CA1049641A - Laser cavities with gas flow through the electrodes - Google Patents

Abstract

LASER CAVITIES WITH GAS PLOW
THROUGH THE ELECTRODES

ABSTRACT OF THE DISCLOSURE
A uniform field electrode arrangement for exciting a laser gas in a high pressure pulsed gas laser apparatus so constructed to facilitate discharge initiation and smooth laminar gas flow. The geometry of the electrode assembly provides for laminar gas flow through the discharge volume providing an optically homogeneous lasing medium. Discharge initiation for pulsed operation is achieved using, for example, ultraviolet irradiation of the electrode assembly, corona discharge devices and radioisotope irradiation of the electrode assembly. Independent of the type of initiatory discharge device used, the geometry of the assembly allows for efficient supply of initiating electrons to the discharge gap without interference with smooth gas flow through the optical cavity. Mesh electrodes can be used to facilitate gas flow in a direction parallel with the direction of elec-trical discharge.

Description

BACKGROUND OF THE INVENTION
Field Or the Invention:
This invention relates to high pressure pulsed gas laser systems which require discharge lnitiation and gas flow to stabilize the glow discharge. It is specifi-cally related to electrode assembly configurations whlch enhance smooth gas flow with no large scale turbulence while allowing efflcient discharge initiatlonO
Description of the Prior Art:
Prior electrlcally excited gas laser systems oper-; ated at high pressure have in many instances utllized other ''.

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than unlform-rield type elec~rode conf'igurations. A
common transversely excited atmospheric laser found in prlor art literature uses discharge electrodes in a laser cavity which include a number of pin cathodes set opposite a continuous bar anode. This type of` conflguration has the advantage that under chosen conditions of tored energy in the pulse generator, it is possible to create a plur-ality of transient high current discharges between the individual pins and the continuous bar anode wlthout an independent source of electrons to initiate the discharge.
By proper spacing of the pins, the discharges merge lnto a homogeneous and dif'fuse discharge across the entire inter-electrode region. In this type of a system, the electrons f`or the initiation of the glow discharge are prov~ded by field emission at the ends of` the cathodic pins. The c~r-- rent is amplified by collisional ionization in the high field region of' the gap near the pins providing large numbers of free electrons which undergo exciting collisions and popu-late the upper laser levels of the gas medium. This partic-20 ular electrode geometry requires a very rapid current rlse ,~
time and a. short glow duration in order to prevent constricted high ~emperature spark or arc discharge which would terminate - laser action. ~ ~
In an electrode assembly utilizing a non-unif`orm ~' field electrode configuration, uneven distributlon of current densities can result in damage by heating to the electrodes and cause discharge instabilities. Also, with non-un~form . -~, field excitation, parts of the gas volume may not be pro- ~ ;
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perly excited causing losses by absorptlon. Resultant in-homogenlty o~ the glow discharge can have adverse eff'ects

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upon the optical characterlstics of the total laser systemO
In other gas laser systems in which the electrode assembly has been conrlgured to provide a unirorm fleld discharge region, there has been no provlsion for smooth laminar gas flow. One prior art gas laser system utilizes auxiliary electrodes ad~acent the maln electrical discharge electrodes to trigger the discharge O The main electrodes, however, are positioned in an insulatlng enclosure wlth :~
the auxillary electrodes attached to the outer surface of ~ :
the enclosure. Such a configuration does not allow ~or gas ~low within the discharge region.
However, to operate a gas laser in a pulsed mode ~ :~ ?~
utilizlng a uniform field electrode configuration, requires .
smooth laminar gas flow without ma~or gas turbulence and some separate means for supplying initiatory electrons to the discharge region. Thus, the electrode assembly must be ::
designed to both allow for laminar flow of the laslng gas and for efficient in~ection of initiatory electrons into the discharge region without interruption of the gas flow.
SUMMARY OF THE INVENTION
The present invention is a high pressure pulsed gas laser system whlch has an envelope substantially enclos-ing an optical cavity which is comprlsed of an electrode assembly arranged in a unl~orm field configuration, some means for flowing the laser gas at high pressure, generally being greater than 100 Torr, through the electrode assembly, ~ ::` .
; discharge initiation means ad~acent the electrode assembly, ~`
and pulsing means connected to the electrode assembly and ~ controlling the discharge initlation means to electrically excite the gas medium, thereby causing lasing actlon ln the ~:

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optical cavity. The laser system is constructed to facilitate smooth gas flow through the optical cavity while allowing for efficient injection into the cavity of initi.atory electrons from the discharge initiation means.
BRIEF DESCRIPTION OF THE DRAWINGS : : .
Figures lA and lB are sectional views of one embodiment of the present invention utilizing an ultra~
violet lamp to initiate the glow discharge;
Figures 2A and 2B are sectional views of another embodiment of the present invention with gas flow through screen electrodes and utilizing two ultraviolet lamps for discharge initiation; ; .
Figures 3A and 3B are sectional views of another ..
embodiment utilizing corona discharge gaps adjacent the :main electrode assembly to initiate the glow discharge;
. Figure 4 is a cross-sectional view of another embodiment of the present invention;
: Figurc 5 is a cross-sectional view of another 20 embodiment in which initiatory elec-trons are produced by ~
spark discharge between corona wires and a screen electrode; ~:
and .jFigure 6 is a cross-sectional view of another embodiment of the present invention using radioisotope -~irradiation of the electrode assembly for initiati.ng the glow discharge.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS .
In Figures lA and lB there is shown one embodiment of the present invention utilizing an ultraviolet lamp 10 ;30 as a discharge initiatiOn device set behind electrode 12.

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Electrodes 12 and 14 are configured with planar sur~ace portions 16 and 18, respectively, positioned parallel and opposite one another to define a uniform field region therebetween. The planar portion 16 of electrode 12 is o~ a wire mesh construction transmissive to the radlation rrom lamp 10. The edges o~ the electrodes 12 and 14 are :
proriled to eliminate edge effects in the uniform field ~' while providing a nozzle profile to enhance smooth gas ~low. The gas flow is also laminar to the degree that ma~or turbulence which would e~fect the optical homogeneity and efrlciency of the device is absent. The term "laminar" as used herein is not meant to mean the complete absence of such localized turbulence which would have no significant e~fect on laser operation. ~;
The directlon of gas flow as shown by arrow 20 ls transverse to the direction of electrical discharge between electrodes 12 and 14. A sultable lasing gas at high pres-sure is pumped into the discharge region through inlet duct 22 and from the discharge region through outlet duct 24 Recirculating means (not shown) typically includes a heat exchanger ~or cooling the gas and pump to establish the velocity o~ the gas through the excitation region to the .
desired speed ~or the particular gas medlum and electrode geometry. l~
As shown in Figure -Hb~ the laser axis 26 is parallel to and between the planar surface portions 16 and 18 o~ the electrodes 12 and 14, respectively. The optical cavity of the laser system is def~ned by re~lective optlcal elements 28 and 30, located at elther end of the electrode assembly comprised o~ electrodes 12 and 14. In a typical arrangement, .

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:~0~41 one reflective element 28 is partially transmissive and the other element is totally reflective.
In a concave region 38 of electrode 12 enclosed by a portion of envelope wall 32 and ultraviolet lamp 10 is positioned outside the active optical cavity with its longitudinal axis parallel to the laser axis 26 of the system. The lamp 10 comprises a cylindrical envelope 34 on the ends of which are caps such as 36 including the electrical -terminal 35. Customarily, the lamp 10 will be 10 pulsed to obtain the necessary untraviolet radia-tion to ~, , irradia-te planar surface portion 18 of electrode 14 through the mesh screen portion 16 of electrode 12. The ultraviolet radiation causes generation of electrons by photoemission processes and other bulk ionization processes in the gas.
The electrode 14 is connected to a pulse power supply 31 for supplying pulsed energy to the discharge gap between electrodes 12 and 14 which may be synchronized with the pulsed ultraviolet lamp 10.
By using the electrode geometry of Figures lA and 20 lB with the ultraviolet lamp 10 recessed behind the screen surface portion 16 of electrode 12, gas flow is unimpeded -through the optical cavity and in particular is smoo-th and laminar through the excitation region between the electrodes 12 and 14. There is no flow of gas in cavity 3a, the ultraviolet lamp 10 being removed from the stream of gas ; flow out of the active optical cavity.
In Figures 2A and 2B, two ultraviolet lamps 40 and 42 are positioned out of the direct flow of gas outside - the active optical cavity but within the envelope volume.
30 An envelope 44 encloses the electrode assembly comprised of : ., . ~:
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electrodes 46 and 48. Each of the electrodes 46 and 48 have screen mesh portions 50 and 52, respectively, whlch are substantlally planar and parallel one to the other defining a unirorm rield region therebetween. Gas ~low is ln a direction shown by arrows 54 through the electrode screen mesh portions 50 and 52.
The ultraviolet lamps 40 and 42 used to irradiate the electrode 48, and speci~ically the mesh screen portion 52, with ultraviolet radiation are positioned on either side of the discharge region lylng between electrodes 46 and 48. The longitudinal axes of lamps 40 and 42 are essen-tially parallel to the planar screen portions 50 and 52 of electrodes 46 and 48. They are rigidly held in position in insulating wall portions 56 and 58 so as to expose sur~ace ; ;
52 to the radiation through the cylindrical glass envelopes 60 and 62 of lamps 40 and 42, respectively. End caps 64 having a terminal portion 63 thereon are customarily con-nected to a power source so as to obtain the necessary high .
lntensity ultraviolet radiation required to generate elec- ~;
trons by photoemission processed from the sur~ace of mesh screen 52 and by other bulk ionization processes in the gas.
The electrcde 48 is connected to pulse power supply 65 which supplies energy to the discharge region between screen portions 50 and 52 to sustain the transient pulse glow discharge. ;;
Gas flow is in a direction through the screen por~
tions 50 and 52 Or the electrode assembly in a direction ~-~
indicated by arrow 54 through appropriate ducts such as 66 and 68. Gas ~low at a chosen velocity and temperature may be maintained by means of pumping and heat exchange means . . -: ~ '.

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not shown in the ~igures.
It will ~urther be under~tood by one skilled ln the art that optical re~lactive elements can be placed at either end of the elactrode assembly to de~ine an optical resonant cavity as in the embodiment oi Figure lB.
In Figures 3A and 3B another embodiment of the present invention is shown in which dialectric slab elec-trode pair~ 86 and 88 are pul3ed so as to produce a corona discharge in a narrow gap to irradlata ~he electrode sur~ace b~ photons from the corona discharge. Other bulk ionization processes also occur to produce additional ~ree electrons ln the discharge gap region. Ma~n electrodes 70 and 72 are positioned opposite one another wlth planar sur~ace portions 71 and 73 parallel~ me electrode sur~aces can be pro~iled so as to eliminate edge dlstortion to the uni~orm electrlc ~ield while providing a nozzle pro~ile to sustaln a smooth ilow of gas.
Gas ~low indlcated by arrow 74 is tra~sver3e to the direction ~ discharge between el~ctrodes 70 and 72, . .
A nozzle pro~ile in inlet duct 76 causes streamlined ~low and increased veloclty through the dlscharge re~ion between electrodes 70 and 72, Outlet ductwork 78 cycles the ga~ to p~mping and heat exchange means (not shown) and then back to the inlet duct 76, ~ he electrode assembly is partially enclo~ed b~
envelope walls 8~ and 82, El~ctrode 70 is conneated to a pulse power supply 8~ by means o~ 'which the main gap region : ~ i8 pulsed.
Tha discharge initlation means are two pairs 86 and 88 of slab electrodes mad~ o~ a high dielectrlc ¢onstant .
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~l09~6~ -materlal arranged ad~acent and parallel to the sur~ace portion of electrode 72. Each set 86 and 88 o~ ~lab elec-trodes ls po~itioned in an lndented part o~ the insulated ducts 76 and 78, respectively~ me electrode pairs 86 and 88 whichJ for instanceJ can be titanlum dioxide (~utlle), run the len~th o~ the electrode 72 as can more clearly be seen in Figure 3B. One slab electrode o~ each dielectrlc pair is connected ~rom terminals 89 and 91 through leads 90 and 92 to electrode 70 so that they are pulsed synchron~
ouæly with the main gap reglon between electrode~ 70 and 72~ m e slab electrodes of each pair 86 and 8~ can touch at their lnterface as shown in F~gsa 3A and ~B or can deflne a narrow gap region therebetween~ `
The opera~ion o~ the dlelectric electrode sets as discharge initiators is more ~ully explai~ in copending Canadian application Serial No. 2259654, ~iled Aprll 28J 1975 by the same inventors and as~lgned to the same assignee as the present applicatlon.
In operation, corona discharges are generated at the inter~ace o~ the dielectric slab electrodes 86 and 88 such tha* the surface por~ion of electrode 70 is irradiated with photons producing in~.tiatory electrons at the sur~ace ~:
o~ electrod~ 70 through a photoemission proces~. Some ga~
lonization e~fects may also generate additional electrons~
A transient pulse glow discharge can then be main~alned between electrodes 70 and 72 by means o~ the applicatlon of pulse5 to the alectrode 70 from the pulse power supply 84.
By setting the two dlelectric slab electrode sets 86 and 88 into the reces~ed portions o~ ducts 76 and 78, smooth .
30 laminar ga~ flow through the discharga reglon i5 preæerved~ ~

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Uslng thls electrode configuratlon, lamlnar gas ~low to sonic velocities can be maintained with no ma~or turbulence in the discharge region. Good optlcal homogeneity of the laser medium is thereby maintalned and the pulsed dis-charge is stabilized.
In Figure 4, a partial section of an electrode assembly is shown in which dielectric slab electrodes 128 and 129 are used to provide initlatory electrons to the gap region, but set in a different configuration than that 3 A '~, ~, d ~ ~, shown in Figures 3a and 3b. Electrodes 94 and 96 are posi-tioned within an envelope having wall portions 98, 104 and 106. The electrodes 94 and 96 have surface portions 108 ~` and 110, respectively, which are substantlally planar and parallel one to the other. Surface portions 112 and 114 of ~- electrodes 94 and 96 respectively have been constructed with a nozzle profile to facilitate laminar ~low of gas into the discharge region between surfaces 108 and 110.
Gas flow is through two inlet orifices 116 and 118 -ln the general dlrection o~ arrows 120 and 122, respectively.
The ori~ices 116 and 118 run the length of the electrodes 94 and 96 so that the gas volumetric ~low is uni~orm along ; the entlre length of the electrode assembly~ Gas ~low out of the uniform field reglon between surface portions 108 and 110 is through outlet orifice 124 in the direction in-dicated by arrow 126. As has been previously described, continuous gas flow can be malntained by appropriate pump-ing and heat exchange means.
Set ad~acent the gas inlet orifices 116 and 118 are dielectrlc slab electrodes 128 and 129 in a contiguous ; 30relationship at interface 130. The slab electrodes 128 and ' - 10 - , -, ........ . . .
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129 can either abut at the inter~ac~ 130 or define a narrow gap region therebetween. The edges of the slabs 128 and 129 are pro~lled at inter~ace reglon 130 as i8 lnd~cated at 1~2 xo as to facilltate ~rradiation o~ the planar surface region 110 o~ electro~e 96 with ultrav~olet radiation. Free electrons for dischargs initlation are produced by photoemis~on and photo ionization processe~.
me operation of the embodlment ~hown ln Figure 4 i~ qulte sim~lar to that of the embodiment of Figures 3A and 3B. Both the dielectric slab electrode 129 and electrode 96 are connected to a pulsing source 11~ which operates to pulse both the gap region 130 to generate ultra-v~olet radiation o~ the electrode 96 and pulse the main discharge reglon between surface portions 108 and 110 and electrodes ~4 and 96, respecti~ely. By po~ltioning the dielectric slab electrode~ 128 and 129 to one side Or the electrode assembly, the gas ~low throu~h inlet ori~ices 116 and 118 is unimpeded. Consequcn$1y, smooth iamin~r flow i5 experienced in th~ region between the planar sur~ace por-tion~ 108 and 110, Good opticalihomogeneity of the gas la~e~ medium i8 achieved for laser actlon when the ~ppro-priate optical re~lectiva elements are utilized in con~unc-tion with the described e-ectrode as~embly. Optics can be added as shown ln Figure lB.
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Initlatory electron~ can also be supplied directly ~rom a corona ~ource, This type o~ action i8 shown in the ~ ;
device o~ ~igure 5, Two electrodes 132 and 134 are posi-tloned within an envelope havlng wall portlons 136 and 138~
Electrode 1~2 has a planar wire mesh portion 140 ~et opposite and parallel to solid planar surface 142 o~ electrode 134.
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In a cavity region 144 behind the mesh sur~ace portion 140, a series o~ parallel corona wlres 146 are positloned ad~acant and equidistant ~rom planar mesh sur~ace portlon 1~0. The corona wires are negatively biased with respect to electrode 132 by means o~ d~c. source 148. Electrode 134 is connected to a pulse power supply 150, When pulses are applied to the gap region between the sur~aces 140 and 142, corona dl~charges 141 are initiated behind the mesh screen portion 140 between the corona wi-res 146 and the mesh 140. me generated corona discharges 141 produce ~ree electrons along the sur~ace o~ mesh 140 which aid ~n initia-tion o~ transient pul~e ~low discharge in the ma~n'gap re-,~ .
gion. With appropriate optics such as provided in the em-bodiment of Figure lB, the la~er gas is excited to upper lasing ~vels and làsing actlon take3 place within the de~ice.
Gas`~low~into the discharge region i9 through inlet -ori~ice 152 and out through ori~ice 154 in the direction indicated by arrows 156. The coro~a wires 146 are not ex-posed directly to ~he gas ~low~ and there i8 no consequent J
obstructlon to essentially laminar ~low, In Figure ~ an embodiment o~ the present invention is shown in which a radioisotope i9 used to provide frea electrons by alpha partlcle bombardment. Main electrodes 158 and 160 are set within an envelope ha~ing w~ll portions 162 and 168. Planar ~ur~ace portion 174 o~ electrodq 158 i8 positioned parallel to the wire mesh planar sur~ace por-tion 176 o~ electrode 160. me ~olume betwesn the planar , ~urface portions 174 and 176 dePine a ~ubstantially uniform field region.
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ulthin cavity 178 defined by wall portion 168 and the interlor surraces of electrode 160 ls an alpha particle source 180. A typlcal alpha partlcle source used in thls I conriguration is Amerlclum-241. The alpha particle source ls mounted ln insulating brackets 182 and 184 a ~ew centi-meters rrom the screen mesh portlon 176. If Amerlclum-241 foll is used as the alpha particle source, short range ~;
(approximately 3 centlmèters at atmospherlc pressure), ~
alpha particles are produced, and consequently, no special ~ -precautions need be taken in handling the gas.
A repeller plate 186 is mounted below the alpha / g o ~ -particle source 1~ and held in place by the end brackets 182 and 184 so as to repel particles into the dlscharge gap region between sur~ace portions 174 and 176 Or electrodes 158 and 160, respectively. The repeller plate 186 is held `~
, at a low bias voltage with respect to electrode 16~ by means ~
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;~ o~ d.c. supply 188. The electrons generated by alpha parti- ~ ;
cle collision will tend to flow into the main gas flow re- ;
gion as a result of the polarity of the field.
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Gas flow is in a direction transverse to the uni-rorm electric field between electrodes 158 and 160 as indi-cated by arrows 190. Inlet and outlet orifices 192 and 194, respectively, are located on each side of the electrode assembly comprised o~ electrodes 158 and 160 and have a lateral dimension equlvalent to the length of the total `~ electrode assembly. The profiling of the electrode edges aid both in eliminating edge ef~ect distortion in the elec-tric ~ield and also in enhancing smooth gas ~low.
As in the previous embodiments, the main electrode gap between sur~ace portions 174 and 176 is pulsed by a _ 43,590 .

pulse power supply 196. The electrons inJected lnto the gap reglon by alpha particle collision lnltiate the dis-charge which is then sustained by the power dumped lnto the gap reglon by the pulse power supply ~ , thereby re-sultlng in transient pulse glow discharge action.

With the proper optical elements to deflne an - IA l~
optical cavity, as was shown in Figures ~ and -~, the de-vice of Figure 6 can operate to ralse the laser gas medium ;~
to upper lasing levels resulting in lasing action.
The use of an alpha particle source as ln the conflguration of Flgure 6 insures a steady supply of elec-trons at the surface portion 176 of the electrode 160.
However~ the electrons are produced in pulses of approxi~
mately 105 electrons in intervals of approximately 100 microseconds at the surface of the Americium-241 foil. The burst of electrons is diffused into a quasi-steady supply by the time they reach the surface portion 176 of electrode 160.
The several embodiments disclosed above have the general advantages of a uniform field dlscharge cavity. By providing an adequate supply of electrons by the use of various discharge initiation means, a stable and optically homogeneous glow discharge is achleved. The particular i configuration and arrangement of the electrode assembly in each embodiment facilitates an essentially laminar gas flow while allowing for a sufflcient supply of initiatory elec-trons in the discharge gap region.

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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A high pressure pulsed gas laser apparatus comprising:
a resonant optical cavity including optical reflective elements passively terminating each end of said cavity, a gas medium at high pressure suitable for lasing action, an envelope volume substantially enclosing the resonant optical cavity, an electrode assembly positioned within said envelope volume including first and second electrodes arranged in a substantially uniform field configuration and defining a discharge gap region therebetween, means for producing a flow of said gas through a first portion of said envelope volume including said discharge gap region and said optical cavity where the gas flow is in a direction transverse to the optical axis of said optical cavity, said electrode assembly further including discharge initiation means adjacent said first and second electrodes in a second portion of said envelope volume for supplying initiatory electrons into said discharge gap region by means of photo-emission and bulk gas ionization processes, said first and second electrodes including portions which are substantially planer and parallel to one another whereby said gas flow is substantially laminar to sustain a uniform and homogeneous discharge in said discharge gap region and said optical cavity, and pulsing means operatively connected to said elec-trode assembly to supply energy to said discharge volume for sustaining a glow discharge for exciting said gas medium to upper energy levels to induce lasing action and to con-trol said discharge intiation means for supplying electrons to said discharge gap region.

2. The high pressure gas laser apparatus of claim 1 wherein said discharge initiation means includes an ultra-violet lamp so positioned within said envelope to irradiate a surface of said first electrode for generating electrons.

3. The high pressure gas laser apparatus of claim 2 wherein said second electrode has a concave sur-face portion spanned by a planar mesh screen and wherein said ultraviolet lamp is positioned between said concave surface portion and said planar mesh screen.

4. A high pressure pulsed gas laser apparatus having an optical cavity and an envelope volume substantially enclosing the optical cavity, comprising:
an electrode assembly positioned within said envelope volume including first and second electrodes, said first and second electrodes including mesh portions which are substantially planar and parallel to one another and defining a discharge gap region therebetween, means for flowing a laser gas through said mesh portions of said first and second electrodes, said flow being substantially perpendicular to said mesh portions, said electrode assembly further including discharge initiation means adjacent said first and second electrodes for supplying initiatory electrons into said discharge gap region by means of photoemission and bulk gas ionization processes, and means operatively connected to said electrode assembly to supply energy to said discharge volume for sustaining a glow discharge and to control said discharge initiation means.

5. The high pressure gas laser apparatus of claim 4 wherein said discharge initiation means includes a first and second ultraviolet lamp so positioned in said second portion of said optical cavity to irradiate a surface of said first electrode for causing photoemission of electrons.

6. A high pressure pulsed gas laser apparatus having an optical cavity and an envelope volume substantially enclosing the optical cavity, comprising:
an electrode assembly positioned within said envelope volume including first and second electrodes arranged in a substantially uniform field configuration and defining a discharge gap region therebetween, means for flowing a laser gas through a first portion of said envelope volume including said discharge gap region in a direction orthogonal to the optical axis of said optical cavity, said electrode assembly further including discharge initiation means adjacent said first and second electrodes in a second portion of said envelope volume for supplying the initiatory electrons into said discharge gap region, pulsing means operatively connected to said electrode assembly to supply energy to said discharge volume for sus-taining a glow discharge and to control said discharge initiation means for supplying electrons to said discharge gap region, said discharge initiation means including at least one rutile slab electrode pair positioned contiguous to said second electrode for producing corona discharges when pulsed by said pulsing means to irradiate said first electrode with ultraviolet radiation, said first and second electrodes including portions which are substantially planer and parallel to one another whereby said gas flow is substantially laminar to sustain a uniform discharge in said discharge gap region.

7. A high pressure pulse gas laser apparatus having an optical cavity and an envelope volume substantially enclosing the optical cavity, comprising:
an electrode assembly positioned within said envelope volume including first and second electrodes arranged in a substantially uniform field configuration and defining a discharge gap region therebetween, means for flowing a laser gas through a first portion of said envelope volume including said discharge gap region in a direction orthogonal to the optical axis of said optical cavity, said electrode assembly further including discharge initiation means adjacent said first and second electrodes in a second portion of said envelope volume for supplying initiatory electrons into said discharge gap region, said first and second electrodes being profiled in a tapered nozzle-like configuration to enhance rapid laminar flow through the discharge volume and wherein said discharge initiation means is located adjacent the broadest cross section of said tapered nozzle-like configuration, and means operatively connected to said electrode assembly to supply energy to said discharge volume for sustaining a glow discharge and to control said discharge initiation means for supplying electrons to said discharge gap region.

8. The high pressure gas laser apparatus of claim 7 wherein said discharge initiation means includes a rutile slab electrode pair for generating a corona dis-charge to irradiate a surface of said first electrode with ultraviolet radiation for generating electrons.

9. A high pressure pulsed gas laser apparatus having an optical cavity and an envelope volume substantially enclosing the optical cavity, comprising:
an electrode assembly positioned within said envelope volume including first and second electrodes, said first and second electrodes including portions which are substantially planar and parallel to one another and defining a discharge gap region therebetween, means for flowing a laser gas through said surface portions of said first and second electrodes, said flow being in a direction substantially transverse to the optical axis of said optical cavity, said electrode assembly further including discharge initiation means adjacent said first and second electrodes for supplying initiatory electrons into said discharge gap region, and means operatively connected to said electrode assembly to supply energy to said discharge volume for sustaining a glow discharge and to control said discharge initiation means.