tied to a common bus 10 which is 
attached to a pulsed high voltage source 17. It is anticipated, that arcs 
will be prevented in the same manner as described in the previous 
paragraph. 
FIG. 2 is a schematic representation of the electrical circuitry used to 
energize the apparatus of the present invention for its use as an excimer 
laser. Here the pulsed high voltage source includes a dc supply 25, and a 
thyratron 31 triggered by a pulse generator 39. The capacitor bank 33 is 
constructed of discrete components because ultra fast circuits are no 
longer necessary. The combination of capacitors 35 and inductors 34 are 
used to make a pulse forming network. The peaking capacitors 36 are 
necessary because the corona preionization exists only during the rise 
time of the voltage pulse before gas breakdown. A fast current rise time 
is required so that a uniform current distribution is established in the 
discharge volume before the preionization electrons disappear. The 
inductor value chosen for each of the 87 segmented cathode sections was 
0.15 .mu.H, giving a total inductance in series with the discharge of only 
1.7 nH. This value is comparable to the electrode inductance resulting 
from its size and shape, and thereby does not effect the overall circuit 
parameters to any significant extent. The main discharge is fired by 
switching thyratron 31 with trigger 39. 
FIG. 3 shows the temporal development of the voltage, fluorescence, and 
laser output when the subject apparatus was used as a XeCl laser. The gas 
mixture was 0.13% H.sub.2 --HCl/0.2% Xe in helium buffer where the ratio 
of H.sub.2 :HCl=1.4 The filling pressure was 35 psia, and the output 
coupler was 75% R. The voltage was measured across both the discharge and 
the inductor array. The lasing gas mixture was poorly impedance matched to 
the impedance of the peaking capacitor 36 of FIGS. 1a and 1b and the laser 
head inductance which includes the inductor array 15 of FIGS. 1a and 1b, 
and the single-pass energy deposition time was only approximately 40 ns 
long. However, the lack of impedance matching caused multiple reflections 
and since the discharge remained stable, these reflections contributed to 
the energy deposition and resulted in total lasing times of greater than 
120 ns. From the fluorescence and laser output temporal traces in FIG. 3 
for this gas mixture and pressure, it is seen that as many as four voltage 
reflections contributed to the energy deposition. As an example of the use 
of the apparatus of the present invention, Table I shows the best 
efficiencies and energies measured for KrF, XeCl, and XeF lasers. 
Parametric studies have shown the effect of discharge impedance on the 
total lasing time. Generally, high impedance of the discharges have short 
pulse lengths. This is because the higher the impedance discharge, the 
closer is the matching to the source impedance which allows more energy to 
be deposited into the gas in the first ring of the circuit. Therefore, 
less energy is available for subsequent rings of the discharge circuit 
which contribute to long pulse operation. Table II gives estimates of the 
arc-free energy deposition and energy extraction from the excimer 
mini-laser example of the present invention as compared with those 
obtained for a commonly used unstabilized discharge excimer laser. By use 
of the apparatus of the present invention, more than 300 
J/liter-atmosphere of energy may be deposited in an arc-free discharge at 
3 atm filling pressure as opposed to the 60-70 J/liter-atmosphere 
obtainable by unstabilized devices. Little benefits, however, is derived 
from energy deposition beyond the 100 J/liter-atmosphere level in the 
present time scale for energy deposition since rapid saturation of the 
output energy as well as the pulse width are observed. 
It is known that residual ions and thermal instabilities limit the pulse 
repetition rate of a laser. In a small lasing device such as that used to 
demonstrate the apparatus of the present invention, the stabilized laser 
was operated at a pulse rate in excess of 70 Hz without gas flow, while 
similar unstabilized devices are generally limited to about 1 Hz. However, 
the present laser is 
TABLE I 
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Best Output Lasing Time 
Total 
Efficiency Coupling Energy FWHM (ns) 
(ns) 
______________________________________ 
KrF 1.07% 40% R 6.8 mJ 
40 50 
80% R 3.7 60 80 
XeCl 0.57% 55% 3.1 40 85 
75% 1.7 80 90 
XeF 0.36 48% R 2.3 40 80 
87% R 2.0 62 120 
______________________________________ 
TABLE II 
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Inductively stabilized 
Levatter-Lin type 
______________________________________ 
Energy 300 J/1 atm at 45 psia 
60-70 J/1 atm at 
Deposition 
385 J/1 atm at 35 psia 
30 psia 
Energy 1.2 J/1 atm in KrF 
1.4 J/1 atm in XeCl 
Extraction 
0.5 J/1 atm in XeCl 
______________________________________ 
generally operated at 10 Hz because of thermal distortions observed at 
higher pulse repetition rates. These distortions, which are pressure 
dependent, should be correctable with proper index correcting optics once 
the thermal index changes are measured at a particular pulse repetition 
rate. 
FIG. 4 shows the performance characteristics for KrF for two different gas 
mixtures having neon as a buffer gas at 45 psia filling pressure. The 
optimum gas fill depends on what value of the reflectivity is chosen for 
the output mirror. Curves a and b in FIG. 4 represent data taken with a 
0.1% F.sub.2 /5% Kr mixture in neon buffer gas, while Curves c and d 
represent data taken with a 0.2% F.sub.2 /5% Kr mixture again with neon as 
a buffer gas. For long pulse operation, the 0.1% F.sub.2 mixture is 
clearly better with the 80% R output coupler because the impedance 
matching is poorer, thereby giving rise to longer pulses. For the highest 
energy output, however, regardless of pulse length, the 0.2% F.sub.2 
mixture with 38% R output coupler is most effective. Arcing was observed 
for KrF laser mixtures with helium diluent and the resultant pulse length 
was substantially shorter. 
FIG. 5 compares results of a XeCl lasing for neon and helium buffer gases 
as a function of output coupling. Curves e and f in FIG. 5 represent data 
taken for 0.15% H.sub.2 --HCl/0.5% Xe mixture in neon buffer gas, while 
Curves g and h represent data taken with a 0.2% H.sub.2 --HCl/0.2% Xe gas 
mixture with helium buffer gas. The filling pressure was 45 psia, and the 
ratio of hydrogen to hydrogen chloride was 1:4. The best performance 
obtained for KrF was 6.8 mJ per pulse with 40 ns 
full-width-at-half-maximum and a total lasing time of greater than 50 ns, 
and for long pulse operation, 3.7 mJ per pulse with 60 ns 
full-width-at-half-maximum and a total lasing time of greater than 80 ns. 
Further, the best performance obtained for XeCl was 3.1 mJ per pulse with 
a 40 ns full-width-at-half-maximum and a total lasing time of greater than 
85 ns and 1.7 mJ per pulse with 60 ns full-width-at-half-maximum and a 
total lasing time of greater than 90 ns. The storage capacitors were made 
up of five banks of twelve 0.5 nF barium titanate 20 kV capacitors. At 16 
kV charging voltage, the stored energy in the capacitors was 2.7 J using 
all five capacitor banks and 0.54 J using only one capacitor bank. 
Approximately 85% of the lasing energy was obtained with one bank in 
contrast to using all five capacitor banks. This gives an efficiency of 
approximately 1.07% for KrF including all losses. The 0.6% efficiency 
obtained for XeCl is a factor of about twenty more efficient than the 
resistively stabilized devices previously reported. 
FIG. 6i shows the total lasing time, while FIG. 6j shows the laser output 
energy per pulse as a function of gas pressure for XeCl. A mixture of 
0.13% H.sub.2 --HCl/1.9% Xe in neon buffer gas was used with a 75% R 
output coupler. To be noticed is the decrease in pulse length with 
increasing pressure which reflects the increase in gas discharge impedance 
as discussed hereinabove. 
The foregoing description of the two preferred embodiments of the invention 
has been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and obviously many modifications and variations are possible in 
light of the above teaching. For example, a solid elongated cathode could 
be employed with inductive elements spaced apart along its longest 
dimension. The second terminals of these inductive elements would be tied 
forming a common bus to which the high voltage would be applied. A similar 
limitation of current would occur which would reduce the tendency for arc 
formation from any area of the cathode. The embodiments were chosen and 
described in order to best explain the principles of the invention and its 
practical application to thereby enable others skilled in the art to best 
utilize the invention in various embodiments and with various 
modifications as are suited to the particular use contemplated. It is 
intended that the scope of the invention be defined by the claims appended 
hereto.