CA1040735A - Generation of corona for laser excitation - Google Patents

Abstract

GENERATION OF CORONA FOR LASER EXCITATION
ABSTRACT OF THE DISCLOSURE
A gas laser tube apparatus for producing a high-current glow, arc or spark which generates initiatory electrons and charged particles for establishing excitation of gas molecules in a separate region of the tube. Separate regions of ionization and excitation are established so that uniformity of excitation, optimization of the ratio of electric field intensity to electron density (E/N) and uniform intensity of excitation can be obtained in a high pressure gas laser system independent of ionization or initiation of discharge which also may need to be opti-mized. The separate fields are achieved by proper con-figuring of the electrodes with appropriate voltages applied thereto.

Description

BACKGROUND OF THE II~NTION
F~eld of ihe Invent~on:
Thls lnvention relates to gas laser tubes and more particularly to those laser ~ystems operating in a pulse' mode at high pres;sure.
20- Description of the Prlor Art:
One of the sources for high ave'rage power laser output ls the high pressure transversely exc~ted laser.
A typical arrangement for such a laser provides for two long electrodes ~ oft~e~ including a number of pln cath-odes and the other being a continuous anode) a few centi-meters wide and spaced a few centimeters apart. The opti-cal axis of the laser is parallel to the electrode surfaces and transverse to the electric field esta~lished between the ~electrodes. The lasing gas is clrculated through the , 1 :-'', :.~
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electrode gap in a direction transverse both to the optical axis and the electric field at typical velocities in the .
range of 15 ~/sec to 120 m/sec.
In such lasers, population inversion is obtained by a pulsed glow discharge generated by applylng a pulsed voltage across the electrode gap. Complete breakdown of the gap when the pulse ls applled must be avolded to have proper laser operation. By tallorlng the duratl~n and ; sh'ape of the pulses to avoid overheatlng of the gas the ~ ch~c~
: A lo ~h~ of complete breakdown can be reduced although the most efflcle~t lasing cond;itions may not be achleved since the value of E/N cannot then be lndependently optlmlzed :
One reason for breakdown ln prlor laser config-~i urations has been lack of control over the ratlo of elec-.~l trlc f.leld s.t~rength to the denslty of gas molecul.es (E/N) ~ in the reglon where excitlng colllslons occurO The fleld, -~ to obtain efflcient io~izatlon of the gas, must be hlgh ..
. ~ while optlmum excitatlon conditions requIre'a reduced ' value of E/No If ionlzatilon and excitatlon occur ln the :' 1 20 same or overlapping regions, opt.lmum conditions for both '~l lonization and exc'ltation cannot be achievedO

.`l One method of separating the lonization and ex-cltatlon reglons.in a low pressure CW laser ls by utillz-, lng a coaxial electrode configuratlon. Such a conflgura-~! tlon ls operatlve for low voitage D.C., static g~s, and ~: low pressure condltion~ Under hlgh voltage and hlgh pres-sure condltlons, however, overheatlng Or the gas would occur and breakdown of the gap would result since no allowanoe ls made for gas flow.

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, 4z ,164 SUMMARY OF THE INVENTION
- Accordlng to the present inventlon, a high ~' pressure gas laser tube is provided having at least two ,, electrodes so conflgured to pro-vide separate reglons for lonizatlon Or the gas and for excitatlon of the gas mole-- cules to a lasing state~ A typical con~lguratlon allows : for a glow, spark or arc,dlscharge behind a screen or mesh ~`
electrode thr~ugh which the electrons gcnerated in the dis-charge are in~ected into an excitation region in a dlrec-tion toward an anode electrode set opposlte the screen ' "J electrode. By providlng gas,rlow through thc dlscharge ~ ' and excltation reglons and by applying hlgh voltage pulses ! ~ to the eIectrode asscmbly to produce large numbers of '"`'' 'I , .
electro~s by lonization in the hlghly stressed pin-screen "~3, gap while maintaining a D~C. or pulsed blas a~ross the ;~';.~ excitation reglon be,'tween the mesh and plane lectrodes, the gas molecules in the exc,itatlon..region can be raised to a lasing level by colllslon wlth the inltlatory electrons.
~ Since the E/N ratio can be optimized in the excitation ,:31 20 .r.eglon, the resulting operation ls more efflclent and ~! stable than previous configurations for high pressure use having reduced the llkelihood of total breakdown ln the gas.
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.~ BRIEF DES~RIPTION OF THE,DRAWINGS, ,~ ~ ;, .
.~.,} ' Figure l ls a,schematic diagr,am of a prior art '::: . .
pu~sed gap electrode co~fi~uration for a high pressure .: gas l,aser system.

,~ Fig. 2 ls a schemati,e clrcuit dlagram of a pre-ferred e~bodiment of t~e present i1nvention.

~ 30 Fig~ 3 ~s a cutaway sectional view,of the,embodi-.,: . -3-;r - .

`~ 42,164 , ~

,,, ~040735 ment of Flgure 2.
Flg. 4 is another embodlment havlng unlform fleld gap wlth corona pins.
. Fig. 5 is another embodiment having quasi-unl-form field gap with corona pins.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In Flgure l a prior art electrode conflguration which can be used in a laser tube is shown wherein a pulse voltage applled to the electrodes 2 and 6 controls the hlgh current glow.bet,ween the electnode~. A cathode

2 having a plurallty of pins 4 1s shown ad~acent a planar anode 6 which together c~mprlse a pin-to plane electrode assembly. When a p.,ulse from pulse generator 8 is applled across the~gap defined by the plns 4 and anode ~., a non-uni,form fleld is established. Provided t,hat t;he pulse is controlle,d, to the proper shape and duration, a high current glow,dlscharge can be established ln a hi'gh pressure environ-ment,. without overheatlng the gas and wlthjou,t subsequent breakdown. Under the proper pulslng conditions a "shower"
discharge can be obtained between the cathode pins 4 and the apode 6 havlng peak currënts of several amperes.
The electrode configuratlon of Flgure l, however, does not allow for control of the E/N ratlo at an,optlmum value ln that region of th.e gap where exci,tlng collisions -, occur. An improvement to the conflguratlon of Flgure l is to provlde çontrol o~ the excl,tatlon reglon by separate , electric, fields; one high voltage fleld glvlng rise to a A hlgh current glow~spark or arc dlscharge ln the lonizatlon reglon to produce free electrons and a second unlform elec-tric field to produce an opt,imum E/N ratlo for ex,citatiDn . ; _4_ 1040735o~ the gas molecules to the required vibrational energy levels for laser action.
The electrical discharge in the ionization region can take the form o~ a glow, spark, or arc dis-charge provided the discharge is separate from the exci-tation region so as not to interfere with lasing action.
For each type of discharge, electrons are generated by the discharge ~or utilization in the excitation region although for each the physical phenomenon which occurs is di~i~erent. Under glow discharge conditions there is high potential across the gap with relatively little heating of the gas while the energy supplied to the ionization region is below some maximum amount. A unii~orm discharge occurs across the entire region. Arc discharge occurs when the energy ~upplied to the ionization region exceeds some given amount causing nonuniformity o~ discharge between electrode~.
e arc or arcs that are i~ormed carried heavy currents between the electrodes, and the potential drops to near zero. In the spark type o~ discharge, a spatial discontinuity de-velops in the discharge and a sputtering or inte~mittentdl~charge occurs.
In Figure 2 a gas tube 22 is shown which is oper-ative at high pressure with a continuous gas i~low through the electrode gap regions (Figure 3). Dl~ferent electrode geometries including planar, pin or shaped surface types can be used to obtain separate ionization and excitation regions but allowance must be made in each case for gas n ow.
In Figure 2 the gas tube 22 is connected to a D.C. bias supply circuit 24 and a pulse generator circuit 26. The gas tube 22 has an essentially totally re~lecting ,., ~

42,164 . .

~040735 optical element 28 and a partially transmitting optical .lement 30 positioned opposite one another and orthogonal to the optical axis 42 of the laser. Side walls 32 are sealed hermetically to the opt$cal elements 28 and 30 to provide an integral enclosure for the gas medium of the laser. It will be understood that these elements Or the : tube 22 can be modlfied without effecting the claimed.in-.. vention, tube 22 being.merely typical in the art.
Once t.he gas tube 22 has been evacuated it is fllled wlth a suitable gaseous lasing medium as for ex-ample CO, CO2, a metalllc vapor, or some other atomic or molecular gas which can supply an active partlcle for l~l laslng~
Wlthin tube 22, a first electrode cathode 34 having a plurality of pins 36, a second electrode, grid . 38 of a screen or mesh construction which ls pervious ;.;~ to electrons and photons, and a third electrode anolde .. i 40, are positioned parallel one to another and to the , . .
. optical axis 42 of the gas tube 22. .Support for the elec-. .
.l 20 trodes, to rigidly maintain each fixed relative to the sidewalls 32 and to one another, is provided by support means 44a, 44b, 44c, and 44d.
Gas flow is in a direction orthogonal to both the optical axls 42 and the electric.al discharge between electrodes. It ls best shown in Figure 3 by arrows 46.
~c~tual gas supply means, cooling means and recirculating means for the gas are not shown in that they are well : known ln the art~
It will also be noted from Figures 2 and 3 that the multiple pins 36 are arranged in a series of rows ` 42,164 ~ ~0407;~5 which are aligned with the optical axis of the laser system. A single row of pins would also be sufficient for the arc or glow dlscharge function in a given laser system.
Referrlng agaln to Figure 2, the D.C. bias supply circuit 24 is shown connected across the grid 38 and the anode 40 of the gas tube 22. The circult 24 in-. .
~- cludes a D.C. bias supply 46 connected from its negative terminal through resistance 48 to the grid 38. The posi-tive side of the D.C. bias supply 46 is connected to the ~! anode 40. The DC bias supply 46 established a uniform l electrlc fleld across the gap between the grld 38 and the . 1 anode 40 resulting ln a region Or unlform electric fleld which can be controlled to obtaln a~ optimized E/N ratio ~ ' whlch may be somewhat lower than that necessary to ini-:
tiate sparks or arcs, for a given flow and gas pressure.

With a low E/N ratio in the gap between grid 38 and anode . . ., ~
40 the requlred exciting collislons between electrons and ~i molecules take place giving rise to laser action. The unlform electric field E can have a higher average valuebefore breakdown is experienc~d than for a non-uniform ~ field across a comparable gap thereby enhancing the condl-; tions for excltation of the gas molecules wlthout break-down.

Reversal of the relatlve polarity of the uniform field electrodes 38 and 40 is a posslble alternative arrange-ment to that shown in Figure 2. In such an arrangement ultravlolet lrradlatlon of the electrode 40, which effec-tively would act as a cathodc relative to electrode 38, causes photoelectron production. Excitation of the gas --42,164 104~73S
molecules ls then triggered by the photoelectrons.
The pulse circuit 26 includes a pulse generator 50 connected across cathode 34 and anode 40. A capacitor 52 is connected between pulse generator 50 and cathode 34.
The D.C. blas voltage from the DC bias circuit 24, applied to grid 38, is coupled to the multi-pin cathode 34 through a high impedance element 54. Initially, before a pulse is generated by generator 50, the discharge gap between the multi-pin cathode 34 and the grld 38 ls a fleld free regiont An arc, 8park or glow discharge ls then initiated in tho gap between the multl-pin cathode 34 and the grid 38 by applylng a pulse voltage from pulse generator 50 to cathode 34 t The dlscharge causes lonlzation of the gas and free electrons are generated which dri~t through the ,: ~
I mesh-or-screenllke structure o~ grid 38 into the excita-i.~, tion region of the uniform field gap between grid 38 and anode 40. The gas tube 22 of Flgures 2 and 3 can thereby be operated to give high current glow or arc discharge or even a spark discharge while maintainlng a separate uni-: form field region in which ~/N is more easily ad~usted to some optlmum value for laslng operation.
The DtC. bias circuit 24 can be replaced with ~ a pulse bias circuit operated in synchronism with pulse :~ clrcuit 26. In such an arrangement the uniform field between ~hYi 38 and anode 40 could be pulsed on to coin-cide with the ln~ec,tion of electrons from the discharge gap between cathode 34 and grid 38. The effect would be slmllir to that obtained wlth a constant D.C. blas.
Referring to Figure 4 another electrode conflg-~" 42,164 uration ls shown in which a separation of ionization and ~,' excitation Punctions is achieved. It will be appreciated ~ that the electrode structure illustrated ls positioned ,~ wlthin a laser gas tube such as was shown in Flgure 2 but "I which ls here indicated by dotted llne 56. Gas flow in . ., the tube, although not shown in Flgure 4, would normally ,, be in a direction parallel to the planar uniPorm field ~',, electrodes 58 and 60.

'~! In Figure 4 the pi,n electro,des 62 are essential-"~ 10 ly in the same plane as the uniform Pield electr,ode 58 ''~ but insulated therefrom by insulating means 64. Only two : ! c~rona pins are shown for ease oP representation but more '1 ,can typically be used. The eIectrode 58 has a pl,anar ~ surPace and acts as a cathode relatlve to electrode 60.
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~ A uniPorm Pield is established between electrodes 58 and ". ~ .
~ll 60 by application oP a D.C. bias voltage from D.C. volt-l age source 66. By applying a high voltage pulse to the ,', corona pins from the pulse generator 68 a corona dis-, charge can be initiated at the corona pins 62 next ad~a-cent the electrode $8 which ionizes the gas molecules in that immedlate vicinity. Electrons are generated which are accelerated by the uniform field to collide with gas ~i' molecules initiating the avalanche ePfect required to raise the gas to the necessary vibrational energy levels ~' for lasing.
The electrode configuration oP Figure 4, although ~, possibly introducing some disturbance of the uniform field in t,he immediate region oP the electrode 58 due to the corona discharge, does allow for in,dependent control of E/N for a given flow and pressure in the reglon of exci-~ ' , , . ' . .

42,164 tatlon by means of the D.C. bias supply 66. In some :~, cases it mlght also allow for easier construction of the -~' electrode assembly in that the pin.s 62 would be firmly affixed through electrode 58.
In Figure 5 a slight mo.dlficatlon to the scheme ~, of Flgure 4 is shbwn~ Rather than a unlform fleld being ... .
' establlshed across the entlre gap between electrodes 70 ''' and 72, what can rather be termed a quasi-unlform fleld '~ ls establlshed. Electrode 70 is shaped so that the ., 10 fleld ncar the corona pin 74, when pin 74 is sub~ected to a hlgh voltage pulse from pulse generator 75, becomes "i an ionization region creating free electrons whlch drift ,,li into ad~acent regions of essentially uniform fields 76 and 78 which consequently are also regions of constant E/N. The electrode 70 has a shaped surface facing electrode 72. The portions of that surface in the regions C / oS ~ r A of unlform fields 76~and 78 are olosurc to the electode'72 th,an the portfon of the surface surroundlng the corona Pln 74. Agaln the E/N ratio is lndependently controlled 20 for a glven flow,and pressure by bias supply 80. The corona pin 74 can be insulated from electrode 70 by some suitable means 82, but it will be readily obvlous that lnsulation is not essential to operation of this electrode assembly.~ AThe'electrode assembly would also_norma,lly be ; positioned in some tube similar to that shown in Figure 2 and indicated in Figure 5 by dotted llne 56 with allowance for gas flow between electrodes 70 and 72.
By separatlng the reglons for ionlzatlon and excltation of the gas medlum and by provlding independent control of the excitation region to obtain a constant, ` 42,164 ~;: 104~)735 ~ optimum value of E/N for given pressure and flow condl- ~
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~: 1 tions, laser excitation in a high pressure gas laser is :~ much improved~

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Claims (2)

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

1. A high-pressure gas laser apparatus for use in producing a laser output 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 having a first field gap region and a second field gap region, wherein said electrode assembly includes first, second and third electrodes, said second electrode being at least one corona pin protruding through and insulated from said first electrode, said first electrode having a substantially planar surface consisting of a surface area immediately adjacent to said second electrode and a remaining surface area, said third electrode having a substantially planar surface and parallel to said substantially planar surface or the first electrode, said first field gap region being determined between the second electrode and the surface area of said first electrode immediately adjacent to said second electrode, said second field gap region being determined between said third electrode and the remaining surface area of said first electrode, pulsing means coupled to said first gap region for establishing an electrical discharge in said first region for generating free electrons, and D.C. bias means, separate and independent from said pulsing means, coupled to said second field gap region for establishing a predetermined uniform electric field for accelerating said free electrons from said first field gap region to said second field gap region for excitation of said gas medium to lasing vibrational energy levels by electron collision.

2. A high pressure gas laser apparatus for use in producing a laser output 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 cavity, an electrode assembly positioned within said envelope and having a first field gap region and a second field gap region, said second field gap region being separate and apart from said first field gap region, pulsing means coupled to said first field gap for establishing an electrical discharge in said first region for generating free electrons, D.C. bias means, separate and independent from said pulsing means, coupled to said second field gap region for establishing a predetermined uniform electric field in said second region and causing said free electrons to drift from said first region into said second region for excitation of said gas to lasing vibrational energy levels by electron collision and providing a glow discharge, said electrode assembly including first, second or third electrodes, said second electrode being at least one corona pin protruding through and affixed to said first electrode, said first electrode having a first surface area facing and spaced at a first distance from said third electrode, said first surface area being in close proximity to said second electrode, said first electrode having a second surface area substantially planar, facing and spaced at a second distance from said third electrode, said third electrode having a substantially planar surface and parallel to the said second surface area of the first electrode, said first field gap region being determined between the second electrode and said first surface area of said first electrode, said second field gap region being determined between said third electrode and said second surface area of said first electrode.