Abstract:
A carbon-dioxide (CO 2 ) gas-discharge slab laser includes elongated discharge-electrodes in a sealed enclosure. Radio Frequency (RF) power is supplied to the electrodes via an impedance matching network and a co-axial electrical low inductance transmission line feed-through sealed to the enclosure. The feed-trough includes two spring contacts which are configured to be spring compression push-fit in grooves in edges of the discharge-electrodes. A central conductor of the feed-through is fluid cooled. A capacitor of the impedance matching network is assembled on the central conductor as an integral part of the feed-trough.

Description:
PRIORITY CLAIM 
     This application claims priority of U.S. Provisional Patent Application No. 61/114,333, filed Nov. 13, 2008, the complete disclosure of which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to carbon dioxide (CO 2 ), radio frequency (RF) excited gas discharge lasers. The invention relates in particular to providing RF power from an RF power supply through a hermetic sealed feed-through discharge-electrodes within a sealed off laser chamber including a lasing gas mixture. 
     DISCUSSION OF BACKGROUND ART 
       FIG. 1  is an electrical schematic drawing depicting a prior-art arrangement 10 of an (RF) excited gas discharge laser. The arrangement includes an RF power supply 12, an impedance matching network 14 and a sealed laser housing 16 including discharge-electrodes 28 and 30 and a laser gas mixture. An RF feed-through 22 (in electrical schematic form) for conducting RF power into the laser housing is outlined by a dashed line, as the feed-through from its function can be considered as being both within and without the laser enclosure. The feed-through has a center conductor 24 passing coaxially through a feed-through body 26 which is grounded. The center conductor is the live or “hot” conductor and is connected within the laser enclosure to the hot electrode 28. The feed-through body is connected to the ground electrode 30 and is grounded to the enclosure. All enclosures are RF shielded and grounded. 
     In an RF power supply the output from a plurality of RF amplifiers is connected via coaxial leads 18 to a common point 19. The RF power is transmitted from point 19 into the impedance matching network via short coaxial structure 20. The ratio of the diameter of a center conductor of the coaxial structure and the inside diameter of the outer conductor are chosen such that the coaxial structure has an impedance as Z p  to match the output impedance of the power supply. The impedance matching network includes two L-shaped networks, one thereof including an inductor L 1  and a capacitor C 1 , the other including an inductor L 2  and capacitor C 2 . The L 1 -C 1  network increases the impedance from Z P  at structure 20 to Z N  at point 21 where the inductor and capacitor are connected. The L 2 -C 2  network matches impedance Z N  to the load impedance of the laser Z L . 
     Certain problems were encountered in configuring and using an example of an arrangement such as arrangement 10 for a laser having an average power output of about 1 kilowatt (kW) or greater necessitating an RF power input of about 10 kW or greater. The RF feed-through selected was of a type described in U.S. patent application Ser. No. 12/069,939 (U.S. Pre-Grant publication No. 2008/0205473), filed Feb. 14, 2008 assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference. This feed-through had previously been successfully used for a CO 2  laser having an average output power up 500 Watts (W). 
     One problem encountered in the higher power laser was that it was difficult to realize practical values for the inductor L 2 , capacitor C 1  and especially for capacitor C 2 , to match to the low impedance of the laser discharge impedance Z L , and also provide components small enough to fit within an RF shielded enclosure of convenient dimensions. Another problem was that large RF currents flowing within the electrically hot central conductor of the feed-through generated excessive heating in the feed-through which caused problems in using rubber of indium vacuum seals. Yet another problem was that corona discharge occurred between connections from the feed-through to the electrodes within the laser housing. At an RF power level of 20 kW, arcing occurred between metal plates of capacitor C 2  There is need to solve these problems in order to provide reliable sealed-off CO 2  lasers having an average power of 1 kW or greater. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, gas discharge laser apparatus comprises first and second spaced-apart parallel discharge-electrodes in a laser enclosure. The first discharge electrode serves as a live discharge-electrode and the second electrode serves as a ground discharge-electrode. A radio frequency (RF) feed-through is provided for transmitting RF power from without the enclosure to the first and second discharge-electrodes within the enclosure, each of the discharge electrodes has a groove in an edge thereof facing the RF feed-through. The RF feed-through includes first and second springably compressible connecting members on a side thereof within the enclosure and attached respectively to a live electrode and a ground connection of the feed-through. The connection members and the discharge-electrode grooves are cooperatively configured such that the first and second connection members can be compressed to a push-fit into the grooves in respectively the first and second discharge-electrodes and retained in electrical contact with the electrodes by the spring compression of the connection members against sides of the grooves. 
     In another aspect of the present invention, the electrical feed-through has a hollow, metal feed-through body having first and second opposite ends, the first end intended to be without the enclosure and the second end intended to be within the enclosure. A plate of an insulating material closes the second end of the feed-through body. The live electrode of the feed-through extends through the feed-through body and through an aperture in the insulating-material plate. An arrangement is provided for flowing a coolant fluid within the live electrode. 
     In yet another aspect of the present invention, the RF-power is supplied to the feed-through via an impedance matching network including at least one capacitor. The capacitor is assembled on the end of the feed-through without the enclosure as an integral part of the feed-through. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
         FIG. 1  is an electrical schematic drawing of a prior-art CO2 laser including a laser enclosure containing two discharge-electrodes and an RF feed-through for conducting RF power to the discharge-electrodes, the RF power being supplied to the feed-through via an impedance matching network including two inductors and two capacitors. 
         FIG. 2  is an isometric view, partly in cross-section, schematically illustrating one preferred embodiment of an RF feed-through in accordance with the present invention suitable for use in a laser arrangement similar to the laser arrangement of  FIG. 1 , the feed-through having a feed-through body including a flanged portion and a hollow cylindrical portion in a single piece and having an insulating disc at one end of the cylindrical portion through which a fluid-cooled central electrode extends, the cylindrical body and the electrode furnished with electrical connectors designed to provide spring push-fit into grooves provided in the discharge electrodes, and a capacitor of the impedance matching network assembled on the electrode at the flange-end of the feed through body. 
         FIG. 2A  is an isometric view from the front of the feed-through of  FIG. 2 , schematically illustrating further detail of the push-fit connectors. 
         FIG. 2B  is an isometric view from the rear of the feed-through of  FIG. 2 , schematically illustrating a coolant fluid manifold for the fluid-cooled electrode and further detail of the integral capacitor. 
         FIG. 2C  is an isometric view, partly in cross-section, schematically illustrating details of the attachment of the feed-though to an enclosure wall, and details of the push fit of the connectors into the electrodes. 
         FIG. 3  is an isometric view, partly in cross section, schematically illustrating another preferred embodiment of an RF feed-through in accordance with the present invention, similar to the feed through of  FIG. 2  but wherein the feed-through body is assembled from three parts and the insulating disc has a different configuration from that of the feed-through of  FIG. 2 . 
         FIG. 3A  is an isometric view from the front, schematically illustrating further detail of the feed-through of  FIG. 3 . 
         FIG. 4  is an isometric view, partly in cross section, schematically illustrating yet another preferred embodiment of an RF feed-through in accordance with the present invention, similar to the feed through of  FIG. 3  but wherein the insulating disc has a simpler configuration from that of the feed-through of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 2  schematically illustrates one preferred embodiment  40  of an RF feed-through in accordance with the present invention. Reference is made in addition to  FIGS. 2A-2C  which depict the inventive feed-through from other viewpoints. 
     Feed-through  40  has a body  42 , preferably of aluminum having a cylindrical aperture  44  extending therethrough for an atmospheric-pressure side  46  to a low pressure (laser enclosure interior) side  48  of the feed-through. Body  42  has a flange  50  for sealing the low pressure  48  side from the atmospheric-pressure side  46 . In use, all of the feed-though body except for flange  50  will be in the laser enclosure. The body is clamped to a side-wall of the enclosure by screws (not shown) extending through holes  52  in the flange with sealing of the body to the enclosure being effected by clamping pressure bearing on an C-metal seal (not shown) in a groove  54  in flange  50 . 
     A ceramic insulator disc  56 , having an aperture  58  extending therethrough, is seated in a shallow, circumferentially-recessed portion  44 A of aperture  44  at the low-pressure end of body  42 . A clamping ring or ground-ring  60 , preferably of aluminum, is clamped to the low pressure end of body  42  by screws  62 . The clamping force bears on and distorts a sealing ring (not shown) in a circumferential, sloping-sided recess  64  of body  42 . This forces the sealing ring onto the edge of ceramic disc thereby sealing the disc to body  42  in one step. It is recommended that the edge of disc  56  be polished to optimize the seal. 
     An elongated central, electrically hot, electrode  66  extends through aperture  58  in ceramic disc  66  and out of the atmospheric-pressure end  46  of body  42 . A flanged portion  68  on the low-pressure end of electrode  66  has a sealing ring (not shown) in a groove  70  in the flanged portion. A retaining ring  72 , preferably of brass, is seated in a circumferential groove (shown but not numerically designated) in electrode  66  on the opposite side of disc  56  to electrode flange  68 . Screws  74  extend through threaded holes in retaining ring  72  and bear on a flat washer  76 , preferably of copper, against disc  56 . Screws  74  are tightened to compresses the seal (not shown) in groove  70  of flange  68  of the hot electrode. This completes the seal of the disc to feed-through body  42 . 
     Screws  74  are contained within an air cavity  75  whose sides are formed by the ceramic disc  56  and metal plug  80 , preferably of aluminum, inserted into the atmospheric press end of aperture  44  of body  42 . Hot electrode  66  extends though a Teflon insulating sleeve  82  extending in turn through an aperture  84  in plug  80 . 
     The combination hot electrode lead  66  and plug  80  can be considered as a sort of re-entry type transmission line. The thinner sleeve  82  separating the hot electrode  66  lead and plug  80 , the smaller is the series inductance (reactance) presented by the re-entry type transmission line. The sleeve of course must be thick enough to provide effective electrical insulation without suffering breakdown. A preferred thickness for this sleeve is about  0 . 30  inches. 
     In the above described arrangement of  FIG. 2  the inductance is comparable to, or lower than the impedance presented by the discharge between the discharge electrodes when the laser is operating. This allows the inventive feed-through to be smaller, more reliable, and more efficient than a prior-art RF feed-through. 
     The discharge impedance of a 1000 W CO 2  laser can be less than a few Ohms. The use of this re-entry transmission line design resulted in the reduction of the series inductance from about 25 nano-Henrys in a prior art design to about  10  nano-Henrys yielding an impedance of about 6.28 Ohms at 100 Mz. This low impedance reduces by two-and one-half times the voltage required at the entrance of the transmission line required to drive the discharge. This lower voltage prevents corona discharges from occurring at the entrance to the transmission line at the atmospheric-pressure side  46  of the laser housing. 
     A flange portion  86  of plug  80  bears on flange  50  of feed-through body  42  and is clamped against flange  50  by screws (not visible). This provides a reasonably good RF ground for the feed-through body, in addition, of course, to retaining the plug in the body. An additional good RF ground contact is made by a canted spring  88  compressed between the plug and the feed-through body in a circumferential groove  90  in plug  80 . 
     Returning now to a further description of hot (live) electrode  66 , the electrode has a hollow interior formed by a bore  92  extending co-axially through the electrode into the portion thereof surrounded by ceramic disc  56 . A coolant introduction tube  94  is inserted coaxially within bore  92 . The tube is held in alignment by a manifold (not shown in  FIG. 2 ) which fluid-sealed to the electrode and the coolant tube. The manifold is depicted as manifold  96 , partially, in  FIG. 2A , and, fully, in  FIG. 2B . A description of the construction of the manifold is manifold is not necessary for understanding principles of the present invention and is not presented herein. 
     A coolant fluid, which may be simply water, is flowed into electrode  66  through tube  94  and back out through coaxial space  92  between the coolant tube and the electrode body. This coaxial cooling arrangement is an important feature of the inventive feed-through and has been found to solve the above-discussed overheating problem encountered with a prior-art feed-through at 20 kW RF-power. 
     Another important feature of the inventive feed-through is an integrated series capacitor arrangement  100 . The equivalent of two series-connected capacitors is provided by two copper discs  102  and  104 , two thin dielectric plates  106  and  108  and a thicker dielectric plate  110 . Holes in the discs allow the discs to be threaded over Teflon sleeve  82  around the electrode body. The discs are clamped in place against flange portion  86  and plug  80  by a pinch clamp  112  in electrical contact with hot electrode. The pinch clamp and capacitor arrangement are partially depicted in  FIG. 2  and completely depicted in  FIG. 2B . The net value of the series capacitance can be varied, if desired, by providing machined surface recesses (not shown) in the copper plates and selectively rotating them with respect to each other. This technique is described (for stand-alone capacitors) in U.S. patent Application Ser. No. 12/051,232 (Pre-Grant Publication Number 2009/0004918), filed Mar. 19, 2008, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference. In this inventive arrangement of integrated capacitors, minor capacitance adjustments can also be made by adjusting the amount of pressure applied by pinch clamp  112  against plate  102  then locking the clamp using securing screw  114  of the clamp. 
     The provision of the two capacitors in series reduces the voltage across the individual capacitors which has been found, at 20 kW RF-power into the laser discharge, to solve the above-discussed corona and arcing problem with capacitors in a prior-art impedance-matching network. Further the integrated series capacitor arrangement and an additional capacitance resulting from Teflon sleeve  82  between plug  80  and electrode  66  provide the value of capacitor C 2  in the prior-art impedance-matching arrangement of  FIG. 1 . This eliminates the above-discussed problem of accommodating impedance-matching network components in an enclosure of convenient dimensions. 
     Continuing with reference to  FIG. 2  and with reference in addition to  FIG. 2A  and  FIG. 2C , arrangements for making electrical contact between the inventive feed-through and laser discharge-electrodes include two, here, identical, contacting members  120  and  122 . Each contact member includes a base block  124  from which protrude two spaced-apart, parallel rows  126 A and  126 B of metal-spring fingers  128 , preferably of beryllium-copper (Be—Cu). Contact member  120  is attached to a platform  69  extending from flange  68  of hot electrode  66  by screws  63 . Contact member  122  is attached by screws  63  to a platform  61  extending from ground ring  60 . 
       FIG. 2C  schematically illustrates how the contact members, depicted fully in  FIG. 2A , make electrical contact with cooperatively designed discharge-electrodes  130  (the electrically hot electrode) and  132  (the ground electrode). The electrodes are separated by ceramic spacers  134  (only one shown) to define a discharge gap  138  therebetween. Both the hot electrode  130  and the ground electrode  132  have channels  136  therein to allow passage of a cooling fluid. A wall  139  of a sealed laser enclosure for the electrodes is depicted to illustrate the manner in which feed-through  40  is attached to the enclosure. The enclosure itself is represented by space-designation numeral  48 . 
     Those familiar with the CO 2  laser art will be aware that the electrodes in practice are elongated electrodes arranged parallel to each other characteristic of electrodes in a slab-laser. Only a sufficient portion of each electrode is shown to allow a drawing having a scale adequate to understandably depict the manner of connecting the electrodes to the inventive feed-through. Only a portion of one enclosure wall  139  is shown as those familiar with the art will be familiar with how laser housings are constructed. Axes X, Y, and Z depict the laser orientation with Z being the propagation axis for the laser beam and X and Y being transverse (free-space and waveguide, respectively) axes. 
     Electrodes  130  and  132  each include a groove  131 / 133  along an edge of the electrode. Each groove has a width sufficient to accept spaced-apart spring finger rows  126 A and  126 B of a contact member in a push fit compressing the spring finger rows toward each other, with spring force of the fingers maintaining electrical contact with the contact member and the electrode. The inventive feed-through is configured such that the Y-axis spacing of the contact members thereon is equal to the Y-axis spacing of the grooves in the electrodes. This contacting arrangement was found to eliminate the above-discussed corona discharge problem around contact electrodes in a prior-art feed-through at 20 kW RF-power. 
       FIG. 3  and  FIG. 3A  schematically illustrate another preferred embodiment  140  of an RF feed-through in accordance with the present invention. The feed-through is assembled on a flange  142 , preferably of stainless steel, and intended for clamping and sealing the feed-through to a wall of a laser enclosure (not shown). Flange  142  has a groove  144  therein for seating an O-ring seal or the like for effecting the feed though-to-enclosure seal. 
     In  FIGS. 3 and 3A  only details on the low pressure side  48  of the inventive feed-through are depicted. It is intended that on the atmospheric-pressure side the aluminum plug, capacitor assembly, insulating sleeve, coolant feed tube, pinch clamp, and coolant manifold will be assembled, generally as depicted for feed-through  40  of  FIG. 2 , with dimension changes where necessary. A description of these components is omitted in  FIGS. 3 and 3A  to avoid a repetitious description, and for simplicity of illustration. From the detailed description of the components provided above and the detailed description of feed-through  40  provided below, it will be clear to those skilled in the how the atmospheric-pressure side components are assembled to complete feed-through  140 . 
     Feed-through  140  includes a hollow central conductor (electrode)  146 , preferably of copper, having an extension  145  brazed thereon, on which it is intended that a connector assembly similar to connector assemblies  120  and  122  of feed-through  40  be mounted. Electrode  146  is inserted through an aperture  148  in a thick ceramic disc  149 . The outer surface of electrode  146  is attached permanently, for example by brazing, for a short distance into aperture  148  from the low-pressure side. A distance of about 0.2 inches has been found sufficient for attachment and sealing. 
     A flared portion  150  of ceramic disc  149  is brazed to a mating tapered portion  154  of a gold-plated copper adaptor tube  152 . The adapter tube in turn is brazed into and in electric contact with a gold-plated copper ground-connection tube  158 . Tube  158  is brazed into a recess  143  in flange  142 . The ground-tube and ground-tube adaptor can be compared to the cylindrical portion of the one-piece body  42  of feed-through  40  of  FIG. 2 . Tube  158  has an extension on a bottom portion  157  thereof, on which extension is a platform  159  on which it is intended that that a spring finger contact assembly in accordance with the present invention is mounted. The relatively long distance between the ground adaptor tube  152  and platform  157  provides that arcing is avoided in operation. 
     Ceramic disc  149  has a circular trench  151  machined in the atmospheric-pressure side thereof. The trench  151  being located on the high pressure side serves to reduce the electrical field strength seen on the low pressure side at metal to ceramic brazing interface  155 . By minimizing the electric RF field at this metal to dielectric interface, detrimental parasitic discharges in low pressure region are avoided. 
       FIG. 4  schematically illustrates yet another embodiment of an RF feed-through  180  in accordance with the present invention. Feed-through  180  is similar to feed-through  140  of  FIG. 3 , with an exception that thick, channeled ceramic insulating disc  149  of feed-through  140  is replaced feed-though  180  by a relatively thin ceramic insulating disc  182  not having a machined trench therein. Feed-through  180  is somewhat simpler and less expensive in construction than feed-through  140  due to the simpler configuration of the ceramic insulator, particularly the elimination of the trench. It can be expected, however, that feed-through can not tolerate as high an RF power as above-described feed-through embodiments  40  and  140  before spurious arcing or corona discharges occur. 
     One advantage of thin ceramic disc  182  is that the length of the space defined at the interface between the ceramic disc and the conductor  146  is reduced compared with that of disc  49  in feed-through  140 . This provides for a shorter out-gassing time in manufacture. Out-gassing (by baking under vacuum) for all brazed-together components wherein unfilled spaces may trap impurities is recommended. If not eliminated by the out-gassing these impurities could eventually find passage into the laser housing and potentially contaminate the laser gas mixture and shorten the sealed-off lifetime of the laser. 
     In ceramic disc  182  it is preferable that surface  184  on the low-pressure side thereof is machined into an inverse-conical shape as depicted in  FIG. 4 . Preferably the surface is machined such the angle (p between the surface and the surface of electrode  146  is about 70°. This inverse conical surface provides a longer surface pathway along the ceramic, spreading electric field lines between tapered portion  154  of ground adaptor tube  152  and thereby reducing electric field intensity at the surface junctions between the ceramic disc, the ground adaptor tube and the central conductor (electrode)  146 . 
     It should be noted here that while preferred materials and attachment methods are discussed above with reference to embodiments of the present invention, these materials and techniques should not be considered limiting. Those skilled in the art may substitute other materials and methods without departing from the spirit and scope of the present invention. Regarding exemplary dimensions it is contemplated that all above described embodiments of the inventive feed-through, designed for operation up to 20 kW RF power, be assembled on a flange, such as flanges  50  of feed-through  40 , and flange  142  of feed-through  140  and feed-through  180 , having a diameter of about 2.65 inches. As drawings of embodiments of the invention are isometric projections, with components depicted being relatively to scale, dimensions of other components can readily be estimated. Here again, those skilled in the art may reproduce above-described embodiments with different absolute and relative dimensions without departing from the spirit and scope of the present invention. 
     In summary, the present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.