Abstract:
A movable electrode assembly for use in laser system includes a first electrode, a second electrode arranged opposite from the first electrode, the second electrode being spaced apart from the first electrode by a discharge gap and a discharge gap adjuster interfaced with at least one of the second electrode or the first electrode, the discharge gap adjuster configured to adjust the discharge gap. A movable electrode assembly for integration into a housing of a laser system includes a first electrode having a discharge surface, a second electrode having a discharge surface, such that the discharge surface of the first electrode and the discharge surface of the second electrode face each other in a spaced apart setting that defines a desired discharge gap, and a mechanism for moveably adjusting the spaced apart setting toward the desired discharge gap. A method of adjusting a discharge gap is also disclosed.

Description:
PRIORITY CLAIM 
     This application is a continuation of and claims priority from U.S. Pat. No. 7,856,044, filed on Apr. 16, 2007 and issued on Dec. 21, 2010, and entitled “Extendable Electrode For Gas Discharge Laser,” which is incorporated herein by reference in its entirety. The U.S. Pat. No. 7,856,044 is a continuation-in-part application of co-pending, co-owned U.S. Pat. No. 7,218,661, Entitled “Line Selected F.sub.2 Two Chamber Laser System” which issued on May 15, 2007 and which is a continuation of U.S. Pat. No. 6,801,560, Entitled “Line Selected F.sub.2 Two Chamber Laser System”, which issued on Oct. 5, 2004, which is a continuation-in-part of U.S. Pat. No. 6,414,979, Entitled “Gas Discharge Laser with Blade-Dielectric Electrode”, which issued on Jul. 2, 2002, the entire contents of each of which are hereby incorporated by reference herein for all purposes. 
     RELATED APPLICATIONS AND PATENTS 
     The present application is related to U.S. Pat. No. 6,466,602, Entitled “Gas Discharge Laser Long Life Electrodes”, which issued on Oct. 15, 2002, the entire contents of each of which are hereby incorporated by reference herein for all purposes. 
    
    
     BACKGROUND 
     The present application relates generally to gas discharge laser systems. The present application is particularly, but not exclusively useful as an extendable electrode system for a transverse discharge gas laser. 
     Electrode erosion in high-pressure transverse discharge lasers is usually the mechanism that limits their operational lifetime. The erosion of one or both of the electrodes is typically caused by the combined attack of fast ions and electrons from the current discharge. As the electrodes wear, the inter-electrode spacing increases to the point where the operational characteristics of the laser are so severely affected that laser operation must be stopped. The gain generator must then be refurbished with new electrodes in order to re-establish the correct electrode spacing. 
     In an attempt to increase laser lifetime, Japanese Patent Application JP06-029592 filed on Jun. 10, 1991 and titled “Discharge-Pumped Laser” discloses a scheme “to regulate an interval between electrodes in accordance with consumption of a discharge part of the electrode and to always hold a discharging width constant by providing moving means for at least one of discharge electrode pair toward the other electrode.” However, to applicant&#39;s knowledge, such a relatively simplistic system has yet to be successfully commercialized. 
     Since 1991 when Japanese Patent Application JP06-029592 was filed, gas discharge lasers have evolved substantially. Modern transverse discharge lasers are now designed to produce a relatively high power output (having both a relatively high pulse energy and high pulse repetition rate) with relatively tight specifications on beam properties such as bandwidth and pulse-to-pulse energy stability, to name just a few. To achieve this performance, modem transverse discharge lasers typically include complex, highly engineered discharge chambers. For example, a relatively low impedance, low inductance current path geometry is typically provided in the chamber to conduct the extremely high peak currents that are generated by an electrical drive circuit to the electrodes. Also, the chamber may need to provide suitable heat transfer paths, for example, to prevent component overheating, and in particular, electrode overheating. In addition to heat transfer paths, the chamber may need to provide suitable gas flow paths to reduce gas flow turbulence and ensure that a fresh quantity of laser gas is positioned between the electrodes prior to the initiation of the next discharge. Concurrent with the above-described engineering constraints, the chamber may need to provide suitable component geometries which prevent or minimize the impact of reflected acoustic waves which can reach the discharge area and adversely affect properties of the output laser beam such as bandwidth, divergence, etc. 
     With the above considerations in mind, Applicants disclose an extendable electrode system for a gas discharge laser. 
     SUMMARY 
     Disclosed herein are systems and methods for extending one or both of the discharge electrodes in a transverse discharge laser chamber in which the electrodes are subject to a dimensional change due to erosion. Electrode extension can be performed to increase the chamber life, increase laser performance over the life of the chamber, or both. Operationally, the inter-electrode spacing may be adjusted to maintain a specific target gap distance between the electrodes or to optimize a specific parameter of the laser output beam such as bandwidth, pulse-to-pulse energy stability, beam size, etc. 
     As disclosed herein, control of the inter-electrode spacing may be effectuated in several different ways. In one implementation, the inter-electrode spacing may be visually observed and the observation used to move one or both of the electrodes. For example, a technician may manually instruct a laser system controller via keypad or graphic user interface to signal an actuator, which in turn, produces the desired inter-electrode spacing adjustment. 
     In another implementation, the inter-electrode spacing may be adjusted using a feedback loop. For example, a controller may be provided to monitor a device parameter and generate a control signal indicative of the parameter. For use with the controller, an actuator may be operably coupled with one or both of the electrodes, the actuator responsive to the control signal to move one or both of the electrodes and adjust the inter-electrode spacing. For this implementation, the parameter may be provided to the controller by an on-board measuring instrument or other laser component as described below. The parameter can include, but is not necessarily limited to wavelength, bandwidth, pulse-to-pulse energy stability, beam size, accumulated pulse count, average historical duty cycle, a measured relationship between discharge voltage and pulse energy, or combinations thereof. 
     In a particular implementation, a controller may be programmed to scan the inter-electrode spacing over a pre-determined spacing range. During the scan, a measuring instrument or other laser component may provide one or more parameter inputs to the controller allowing the controller to determine a relationship between the parameter and the inter-electrode spacing. From the relationship, the controller may deduce an optimum inter-electrode spacing and thereafter adjust the inter-electrode spacing accordingly. 
     Several mechanisms capable of being coupled to an electrode to produce an actuator-driven, electrode movement are disclosed herein. In one mechanism, a first elongated rigid member having sawtooth ramp structure and a second elongated rigid member having complimentary sawtooth ramp structure are provided. The ramp structures are aligned longitudinally and placed in contact with each other. The first rigid member may be attached to an electrode and the second rigid member attached to an actuator such that movement of the actuator translates the second rigid member in the direction of member elongation. With this structural arrangement, longitudinal movement of the second rigid member causes a movement of the first rigid member (and the attached electrode) in a direction normal to the direction of member elongation. Other electrode movement mechanisms are disclosed in further detail below including a cam-operated mechanism and a screw-operated mechanism. 
     For use in conjunction with one or more of the electrode movement mechanisms described above, a conductive, flexible member may be provided for electrically shielding moving parts and/or contact surfaces of the mechanism from the fields generated during an electrode discharge. For example, the flexible member may extend from a first flexible member edge that is attached to one of the electrodes for movement therewith to a second flexible member edge that is held fixed relative to the housing. In some cases, the flexible member may be formed with one or more convolutions that are aligned parallel to the direction of electrode elongation to impart flexibility to the member. In one embodiment, the second edge of the flexible member may be electrically connected to a plurality of so-called “current tines” which provide a low impedance path from the moveable electrode to a pulse power supply. 
     In another implementation, a movable electrode assembly for use in laser system includes a first electrode, a second electrode arranged opposite from the first electrode, the second electrode being spaced apart from the first electrode by a discharge gap and a discharge gap adjuster interfaced with at least one of the second electrode or the first electrode, the discharge gap adjuster configured to adjust the discharge gap. 
     In another implementation, a movable electrode assembly for integration into a housing of a laser system includes a first electrode having a discharge surface, a second electrode having a discharge surface, such that the discharge surface of the first electrode and the discharge surface of the second electrode face each other in a spaced apart setting that defines a desired discharge gap, and a mechanism for moveably adjusting the spaced apart setting toward the desired discharge gap. 
     In another implementation, a method of adjusting a discharge gap includes moving a first elongated member longitudinally relative to a second elongated member, the first elongated member having a first inclined face, the first inclined face being inclined longitudinally along the first elongated member, the second elongated member having a second inclined face, the second inclined face being inclined longitudinally along the second elongated member, the second inclined face being substantially complimentary to the first inclined face, wherein a second electrode is coupled to the first elongated member and a first electrode is opposite from the second electrode, the second electrode being separated from the first electrode by a discharge gap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. 
         FIG. 1  shows a simplified, perspective, partially exploded view of a transverse discharge gas laser. 
         FIG. 2  shows a simplified schematic view of a multi-stage laser system. 
         FIGS. 3A-D  each schematically show a pair of electrodes viewed as seen along line  3 A- 3 A in  FIG. 1  with  FIG. 3A  showing the electrodes in their initial positions prior to erosion,  FIG. 3B  showing the electrodes after erosion,  FIG. 3C  showing the electrodes after erosion and after one of the electrodes has been moved to adjust the inter-electrode spacing and  FIG. 3D  showing the case where one electrode is moved into the initial electrode gap to accommodate erosion of the other electrode. 
         FIGS. 4A-G  show the components of a mechanism that may be coupled to an electrode to produce an actuator-driven, electrode movement, where  FIGS. 4A and 4B  schematically show a pair of electrodes viewed as seen along line  3 A- 3 A in  FIG. 1  with  FIG. 4A  showing the electrode in a retracted state relative to the electrode support bar and  FIG. 4B  showing the electrode in an extended state relative to the electrode support bar;  FIGS. 4C and 4D  show perspective, simplified views of a rigid sawtooth structure and a complementary rigid sawtooth structure, respectively;  FIGS. 4E and 4F  show a moveable electrode viewed as seen along line  4 E- 4 E in  FIG. 1  with  FIG. 4E  showing the electrode in a retracted state relative to the electrode support bar and  FIG. 4F  showing the electrode in an extended state relative to the electrode support bar; and  FIG. 4G  shows a linkage including a push rod and pivoting lever for establishing a mechanical path between an actuator and a rigid sawtooth structure. 
         FIG. 5  shows a perspective view of a moveable electrode assembly illustrating a flexible conductive member electrically connecting the moveable electrode to a plurality of current return tines. 
         FIGS. 6A and 6B  show the components of another mechanism having a camshaft that may be coupled to an electrode to produce an actuator-driven, electrode movement, where  FIGS. 6A and 6B  schematically show a pair of electrodes viewed as seen along line  3 A- 3 A in  FIG. 1  with  FIG. 6A  showing the electrode in a retracted state relative to the electrode support bar and  FIG. 6B  showing the electrode in an extended state relative to the electrode support bar. 
         FIGS. 7A-E  show the components of drive screw mechanisms that may be coupled to an electrode to produce an actuator-driven, electrode movement, where  FIGS. 7A and 7B  schematically show a pair of electrodes viewed as seen along line  3 A- 3 A in  FIG. 1  with  FIG. 7A  showing the electrode in a retracted state relative to the electrode support bar and  FIG. 7B  showing the electrode in an extended state relative to the electrode support bar;  FIGS. 7C-7E  show a moveable electrode viewed as seen along line  4 E- 4 E in  FIG. 1  with  FIG. 7C  showing a mechanism having a single drive screw,  FIG. 7D  showing a mechanism having two drive screws, and  FIG. 7E  showing a mechanism having a three drive screws. 
         FIGS. 8A and 8B  show the components of a device having moveable flow guides to accommodate extension of electrodes having non-parallel sidewalls, where  FIGS. 8A and 8B  schematically show a pair of electrodes viewed as seen along line  3 A- 3 A in  FIG. 1  with  FIG. 8A  showing the electrode in a retracted state relative to the electrode support bar and  FIG. 8B  showing the electrode in an extended state relative to the electrode support bar. 
         FIG. 9  shows a moveable electrode viewed as seen along line  4 E- 4 E in  FIG. 1  having an electrode end contour to accommodate electrode extension. 
     
    
    
     DETAILED DESCRIPTION 
     Referring initially to  FIG. 1 , a simplified, partially exploded view of portions of a transverse discharge gas laser device are shown and generally designated  20 . For example, the device  20  may be a KrF excimer laser, an XeF excimer laser, an XeCl excimer laser, an ArF excimer laser, a molecular fluorine laser or any other type of transverse discharge gas laser known in the pertinent. As shown, the device  20  may include a two-part chamber housing  22   a,b  being formed of a chamber wall that may be made of a conductive, corrosion resistant material, e.g., nickel-plated aluminum. As further. shown in  FIG. 1 , window assemblies  24   a,b  may be provided at each end of the chamber housing  22   a,b  to allow light to enter, exit and pass through the chamber housing  22   a,b  along a common beam path. With this structure, the hollow chamber housing  22   a,b  and window assemblies  24   a,b  may surround a volume which holds a laserable gas medium under pressure together with other components suitable to create a discharge in the medium. These other components may include, for example, a pair of discharge electrodes (not shown in  FIG. 1 ), a fan to circulate the gas (not shown in  FIG. 1 ), heat exchangers to cool the gas (not shown in  FIG. 1 ), etc. It is to be appreciated that the chamber housing  22   a,b  may also be formed with a number of sealed inlets/outlets (not shown in  FIG. 1 ), to allow gas to be introduced/extracted from the chamber, to allow conductors  26  to deliver an excitation voltage to the electrodes, etc. 
     In addition to the chamber,  FIG. 1  shows that the device  20  may also include a beam reverser  28  and outcoupler  30  cooperatively arranged to form an optical cavity. For the device  20 , the beam reverser  28  may be as simple as a flat, fully reflective mirror or as complex as a grating-based line-narrowing unit. It is to be appreciated that the use of a moveable electrode is not limited to the stable, standing wave cavity alluded to above. Instead, a transverse discharge gas laser chamber having one or more moveable electrodes may be employed within other optical arrangements such as a one-pass amplifier, multi-pass amplifier, traveling wave amplifier such as a ring amplifier, unstable cavities, etc. 
     Continuing with  FIG. 1 , the device  20  may also include a pulse power system delivering electrical pulses to electrodes located within the chamber housing  22   a,b  via conductors  26 . Although the description that follows will be provided with reference to a pulsed laser device, it is to be appreciated that some or all of the concepts disclosed herein may be equally applicable to continuous discharge gas laser devices which have electrodes that suffer a dimension change due to erosion or some other phenomenon.  FIG. 1  further illustrates that during operation of the device  20 , a laser beam  34  is created which exits the optical cavity via the outcoupler  30 . 
       FIG. 2  shows a multi-stage gas discharge laser device, generally designated  20 ′ to illustrate that the inter-electrode spacing may be independently (or in some cases dependently) adjusted in one, both or all of the laser device chambers of a multi-stage device. For example, the first stage may be either a power oscillator, PO or a master oscillator, MO. Typically, an oscillator is referred to as an MO if more than about a third of the total laser output power is produced in the initial oscillation cavity and is referred to as a PO if less than about a third of the total output power is produced in the initial oscillation cavity. Subsequent stage(s) may be, for example, a one-pass power amplifier, a multi-pass power amplifier, a power oscillator or a traveling wave amplifier such as a ring amplifier. It is to be appreciated that a multi-stage device may include some or all of the components shown in  FIG. 2 , depending on the configuration. The components shown in  FIG. 2  include a beam reverser  28 ′, first stage chamber  50 , first stage outcoupler  30 ′, turning optics  52   a,b , incoupler  54 , second stage chamber  56  and second stage outcoupler  58 . 
     Inter-Electrode Spacing Adjustment 
       FIGS. 3A-D  illustrate how electrode dimensional changes associated with erosion can affect the inter-electrode spacing and how the movement of one electrode relative to the other may re-establish a more desirable inter-electrode spacing. In more detail,  FIG. 3A  shows the initial electrode positions (prior to erosion) with electrode  60  spaced from electrode  62  to establish an initial inter-electrode spacing  64 .  FIG. 3B  illustrates the electrodes  60 ,  62  after significant electrode erosion has occurred resulting in inter-electrode spacing  66  (note initial inter-electrode spacing  64  is shown for reference purposes).  FIG. 3C  illustrates the electrodes  60 ,  62  after significant electrode erosion has occurred ( FIG. 3B ) and after electrode  62  has been moved in the direction of arrow  68  resulting in an inter-electrode spacing that is close to the initial inter-electrode spacing  64 .  FIG. 3D  illustrates that the electrode  62  may be moved to a position where its discharge surface extends into the initial electrode gap (illustrated by the dotted lines) to accommodate erosion of electrode  60 . 
       FIGS. 3A-D  illustrate the case of asymmetric electrode erosion. In particular, it is clear from  FIG. 3B  that electrode  62  has eroded about 10 times more than electrode  60 . For this case, movement of electrode  62  may be sufficient by itself (movement of electrode  60  may not be required) to provide the desired inter-electrode spacing correction. This type of asymmetric electrode wear is common in certain types of transverse discharge gas lasers such as some high-power, high repetition rate, excimer lasers where the anode (the electrode electrically connected to the housing) typically erodes at a rate much greater than the cathode. Although  FIGS. 3A-D  illustrate asymmetric electrode erosion, it is to be appreciated that one or both of the electrodes may be moved to provide an inter-electrode spacing correction for a device which experiences symmetric electrode wear. For systems where both electrodes are moveable, the electrodes may be moved to set a desired inter-electrode spacing and/or may be used to move the discharge region relative to the other optics and apertures in the system. Thus, the electrode movement system may be used as an alignment tool to adjust the beam footprint relative to one or more system apertures/optics. 
     Inter-Electrode Spacing Control 
     For the device  20  shown in  FIG. 1 , the control of the inter-electrode spacing may be effectuated in several different ways. In perhaps the simplest implementation, the inter-electrode spacing may be visually observed, for example by looking through one of the windows  24   a,b , and the observation used to move one or both of the electrodes. For example, a technician may use the observation to instruct a laser system controller  70  via keypad or graphic user interface (or any other controller input device known in the art) to signal an actuator  72 , which in turn, may drive a mechanism (see description below) to produce the desired inter-electrode spacing adjustment. For this purpose, a linkage  74  may pass through the wall of the chamber housing  22   a,b , and a flexible bellows  76  (or other suitable arrangement known in the pertinent art) may be provided to prevent laser gas from exiting the chamber housing  22   a,b . It is to be appreciated that portions (memory, processor, etc) or all of the controller  70  may be integral with (e.g. shared) or separate from a main laser system controller which controls other laser functions such as discharge voltage, timing, shutter activation, etc. 
     In another implementation, the inter-electrode spacing may be adjusted based on a monitored device parameter. For example, the device  20  may monitor one or more device parameters such as accumulated pulse count, average historical duty cycle, wavelength, gas pressure, running voltage, bandwidth, pulse-to-pulse energy stability (sometimes referred to as sigma), beam size, or a measured relationship between discharge voltage and pulse energy. The device parameter(s) may be selected to predict the extent of electrode erosion (pulse count, average historical duty cycle, etc.) and/or may be selected to tune the laser device to produce an output beam having a desired characteristic (bandwidth, pulse-to-pulse energy stability, etc.). 
     As shown in  FIG. 1 , one or more of these device parameters may be monitored by measuring a property of the output laser beam  34  using a measuring instrument  78 . A control signal indicative of the device parameter may then be output from the instrument  78  and transmitted to the controller  70 , which in turn, provides a signal to the actuator  72 . Some device parameters, such as accumulated pulse count, average historical duty cycle, etc, may be provided to the controller  70  or generated within the controller  70  without the use of a measuring instrument. Thus, in at least some implementations envisioned herein, a measuring instrument  78  may not be required. One the other hand, more than one parameter (i.e., a plurality of device parameters) may be communicated to, or developed within, the controller  70  for processing in an algorithm to determine an appropriate inter-electrode spacing adjustment. 
     In a particular implementation, a controller may be programmed to scan the inter-electrode spacing over a pre-determined spacing range. Thus, the inter-electrode spacing may be adjusted either continuously or incrementally while the laser is operating and outputting laser pulses. During the scan, a measuring instrument or other laser component may provide one or more parameter inputs to the controller allowing the controller to determine a relationship between the parameter(s) and the inter-electrode spacing. From the relationship, the controller may then deduce an optimum inter-electrode spacing and thereafter adjust the inter-electrode spacing accordingly. 
     Inter-Electrode Spacing Mechanisms 
       FIGS. 4A-E  show the components of a first mechanism that may be coupled to an electrode  80  to produce an actuator-driven, electrode movement. For the mechanism, a first elongated rigid member  82  having sawtooth ramp structure and a second elongated rigid member  84  having complimentary sawtooth ramp structure are disposed within a channel formed in an electrode support bar  86 , as shown in  FIG. 4A  (which shows the electrode  80  in a fully retracted position) and  4 B (which shows the electrode  80  in a fully extended position). For the device, the electrode support bar  86  is typically elongated, like the electrode and is affixed at its ends to the wall of the housing  22   a,b  (see  FIG. 1 ). 
     Perspective views of elongated rigid members  82  and  84  are shown in  FIGS. 4C and 4D , respectively. As seen there, elongated rigid member  82  is formed with a plurality of inclined, parallel surfaces, (of which surfaces  88   a - c  have been labeled) and an opposed flat surface  90 . Somewhat similarly, elongated rigid member  82  is formed with a plurality of complementary, inclined, parallel surfaces, of which surfaces  92   a - c  have been labeled) and an opposed flat surface  94  which includes a raised flat portion  96  onto which the electrode  80  may be affixed (see  FIGS. 4A and 4E ). Although  FIG. 4C  illustrates a rigid member  82  having about 20 inclined surfaces for a 30 cm electrode, it is to be appreciated that more than 20 and as few as one inclined surface may be used. 
     For the mechanism, as best seen in  FIGS. 4E and 4F , the rigid members  82 ,  84  are aligned longitudinally (i.e., each aligned parallel to the direction of electrode elongation shown by arrow  98 ) and placed in contact with each other. Specifically, each inclined surface  88   a - c  of rigid member  82  is placed into sliding contact with a corresponding inclined surface  92   a - c  of rigid member  84 . As further shown in  FIG. 4E , spring assemblies  100   a,b  may be employed to maintain a preselected contact pressure between the inclined surfaces  88   a - c  of rigid member  82  and inclined surfaces  92   a - c  of rigid member  84 . 
       FIGS. 4E and 4F  illustrate the movement of electrode  80  in response to a movement of the elongated member  82  relative to the electrode support bar  86  with  FIG. 4E  showing the electrode  80  in a fully retracted position and  4 F showing the electrode  80  in an extended position. Comparing  FIG. 4E  with  4 F, it can be seen that a movement of elongated rigid member  82  relative to the electrode support bar  86  in the direction of arrow  102  will result in a movement of the electrode  80  and elongated rigid member  84  in the direction of arrow  104 . Similarly, movement of elongated rigid member  82  relative to the electrode support bar  86  in the direction opposite arrow  102  will result in a movement of the electrode  80  and elongated member  84  in the direction opposite arrow  104  with the spring assemblies maintaining contact between the inclined surfaces  88   a - c ,  92   a - c.    
       FIG. 4G  shows a mechanism linkage which includes a substantially straight push rod  106  and an L-shaped pivoting lever  108  for establishing a mechanical path between the actuator  72  and rigid sawtooth structure  82 . With this arrangement, movement of the push rod  106  in the direction of arrow  110  will cause the rigid sawtooth structure  82  to move in the direction of arrow  102  (arrow  102  also shown in  FIG. 4F ). This in turn will cause a movement of the electrode  80  in the direction of arrow  104  as shown in  FIG. 4F . Note: the rigid member  82  is disposed in a similarly sized channel formed in the support bar  86  and as such is laterally constrained therein (see  FIG. 4A ). 
       FIG. 4G  further shows that the actuator  72  may be affixed to the wall of the chamber housing  22   a  and operable attached to a first end  112  of the push rod  106 . Push rod  106  then extends through an opening in the wall of the chamber housing  22   a  to a second push rod end  114  which is disposed inside the chamber. Flexible bellows  76  may be provided to maintain gas pressure within the chamber while allowing translation of the push rod  106 . Also shown, second push rod end  114  may be pivotally attached, for example using a pin/cotter key arrangement (or any similarly functioning arrangement known in the art), to the L-shaped lever  108 , which in turn is pivotally attached near its midsection to the electrode support bar  86  at pivot point  116 . End  118  of lever  108  may be pivotally attached to rigid member  82 , as shown. A simpler design may be employed in which the push rod is aligned parallel to the rigid member and attached directly thereto, however, use of the L-shaped lever  108  allows for motion amplification/de-amplification depending on the relative lengths of the lever arms. If desired, the actuator may be replace by a drive screw (not shown) or similar component allowing for manual adjustment of the inter-electrode spacing. Another alternative to the push rod/lever system is to use a pulley/cable system to move the rigid member  82  within channel. For this alternative, the member  82  may be biased away from the pulley using a spring attached to the support bar. 
     Flexible Conductive Member 
     As best seen cross-referencing to  FIGS. 4A and 5 , a conductive, flexible member  120  may be provided for electrically shielding some or all of the inter-electrode spacing mechanism components and/or electrically connecting the electrode  80  to the current return tines  122   a - c  and/or constraining the electrode  80  and rigid member  84  from longitudinal movement (i.e., movement in the direction of arrow  98  in  FIG. 4E ) and/or to provide a thermally conductive path allowing heat to flow from the electrode to the support bar. In some applications, contacting surfaces of the electrode spacing mechanism may arc, and in extreme cases weld together, if unshielded in the presence of the electric fields generated by discharge. 
     As shown in  FIGS. 4A and 5 , the flexible member  120  may have a first flexible member edge  124  that is attached to electrode  80  and/or rigid member  84  for movement therewith (note: for the embodiment shown, the edge  124  is clamped between electrode  80  and rigid member  84 ). Typically, the flexible member  120  is made of a conductive metal such as copper or brass allowing the flexible member  120  to conduct heat and/or electricity from the electrode  80  to the support bar  86 /current return tines  122   a - c.    
       FIGS. 4A and 5  also show that the flexible member  120  may have a second edge  126  that is attached to the support bar  86  and thus, may be held fixed relative to the housing  22   a  (see  FIG. 1 ).  FIG. 4A  further illustrates that the edge  126  of the flexible member  120  may be electrically connected to the current return tines  122  establishing an electrical path from the electrode  80  to the tines  122 . The current tines, in turn, provide a relatively low impedance path from the flexible member  120  to a pulse power system  32  (shown in  FIG. 1 ). For the device shown, the flexible member  120  may be formed with one or more convolutions  128   a - c , e.g. bends, that are aligned parallel to the direction of electrode elongation (i.e. the direction of arrow  98  in  FIG. 4E ) to impart flexibility to the member. With this arrangement, the flexible member  120  may be described as having a planar, corrugated shape. 
     As described above, the flexible member  120  may function to electrically shield some or all of the inter-electrode spacing mechanism components and/or electrically connect the electrode  80  to the current return tines  122   a - c  and/or constrain the electrode  80  and rigid member  84  from longitudinal movement. Although a flexible member  120  may be designed to achieve all of these functions, it is to be appreciated that some applications may not require all three functions. For example, for some discharge power levels, shielding may not be required. Moreover, one or more of the three functions may be performed by another component. For example, longitudinal constraint of the electrode  80  may be performed in a different manner allowing a flexible member  120  which lacks the strength necessary to constrain the electrode  80 . Other arrangements may be provided which perform one or more of the functions described above including a member whose flexibility is derived from its thickness, a plurality of spaced apart flexible members and tines having one or more convolutions. 
     One feature of the structural arrangement shown in  FIGS. 4A-G  and  5  is that the inter-electrode spacing can be adjusted without moving the electrode support bar  86  relative to the other laser components, e.g., fan, housing, etc.). This allows a close tolerance between the support bar  86  and other structures to be maintained. For example, in some applications, a close tolerance between the support bar  86  and a fan (not shown) may be maintained allowing the fan to run more efficiently. 
     Another feature of the structural arrangement shown in  FIGS. 4A-G  and  5  is that a substantial heat transfer path is provided between the electrode  80  and the support bar  86 . In particular, the relatively large contact area between the rigid member  82  and rigid member  84  and the relatively large contact area between the rigid members  82 ,  84  and the support bar cooperate to provide a substantial heat transfer path. For some applications, this path may be useful in preventing overheating of the electrode  80 . 
     Another feature of the structural arrangement shown in  FIGS. 4A-G  and  5  is that it maintains a relatively good parallelism between electrodes over the range of electrode movements. 
       FIGS. 6A and 6B  show an alternative mechanism in which a camshaft  150  may be rotated about a rotation axis  152  (which may be generally parallel to the direction of electrode elongation) to provide electrode extension with  FIG. 6A  showing the electrode  154  in a fully retracted position and  6 B showing the electrode  154  in an extended position. For the mechanism, the camshaft  150  may be in direct contact with the electrode  154  (with or without a thermally conductive rigid member establishing a heat path from the electrode  154  to the support bar  158 ) or, as shown, a thermally conductive rigid member  156  may be interposed between the electrode  154  and camshaft  150  and used to conduct heat from the electrode  154  to the support bar  158 . For the mechanism shown in  FIG. 6 , a flexible member  160  (as described above may be used to electrically shield some or all of the inter-electrode spacing mechanism components and/or electrical connect the electrode  154  to the current return tines  162  and/or provide a heat conduction path from the electrode  154  to the support bar  158 . For the device, the camshaft  150  may be rotated manually or by an energized actuator and may be controlled by any of the techniques/structural arrangements described above. 
       FIG. 7A-7E  show alternative mechanisms which include one or more drive screws  170  to provide electrode extension with  FIG. 7A  showing the electrode  172  in a fully retracted position and  7 B showing the electrode  172  in an extended position. For the mechanism, the drive screw(s)  170  may be in direct contact with the electrode  172  (with or without a thermally conductive rigid member establishing a heat path from the electrode  172  to the support bar  174 ) or, as shown, a thermally conductive rigid member  176  may be interposed between the electrode  172  and drive screw(s)  170  and used to conduct heat from the electrode  172  to the support bar  174 . 
     For the mechanism shown in  FIGS. 7A-7E , a flexible member  178  (as described above may be used to electrically shield some or all of the inter-electrode spacing mechanism components and/or electrical connect the electrode  172  to the current return tines  180  and/or provide a heat conduction path from the electrode  172  to the support bar  174 . Cross-referencing  FIGS. 7A and 7C , it may be seen that the drive screw(s)  170  may extend through the wall  182  and be threaded through a prepared hole (i.e., drilled, reamed and tapped) in the support bar  174 . Alternatively, a prepared hole may be provided in the wall  182  or some other structure or a nut (not shown) may be affixed to the wall  182  or support bar  174 . A flexible bellows as described above (not shown) may be employed at the wall  182  to prevent gas leakage from the chamber. For these mechanisms, each drive screw  170  may be rotated manually (from outside the chamber) or by an energized actuator  184  (shown with dashed lines to indicate an optional component) and may be controlled using one or more of the techniques/structural arrangements described above. Springs  186   a,b  may be provided to bias the electrode  172  relative to the support bar  174  as shown in  FIG. 7C . 
       FIG. 7D  illustrates a mechanism having two drive screws  170   a,b  that are spaced apart along the length of the electrode  172 ′ with each drive screw  170   a ,  170   b  independently rotatable manually (from outside the chamber) or by energized actuators  184   a ′,  184   b ′, respectively (shown with dashed lines to indicate an optional component). 
       FIG. 7E  illustrates a mechanism having three drive screws  170   c,d,e  that are spaced apart along the length of the electrode  172 ″ with each drive screw  170   c,d,e  independently rotatable manually (from outside the chamber) or by energized actuators  184   c′, d′, e ′, respectively (shown with dashed lines to indicate an optional component). 
     For the mechanisms having two or more drive screw(s)  170  ( FIGS. 7D and 7E ), each of the drive screws may be independently adjusted to adjust inter-electrode parallelism and/or inter-electrode spacing. Specifically, the drive screws may be independently adjusted until the electrode  172 ″ is parallel to the other electrode in the discharge pair (dotted lines in  FIG. 7D  showing an unaligned electrode and solid lines showing an electrode after alignment into parallel with another electrode). 
     For mechanisms having three or more drive screw(s)  170  ( FIG. 7E ), each of the drive screws  170  may be independently adjusted to adjust electrode parallelism (as described above) and/or electrode curvature and/or inter-electrode spacing. Specifically, the drive screws may be independently adjusted until the electrode  172 ″ has a desired curvature such as straight or having a curvature matching the other electrode (dotted lines in  FIG. 7D  showing a non-desired curvature and solid lines showing an electrode after a curvature adjustment). 
     Movable Flow Guides 
     Although the electrode  80  shown in  FIG. 4A  has substantially straight, parallel sidewalls, it is to be appreciated that other electrode shapes may be used in the devices described herein. For example,  FIGS. 8A and 8B  show an electrode  200  having a tapered construction (in a plane normal to the direction of electrode elongation) in which the electrode width, ‘w’, decreases gradually from the electrode base  202  to the initial discharge surface  204 . Other electrode designs can include an hourglass shape (not shown) in which the electrode width decreases from the base to a minimum and increases thereafter to the initial discharge surface. 
       FIGS. 8A and 8B  also show that flow guides  206   a,b , which may be made of an insulating ceramic material may be disposed surrounding the electrode  200  on each side to control the flow of gas over the tip of the electrode  200  and prevent the discharge from striking metal structures adjacent to the electrode  200 . For the case of electrodes having parallel sidewalls ( FIG. 4A ) these guides may be affixed to the support bar and may remain stationary with respect thereto. Comparing  FIG. 4A to 4B , it can be seen that extension of the electrode  80  with parallel sidewalls does not affect the spacing between the electrode sidewalls and stationary flow guides  206   a ′,  206   b ′. On the other hand, for electrodes having non-parallel sidewalls, such as electrode  200  in  FIG. 8A , electrode extension may affect the spacing between the electrode sidewalls and stationary flow guides  206   a,b.    
       FIGS. 8A and 8B  illustrate an arrangement in which the flow guides  206   a,b  are moveable attached to the support bar  208  allowing the flow guides  206   a,b  to move apart (in the direction of arrows  210   a,b  from one another as the electrode  200  is extended (in the direction of arrow  212 ). To effectuate this flow guide movement, each flow guide  206   a,b  is form with a surface  214  that contacts the electrode  200  and is inclined at an angle relative to the direction of electrode movement (arrow  212 ) For the arrangement shown, one or more springs (not shown) may be provided to bias each flow guide  206   a,b  toward the electrode  200 . 
     Electrode End Contour 
       FIG. 9  shows a pair of electrodes  218 ,  220  and illustrates an end contour for a moveable electrode  220 . As shown, electrode  218  is formed with a relatively flat portion  222  where discharge is desired and begins to curve away from the discharge region at point  224 . Electrode  220  is shown with the solid line indicating its initial electrode shape and the dashed line indicating it end-of-life shape. As shown, the electrode  220  is initially formed with a relatively flat portion  226  where discharge is desired, a curved transition section  228  and a second flat section  230 . As shown, the flat section  230  may be spaced at a distance ‘d.sub.1’ from the electrode base  232 , the beginning-of-life flat section  226  may be spaced at a distance ‘d.sub.2’ from the electrode base  232 , and the end-of-life flat section  234  may be spaced at a distance ‘d.sub.3’ from the electrode base  232 , with d.sub.2&gt;d.sub.1&gt;d.sub.3. In a particular embodiment, the electrode  220  is formed with d.sub.1=d.sub.3+n(d.sub.2−d.sub.3), where n is typically in the range of about 0.25 to 0.75, placing the flat section  230  between the average height of the electrode  220  over the electrode&#39;s life. For example, d.sub.2−d.sub.3 may be about 3 mm. One feature of the arrangement shown is that it confines the discharge to a selected discharge region (ending near point  224 ) over the life of the electrode  220 . 
     While the particular embodiment(s) described and illustrated in this patent application in the detail required to satisfy 35 U.S.C. .sctn.112 are fully capable of attaining one or more of the above-described purposes for, problems to be solved by, or any other reasons for or objects of the embodiment(s) above described, it is to be understood by those skilled in the art that the above-described embodiment(s) are merely exemplary, illustrative and representative of the subject matter which is broadly contemplated by the present application. Reference to an element in the following Claims in the singular is not intended to mean nor shall it mean in interpreting such Claim element “one and only one” unless explicitly so stated, but rather “one or more”. All structural and functional equivalents to any of the elements of the above-described embodiment(s) that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present Claims. Any term used in the Specification and/or in the Claims and expressly given a meaning in the Specification and/or Claims in the present Application shall have that meaning, regardless of any dictionary or other commonly used meaning for such a term. It is not intended or necessary for a device or method discussed in the Specification as an embodiment to address or solve each and every problem discussed in this Application, for it to be encompassed by the present Claims. No element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the Claims. No claim element in the appended Claims is to be construed under the provisions of 35 U.S.C. .sctn.112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited as a “step” instead of an “act”.