Note: Descriptions are shown in the official language in which they were submitted.
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
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~ 42,164
1~40735 :
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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104073S
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.
,.;
.~ 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-
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~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
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42,164
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~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
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~ ~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
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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-
-8-
~" 42,164
104CI1735
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.
.,: ,1 . .
~ 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
104Q735
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,
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` 42,164
~;: 104~)735
~ optimum value of E/N for given pressure and flow condl- ~
, . .
~: 1 tions, laser excitation in a high pressure gas laser is
:~ much improved~
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