Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACXGROUND OF THE INVENTION
1. Field of the Invention:
This invention relates generally to high pressure
gas lasers which are repetitively pulsed and more particu-
larly to the use of a uniform field electrode configuration
utilizlng cathode irradiation by corona discharge to provide
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~ 0430(39
~nitiatory electrons.
2. Description o~ the Prior Art:
A high power stimulated emis~ion o~ radiation
device uses transverse electric ~ield excitatio~ of a
gaseous medium having molecules with vibrational ~nd
rotational energy levels, between two parallel electrode~
capable of operating at atmospheric pressures and above.
Prior to tha above device, there was the problem that
the electrical discharge in the ga~eous medium tended
to occur in the form o~ an arc as the pressure in the
laser device increased above some lc~ value, typically
20 to 50 torr. Such an arc would be supported by a very
limited portion o~ the gaseous medium in a small column
around the arc and the gain consequently would not be suf-
ficient to cause laser action. The result was localized
heating of the gas generally preventing laser operation
entirely.
In this device means ~or irradiating the
cathode with ultraviolet light for producing free
electrons by photoelectric emission from the cathode i5
included. The free electrons in turn produce a glow
discharge uniformly over the surface o~ the electrodes
which is self-sustaining when the conditions o~ voltage and
electrode gap dimensions are such that each electron leaving
the cathode establishes secondary processes whereby it is
replaced by a new electron leaving the cathode. The free
electrons procluced in the glo~l discharge near the cathode
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engage in exciting collisions with ions, atoms or molecules
of the gas~ous medium in regions more remote from the
cathode. In these excitation regions there is a lower ratio
of electric field to particle density ~)and an amplif~ing
action results by the further interchange of energy between
free electrons and unexcited particles and between excited
and unexcited particles of the gas.
The geometry of the electrode configuration and
the ratio of ~ are important in the above type laser appara-
tus to establish the proper conditions ~or transfer ofenergy from the electrical energy source to the gaseous
medium. m e choice of electrode geometry and ~ ratio can
improve the efficiency of energy transfer from the
electrical energy source into selected modes of excitation
of the gas~ous medium which in turn can increase the over-
all efficiency of the laser device for a selected output fre-
quency.
Previous devices have included a means in the
form of an ultraviolet light source for irradiating the
cathode for producing free electrons by photoelectric
emission from the cathode ~ausing a uniform glow discharge
- over the surfaces of the electrodes. As previously noted
the glow discharge is self-sustaining when the conditions of
voltage and dimensions of the gap between the e~ectrodes are
such that each electron lea~ing the cathode establisheæ
secondary processes whereby it is replaced by a new electron
leaving the cathode. ~he free electrons in such a glow
discharge are accelerated by the electric field and excite
the ions, atoms or molecules of the gaseous medium in
inelastic collisions in regions
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remote from the cathode thereby causing an amplifying
action by way of the further exchange of energy between
free electrons and unexcited particles and between excited
and unexcited particles of the gas. Since there is an
infinite number of points on the planar plate electrode,
a substantially uniform diffused glow discharge can be
maintained for a limited time between the plates so that
energy may be transferred from the electric field to the
molecules of the active gaseous medium. This makes it
possible to operate a gas laser in a pulsed mode at pres-
sures higher than what was generally considered the cut-of~
threshold pressure for such devices (in the ne~ghborhood of
20 Torr).
Another prior art technique for providing free
electrons to initiate the diffused glow discharge in a
high pressure gas laser having parallel plate electrodes
is to use auxiliary electrodes ad;acent each main parallel
plate electrode for discharge initiation. The auxiliary
electrodes closest to the cathode initiate a trigger dis-
charge between cathode and auxiliary electrodes to providethe initiatory electrons for the main gap which is pulsed
from the same source. The auxiliary electrodes nearest the
anode are said to "focus" the beam in the vicinity of the
anode.
In another prior art device in which a plurality
of pins comprising one electrode means is positioned op-
posite a bar electrode, triggering of the glow discharge is
accomplished by field~emission at the pins when an impulse
voltage is applied. Initiatory electrons are thereby
provided because of the inherent physical properties of the
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nonuniform field configuration.
~1, 5
A In an analogous art arealPatent No. 2,990,492
issued to Wellinger et al teaches the application of a
solid state radioactive medium to lightning arresters where-
by the radioactive medium supplies free electrons to initiate
a power absorbing arc stream or discharge for the purpose of
protecting power lines. This patent is representative of
the art dealing with those protective devices whose primary
purpose is to develop an arc stream or discharge. Such
devices are the antithesis of the present invention because
it is the prevention of an arc stream or discharge which
must be accomplished while producing a very fast rise time
in a diffused glow discharge. An arc stream or discharge
in a gas laser causes the device to cease operating
immediately and can cause serious damage to the electrodes.
The present invention relates to the broad field
of producing an improved transfer of energy from the electric
field to the selected rotational vibrational modes of
molecules of a gas medium in a laser system. Free electrons
are generated and accelerated by the electric field applied
to the gaseous medium. Since the energy that each electron
receives from the electric field depends upon the strength
of the field and the distance through which the electron
is accelerated by the field between collisions, energy losses
can be minimized in relation to the total input energy to
the electric field by ad~usting certain parametersO Also,
the transfer of energy from the electrons to the selected
vibrational rotational modes of the gas molecules can be
maximized.
Generally speaking, there are two basic systems
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that have been used in the prior art to which the present
invention relates as far as electron excitation of the
molecules is concerned. The gaseous medium can be excited
by RF energy with electrodes placed external to the gaseous
medium container or it can be excited by applying a voltage
either DC or AC, across a pair of electrodes immersed in
the gaseous medium. As a practical matter, the excitation
process has a significant influence on the operation of
a gas laser device. The optimum operating pressure is
limited by thermal consideration. As the pressure increases
above approximately 20 Torr in a static volume of gas there -
is a tendency for a discharge to occur in the form of an
arc streamer. This creates heat with a very high thermal
gradient which adversely affects the lasing operation. The
ob~ective in this art is then to create a self-sustaining
diffused glow discharge and maintain it in this mode as
long as possible before thermal effects cause an arc streamer.
An arc streamer discharge will cause constriction of the
discharge, rapid temperature rise, and immediate cessation of
the lasing opçration.
SUMMA~Y OF THE INVENTION
Briefly, the present invention is an improvement
in the high pressure gas laser art for generating a dif-
fused glow discharge for the purpose of generating electrons
which in turn excite the gaseous laser medium by collision
with the gas molecules. The present invention provides a
laser in which the gas medium is excited by the diffused
glow discharge between two electrodes having continuous
surfaces, the surfaces being so configured that there are
no angular edge configurations to distort an essentially
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uni~orm electric f~eld between the electrodes. Reglons of
uniform electric fiela density are thereby developed when
a pulsed voltage is repetitively applied to the electrodes.
The specific improvement to which the present invention ls
directed is the means for provlcling ~ree alectrons to
initiate the diffused glow dlscharge mode of operation of
the laser. The present invention provides for a plurality
of member pairs made of a material having a high dielectric
constant such as titanium dioxide (rutile) which are posi-
- 10 tioned between or adjacent the main gap region and in in-
timate contact with the two main electrodes. A voltage
pulse applied to the main electrodes causes a very high
field to appear at the edges of the members because of the
high dielectric constant of the material. This generates
an intense burst of corona which provides ~ree electrons
directly and irradiates the cathode with ultraviolet radia-
tion thereby generating additional electrons. These
initiatory electrons trigger a pulsed glow discharge for
exciting the gas to lasing energy levels.
BRIE~ DESCRIPTION OF THE DRAWINGS
Figures 1 and 2 are schematic diagrams of prior
art electrode con~igurations for supplying free ele~trons
to the dlffused glow discharge;
Fig. 3 is a schematic diagram o~ a prior art
lightnlng arrester type gap;
Fig. 4 is a cross-sectiQn of the device of
Figure 3 taken on the line IV-IV;
Flg, 5 is the current and voltage waveform for
the annular uniform field gap of the device shown ln Figure
3 with a gas laser mixture at high pressure;
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104;~
Fig. 6 is an elevational view o~ a laser
according to the present invention;
Fig. 7 is a ~ection o~ the laser illustrated
in Flgure 6, the section being t,aken on the lines VII VII;
Fig, 8 is an elevational view o~ a multipath
laser according to the present invention;
Fig. 9 is a section of the laser o~ Figure 8,
the section being taken on the line IX-IX,
Fig.10 is a plan view o~ the multipath laser
of Figure 8 including the folded optics.
DETAILED DESCRIPTION OF THE DRA~INGS
Referring now to the drawings, spec~ically
Figure 1, a prior art uniform field laser cavity 10 defined
by walls 12 is shown. Main planar electrodes 14 and 16
are situated inside cavity 10. Electrode 16 acts as a
cathode and electrode 14 acts as an anode. Auxiliary
electrodes 18 and 20 are af~ixed to the exterior wall 12
adjacent opposit2 edges of electrode 16. Auxiliary
electrodes 22 and 24 are affixed to the exterior walls 12
adjacent opposite edges of electrode 14.
Auxilisry electrodes 18 and 20 and main electrode
14 are connected to ground potential. Auxiliary electrodes
22 and 24 and maln electrode 16 are connected to pulslng
circuit 26.
The princ~ple of operation of the device shown
in Figure 1 is not completely understood, but it appears
to depend upon the initiation of a trigger discharge between
the cathode electrode 16 and the auxiliary electrodes 18 and
20. This initial discharge pro~ides large numbers o~ elec~
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043a~a~9
trons which then initiate a di~fused glow discharge in the
main gap between electrodes 14 and 16. The function of
the auxiliary electrodes 22 and 24 is evidently to confine
the discharge to the main gap and to focus it toward the
anode 14.
Figure 2 shows a second prior art gas laser ap-
paratus in cross section. A gas tube 28 is shown hav~ng
an essentially totally reflecting optical element 30 and
a partially transmitting optical element 32 positioned
opposite one another and orthogonal to the optical axis 34
of the laser. Side wall 36 is sealed in an air tight
manner to the optical elements 30 and 32 to provide an
integral enclosure for the gas medium o~ the laser. It will
be understood that the elements of the tube 28 can be
modified without affecting the claimed invention, tube 28
being merely typical in the art.
Once the gas tube 28 has been evacuated it is
filled with a suitable laser gas mixture, which for example
might include CO or CO2, at a high pressure, ty~ically
above 50 torr.
~ lithin the tube 28 a cathode 38 comprised of
a plurality of pins 40 and an anode 42 typically a continu-
ous bar type electrode, are positioned opposite one anothe-r-~
within the tube 28. The pins 40 of cathode 38 are trans-
verse to the optical axis 34 while the surface Or anode 42
lies parallel to the optical axis 34. The support within
the tube 28 for the electrodes 38 and 42 is provided by
support means 44.
The electrodes 38 and 42 are connected to a
pulsing network 46.
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Under certain conditions of stored energy in the
pulsing network it is possible upon acquiring an impulse
voltage to the configuration shown in Figure 2 to create
a diffuse, transient, high current glow discharge between
the pins 40 and electrode 42. ~lectrons for initiation of
the pulse glow are provided by field emission at the pins
and the current is amplified by collisional ionization in
the high field region of the gap near the pins thereby
providing large numbers of electrons in the gap between the
pins 40 and anode 42. In the lower field region of the
gap near the anode 42 the electrons undergo exciting colli-
sions to achieve the required vibrational levels of the gas
medium. In order to obtain the required diffuse discharge
mode and avoid a constricted high temperature spark dis-
charge which would terminate the lasing action, it is
necessary in this prior art device to have a very rapid
current rise time typically less than 100 nanoseconds. The
glow duration in this particular configuration would be
typically of the order of 0.5 microseconds.
While the device of Figure 2 represents a high
pressure laser system in which high repetition rakes and
high average powers can be obtained, it is desirable to
increase the glow duration while reducing the peak pulse
power. Switching problems are reduced and the output
characteristics of the laser improved. To achieve such
improved performance electrodes utilizing uniform field
geometry can be implemented in place of the multiple pin
geometry provided that large numbers of initiatory electrons
can be generated to produce the high current diffuse glow
required.
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In Figures 3 and 4 a lightning arrester type
gap found in the prior art is shown in which a concept for
generating initiatory electrons is used. The device is
comprised of two electrodes 48 and 50 having spaced apart
surfaces 52 and 54. An exterior dielectric wall 56
separates the electrodes 48 and 50 and effectively seals
the gas discharge volume between the electrodes 48 and 50.
At the center of each electrode a button of high dielectric
constant material such as titanium dioxide (also identified
as rutile) pro~ects into the discharge volume. The buttons
58 and 60 abut at interface 62, each being held firmly in
place by a spring mechanism 64.
The main gap between surfaces 52 and 54 as shown
ln Figure 3 is of interest in that it utilizes a configura
tion similar to that of the invention taught and claimed
herein but which is applied quite differently. In a
lightning arrester application it is essential that
abnormally high potentials, caused usually by lightning
transients, may be relieved by flashovers in the air or a
- 20 gas rather than over the surface of ~orcelain insulating
elements. For this reason, it is necessary to have a well
defined sparkover voltage path~ Using the gap of Figures 3
and 4, a well defined sparkover voltage under ramp voltage
conditions is possible, thereby providing rapid release
from the high potential appearing across the electrodes in
the form of an arc discharge.
Using an annuIar uniform field gap configuration
similar to that of the device of Figures 3 and 4 with a gas
laser mixture at high pressure, current and voltage responses
as shown in Figure 5 have been obtained while generating a
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glow discharge with no spark or arc discharge. The current
and voltage measurements across the gap region were made
at a gas pressure of 400 torr and for an applied pulse of
30 kv for a gas mixture of 6 parts He: 1 part N2: 1 part C0
The current and volt;age waveforms shown in
Figure 5 reflect that ~or the period o~ current flow
(approaching 2~ sec) there was no arc discharge. An arc
discharge would h~ve been characterized by ~ sudden drop
of voltage. It has generally been ~ound that this con-
figuration speclfically utilizing the rutile buttons 58and 60 for initiatory corona discharge allows ~or a lon~er
glow duration as compared to prior art devices such as
the pin-plane electrode assembl~ of Fig. 2.
In Figures ~ and 7 and elevational view and a
sectional ~iew respectively are shown o~ a single pass
laser utilizing the present invention. In Figure 6,
contoured planar electrodes 66 and 68 are connected to
pulsing network 77 and are spaced apart with sur~aces
parallel to the optical axis 74 defining a uni~orm field
reglon. Reflect~ng optical elements 70 and 72 are
disposed at either end of the electrodes 66 and 68 to form
a resonant cavity for the stimulated radiation. Re~lector
72 is partially tr~nsm~ssive in order to couple out a por-
tion of the stimulated radiation. The laser beam axis 74
is parallel to and lies between the surfaces of the
electrodes 66 and 68.
The sectional view of Figure 7 shows a set o~
two elongated members 76 attached at either side of the
electrode 68 and projecting into ~he discharge gap in the
general direction of the uniform field. A second set of
two elongated members 78 are similarly attached to electrode
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66, each projecting toward a corresponding member 76 in a
contiguous relationship. Corresponding members 76 and 78
can be in direct contact or the surfaces thereof may de~ine
a narrow gap region. It will be recognized that each set
can be comprised of one or more members.
The members 76 and 78 are constructed o~ a high
dielectric constant material such as titanium dioxide
(rutile). When a voltage pulse from pulslng network 77 is
applied to the gap between the electrodes 68 and 66, a very
high field appears at the button interface generating an
intense burst of corona. The burst of corona provides
both free electrons directly to the main discharge gap and
irradiates the cathode with ultraviolet radiation thereby
generating additional electrons by photoemission ~t the
cathode and by gas photoionization.
The dielectric members 76 and 78 function as
dielectric discharge initiators ~or purposes of pulsed
laser operations. They can be in the shape of elongated
bars as shown in Figure 6 having abutting surfaces
essentially o~ a rectangular shape. Or, the members can be
button shaped with abutting circular surfaces. Indeed the
buttons can even be hollow and closed at the ends having
me$allic pieces extending into them. It will readily be
recognized that the particular configuration o~ buttons
used and dimensions chosen will be dependent upon the
e~iciency o~ irradiating the cathode with ultra~iolek
radiation while maintaining an essentlally uniform fleld
between the electrodes. It will also be appreciated that
when using the cylindrically shaped dielectrlc discharge
initiators or buttons that they can be arranged in a single
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or a plurality of rows, the particular arrangement being
one factor in determining the uniformity and optical
homogeneity of the pulse glow discharge.
In Figures 8, 9 and :LO several views o~ a multi-
path laser using folded optics is shown. Figure 8
shows one pa~h of the beam along the laser axis 80 which
runs parallel and between the sur~aces o~ planar electrodes
82 and 84. ~ielectric members 90 and 92 abut at their
interface. Optical reflecting elements 86 and 88 are
disposed at either end of the planar electrodes to define
a resonant cavity ~or the stimulated radiation. Optical
element 86 is partially transmiss~ve coupling the radiation
out of the cavity.
Figure 9 shows in cross-section the multipath
laser which is identical to that cavity shown in
Figure 7 except for having three lnstead o~ a single excita~
tion gap region. The members or dielectric discharge
lnitiators 90 are attached to the electrode 82 and pro~ect
into the gap region. The members or dielectric discharge
initiator~ 92 are attached to the electrode 84 and pro~ect
into the gap region abutting against correspondlng dielectric
discharge initiators 90 in the viciniky of the mid-gap
region. The end edge sur~aceæ of members gO and 92 are
beveled to expose greater discharge volume to the corona
discharge.
In Figure 10, a sectional plan view i8 shown o~
the multipath laser. Optical re~lective elements
86, 88, 94, 95~ 98 and 100, are so arranged as to fold the
laser beam along the multlple paths de~ined by the elec-
trodes 82 and 84 and the ~ielectric discharge initiators
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90 and 92. The optical element 86 being partially trans-
missive couples some of the radiation out o~ the cavity.
The use of the dielectric members or dielectric
discharge initiators for supplying the initiatory electrons
permits the use of planar electrodes to develop a uniform
field in place of the prior art pin cathode configuration.
The advantages achieved include a longer glow duration than
normally can be attained in the pin-cathode system. This
is achieved with lower peak current in the pulse generator
switching element and lower output peak powers for a given
average power.
me current rise time limitations are less
critical for this electrode geometry. This essentially
means that the pulsing network 91 switching element does
not need to meet as high a requirement for the rate of
change of current.
In the laser application it is mosb desirable
that the glow be uniformly distributed over the sur~ace of
the planar electrode. By proper arrangement of the dielectric
discharge initiators a uniform inJection of initiatory
electrons into the discharge region is achieved ei-~her
directly or by ultraviolet radiation from the corona dis-
charge which generates additional electrons by photoemission
at the cathode and by gas photoionization, and a homogeneous
glow discharge results.
The uniform field configuration of this invention
is simpler than that developed in the prior art and shown
in Figure 1 in that no auxiliary electrodes are required.
Additionally, there is no danger o~ damage to the cavity
walls by a trigger discharge.
