Abstract:
A cooling structure ( 16 ) for use inside a ceramic cylindrical tube ( 11 ) of a metal vapor laser ( 10 ) to cool the plasma in the tube ( 11 ), the cooling structure ( 16 ) comprising a plurality of circular metal members ( 17, 31 ) and mounting members ( 18, 34 ) that position the metal members ( 17, 31 ) coaxially in the tube ( 11 ) to form an annular lasing volume, with the metal members ( 17, 31 ) being axially spaced from each other along the length of the tube ( 11 ) to prevent the metal members from shorting out the current flow through the plasma in the tube ( 11 ) and to provide spaces through which the heat from localized hot spots in the plasma may radiate to the other side of the tube ( 11 ).

Description:
This is a Continuation of application Ser. No. 08/088,090 filed Jul. 7, 1993 now abandoned. 
    
    
     STATEMENT OF GOVERNMENTAL RIGHTS 
     The Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the U.S. Department of Energy and the University of California. 
    
    
     BACKGROUND OF THE INVENTION 
     The copper vapor laser is an efficient source of visible radiation of high average power. Because the laser operates on a transition, the lower level of which is metastable, efficient operation requires a rapid pumping current pulse, and repetitive operation depends on depopulation of the metastable level primarily by collisional means. The majority of high power and efficient copper vapor lasers have utilized discharges in ceramic cylindrical tubes that are up to several meters in length. 
     The gas discharge, with an inert gas such as neon or helium as the buffer gas, takes place longitudinally along the length of the lasing volume in the ceramic tube between two electrode assemblies located at each end of the ceramic tube. Typically, a metal housing which serves as a coaxial return for the discharge current surrounds the ceramic tube. Near-normal incidence quartz windows held in water cooled mounts terminate the two ends of the vacuum enclosure and serve as the optical aperture windows. Copper vapor is introduced into the discharge at reservoirs located around the inside of the ceramic tube and the extent of the lasing region is determined by the axial temperature profile of the ceramic tube. The laser operates in the self-heated mode in which the discharge power that excites the laser medium also acts as the power source to heat the tube to its operating temperature. 
     Attempts have been made to increase the power output of such a laser by increasing the diameter of the ceramic tube to thereby increase the lasing volume within the tube. However, as the diameter is increased, the gas temperature increases. This trend continues until a diameter is reached where no further increase in output power is possible because the gas temperature has increased to a level wherein the metastable energy level is thermally populated to such a level that the population inversion necessary for lasing is significantly reduced or not even achieved in much of the lasing volume. 
     In an attempt to overcome the heating problem in large diameter lasing tubes, an inner ceramic tube has been placed inside the ceramic lasing tube, coaxially therewith, so that an elongated annular lasing volume is formed between the two tubes. Such a structure allows cooling of the gas by wall collisions and gas conduction to the inner ceramic tube. This tube is cooled by thermal radiation to the externally cooled outer ceramic tube. This allows a higher gas discharge volume with increased power density while lowering the gas temperature, and therefore provides more laser output power. 
     The inner and outer tubes are typically made of ceramic material for two reasons. First, ceramic materials have very high electrical resistivity and thus will confine the current-flow between the electrodes to the annular volume between the two tubes without electrically shorting out such flow. Secondly, suitably chosen ceramic materials will normally resist the high operating temperature, in the order of 1500° C., in a copper vapor laser. 
     Although the coaxial tube arrangement does provide cooling of the gas between the tubes so that the system can be scaled up in size for increased power output, significant mechanical problems exist because of breakage of the inner and/or outer ceramic tubes. One reason for such failure is that the lasing is often not uniform around the annular lasing volume, but can be concentrated through a small portion of the annulus. Such concentration produces localized heating of the inner and outer tubes. The unevenness of heating causes cracking and eventual destruction of one or both tubes. Another cause of failure is thermal cycling on start up and shut down. This induces thermal shock failures, such as cracking of the ceramic tube. 
     SUMMARY OF THE INVENTION 
     It is the principal object of the invention to provide a metal vapor laser having an elongated ceramic tube with an internal structure that will provide an annular lasing volume and will have a much greater resistance to mechanical failure than the inner structures heretofore used. 
     Additional objects, advantages and novel features will be set forth in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations pointed out in the appended claims. 
     To achieve the foregoing and other objects, and in accordance with the present invention as described and broadly claimed herein, an inner structure is provided for a metal vapor laser having an elongated ceramic cylindrical tube whose inner surface defines the outer boundary of a lasing volume extending along the length of the ceramic tube, said laser also having electrode assemblies at each end of the ceramic tube, the inner structure comprising a plurality of circular metal members each having a diameter less than the diameter of the inner surface of the ceramic tube, and means for mounting the metal members coaxially in the ceramic tube, with the metal members being spaced physically and electrically from each other along the length of the ceramic tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form part of the application, together with the description, serve to explain the principles of the invention. 
     FIG. 1 is an elevational sectional view of a ceramic lasing tube, with an inner cooling structure in accordance with the present invention. 
     FIG. 2 is a sectional view of the apparatus of FIG. 1, taken on line  2 — 2  thereof. 
     FIG. 3 is a perspective exploded view of a disk and support of FIGS. 1 and 2. 
     FIGS. 4 and 5 compare the radial temperature profile of a cylindrical gas discharge to that of an annular gas discharge. 
     FIG. 6 represents the minor axial temperature variations caused by the spaced metal disks. 
     FIG. 7 is a simplified sectional view of the apparatus of FIG. 1, illustrating how the spaced disks allow cooling of localized hot spots by thermal radiation transport. 
     FIG. 8 is an elevational sectional view similar to FIG. 1, with another embodiment of the inner cooling structure of the present invention. 
     FIG. 9 is a sectional view of FIG. 8, taken on line  9 — 9  thereof. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and in particular to FIGS. 1-3, the laser  10  includes an elongated ceramic cylindrical tube  11  whose inner surface  12  defines the outer boundary of a lasing volume extending along the length of the tube  11 , and further includes electrode assemblies, indicated generally at  13  and  14 , at each end of tube  11 . For purposes of simplification, the other portions of an operating laser, such as the vacuum envelope surrounding tube  11 , the quartz windows at each end of the laser, the means to cool the exterior of tube  11 , the means to feed a buffer gas into tube  11 , and so forth, are not shown herein since the conventional details thereof form no part of the present invention. 
     In accordance with the present invention, a cooling means  16  disposed inside ceramic tube  11  comprises a plurality of metal disks  17  each having a diameter less than the diameter of the inner surface  12  of tube  11 , and a plurality of ceramic support blocks  18 . The metal disks  17  are notched at  19 , so that the disks will fit into grooves  21  of the blocks  18  with the downwardly extending portions  22  of disks  17  embracing the sides  23  of blocks  18 , as best seen in FIG.  3 . To increase the stability of the cooling means  16  in tube  11 , metal pins  24  extending through the blocks  18  may be used to engage the inner surface  12  of tube  11  and thereby maintain the blocks  18  upright, with the disks  17  being held concentrically in tube  11 . 
     Merely for purposes of illustration, a ceramic tube  11  used as a copper vapor laser may have an internal diameter of 14 cm and a length of 3 meters, while the disks  17  have a diameter of 7-10 cm and are spaced from 1-2 cm apart along the length of the tube  11 . 
     In operation, the electrodes  13  and  14  will be repeatedly pulsed, at an operating potential of several tens of kilovolts, to cause repeated gas discharge along the length of the lasing volume in tube  11 . FIGS. 4 and 5 illustrate typical radial temperature profiles  26  and  27 , respectively, within a copper vapor laser when operating with cylindrical and annular gas discharges. In either case, the ceramic tube  11  will be cooled to maintain the inner surface  12  at an operating temperature in the order of 1500° C. When operating with a cylindrical gas discharge, the temperature in the center of the tube will be in the order of 3800° C. When operating with an annular gas discharge, as in the present invention, the disks  17  will be heated by the plasma in tube  11 , but will radiate to the cooler surface  12  of tube  11 , with the temperature of the disks being maintained at about 1550° C. The temperature of the plasma in tube  11  along a line halfway between the inner surface of tube  11  and disks  17  will be in the order of 2300° C. FIG. 6 illustrates the effect of the spaced metal disks  17  on the axial temperature profile in the annular gas discharge. At points between disks  17 , the cooling of the plasma by the disks will be reduced, and the temperature of the plasma between the disks will increase. 
     As is thus apparent, the maximum axial spacing between disks  17  must be such that the plasma between the disks is sufficiently cooled by the disks so that the metastable energy level is not populated to such a level that a population inversion is significantly reduced. On the other hand, if the axial spacing between disks is too small, then the cost of the structure will increase without producing a commensurate increase in output power, and the increased number of disks  17  will increase the possibility of component failure. 
     The optimal diameter of the disks  17  relative to the inner diameter of tube  11  is determined by the same considerations as are involved in the prior use of a ceramic inner cylinder. If the diameters of the disks  17  are too small, then insufficient cooling will be provided. On the other hand, if the diameters of the disks are too great, then the alignment problems will become untenable. 
     FIG. 7 illustrates a significant advantage of the use of spaced-apart disks  17  within tube  11 . As often happens, the lasing in tube  11  may be concentrated in a small portion of the lasing volume, as indicated at  28  in FIG.  7 . In the prior use of ceramic inner tubes, such a hot spot would locally heat the inner and outer tubes, causing undesirable circumferential temperature differences in each tube that lead to cracking and subsequent failure. In the present invention, the hot plasma at  28  will increase the local temperature of tube  11 , which can radiate (as illustrated by arrows  29 ) freely through the open space between the disks to all circumferential portions of the ceramic tube  11 . 
     The metal disks  17  have a much greater mechanical strength and much greater resistance to thermal shock than the heretofore used ceramic inner tubes and are consequently much less prone to failure. The heat conductivity of metal disks is also much higher so that the disks will not become locally heated by hot spots of plasma. Instead, a locally heated disk will have substantially the same temperature throughout itself, and will radiate such heat in all directions to the surrounding surface of tube  11 . 
     Although the metal disks  17  are electrically conductive, the disks are electrically isolated from each other to prevent them from shorting out the plasma between the electrodes  13  and  14 . 
     For use in copper vapor lasers, the metal disks  17  are preferably made of a refractory metal such as molybdenum, tungsten, tantalum, or niobium, in order to withstand the high operating temperatures. Disks used in a metal vapor laser operating at a lower temperature may be made of other metals or alloys, as long as the disks can retain their shape and are chemically compatible at such operating temperatures. 
     FIGS. 8 and 9 illustrate another embodiment of the invention wherein the metal members  31  are can-shaped with a cylindrical portion  32 , at least one closed end  33 , and a hollow interior. Tee-shaped mounting members  34 , of a suitable refractory metal, have a narrow portion  36  passing through diametrically-opposed slots  37  through the cylindrical portion  32  of the metal members  31  and shoulders  38  on which the cylindrical portion  32  rests. The upper end  41  of each mounting member  34  fits within a groove  42  formed in the inner surface of tube  11  and extending length-wise thereof, while the lower end  43  of member  34  fits within a similar groove  44  in ceramic tube  11 . 
     The embodiment of FIGS. 8 and 9 will provide a greater cooling surface inside the tube  11  than the embodiment of FIGS. 1 and 2, but will have less open space between the cooling members through which the heat from hot spots in the plasma can be radiated to opposite inner surfaces of tube  11 . 
     As seen from FIGS. 2 and 9, the cooling structure of FIGS. 8 and 9 will provide less blockage of the annular lasing volume. However, the tube  11  of FIGS. 8 and 9 is more expensive to fabricate because of the grooves  42  and  44  along the inner surface thereof. The cooling structure of FIGS. 1 and 2 is less costly to fabricate, and has an advantage in that the blocks  18  simply sit in the bottom of tube  11 . 
     The foregoing description of the preferred embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms described, and obviously many other modifications are possible in light of the above teaching. The embodiments were chosen in order to explain most clearly the principles of the invention and its practical applications, thereby to enable others in the art to utilize most effectively the invention in various other embodiments and with various other modifications as may be suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.