RCA 66,423 10~4151 1 Background of the Invention This invention relates to a gas laser apparatus, and particularly to a high power gas laser apparatus in which the glow discharge is assisted through the use of a preionization means. 1ntercst in the production of high prcssure glow discharges ror industrial and military applications has increase~ considerably in connection with the development - of high power, electric discharge gas lasers. High pressure glow discharges are attractive since the power output of the discharge increases as the pressure is increased, i.e., as the number of active molecules per unit volume is increased. As is employed in the art, the connotation high pressure means a pressure above which a glow discharge can only be produce~ with the aid of some form of plasma con~ ditioning, e.g., preionization of the laser gas mixture. Typically, some means of preionization becomes necessary : at a pressure of about 30 torr. Heretofore, such high pressure glow discharges for laser discharge have been difficult to achieve. For example, the excitation of a C2 laser gas mixture is most efficiently performed, relative to laser output power, by means of an electric discharge having an optimum efficiency require~ent of an electric field strength to pressure ratio of about 3 to 4 volts per cm-torr. 1hus, to sca1e the C02 laser into the multikilowatt-megawatt output rangc requires gas pressures of the order of l atmosphcre and gas flow speeds which are commensurate with the gas coo1ing requirement. Characteristically, the dis- - charge in such a system has a tendency to constrict, and to 3 cventu~1ly arc, i.e., change from the normal glow discharge ~ - RCA 66,423 1~i4151 1 to a low vo1tiage discharge, as the discharge loading isincreased. r1ectron beam excitation has been proposed for use in high pressure gas lasers wherein an external electron gun supplies the electrons and a DC supply sustains the discharge. Typically, in electron beam excitation, the energetic elec- trons are produced in an evacuated electron accelerating structure, i.e., a gun. The energetic electrons are then directed into the discharge cavity through a foil window 10 which approximates the required cross section of the electron beam, e.g., lOcm by lOOcm or greater. The window, typically of titanium or aluminum, should be strong enough to support cavity pressure of several atmospheres, yet thin enough to pass the electron beam without absorbing too much energy. Thus, the electron beam is produced by an external electron gun which must be of high energy, i.e., approximately lO0 KeV, so as to possess sufficient energy to pass through the foil window. The use of high energy electron beam excitation may cause problems associated with X-ray emission as well as possible harmful effects of the window material. Thus, it would be desirable to develop a simpler and more convenient high power gas laser apparatus capable of producing a non-arcing glow discharge. Summary of the Invention A gas laser apparatus is provided with a region ror maintaining a glow discharge in a flowing laser gas mixturc. Thc gas laser apparatus includes preionization mcans for providing a plasma gas stream, the plasma gas being other than the laser gas. The preionization means directs the plasma stream into the flowing laser gas mixture such that the plasma stream assists the glow discharge. The --3- l~(~ ()(,~2~ 10~i41Sl l preionization means substantially prevents arc formation in the glow discharge region. Brief Description of the Drawings llGIlRE 1 is a schematic drawing of one form of lascr upp,lratus of thc prcscnt invention. i:lGURI' 2 is a schcmatic view of one form of pre- ionization means suitable for use with the laser apparatus of FIGURE 1. FIGURES 3 and 4 are schematic views showing alter- nate orientations of the preionization means of FIGURE 2 in relation to the' laser apparatus of the present invention. FIGURE 5 is a schematic view of another form of laser apparatus of the present invention. netailed Discussion Referring initially to FIGURE l, one form of a gas laser apparatus of the present invention is generally designated as 10. The gas laser apparatus lO includes a tube ~2, preferably of glass, having a region 14 for main- taining a glow discharge, i.e., the main discharge region. The tube 12 includes a projection 13. A mirror 16 is located at one end of the tube 12, and a window 18 at the other end is sealed at the appropriate Brewster angle. As is known, the Brewster angle window decreases reflective losses and permits a higher gain per pass of the light beam, resulting in hi!ghcr power output. A second mirror 19 is mounted cxtcrnally at the other end of the tube 12. The mirrors 16 and l9 arc preferably coated so as to provide maximum reflectance at the emitted wavclength, although broad-band mirrors may be utilized if it is desired to obtain more than one emitted frequency. The second mirror l9 is not -4- . RC~ 66,423 10tj4~51 1 entirely reflective so as to provide a transmittance of approximately 5%. The mirrors 16 and 19 are preferably separatcd hy a distance equal to an even multiple of the wave length which it is desired to amplify, thereby providing the dcsired resonant cavity effect. The tube 12 includes an entrance 22 and an exit 24 for communicating a flowing laser gas mixture therethrough. The entrance 22 continues into and through the projection 13 and into the glow discharge region 14. The tube 12 also includes a cathode region 26 and an anode region 28 which can be energized by a conventional high energy power supply 30, e.g., a 50 KW DC supply capable of operation at 50 KV and 1 amp. The gas laser apparatus 10 includes a ballast load resistance RL in order to provide current stabilization. Located in the projection 13, and in intimate relation with the glow discharge region 14, is an arc plasma source 32, i.e., preionization means, for providing a plasma stream which is rich in electrons andjor metastables species. The arc plasma source 32 includes a chamber 33 which houses an arc cathode 37, as shown in FIGURE 2. The arc plasma source - 32 includes an entrance opening 34 and an exit opening 36 for communicating a carrier gas therethrough. The exit opening 36 of the arc plasma source 32 faces into the discharge rcgion 14 of the tube 12. The exit opening 36 is preferably located near the end portion 22a of the tube entrance 22. lo keep the gas transit time small and to reduce electron recombination losses, the exit opening 36 of the arc plasma source 32 is provided with a nozzle 38 having a length as small as possible but providing sufficient space for expansion cooling. It is preferable to locate the exit opening 36 ~ A ()(),~2~ 1064151 l of the arc plasma source 32 near the cathode region 26 of the tube 12. In some cases it may be desirable to include a grid near the arc source exit opening, i.e., the grid can be used to control the subsequent glow discharge. The operation of one form of the high power gas laser apparatus of the present invention can be described by referring again to F~GURI.~S 1 and 2. rn one form of thc g;lS laser apparatus lO Or the present invention, the arc plasma source 32 is a small, e.g., 1-2 KW, argon arc operating at about 2 atmospheres of pressure and at a temperature of about 5000K. The arc cathode 37 is connected to a suitable power source (not shown) such that the arc cathode 37 is provided with a negative electrical potential while the arc source exit opening 36 and nozzle 38 are held at a positive ground potential whereby the arc cathode 37 is negative with respect to the exit opening 36 and the nozzle 38. The arc ~plasma source 32 can use any other carrier gas that is not harmful to the laser action, e.g., helium or nitrogen. The argon arc operates with a relatively low mass flow rate of the order of 0.453 gm/sec. which is directed into the arc plasma source entrance opening 34. The resultant arc produces a plasma stream which is rich in thermal electrons and metastable species which then flows out through the exit opening 36 of the arc plasma source 32 through the nozzle 38 and into the glow discharge region 14. A typical laser gas mixture, e.g., C02+N2+Ne, in a l:7:60 ratio hy mole fraction, is directed at high pressure, e.g., 1 atm, and high flow rate, e.g., 22 gm/sec., through the tube entrance opening 22 and into the discharge region 14 bounded by the cathode and anode regions 26 and 28. At I~(A 6(),423 106415~ l the same time, the plasma stream flows through the nozzle 38 and into the glow discharge region 14. Thus, the laser gas mixture and the plasma stream flow together into the discharge region 14. Typically, the flow rate of the laser gas mixture is about fifty times as great as the flow rate of the plasma stream. Consequently, the plasma stream - does not heat the laser gas mixture in the discharge region 14. The end portion 22a of the tube entrance opening 22 for the laser gas mixture may be concentric with the exit opening 36 of the arc plasma source 32, as in FIGURE 1, such that the laser gas mixture enters into the discharge region 14 along with the electron rich plasma stream. Under these conditions the plasma stream corresponds to an electron injection, excluding recombination losses, of about 7 x 1014 electrons per sec. and having energies of about leV. Under these conditions, the arc input energy is less than 10~ of the main discharge load. With a sufficient electron density (which is controlled by the arc source) injected into the cathode region 26, a nonarcing continuous discharge can be attained in the glow discharge region 14 through Townsend avalanche. The full discharge load depends on the laser gas mixture and the flow rates utilized. For example, the typical optimum or full power loading is about 0.441 to 0.661 KW/gm/ sec. The input power into the glow discharge region 14 for full load may be approximately 15KW for a flow rate of 22 gm/sec., 6.75KW for 1l.34 gm/sec., and 32.4 KW for 54.4 gm/ sec. TllC electric field provided between the cathode and anode regions 26 and 28 functions to produce the electric discharge, i.e , the desired glow discharge. Since the arc A 6G,~23 . 1~)64~51 l plasma source 32 provides additional electrons for use in the electric discharge, the electric field provided by the cathode and anode regions 26 and 28 can be relatively small in relation to the electric fields conventionally required in high powcr lasers, i.e., the plasma stream assists the -- giow discllargc. In thc laser apparatus 10 Or the present invention the arc p]asma source 32 providcs a stream of electrons and metastable species in the form of a plasma jet afterglow which spreads and penetrates into the glow discharge region 14 so as to prevent contracting to an arc formation in the glow discharge region 14. The coupling of the electrons emitted from the arc plasma source 32 to the metastables, which have a relatively long lifetime, functions to increase the penetration of these electrons into the main discharge region 14. The presence of metastables aids in offsetting the recombination losses of the electrons which would other- wise limit their adequate penetration into the main discharge region 14. Also, ionization from metastable energy levels in the main discharge region 14 are an important source of electrons in addition to the electrons carried by the plasma stream. The penetration of these electrons into the glow discharge region 14 provides means for supplying the necessary clectron density as the volume preionization, i.e., ionization which is uni~orm throughout the glow discharge region, required for the sustaincd opcration of the discharge. Thus, once the volume ionization is provided by the arc plasma source 32, the main discharge region 14 can be power loaded using a separate cathode. Since the flowing plasma is capable of producing volume ionization sufficient to initiate a glow discharge, no tr;ggering is necessary, e.g., the use of a tesla coil isinot nece-sary. In fact, if d~sired, the plasma stream ~an provide - RCA 66,423 10~i4151 all the ionization required in glow discharge region, i.e., l no ionization need be provided by the electric field in the glow discharge region. Although one form of laser apparatus 10 of the present invention has been heretofore described as having a particular structure, other structures are possible, e.g., the laser optical cavity, i.e., optic axis, may be parallel to or transverse to the gas flow and/or discharge region. For example, as shown in FIGURE 3, the optic axis is per- pendicular to the gas flow and to the discharge region. As shown in FIGURE 4, the optic axis is parallel to both the the gas flow and the discharge region. In addition, the arc plasma source 34 may be at right angles to the tube 12, as shown in FIGURES 3 and 4. Consequently, the relative orientations of the gas streams and discharge may be in-line, lS angled, with or without cross-flow patterns. Another form of gas laser appar~tus of the present invention is shown in FIGURE 5 in which the laser apparatus 20 includes two arc plasma sources 132 for providing the desired preionization. The arc plasma sources 132 are located in projections 113 which extend from the tube 112. The gas laser 20 of FIGURE 5 is substantially the same as the gas laser 10 of FIGURE 1 except for the use of an additional arc plasma source 132. The gas laser 20 of FIGURE 5 includes two internal mirrors 116 and 117 for design convenience, as opposed to the external mirror l9 shown in FIGURE 1. Also, for design purposes, the exit openings 136 of the arc plasma sources 132 of FIGURE 5 are shown angled toward the glow discharge region 114. Although only two arc plasma sources 132 are utilized in FIGURE 5, it is apparent that a plurality of these modules, i.e., electron and/or metastable sources, RCA ~f),42.~ 1~6~15~ 1 would also be successful in multiplying the output of a laser discharge. rn addition to CW operation, as hereinbefore discussed, the gas laser apparatus of the present invention can bc operated in a high repetition rate pulsed mode by controlling thc arc plasma stream, e.g., through the use of a grid near the arc source exit (not shown). In this embodiment, the plasma stream controls the glow discharge. Also, the gas laser can be mode locked, if desired, by mod- ulation of the arc plasma stream through means of grids oryokes. The gas laser of the present invention is n~t limited to the C02 laser, but is useful in any laser gas system which requires an electric discharge either for direct excitation of the laser levels, e.g., C0, HF lasers, or for initiating chemical processes, e.g., chemical lasers, and for metal vapor lasers. Also, although the gas laser of the present invention has been described as having an arc plasma source, other plasma sources are also suitable, e.g., rf discharge, thermal or nuclear heat sources. Also, although the primary purpose of the arc plasma source is to provide electrons and/or metastables, any ultraviolet emission from the arc is advantageous in preionization and/or for use for photoelectron emission from a metal surface, i.e., the anode. ~ - Thc gas laser of the present invention utilizes thc pl.ls~ strcam to provide thc desired volume preionization ~o as to prevent an arcing glow discharge. Thus, the glow discharge region can be power loaded using a separate cathode. Ihe gas laser can also be operated wherein all the necessary ionization is provided by the plasma stream. Thus, there is provided by the present invention, a high power gas laser apparatus capable of producing a non-arcing glow discharge. - 10-