Abstract:
An slab CO2 laser includes spaced-apart elongated slab electrodes. A lasing gas fills a discharge gap between the electrodes. An RF power supply is connected across the electrodes and sustains an electrical discharge in the lasing gas in the discharge gap. Either one or two ceramic inserts occupy a portion of width of the electrodes and in contact with the electrodes. A discharge gap is formed between the portions of the width of the electrodes not occupied by the insert or inserts. Provision of the ceramic insert or inserts increases the resistance-capacitance (RC) time constant of the electrode impedance by increasing the capacitive component of the time constant. This hinders the formation of arcs in the discharge, which, in turn enables the inventive laser to operate with higher excitation power or higher lasing-gas pressure than would be possible without the dielectric insert. The ceramic insert also decreases the difference in impedance of the electrodes with and without a discharge. This leads to a better-behaved discharge, and a discharge that is easier to light.

Description:
CLAIM OF PRIORITY  
       [0001]     This application is a continuation-in-part of application Ser. No. 10/773,506, filed Feb. 6, 2004, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to carbon dioxide (CO 2 ) slab lasers. The invention relates in particular to a slab laser having a dielectric coupling-element between metal slab electrodes.  
       DISCUSSION OF BACKGROUND ART  
       [0003]     CO 2  lasers are commonly used in commercial manufacturing for operations such as cutting or drilling, in particular, in nonmetallic materials. One form of CO 2  laser suited for such operations is known to practitioners of the art as a “slab” laser. Such a laser has an assembly including a pair of elongated, slab-like planar electrodes arranged face-to-face and spaced apart to define a gap between the electrodes. The electrodes are usually contained in a gas tight enclosure. The enclosure and the gap between the electrodes are filled with a lasing gas mixture including CO 2 . A radio frequency (RF) potential is applied across the electrodes to cause an electrical discharge in the CO 2  laser gas mixture. The discharge energizes the CO 2  lasing gas. A pair of mirrors is arranged, with one thereof at each end of the pair of electrodes, to form a laser resonator. A preferred type of resonator is an unstable resonator. The energized CO 2  lasing gas provides optical gain allowing laser radiation to be generated in the resonator. The electrodes form a waveguide or light guide for the laser radiation in an axis of the resonator perpendicular to the plane of the electrodes. This confines the lasing mode of the resonator in that axis. The mirrors define the lasing mode in an axis parallel to the plane of the electrodes. In an unstable resonator arrangement, laser radiation is delivered from (in effect, spilled out of) the resonator by bypassing one of the resonator mirrors.  
         [0004]     In a slab laser used for drilling, cutting, and other machining operations a high output power, for example, greater than about 100 Watts (W), and maximum possible efficiency are important. In any given slab laser configuration, available output power generally increases with increasing gas pressure, provided that there is sufficient RF power to maintain a full discharge. Further, when operating in a pulsed-mode, faster rise and fall times for the pulses are possible at the higher pressure. A common problem limiting the output power of a slab laser is instability of the RF discharge. As RF power to the discharge (pump power) is increased to increase output power, the discharge eventually becomes unstable and is constricted into arcs. This adversely affects mode quality and efficiency of the lasers. This problem is exacerbated by higher gas pressures. Another problem in RF-energized slab lasers results from a substantial difference in RF impedance across the electrodes when there is no discharge (an “unlit” condition) from the RF impedance across the electrodes when there is a discharge (a “lit” condition). This impedance difference causes a change (a drop) in the resonant RF frequency when the discharge is ignited, i.e., the laser is changed from the unlit to the lit condition. Further, increasing gas pressure increases the difficulty of igniting the discharge, i.e., in turning on the laser. There is a need for an improvement of discharge stability in high peak power slab lasers.  
       SUMMARY OF THE INVENTION  
       [0005]     In one aspect a laser in accordance with the present invention comprises first and second elongated electrodes arranged spaced apart and face-to-face. At least one solid dielectric insert extends longitudinally along the length of said electrodes. A first portion of the insert is located between the electrodes in contact therewith and a second portion of the insert extends laterally beyond corresponding edges of the electrodes. The first portion of the insert has a width less than the width of said electrodes, thereby leaving an elongated gap between said electrodes. The gap is filled with a lasing gas. A pair of mirrors is configured and arranged to define a laser resonant cavity extending through said elongated gap. Means are provided for exciting the lasing gas to create a gas discharge, thereby causing laser radiation to circulate in the resonant cavity. The height of the gap is selected such that the gap forms a waveguide for the laser radiation in the height direction. The width of said gap is selected such that the laser radiation is allowed to propagate in free space in the width direction of the gap in a manner controlled by the configuration and arrangement of the mirrors.  
         [0006]     The dielectric insert increases the resistance-capacitance (RC) time constant of the electrode impedance by increasing the capacitive component of the time constant. This hinders the formation of arcs in the discharge, which, in turn enables the inventive laser to operate with higher excitation power or higher lasing-gas pressure than would be possible without the dielectric insert. The ceramic insert also decreases the difference in impedance of the electrodes with and without a discharge. This leads to a better-behaved discharge, and a discharge that is easier to light. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     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 the principles of the present invention.  
         [0008]      FIG. 1  schematically illustrates one preferred embodiment of a CO 2  slab laser in accordance with the present invention including first and second elongated metal slab electrodes arranged face-to-face and spaced apart by two ceramic inserts extending laterally partway between the electrodes and longitudinally along the length of the electrodes, and also including two mirror forming a one-axis unstable resonator extending between the electrodes.  
         [0009]      FIG. 1A  schematically illustrates the laser of  FIG. 1  further including ceramic mirror-shields located between the ends of the electrodes and the mirrors.  
         [0010]      FIG. 2  schematically illustrates another preferred embodiment of a CO 2  slab laser in accordance with the present invention, similar to the laser of  FIG. 1 , but wherein there is only one ceramic insert extending laterally partway between the electrodes and extending along the length of the electrodes.  
         [0011]      FIG. 3  is a cross-section view seen generally in the direction  3 - 3  of  FIG. 4  schematically illustrating still another embodiment of a CO 2  slab laser in accordance with the present invention similar to the laser of  FIG. 2  but wherein one of the slab electrodes is provided by a sealed enclosure surrounding the other electrode and the ceramic insert.  
         [0012]      FIG. 4  is a three-dimensional view schematically illustrating details of the electrode and ceramic slab arrangement of  FIG. 3 .  
         [0013]      FIG. 5  is a graph schematically illustrating measured maximum RF peak power input to the electrodes as a function of lasing-gas pressure for an example of the laser of  FIG. 1 , an example of the laser of  FIG. 2  and similarly configured prior-art laser without any ceramic inserts.  
         [0014]      FIG. 6  illustrates still yet another preferred embodiment of a CO 2  slab laser in accordance with the present invention including first and second elongated metal slab electrodes each having a single-stepped cross-section stepped and arranged face-to-face with a ceramic insert extending laterally partway between the electrodes, longitudinally along the length of the electrodes and having a height less than the step height of the electrodes.  
         [0015]      FIG. 7  illustrates a further preferred embodiment of a CO 2  slab laser in accordance with the present invention including first and second elongated metal slab electrodes each having a double-stepped cross-section and arranged face-to-face with ceramic inserts laterally partway between the electrodes, longitudinally along the length of the electrodes an each having a height less than the step height of the electrodes.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     Turning now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates one preferred embodiment  10  of a CO 2  slab laser in accordance with the present invention. Laser  10  includes upper and lower elongated slab electrodes  12  and  14 , respectively, arranged spaced-apart and face-to-face. The electrodes have an overall width E. The electrodes are spaced apart by dielectric (ceramic) inserts or spacers  16  having a height H, and extending laterally, i.e., in the width direction of the electrodes, partway between the electrodes. Inserts  16  preferably also extend laterally beyond longitudinal edges respectively  12 E and  14 E of the electrodes by at least about 2.0 millimeters (mm). Most preferably, inserts  16  extend at least about 5.0 mm beyond the electrode edges. Inserts  16 , here, further extend along the entire length of the electrodes. This lateral extension of ceramic inserts  16  increases the surface path length from electrode  12  to electrode  14  across the ceramic insert, thereby increasing the surface resistance across the ceramic to minimize the possibility of arc or discharge formation between the electrode edges.  
         [0017]     The assembly of electrodes and inserts is held together by ceramic clamps  18  attached to electrodes  12  and  14  by screws  20 . Preferred ceramic materials for inserts  16  include alumina (aluminum oxide—Al 2 O 3 ), beryllia (beryllium oxide—BeO), zirconia (zirconium dioxide ZrO 2 ) and zirconia and alumina mixtures. Alumina is also a preferred material for ceramic clamps  18 .  
         [0018]     The cross-section configuration of electrodes  12  and  14 , and the thickness of inserts  16  is selected such that there is a gap  26  (the discharge gap), having a height G, between thick central portions respectively  13  and  15  of electrodes  12  and  14 . The electrode surfaces  12 B and  14 A bounding the gap are parallel to each other. Gap  26  has a width W, here, determined by the width of the thick portions of the electrodes. Width W of course, is less than the total width E of the electrodes. The stepped cross-section shape of the electrodes, with thinner portions of the electrodes on either side of gap  26 , provides that height H of inserts  16  can be greater than the height G of discharge gap  26 . This provides that the corresponding edges of the electrodes are further apart than electrode surfaces  12 B and  14 A of the electrodes forming the gap. This minimizes the possibility of a discharge forming between the electrode edges. The capacitance (Cx) added by the ceramic inserts can be approximated to a first order by an equation: 
 
 C   X =ε( E−W ) L/H   (1) 
 
 where ε is the dielectric constant of the ceramic and L is the length of the electrodes. The percentage of space between the electrodes occupied by the ceramic (P C ) is approximated by an equation: 
 
 P   C =100( E−W )/ E   (2) 
 
 and P C  is preferably at least about 30%. 
 
         [0019]     The assembly of electrodes  12  and  14 , ceramic inserts  16 , and ceramic clamps  18  and ceramic slab  20  is contained in an enclosure (not shown) filled with a lasing gas mixture including CO 2 . The lasing gas mixture fills gap  26 . Ceramic inserts  16  include apertures  19  extending therethrough to facilitate flow of the lasing gas into gap  26 . An RF potential is applied across electrodes  12  and  14 . Here, the RF potential (supplied by an RF generator designated symbolically in  FIG. 1 ) is applied to electrode  12  (the “hot” electrode), and electrode  14  (the ground electrode) is connected to ground potential. Electrodes  12  and  14  are inductively coupled by inductors  32 . Applying the RF potential across the electrodes sustains an electrical discharge in the lasing gas in gap  26 , thereby exciting (energizing) the laser gas. Electrodes  12  and  14  include channels  34  that allow the passage of a cooling fluid through the electrodes to remove heat generated by the discharge.  
         [0020]     The cooling water passages are typically constructed from materials that will not corrode when water is used as a coolant, for example, copper, nickel, or brass. Cooling channels should be arranged such that the flatness of electrodes  12  and  14  is not distorted by temperature gradients. An example of such an arrangement is described in U.S. Pat. No. 5,237,580, the complete disclosure of which is hereby incorporated by reference.  
         [0021]     Energized CO 2  molecules in the discharge in gap  26  provide a gain medium for laser  10 . Laser  10  includes a hybrid resonator formed including a waveguide resonator and an unstable resonator  36 . Unstable resonator  36  is formed by a concave mirror  38  held in a mirror holder  40 , and a concave mirror  42  (indicated in phantom in  FIG. 1 ) held in a mirror holder  44 . Both mirror  38  and mirror  42  preferably have a reflectivity of about 99.5% or greater at the laser wavelength. The width W of gap  26  is selected such that laser radiation propagates in free space in a direction parallel to the electrodes, i.e., in the width direction of the gap. The mode propagation is determined, inter alia, by the spacing and curvature of the mirrors and the location of straight edge  42 A of mirror  42 .  
         [0022]     In this example, the mirrors of unstable resonator  36  are arranged and configured such that laser radiation circulates through gap  26  between electrodes  12  and  14  in a zigzag fashion, as indicated by dashed lines  46 . The laser radiation exits the resonator around edge  42 A of mirror  42  and then through an aperture  48  in mirror holder  44 . Concave curved walls  17  of ceramic inserts bound gap  26  on opposite sides thereof. It is advantageous to roughen the surface of curved walls  17  of ceramic inserts  16  to avoid the possibility of any waveguide action by these walls that could interfere with the function of mirrors  38  and  42  in determining laser modes in the width direction of the electrodes. The curvature of walls  17  also serves to increase the surface resistance of the ceramic inserts between the electrodes as discussed above.  
         [0023]     The waveguide portion of laser resonator is defined by electrodes  12  and  14  and mirrors  38  and  42  and is perpendicular to the above described unstable resonator portion. Height G of gap  26  is selected such that plane parallel surfaces  12 B and  14 A of electrodes  12  and  14 , respectively, effectively form a waveguide for laser radiation in a direction perpendicular to the plane of electrodes  12  and  14 , i.e., in the height direction of the gap. The waveguide portion of the resonator is completed by mirrors  38  and  42 . Laser radiation propagation modes are restricted, in that direction only, by the waveguide effect. Height G of gap  26  is further selected to provide a desired far field beam profile in combination with maximum laser power.  
         [0024]     It is preferable to space mirrors  38  and  42  at a distance from the ends of electrodes  12  and  14  sufficient that the mirrors are not degraded by the discharge in gap  26 . Preferably the spacing is about 20.0 mm or greater. Such a spacing, however, can lead to optical losses of laser radiation being redirected into gap  26  by the mirrors. One means of minimizing such optical losses is depicted in  FIG. 1A . Here laser  10  includes a ceramic (dielectric) extension  21  at each end of electrode  12 . Extension  21  is attached to electrode  12  by countersunk screws  17  extending through the ceramic extension and into the electrode. The electrode  14  extends under the ceramic extensions. Ceramic extensions  21  preferably have the same cross-section shape as electrode  12 . Spacing between the ceramic extensions and electrode  14  is similar to the spacing between electrodes  12  and  14 . Extensions  21  (in cooperation with opposing electrode  14 ) provides a waveguiding effect similar to that provided by the electrodes. The extensions may extend to within about 5.0 mm of the mirror. Ceramic inserts  16  are correspondingly increased in length to extend at least partway along the length of the extensions.  
         [0025]     Preferred dimensions G, E, W and H depend on desired operating parameters such as the lasing gas pressure, the frequency and power loading of the RF power applied to the electrodes and the output power of the laser. By way of example, for a gas pressure between about 80 and 200 Torr, an RF frequency of about 100 megahertz (MHz) and an output power between about 100.0 and 500.0 Watts (W), G is preferably between about 1.0 and 2.0 mm. Gap width W is preferably between about 20.0 and 80.0 mm for electrode length between 40.0 and 85.0 centimeters (cm). P C  is preferably between about 30% and 70%. The ceramic insert height H is, determined, inter alia, by the dielectric constant of the dielectric material and the desired capacitive loading. Height H is preferably between about 2.0 mm and 6.0 mm for an alumina ceramic.  
         [0026]     It should be noted here that only details of laser  10  sufficient for understanding principles of the present invention are described above. General aspects of CO 2  slab laser construction, such as lasing-gas enclosure, and RF power supplies and connection thereof, are well known in the art to which the present invention pertains and, accordingly, are not described in detail herein. A detailed description of examples of slab lasers is provided in U.S. Pat. No. 5,123,028 the complete disclosure of which is hereby incorporated by reference.  
         [0027]     An object of locating ceramic inserts  16  between electrodes  12  and  14  is to increase the capacitive component of the impedance experienced by the applied RF potential in general, and to limit, in particular, the difference of this impedance in the lit and unlit conditions of the discharge in gap  26 . In an unlit condition, this gas is effectively a dielectric and the electrodes and the gas-filled gap behave as a capacitor. In the lit condition, the gas is electrically conductive, and the capacitive effect of the electrodes and the gap therebetween is minimized. Including inserts  16  in the gap between the electrodes according to principles of the present invention provides a strong capacitive component of the electrode impedance even when the discharge in gap  26  is lit, and also minimizes the capacity difference between the lit and unlit conditions. Preferably the ceramic inserts should have an electrode-covered area greater than or equal to about 30% of the total area of the electrodes and most preferably between about 30% and 70% of the total area of the electrodes as noted above.  
         [0028]     The greater the ratio or percentage area of the ceramic inserts, of course, the more dominant will be the spacers in determining the capacitive component of the electrode impedance and the smaller the difference in impedance in the lit and unlit conditions. If the area of the ceramic inserts exceeds about 70% of the total area of the electrodes the current required to charge and discharge the capacitance during an RF cycle increase to a point where the efficiency of the laser is compromised.  
         [0029]      FIG. 2  schematically illustrates another embodiment  60  of a slab CO 2  laser in accordance with the present invention. Laser  60  includes first and second elongated slab electrodes  62  and  64 . The electrodes are held spaced apart by ceramic clamps  18  as in laser  10 . Electrodes  62  and  64  in laser  60  have the same width. Located between electrodes  62  and  64  between thin edge-portions  62 E and  64 E, respectively, thereof is a ceramic insert  66  extending laterally partway between the electrodes. Ceramic spacer  66  is in contact with both electrodes and extends along the entire length of the electrodes. Spacer  66  also extends laterally beyond corresponding edges of electrodes  62  and  64 , preferably by at least 2.0 mm and most preferably by at least 5.0 mm for reasons discussed above with reference to inserts  16  of laser  10 . Edge portions  63  and  65  of electrodes are thickened to permit, inter alia, insertion of fluid cooling channels  34 . Surfaces  62 B and  64 A of electrodes  62  and  64  respectively, in thickened portions  63  and  65 , respectively, thereof define a discharge gap  26 . An RF discharge is created in gap  26  as described above with reference to laser  10  of  FIG. 1 . Gap  26  here again has a width W less than the overall width E of the electrodes. An unstable resonator  36  is formed by a concave mirror  38  a concave mirror  42  as described above with reference to laser  10  of  FIG. 1 .  
         [0030]     The function of the single ceramic insert  66  is similar to that of the two ceramic inserts  16  of laser  10 . Using only a single insert asymmetrically arranged only one pair of electrode edges leaves discharge gap  26  open along the opposite electrode edges except for a relatively small proportion, preferably less than about 20%, covered by ceramic clamps  18 . This is very effective in facilitating flow of lasing gas into discharge gap  26  and for preventing acoustic resonances from occurring under pulsed discharge conditions.  
         [0031]     In embodiments of the inventive slab laser described above, the assembly of slab electrodes, ceramic spacers and the ceramic insert between the electrodes is structurally independent of any enclosure containing the assembly and a lasing-gas mixture. It is possible, however, to integrate the electrode-ceramic assembly into such an enclosure. A description of one example of such an integrated structure is set forth below with reference to  FIG. 3  and  FIG. 4 .  
         [0032]      FIG. 3  is a cross-section view schematically illustrating an embodiment  70  of a slab CO 2  laser in accordance with the present invention integrated into a metal enclosure  82 .  FIG. 4  is three-dimensional view schematically illustrating laser  70  with enclosure  82  partially cut away. Those skilled in the art will recognize that laser  70  is similar to laser  10  of  FIG. 1  integrated into a water-cooled enclosure. Accordingly, components with a common function in the two lasers are designated by the same reference numeral even though there may be some slight difference in shape therebetween.  
         [0033]     Enclosure  82  is preferably formed from machined aluminum components and is electrically connected to ground potential. Interior  84  of enclosure  82  is filled with a lasing gas via a port  86 , the tip  88  of which can be sealed off to seal enclosure  82  once lasing-gas filling is complete. Cooling channels  34  are provided in the base, sidewalls, and top of enclosure  82 . Cooling fluid is directed into the channels via an inlet port  90  and exits the channels via an outlet port  92 .  
         [0034]     A raised base-portion  94  of enclosure  82  forms a ground electrode for slab laser  70 . A separate top or “hot” electrode  12  is spaced apart from ground electrode  94  by a two ceramic inserts  16 . Electrode  12  has an overall width E. Raised portion  94  of housing  82  (the ground electrode) has a width W. Inserts  16  extend laterally partway between electrodes  12  and  94  and along the entire length of the electrodes. A discharge gap  26  is formed between upper surface  94 A of ground electrode  94  and lower surface  12 B of electrode  12 . The gap-spacing G is defined by ceramic inserts  16 . Gap  26  has a width W defined by the width of raised portion  94  of housing  82 . Apertures  19  extend through the ceramic inserts in fluid communication with discharge gap  26 .  
         [0035]     A discharge is sustained in gap  26  by an RF potential applied across the gap. RF power is supplied by an RF generator designated only symbolically in  FIG. 3 . The generator is attached to a connector  108 , which enters enclosure  82  via an insulated feedthrough  110 . An inductive path to ground is provided by a serpentine inductor  112  spaced apart from electrode  96  by ceramic insulating pads  114 . Inductor  112  is connected to grounded enclosure  82  via low inductance, compressive springs  116 . These springs also provide pressure for assisting clamps  18  hold the electrodes in contact with ceramic inserts  16 , thereby maintaining the spacing of gap  26 .  
         [0036]     Referring in particular to  FIG. 4 , resonator arrangements for laser  80  are similar to those of other above-described embodiments of the inventive laser. An unstable resonator  36  is formed by a concave mirror  38  held in a mirror holder  40 , and a concave mirror  42  (indicated in phantom in  FIG. 4 ) held in a mirror holder  44 . The unstable resonator is arranged such that laser radiation circulates through gap  26  between ceramic slab  20  and electrode in a zigzag fashion as indicated by dashed lines  46  before exiting the resonator via an aperture  48  in mirror holder  44 .  
         [0037]      FIG. 5  is a graph illustrating measured peak RF power as a function of a laser similar in configuration to laser  10  of  FIG. 2  (solid curve) having only one ceramic insert, a laser similar to the laser of  FIG. 1  (long-dashed curve) having two ceramic inserts, and similarly configured laser without any ceramic inserts (short-dashed curve). The electrode length is about 60.0 cm; the electrode total width (E) is about 44.0 mm; and the gap height (G) is about 1.4 millimeters (mm). Mirrors  38  and  42  are spaced apart by about 64.0 cm. Ceramic waveguide extensions about 15.0 mm in length are clamped to each end of the electrodes. In the case of the lasers having the inventive ceramic insert or insert, the insert or inserts occupy a total of about 47% of the electrode width. Lasing gas was a mixture of helium (He) nitrogen (N 2 ) and CO 2  in a ratio 3:1:1. The laser was operated in an RF super-pulsed mode at a 12% duty cycle and at pulse repetition frequencies between 700 Hz and 10 KHz.  
         [0038]     The maximum RF peak power is that maximum RF power applied to the discharge at which it is possible to sustain a stable discharge. At a higher RF peak power, the discharge becomes unstable and arcs begin to form in the discharge. The data of  FIG. 5  indicate that in the case of the prior-art laser without a ceramic insert, it was not possible to sustain a discharge at any pressure greater than about 140 Torr. In the case of the inventive laser having two ceramic inserts (the laser of  FIG. 1 ), it was not possible to sustain a discharge at any pressure greater than about 160 Torr, however, a peak input power of 10.8 kilowatts (KW) was possible at the highest pressure compared with 8.7 KW at the highest pressure for the prior-art laser. In the case of the inventive laser having only one ceramic insert (the laser of  FIG. 2 ), leaving the discharge gap open (unconstrained) along one side thereof, a discharge was still sustainable at a pressure of 180 Torr, at which pressure a peak RF input power of 12.6 KW was possible. Here, a limit was reached because of reaching an upper limit for the output power of the available RF power supply used to power the test laser. Generally the output laser power scales with gas pressure and the input RF power.  
         [0039]      FIG. 6  schematically illustrates yet another embodiment  120  of a slab laser in accordance with the present invention. Laser  120  includes upper and lower slab electrodes  122  and  124  having a ceramic insert  126  therebetween. As in other above described embodiments of the inventive slab laser an unstable resonator  36  is formed by a concave mirror  38  and a concave mirror  42  with laser radiation circulating through a discharge gap  26  between the electrodes. In laser  120  the electrodes and the ceramic insert are arranged such that the discharge gap  26  is open along both sides thereof, so that there is no possibility of interference by the ceramic insert with laser modes determined by the resonator mirrors.  
         [0040]     Each of the electrodes  122  and  124  has a single-stepped cross-section. Electrode  122  (the “hot” electrode) has a thin portion  123  and a thick portion  125 . Electrode  124  (the “ground” electrode) has a thin portion  127  and a thick portion  129 . The electrodes are arranged face-to-face, with the thin portion of one electrode facing the thick portion of the other electrode. Ceramic insert  126  is located between thick portion  125  of electrode  122  and thin portion  127  of electrode  124  in contact therewith. As in other above described embodiments, the ceramic insert extends laterally beyond the electrode edges (here edges  122 E and  124 E) to minimize the possibility of a surface arcing over the ceramic between the electrodes. For this same reason, ceramic insert  126  preferably also extends laterally beyond thick portion  125  of electrode  122  toward thick portion  129  of electrode  124 . Most preferably the width of the ceramic insert is sufficiently greater than the width of thin portion of electrode  124  than the ceramic insert contacts thick portion  129  of electrode  124 .  
         [0041]     Discharge gap  26  is formed between thick portion  129  of electrode  124  and thin portion  123  of electrode  122 . Thick portion  125  of electrode  122  has a plurality of apertures  130  extending therethrough to facilitate flow of lasing gas into discharge gap  26  and to preventing acoustic resonances from occurring under pulsed discharge conditions, as discussed above. Gap  26  is open along the edges of the electrodes formed by thick portion  129  of electrode  124  and thin portion  123  of electrode  122 , although this is entirely visible in  FIG. 6 . Spacing between the electrodes is maintained by ceramic clamps  18 , attached to edges  122 E and  124 E of the electrodes by screws  20 .  
         [0042]     Electrodes  122  and  124  can be defined as having a step height S, being the difference in thickness between the thin and thick portions of the electrodes. Ceramic insert has a thickness C, which is less than the step height S. Preferably the height of ceramic insert S is between about 25% and 75% of the step height. Discharge gap  26  has a width W determined by the width of thick portion  129  of electrode  124 . There is a distance Y between the thin portions of the electrodes, which distance is equal to the step height S plus the height of discharge gap  26 . The total electrode width is E. Thin portion  123  of electrode  122  has a width (W+X). Values of X and Y are selected to minimize the possibility of a discharge occurring between the thin portions of the electrodes. In one example of a laser  120  having a capability of 400 W output power, the length of electrodes  122  and  124  is 82.5 cm, the width (W) of discharge gap  26  is 55.0 mm. The discharge gap height (Y−S) is 1.2 mm, S is 3.8 mm, E is 88.0 mm, Y is 5 mm, X is 5.0 mm. The thickness C of dielectric insert  126  C is 1.0 mm. The width of the dielectric insert  126 , is 29 mm and the length of the dielectric insert is equal to the length of ground electrode  124 . For a given pressure, the output power of the laser will scale directly with the discharge width (W) and the electrode length. The optimum discharge gap (Y−S) dimension will vary inversely with pressure.  FIG. 7  schematically illustrates a further embodiment  140  of a slab laser in accordance with the present invention. Laser is similar in principle to a above-discussed laser  120  but includes slab electrodes  142  and  144  having a double-stepped cross-section.  
         [0043]     Electrode  142  (the “hot” electrode) has a thin portion  143  and two thick portions  141 .  
         [0044]     Electrode  144  (the “ground” electrode) has two thin portions  145  and a thick portion  147 . The electrodes are arranged face-to-face, with the thin portion of electrode  142  facing the thick portion of the electrode  144 . A ceramic insert  126  is located between each of thick portions  141  of electrode  142  and the thin portions  145  of electrode  144 . The ceramic inserts extends at least beyond the electrode edges to minimize the possibility of a surface arcing over the ceramic between the electrodes as discussed above with reference to laser  120  of  FIG. 6 . Discharge gap  26  is formed between thick portion  147  of electrode  144  and thin portion  143  of electrode  142 .  
         [0045]     Thick portions  141  of electrode  142  each a plurality of apertures  130  extending therethrough although the apertures are visible in only one thick portion in  FIG. 7 . These apertures are, as discussed above, to facilitate flow of lasing gas into discharge gap  26  and to prevent acoustic resonances from occurring under pulsed discharge conditions. Spacing between the electrodes is maintained by ceramic clamps  18 , attached to edges  142 E and  144 E of the electrodes by screws  20 . Laser  140  potentially has a higher inter-electrode capacitance and a lesser difference in capacitance between lit and unlit conditions than laser  120  of  FIG. 6 . This is achieved, however, at the expense of not having one side of discharge gap fully open for facilitating gas flow into the gap. For this purpose, reliance, here, is placed primarily on ports  30  in the thick portions of electrode  142 .  
         [0046]     Those skilled in the art to which the present invention pertains will recognize without further illustration that while lasers  60 ,  70 ,  120 , and  140  are depicted without ceramic waveguide extensions between electrodes and mirrors, such extensions may be, and preferably are, incorporated in a configuration similar to that described above with reference to extensions  21  and  23  of laser  10  of  FIG. 1A . Preferably, such extensions for lasers  60  and  70  have the same cross-section shape as the electrodes to which they correspond.  
         [0047]     The present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, by the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.