CA1043009A - High pressure gas laser using uniform field electrode configuration with irradiation by corona discharge - Google Patents

Abstract

HIGH PRESSURE GAS LASER USING UNIFORM
FIELD ELECTRODE CONFIGURATION WITH
IRRADIATION BY CORONA DISCHARGE

ABSTRACT OF THE DISCLOSURE
A molecular gas laser capable of operating at or near atmospheric pressure in which electrical energy is coupled into an active molecular gas medium comprising molecules having vibrational rotational energy levels by means of an electric field transverse to the lasing axis.
By applying an impulse voltage to the electrode configura-tion, high current glow discharges can be created. The pulse discharge takes place between electrodes having parallel planar surfaces facing each other. The lateral edges of these faces are suitably profiled to avoid field concentrations and thus provide a diffused glow discharge in a uniform electric field transverse to the lasing axis.
Initiatory electrons required to produce the high current diffused glow are provided by generating an intense burst of corona in the gap between spacer members having very high dielectric constants which are interposed between the electrodes. Specifically, when a voltage pulse is applied to the gap between the spacer elements a very high field appears at the interface and generates an intense burst of corona which provides ultraviolet irradiation of the cathode resulting in the emission of electrons.

Description

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 . .

~ 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 1C)430~9 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 42,616 ~0430~9 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 ~ -4_ .~ ~ . . .
- .

~ 42,616 --1043Q~
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 42,616 ~.

~0~300~9 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 ' 'I 043009 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;

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~

_~ 423616 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.

L~2,616 1043~)0~
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.

_~ 42,616 104300'9 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 iO43009 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 1~)430~9 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 ~43Q09 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 1~3009 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.

Claims (16)

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 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 electrode assembly having first and second spaced apart electrodes with continuous surfaces defining a uniform field gap therebetween, said uniform field gap traversing said resonant optical cavity, a first set of members of a high dielectric constant material fixedly and conductively attached to the first electrode and extending into said gap, a second set of members of a high dielectric constant material fixedly and conductively attached to the second electrode and extending into said gap so that end surfaces of corresponding members of said first and second sets are in contiguous relationship, pulsing means connected to said first and second electrodes for applying pulsed energy to said gap for stressing said uniform field gap and for generating corona discharges between said first and second sets of members for supplying free electrons to said uniform field gap and for irradiating said first electrode with ultraviolet radiation to cause emission of additional electrons for initiating and maintaining glow discharge between said first and second electrodes for exciting said gas medium to upper energy levels to induce lasting action. 16

2. The high pressure gas laser apparatus of claim 1 wherein said first and second sets of members are made of titanium dioxide.

3. The high pressure gas laser apparatus of claim 1 wherein said members are configured as elongated bars set parallel to said resonant optical cavity and have abutting surfaces of a rectangular configuration.

4. The high pressure gas laser apparatus of claim 1 wherein the abutting surfaces of said members of said first and second sets are circular.

5. The high pressure gas laser apparatus of claim 1 wherein said first and second sets of members are so arranged and configured to provide a plurality of parallel laser beam paths and further including optical means adjacent said electrodes for folding said laser beam from path to path.

6. The high pressure gas laser apparatus of claim 1 wherein said corresponding members of said first and second sets have end surfaces touching.

7. The high pressure gas laser apparatus of claim 1 wherein the end surfaces of said corresponding members of said first and second set define a gap region therebetween.

8. The high pressure gas laser apparatus of claim 7 wherein the distance separating the end surfaces of said corresponding members of said first and second set is small compared to the dimensions of the end surfaces of said corre-sponding members.

9. A high pressure gas laser, capable of pro-ducing stimulated emission of radiation, comprising:

an enclosure, a gas medium at high pressure suitable for lasing action, an electrode assembly having first and second spaced apart electrodes with continuous surfaces defining a uniform field gap therebetween, a first set of members of a high dielectric constant material fixedly and conductively attached to the first electrode and extending into said gap, a second set of members of a high dielectric constant material fixedly and conductively attached to the second electrode and extending into said gap so that end surfaces of corresponding members of said first and second sets are in contiguous relationship, pulsing means connected to said first and second electrodes for applying pulsed energy to said gap for stressing said uniform field gap and for generating corona discharges between said first and second sets of members for supplying free electrons to said uniform field gap and for irradiating said first electrode with ultraviolet radiation to cause emission of additional electrons for initiating and maintaining glow discharge between said first and second electrodes for exciting said gas medium to upper energy levels to induce lasing action, means for stimulating the emission of radiation from said excited gas medium.

10. The high pressure gas laser apparatus of claim 9 wherein said first and second sets of members are made of titanium dioxide.

11. The high pressure gas laser apparatus of claim 9 wherein said members are configured as elongated bars and have abutting surfaces of a rectangular configura-tion.

12. The high pressure gas laser apparatus of claim 9 wherein the abutting surfaces of said members of said first and second sets are circular.

13. The high pressure gas laser apparatus of claim 9 wherein said first and second sets of members are so arranged and configured to provide a plurality of parallel laser beam paths and further including optical means adjacent said electrodes for folding said laser beam from path to path.

14. The high pressure gas laser apparatus of claim 9 wherein said corresponding members of said first and second sets have end surfaces touching.

15. The high pressure gas laser apparatus of claim 9 wherein the end surfaces of said corresponding mem-bers of said first and second set define a gap region there-between.

16. The high pressure gas laser apparatus of claim 15 wherein the distance separating the end surfaces of said corresponding members of said first and second set is small compared to the dimensions of the end surfaces of said corresponding members.