Note: Descriptions are shown in the official language in which they were submitted.
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
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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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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
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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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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
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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
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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.
.
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.
~0 Set behind the wire mesh sur~ace portion 176 and
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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
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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 ~
: -.
;~ 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.
: ;
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
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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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