Abstract:
A gas laser discharge unit is provided. The discharge unit includes an elongated electrode plate having a plurality of spaced-apart holes therein and a plurality of coaxial high voltage ducts. Each duct extends through one of the holes in the electrode plate and includes a central conductive core and an insulator element arranged around the core to electrically insulate said core from the electrode plate. An elongated high voltage electrode is electrically connected to the cores of the ducts. In addition, an elongated ground electrode is positioned to oppose the high voltage electrode and form a gas discharge gap therebetween. The ground electrode is electrically connected to the electrode plate. The gas laser discharge unit may be removably mounted as a module into a gas laser tube, such as an excimer laser tube.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to gas lasers, and more particularly to a discharge unit for a gas laser, wherein a high voltage is supplied to two discharge electrodes. 
     2. Background Of The Invention 
     Excimer lasers provide high intensity laser radiation in the ultraviolet spectral range. This makes them important tools especially for medical and surgical applications as well as for other industrial applications. 
     Excimer lasers are gas discharge lasers that use a rare gas such as argon and a halide gas such as fluor (for example ArF excimer laser) or a gas containing a halide (for example F 2 ) as the laser gas. 
     Generally, in an excimer laser a gas mixture containing the active component and other gases is steadily provided to a discharge gap between a pair of elongated electrodes inside the laser tube by means of a fan or the like. A high voltage applied between the two electrodes causes a gas discharge in said discharge gap, whereby, from the active component of the gas, short-lived excited-state molecules are generated, whose dissociation generates ultraviolet radiation constituting the laser radiation. To increase the homogeneity of the gas discharge, in present excimer lasers a pre-ionization of the laser gas by pre-ionizers is used. As the used laser gas needs to regenerate before it can be re-used, excimer lasers are generally operated in a pulsed operation mode, wherein the laser gas in the discharge gap is being steadily replaced by fresh or regenerated laser gas provided by the fan. 
     The discharge electrodes for an excimer laser are usually located inside the laser tube. 
     The housing of an excimer laser generally consists of a metal tube having openings in a cylindrical wall on the upper side thereof. An insulating plate covers the open upper side. The metal tube and one of the discharge electrodes are grounded. A high voltage is applied to the second discharge electrode via a HV duct extending through the insulating plate. 
     One main problem of excimer lasers, which is still not satisfactorily solved, is the contamination of the laser gas due to the corrosive effect of the active components of the laser gas on many insulating materials which are widely used as insulators, especially on materials containing carbon molecular structures, such as many plastic materials, for example TEFLON®. Due to this contamination the lifetime of the laser gas is reduced, which makes a frequent exchange of the laser gas necessary. To overcome this problem, U.S. Pat. No. 4,891,818 utilizes high-purity aluminum oxide (Al 2 O 3 ) as insulator, on which the corrosive effect of the active components of the laser gas is by far reduced as compared to plastic materials. 
     Another, even more corrosion resistant, class of materials that can be used as insulators is fluorides. 
     Directly related to the above problem is the problem, that exchanging of the gas and maintenance works are expensive and time-consuming. Moreover, they are hazardous activities, as the laser gases for excimer lasers are, besides their corrosive nature, highly toxic. 
     A further problem is to provide an excimer laser with a high pulse repetition rate. U.S. Pat. No. 5,771,258 discloses an aerodynamic chamber design for an excimer laser to provide a high pulse repetition rate. A high repetition rate also requires a high frequency voltage to be efficiently applied to the electrodes, which becomes more difficult with increasing frequency of the voltage. 
     RELATED APPLICATIONS 
     The present invention may be used in conjunction with the inventions described in the patent applications identified below and which are being filed simultaneously with the present application: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                 Serial or 
               
               
                 Docket No. 
                 Title 
                 Inventors 
                 Filing Date 
                 Pat. No. 
               
               
                   
               
             
             
               
                 249/301 
                 A Gas Laser and a 
                 Hans Kodeda, 
                 Feb. 22, 
                 09/511,649 
               
               
                   
                 Dedusting Unit 
                 Helmut Frowein, 
                 2000 
               
               
                   
                 Thereof 
                 Claus Strowitzki, 
               
               
                   
                   
                 and Alexander 
               
               
                   
                   
                 Hohla 
               
               
                 249/302 
                 Dedusting Unit for a 
                 Claus Strowitzki 
                 Feb. 22, 
                 09/510,667 
               
               
                   
                 Laser Optical Element 
                   
                 2000 
               
               
                   
                 of a Gas Laser and 
               
               
                   
                 Method for 
               
               
                   
                 Assembling 
               
               
                 249/303 
                 Shadow Device for A 
                 Claus Strowitzki 
                 Feb. 22, 
                 09/510,017 
               
               
                   
                 Gas Laser 
                 and Hans Kodeda 
                 2000 
               
               
                 249/304 
                 Modular Gas Laser 
                 Claus Strowitzki 
                 Feb. 22, 
                 09/510,538 
               
               
                   
                 Discharge Unit 
                 and Hans Kodeda 
                 2000 
               
               
                 250/001 
                 Adjustable Mounting 
                 Hans Kodeda, 
                 Feb. 22, 
                 09/511,648 
               
               
                   
                 Unit for an Optical 
                 Helmut Frowein, 
                 2000 
               
               
                   
                 Element of a Gas 
                 Claus Strowitzki, 
               
               
                   
                 Laser 
                 and Alexander 
               
               
                   
                   
                 Hohla 
               
               
                 250/002 
                 An Optical Element 
                 Hans Kodeda and 
                 Feb. 22, 
                 09/510,666 
               
               
                   
                 Holding and 
                 Helmut Frowein 
                 2000 
               
               
                   
                 Extraction Device 
               
               
                   
               
             
          
         
       
     
     All of the foregoing applications are incorporated by reference as if fully set forth herein. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a laser discharge unit for a gas laser and in particular for an excimer laser, which minimizes contamination of the laser gas and thus increases the lifetime of the laser gas. 
     A further object of the invention is to provide a laser discharge unit for a gas laser that allows a high frequency voltage to be efficiently applied to the discharge electrodes. 
     Another object of the present invention is to provide a laser discharge unit for a gas laser and in particular for an excimer laser which is easy to handle and yet powerful. 
     The above and further objects of the invention are achieved by a laser discharge unit with an elongated electrode plate made of an electrically conductive material, such as metal, a plurality of waveguide-like coaxial high voltage ducts extending through said electrode plate and comprising respectively a central conductive core and an insulator element made of an insulator, such as a ceramic insulator, and being arranged around said core and electrically insulating said core from said electrode plate, an elongated high voltage electrode and an elongated ground electrode, said high voltage electrode being electrically connected to said cores of said ducts and said ground electrode being electrically connected to said electrode plate, wherein said ducts are arranged spaced apart from each other. 
     The tube of the laser is preferably electrically connected to the ground electrode, and both are preferably grounded. 
     The electrode plate of the discharge unit of the present invention comprises a metal plate, preferably a pure metal plate. An advantage of this is that the laser gas does not corrode the electrode plate to the same extent as it would corrode if it were made from an insulator material such as ceramic; thus contamination of the laser gas is reduced. 
     The electrode plate comprises a plurality of holes, through each of which one of the high voltage ducts is guided. As the electrode plate is grounded, an insulator is required between the conductive cores of the HV ducts and the electrode plate. The number of high voltage ducts (and holes) depends on the size of the laser, in particular on the length of the electrodes. For example, for a typical excimer laser, three high voltage ducts should be used. For a larger laser with longer electrodes more than three ducts should be provided. For smaller lasers with shorter electrodes only one or two ducts may be provided. 
     According to the invention, the conductive cores of the ducts are respectively surrounded by an insulator element. By this construction, as compared to a whole plate made of an insulator, only a small amount of insulator material is in contact with the laser gas. Thus the contamination of the laser gas is clearly reduced. 
     The conductive cores of the ducts and the electrode plate form a coaxial waveguide-like structure, which facilitates the effective coupling of high frequency pulses for pulsed operation mode of the laser to the high voltage electrode. 
     The insulator elements of the ducts are preferably made of an insulating material, such as aluminum oxide (Al 2 O 3 ). Alternatively, the insulator elements can be made from a fluoride material. In this regard, the more expensive fluoride materials, which are more resistant against corrosion by the excimer laser gas, can be used since the amount of insulator material has been reduced to a minimum according to the invention. 
     In principle the insulator elements may have any shape. Preferably the shape of the insulator elements is such that they conically expand towards said high voltage electrode and comprise a corrugated surface, so as to increase a creepage path extending along said surface. This shape is intended to prevent surface flashover between the high voltage electrode and the grounded electrode plate. 
     Preferably the electrode plate carries the high voltage ducts in that the ducts are mounted to the electrode plate by a plurality of screws. Alternatively the ducts can be mounted to the tube of the laser, for example by screws or by rods or plates or the like. 
     The laser discharge unit preferably further comprises a sleeve enclosing the core and insulator of each duct. Each sleeve preferably includes an inner end supported by the electrode plate, and an outer free end. Each core includes an inner end connected to the high voltage electrode plate and a threaded outer free end extending beyond the free end of the sleeve. A nut or other fastening means may be screwed onto the threaded end so as to press the sleeve against the electrode plate and thereby tension the core by pulling it. As those skilled in the art will recognize, any other construction for fixing the high voltage ducts to the electrode via the core or via the insulator element is possible as well. Preferably a stud which comprises a thread at both of its ends, such as a threaded bolt, is used to connect the inner end of the core to the high voltage electrode. Alternatively to the above described construction the high voltage electrode can be carried by, or mounted to, the laser tube, for example via a plurality of plates or rods made of an insulating material that are screwed to the tube. 
     The core and the insulator of each duct are preferably fixed with respect to each other; this may be accomplished, for example, by providing the inner end of each core with a core ring shoulder pressed against the insulator by the tensioned core. A seal is preferably provided between the ring shoulder and the insulator. It is also possible, however, for the core to include a recess into which a ring shoulder of the insulator may be inserted. Alternatively, the core and the insulator may be fixed with respect to each other by some different construction. 
     The insulator element may be pressed against the electrode plate by means of the tensioned core via the core ring shoulder at the inner end of the core and an insulator ring shoulder of the insulator element. Preferably, a seal is provided between the insulator shoulder and the electrode plate. 
     A sealing ring also preferably surrounds each sleeve. The sealing ring should have at its outer circumference a flange that is supported by an outer rim of a hole in the tube through which the respective high voltage duct is inserted. The electrode plate may also be provided with a ring shoulder supported at an inner rim of the tube. The ring and the electrode plate may then be connected, for example, by screws. A seal is preferably provided between the electrode plate shoulder and the inner rim of the tube. 
     The ground electrode is preferably carried by, or mounted to, the electrode plate. Preferably a plurality of flow guides is used for this purpose. The flow guides are preferably made from sheets of metal that extend between the electrode and ground electrode in a plane perpendicular to the longitudinal axis of the electrodes. The flow guides typically comprise an upper flange, a lower flange, and a central flow-guiding portion integrally connecting the upper flange to the lower flange. The upper and lower flanges extend perpendicular to each other and to the central flow-guiding portion. The upper flange is attached to a side face of said electrode plate, and the lower flange is attached to a bottom face of the ground electrode. Preferably the central flow-guiding portion is aerodynamically profiled in order to minimize flow resistance and turbulences for maintaining a substantially laminar gas flow between the flow guides. 
     Alternatively, the laser tube can carry the ground electrode, for example via a plurality of plates or rods or the like made of metal or some other conductive material. 
     Preferably a gas-tight seal is provided between the ducts and the electrode plate. Alternatively a seal can be provided outside the laser tube, for example at the end of the ducts. For practical reasons the holes in the electrode plate and the ducts preferably have a round cross-section. In this case, the gas-tight seals are ring-shaped. The holes can just as well have a square, a rectangular, an oval, an oblong or any other cross-section. As would be apparent to those skilled in the art, the ducts and the gas-tight seals should have a corresponding shape. But ring-shaped seals have the advantage that they are easier to fabricate and to handle, more reliable, and furthermore they are cheaper than for example rectangular seals. On the other hand it is preferred that a metal seal is used. If a ring-shaped seal is used, a commercial metal seal can be used. 
     It is preferred that the ducts are inserted into the electrode plate respectively with a defined tolerance between the insulator element and the respective hole in the electrode plate through which the respective duct is inserted. In this manner, the ducts are held fixed in a defined position. According to an alternative embodiment of the invention this fixing can be achieved solely by fixing elements, such as bolts or screws or a tube, by which the respective duct is held in its hole in a fixed position. 
     The preferred discharge unit further comprises a pair of standard corona pre-ionizers, that is a pair of elongated cylindrical pre-ionizers with a conductive core and a surrounding tube-shaped insulator. The pre-ionizers extend substantially parallel along opposite sides of the electrode. The insulator of the pre-ionizers is preferably a ceramic material such as alumina. It can also be a fluoride material. Alternatively, any other kind of known pre-ionizer can be used. The pre-ionizers are not necessary for the discharge unit to work. Indeed, excimer lasers were known before the invention of pre-ionizers. Pre-ionization, however makes the gas discharge between the high voltage electrode and the ground electrode more homogeneous and thus more reliable. 
     A shadow plate may be mounted between the gas discharge gap and the insulator for protection of the insulator against the laser radiation and discharge light irradiated from the gas discharge gap, and against light from the pre-ionizers. 
     The overall construction of the laser can be such that first a laser tube is provided, and then the high voltage electrode, the ground electrode, the insulator element or elements, the high voltage conductor or conductors and the pre-ionizers are mounted to the tube, one by one. It is preferred, however, that the electrode arrangement is a pre-mounted module-type discharge unit, wherein the electrodes, the shadow plate, and the high voltage ducts are pre-mounted independently of other laser elements. The discharge unit may be mounted to the laser tube as a whole. This provides several advantages. One advantage is that the gas discharge gap between the high voltage electrode and the ground electrode can be adjusted before the discharge unit is mounted into the laser tube. This facilitates an accurate adjustment of the gas discharge gap. Furthermore the construction of the laser can be done in a more efficient manner. 
     The laser gas can, in the case of an excimer laser, be any excimer laser gas, such as KrF, ArF, XeF, XeBr, HgBr, HgCl, XeCl, HCl, F 2 , Ar 2  and the like or any laser gas in case of some other gas discharge laser. 
     Besides the laser gas, a buffer gas comprising a mixture of Helium, Neon and/or Argon is preferably provided in the tube. 
    
    
     Further features and advantages will become apparent from the following detailed description of the preferred embodiment of the invention when read in combination with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cut-away side view of an excimer laser according to a preferred embodiment of the invention; 
     FIG. 2 is a cross-sectional view of the excimer laser in FIG. 1 along Line  2 — 2 ; 
     FIG. 3 a  shows a side view of a discharge unit according to a preferred embodiment of the invention; 
     FIG. 3 b  shows a front view of the discharge unit of FIG. 3 a;    
     FIG. 3 c  shows a plan view of the discharge unit of FIG. 3 a;    
     FIG. 4 shows a detailed cross section of the discharge unit according to a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following a preferred embodiment of the invention will be set forth. 
     FIGS. 1 and 2 show an excimer laser  100  comprising a tube  101 , a discharge unit  102 , a circulation means  201 , and a laser optical system  103 . As is known in the art, gas laser  100  may also comprise means for cooling the laser gas, such as a gas cooler, within laser tube  101 . 
     The circulation means  201  is optional and may comprise, for example, a fan or any other means known for circulating lasing gases in gas lasers. 
     The discharge unit  102  is mounted into the tube  101  and comprises a high voltage electrode  104  and the ground electrode  105 . The high voltage electrode  104  and the ground electrode  105  are spaced apart from each other, thereby defining a gas discharge gap  106 . A high voltage is applied to the high voltage electrode  104  via a plurality of high voltage ducts  107 , which carry the high voltage electrode  104 . Each high voltage duct  107  comprises a conductive core  108  and an insulator element  110  arranged around the conductive core  108 . Each high voltage duct  107  is attached to the high voltage electrode  104 . The high voltage ducts may be attached to the high voltage electrode using any suitable fastener. In the present embodiment, a double threaded stud  112  is used to attach electrode  104  to each conductive core  108  of each duct  107 . 
     Furthermore, discharge unit  102  is provided with an elongated electrode plate  111 . The electrode plate  111  includes holes, through which the high voltage ducts  107  extend so as to be connected to the high voltage electrode  104 . Each high voltage duct  107  is fixed to the electrode plate  111  by an attachment means, such as bolts  113 . Those skilled in the art will appreciate, however, that any suitable attachment means may be used to fix ducts  107  to electrode plate  111 . 
     The insulator elements  110  are preferably made of a ceramic material. Optionally, however, they may be made from other insulative materials, including, for example, a fluoride material. They preferably have a shape that conically expands towards the high voltage electrode  104  and comprise a corrugated surface, so as to increase a creepage path extending along said surface. This helps to prevent surface flashover between the high voltage electrode  104  and the grounded electrode plate  111 . 
     As noted above, insulator elements  110  may be made from fluoride insulator material. These materials have the drawback of being relatively expensive. However, according to the preferred embodiment of the present invention, only a small amount of insulator material is required. Accordingly, the use of fluoride insulator materials is affordable in the present invention. 
     As illustrated in FIG. 2, discharge unit  102  also preferably comprises a shadow plate  210  disposed between the gas discharge gap  106  and the insulator element  110  for protecting the insulator element  110  against the corrosive effect of the laser gas and of the laser radiation. Shadow plate  210  is preferably made out of a metal, such as aluminum. 
     In the present embodiment, shadow plate  210  is interposed between the high voltage electrode  104  and insulator element  110  of the ducts  107 . Preferably shadow plate  210  extends along the entire gas discharge gap  106  and is mounted in such a manner that it at least partially shields the insulator element against the laser radiation irradiated from the gas discharge gap  106 . 
     Referring to FIGS. 2 and 3 a,  shadow plate  210  preferably has an elongated sheet-like shape and comprises a central portion  209 , a first edge portion  211  and a second edge portion  212 . The central portion  209  extends longitudinally in a direction parallel to the gas discharge gap  106  and perpendicular to the cores  108  of the high voltage ducts  107 . The edge portions  211  and  212  are located at the longitudinal edges of the shadow plate  210  and are, with respect to the central portion  209 , preferably bent towards the insulator elements  110  by a small angle of about 20 degrees. Although shadow plate  210  is preferably elongated as described above, it may have a variety of other shapes as well. For example, a plurality of shadow plates  210  could be used instead of one elongated shadow plate that extends the length of the discharge gap. In such a case, the shadow plates would preferably be circular and have a cross-sectional appearance consistent with that shown in FIG.  2 . Thus, a circular shadow plate  210  could be interposed between the electrode  104  and each insulator element  110  of the high voltage ducts  107 . 
     The type of laser gas employed may also affect the shape of the shadow plate  210 . The reason for this is that some laser gases have a substantially lower breakdown voltage than other laser gases. For example, the laser gases used in ArF and KrF excimer lasers have a substantially lower breakdown voltage than the F 2  gas used in fluorine gas lasers. Thus, if laser  100  is a fluorine gas laser, then the shadow plate  210  may come much closer to the insulator element  110  than if laser  100  is an ArF or KrF excimer laser. As a result, bending the edges of the shadow plate  210  toward insulator element  110  may be appropriate. On the other hand, bending the edges of shadow plate  210  toward the insulator element for other excimer lasers may be inappropriate. Indeed, for example, with ArF, KrF, and other excimer lasers employing laser gases with relatively low breakdown voltages, it may be advantageous to bend the edges of the shadow plate away from the insulator element  110  to help ensure that current does not jump the gap between the shadow plate and insulator element. However, this of course, will depend on how far the shadow plate  210  is spaced from the insulator element  110  and the breakdown voltage of the laser gas being used. Similarly, the greater the breakdown voltage of the laser gas, the smaller the diameter of the conical portion of the insulator element  110  may be. 
     As best seen in FIG. 2, the shadow plate  209  may be interposed between the high voltage electrode  104  and inner ends  404  of the cores  108  of the high voltage ducts  107  so that the longitudinal axis of the central portion  209  (or center in the case of a circular shadow plate) coincides with the central axis of the high voltage electrode  104 . 
     Although shadow plate  210  is preferably interposed between high voltage electrode  104  and inner ends  404  of the cores  108  as illustrated in FIG. 2, as those skilled in the art will appreciate, the shadow plate  210  will serve its desired function so long as it is interposed between the discharge gap  106  and insulator elements  110 . Thus, the positioning of shadow plate  210  is not limited to the position illustrated in FIG.  2 . 
     Shadow plate  210  may be interposed between electrode  104  and cores  108  by providing central portion  209  with a plurality of holes  213 , preferably along the longitudinal axis of the central portion  209 , and then attaching high voltage electrode  104  to the cores  108  through holes  213  using a suitable fastener. Thus, the spacing and the number of holes  213  correspond to the spacing and the number of high voltage ducts  107 . In the present embodiment, stud bolts  112  with threads on both ends are used to attach electrode  104  to cores  108 . One end of the stud bolts  112  is inserted into a threaded hole  124  provided in the respective core  108 . The second end of the stud bolts  112  is inserted into a further threaded hole  126  provided in the mating face  128  of the high voltage electrode  104  that faces the inner end  404  of core  108  and the shadow plate  210 . If a circular shadow plate is used, each shadow plate will be provide with a single hole  213  in the center of the shadow plate and one shadow plate will be used for each high voltage duct employed in the laser. 
     The shadow plate  210  preferably has a flow-guiding shape to help guide the lasing gas mixture into the gas discharge gap  106 . 
     A preferred manner of assembling the electrode arrangement of the present invention with the shadow plate is now described. 
     First one end of a stud bolt  112  is screwed into each of the threaded holes provided on the mating face  128  of the high voltage electrode  104 , such that the other end of each stud bolt  112  stands out of the mating face  128 . Then the shadow plate  210  is arranged on the mating face  128  of the high voltage electrode  104  so that the stud bolts  112  are inserted into the holes  213  in the shadow plate  210 . Alternatively, if a circular shadow plate is used, then one shadow plate  210  will be inserted over each of the stud bolts  112 . After the shadow plate  210  is in place, a core  108  of the high voltage ducts  107  is lowered upon the shadow plate  210  such that the end of one of the stud bolts protruding from the electrode partially enters the threaded hole  124  provided in the inner end  404  of the core  108  of the high voltage duct  107 . Subsequently the core  108  is rotated around its longitudinal axis, i.e. around the longitudinal axis of the stud bolt  112 , so as to screw the core  108  onto the stud bolt  112 . As a result, the core  108  is lowered onto the shadow plate  210 , and the shadow plate  210  is finally held between the upper face  128  of the high voltage electrode  104  and the inner end  404  of the core  108 . Additional high voltage ducts  107  comprising cores  108  are attached to the remaining stud bolts  112  in the same way as described above. 
     In the case of an elongated shadow plate, before the cores  108  are tightly screwed to the stud bolts  112 , at least two of the cores  108  are loosely screwed to their corresponding stud bolt  112 . Then, after the shadow plate  210  is correctly positioned, all of the cores  108  are screwed down tightly to lock shadow plate  210  in place. 
     The excimer laser  100  may be, for example, a pulsed fluorine gas (F 2 ) excimer laser with a wavelength of about  157  nanometers. This means that fluorine gas is used for generating the laser beam. However, as those skilled in the art will appreciate, any of the known excimer laser gases may be used in connection with the present invention. 
     By applying a high voltage pulse on the order of 20 kV to the high voltage electrode  104 , the laser gas (e.g., fluorine gas) and additionally helium and/or argon gas as a buffer gas in the discharge gap  106  generate a laser beam which is emitted through the laser optical system comprising a front optical system  103  and a rear optical system  120 . 
     Laser  100  typically further comprises a front optical element  116 , through which the laser beam emits. Optical element  116  may be provided, for example, in an optical system  103  that includes an adjustable mounting means  117  for adjusting the position of the optical element  116  in relation to the tube  101 . Rear laser optical system  120  similarly includes an optical element  116  (not shown) and adjusting means  117 . However, the optical element  116  of the rear laser optical system  120  comprises a totally reflective mirror rather than a partially reflective mirror. As those skilled in the art will appreciate, front and rear optical elements  116  may also be mounted directly in the end walls of the laser tube  101 . Alternatively, they may be mounted on adjustable mounting brackets that are separate from the laser tube  101  as is known in the art. A suitable laser optics system and an adjustable mounting means for use in connection with the present invention as front and rear optical systems  103 ,  120  are described in concurrently filed applications Ser. No. 09/511,648 and 09/510,666, which are hereby incorporated by reference. The filing details of these applications are provided above. 
     FIG. 2 is a cross-sectional view along line  2 — 2  of the excimer laser  100  shown in FIG.  1 . As can be seen in FIG. 2, the excimer laser  100  preferably further includes a circulating means  112 , such as a fan, for circulating the excimer laser gas through the discharge gap  106  and an optional dedusting unit  202  for dedusting the gas flow through the tube  101 . The dedusting unit comprises high voltage wires  203 , separated from each other by U-shaped channels  204  extending along the tube  101 . Furthermore, two guiding plates  205 , which are elongated in the longitudinal direction of the tube  101  are preferably provided for guiding the gas flow through discharge gap  106  and a portion of such gas into dedusting unit  202 . After exiting dedusting unit  202 , the gas returns to fan  201  to be recirculated through the laser  101 . A detailed description of a suitable dedusting unit  202  for use in connection with the present invention is provided in a concurrently filed application bearing Ser. No. 09/511,649 which is hereby incorporated by reference. The filing details of this application are provided above. 
     The ground electrode  105  is preferably carried by, or mounted to, the electrode plate  111  via a plurality of flow guides  209 , which will be referred to again later. 
     Adjacent to the high voltage electrode  104 , two pre-ionizers  206  are provided, which serve to pre-ionize the laser gas to ensure greater homogeneity of the gas discharge in the discharge gap  106 . 
     The pre-ionizers  206  are preferably corona-type pre-ionizers and extend substantially parallel to said high voltage electrode. The pre-ionizers  206  have a coaxial shape with a conductive core  207  surrounded by a tube shaped insulator  208 . 
     The corona-type pre-ionizers can be mounted immediately adjacent to the high voltage electrode. In particular, as shown in FIG. 2, the corona-type pre-ionizers should be mounted at the opposing edges of the high voltage electrode so that it is disposed adjacent the electrode face of the high voltage electrode facing the ground electrode. 
     Although corona-type pre-ionizers are preferred for use as pre-ionizers  206  in connection with the present invention, those skilled in the art will recognize that any of the pre-ionizers known in the art may be used. Furthermore, the insulator of the pre-ionizers is preferably a ceramic material such as alumina. It can also be a fluoride material. Alternatively, any other kind of known pre-ionizer can be used. The pre-ionizers are not necessary for the discharge unit to work. Indeed, excimer lasers were known before the invention of pre-ionizers. Pre-ionization, however makes the gas discharge between the high voltage electrode and the ground electrode more homogeneous and thus more reliable. 
     Referring to FIGS. 3 a  and  3   c  the discharge unit  102  comprises three coaxial waveguide-like high voltage ducts  107 , extending through holes in the electrode plate  111 . The ducts  107  are arranged spaced apart from each other. The holes and the ducts  107  have a circular cross section, as can be seen from FIG. 3 c.  Each of the three ducts  107  is inserted into the respective hole in the electrode plate  111  with a defined tolerance between the insulator element and the hole. As those skilled in the art will appreciate, the number of ducts employed in a particular gas laser  100  will depend on the overall length of the laser. 
     The ground electrode  105  is preferably carried by, or mounted to, the electrode plate  111 . As best seen in FIGS. 2 and 3 a,  preferably a plurality of flow guides  209  are used for this purpose. 
     The flow guides  209  are preferably made from sheets of metal that extend between the electrode plate and the ground electrode in a plane perpendicular to the longitudinal axis of the electrodes  104 ,  105 . The flow guides plates  209  comprise respectively an upper flange  301 , a lower flange  303 , and a central flow-guiding portion  302  integrally connecting said upper flange  301  to said lower flange  303 . Said upper and lower flanges  301 ,  303  extend perpendicular to each other and to said central flow-guiding portion  302 . The upper flange  301  is attached to a side face  304  of electrode plate  111 , and the lower flange  303  is attached to a bottom face  305  of the ground electrode  105 . The central flow-guiding portion  302  is preferably aerodynamically profiled in order to minimize flow resistance and turbulences for maintaining a substantially laminar gas flow between the flow guides. 
     The lower flange  303  preferably includes an oblong hole  306  (shown only at part of the flow guides  209 ). Hole  306  is oblong in a direction perpendicular to the longitudinal axis of the elongated ground electrode  105 . A screw, or other fastening means,  307  is inserted through the hole  306  into a mating threaded hole  308  being provided in the ground electrode  105 . The oblong hole  306  allows for adjustments of the ground electrode  105  with respect to the high voltage electrode  104  essentially in the direction indicated by the double-headed arrow  320  in FIG. 3 c.    
     The upper flange  301  preferably includes an oblong hole  309 . Hole  309  being oblong in a direction perpendicular to the longitudinal axis of the electrode plate  111 . A screw, or other fastening means,  310  is inserted through the hole  309  into a mating threaded hole  311  being provided in the high voltage electrode  104 . The oblong hole  309  allows an adjustment of the ground electrode  105  with respect to the high voltage electrode  104  essentially in the direction indicated by the double-headed arrow  322  in FIG. 3 a.    
     FIG. 4 shows a cross section of the discharge unit  102  according to the preferred embodiment of the invention. In particular, FIG. 4 shows an enlarged cross-sectional view of the discharge unit shown in FIG.  2 . The angle of view is the same as in FIG. 3 b.    
     Each high voltage duct  107  of the laser discharge unit  102  preferably further comprises a sleeve  401  enclosing the core  108  and insulator  110 . Sleeve  401  has an inner end  402  supported by the electrode plate  111 , and an outer free end  403 . The core  108  has an inner end  404  connected to the high voltage electrode  104  and a threaded outer free end  405  extending beyond the free end  403  of the sleeve  401 . A nut  406  may be screwed onto the threaded end  405  as shown in FIGS. 3 c  and  4 , thereby pressing the sleeve  401  against the electrode plate  111  and tensioning the core  108  by pulling it. Preferably a washer  450  is interposed between nut  406  and sleeve  401  to evenly distribute the stresses applied by nut  406  to sleeve  401 . A threaded stud bolt  112  is used to connect the inner end  404  of the core  108  to the high voltage electrode  104 . 
     The inner end  404  of the core  108  is provided with a core ring shoulder  408 , which is pressed against the ceramic insulator element  110  when core  108  is placed under tension. A seal  409  is preferably provided between the ring shoulder  408  and the ceramic insulator element  110 . 
     The ceramic insulator element  110  is also caused to be pressed against the electrode plate  111  by means of the tensioned core  108  via the core ring shoulder  408  at the inner end  404  of the core  108 . Preferably a ring shoulder  410  is provided on the insulator element  110  and another seal  411  is provided between the ceramic insulator ring shoulder  410  and the electrode plate  111 . 
     To provide additional sealing, a sealing ring  412  (see also FIGS. 2 and 3 c ) preferably surrounds each sleeve  401 . Sealing ring  412  may be constructed to have a flange  413  at its outer circumference. Flange  413  is dimensioned so that it is supported by an outer rim  414  of the holes  150  in the tube  101  through which the respective ducts  107  are inserted. Electrode plate  111  is then preferably provided with a ring shoulder  417  facing an inner rim  415  of the tube  101 . A metal seal  416  is preferably interposed between shoulder  417  and rim  415 . As a result, when the ring  412  and the electrode plate  111  are connected by screws  113  a gas tight seal is provided between the shoulder  417  and the inner rim  415  of the tube  101 . 
     All of the seals  409 ,  411  and  416  are ring-shaped metal seals in the present embodiment. However, those skilled in the art will appreciate that the invention is not limited to using ring-shaped seals. 
     As noted above, the ground electrode  105  is carried by the electrode plate  111  via a plurality of flow guides  209 . Flow guides  209  are preferably made from sheets of metal that extend between the electrode plate and the ground electrode in a plane perpendicular to the longitudinal axis of the electrodes. The flow guides comprise an upper flange, a lower flange  303 , and a central flow-guiding portion  302  integrally connecting the upper flange  301  to the lower flange. The upper and lower flanges extend perpendicular to each other and to the central flow-guiding portion  302 . The upper flange  301  is attached to a side face  304  of the electrode plate  111 , and the lower flange  303  is attached to a bottom face of the ground electrode  105 . Preferably the central flow-guiding portion  302  is aerodynamically profiled in order to minimize flow resistance and turbulences for maintaining a substantially laminar gas flow in the gas discharge region  106 . 
     As will be understood by one skilled in the art, the invention may be embodied in other specific forms without departing from the spirit and the scope of the invention. The description of the embodiment is given as an illustrative example only and should not be understood as a limitation of the invention, which is set forth in the following claims.