Abstract:
A laser system and method is disclosed which may comprise a first line narrowed gas discharge laser system producing a first laser output light pulse beam at a pulse repetition rate of ≧2000 Hz; a second line narrowed gas discharge laser system producing a second laser output light pulse beam at a pulse repetition rate of ≧2000 Hz; a beam combiner combining the first and second output light pulse beams into a combined laser output light pulse beam with a ≧4000 Hz pulse repetition rate. The apparatus and method may comprise a compression head comprising a storage device being charged at x times per second; a gas discharge chamber comprising at least two sets of paired gas discharge electrodes; at least two magnetically saturable switches, respectively connected between the compression head charge storage device and one of the at least two sets of paired electrodes.

Description:
[0001]     This application is a divisional of U.S. patent application Ser. No. 10/15,386 entitled “VERY HIGH REPETITION RATE NARROW BAND GAS DISCHARGE LASER SYSTEM” filed Mar. 31, 2004, the disclosure of which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to gas discharge lasers, e.g., used to provide narrow band light, e.g., for integrated circuit lithography purposes, which requires not only narrow band light but also high stability in such things as center wavelength and bandwidth over, e.g., large ranges of output pulse repetition rates and at very high pulse repetition rates.  
       BACKGROUND OF THE INVENTION  
       [0003]     The present application is related to U.S. Pat. No. 6,704,339, entitled LITHOGRAPHY LASER WITH BEAM DELIVERY AND BEAM POINTING CONTROL, with inventor(s) Lublin, et al., issued on Mar. 9, 2004, based on an application Ser. No. 10/233,253, filed on Aug. 30, 2002, U.S. Pat. No. 6,704,346, entitled LITHOGRAPHY LASER SYSTEM WITH IN-PLACE ALIGNMENT TOOL, with inventor(s) Ershov et al., issued on Mar. 9, 2004, based on an application Ser. No. 10/255,806, filed on Sep. 25, 2002, U.S. Pat. No. 6,690,704, entitled CONTROL SYSTEM FOR A TWO CHAMBER GAS DISCHARGE LASER, with inventor(s) Fallon et al., issued on Feb. 10, 2004, based on an application Ser. No. 10/210,761, filed on Jul. 31, 2002, U.S. Pat. No. 6,693,939, entitled SIX TO TEN KHZ, OR GREATER GAS DISCHARGE LASER SYSTEM, with inventor(s) Watson et al. issued on Feb. 17, 2004, based on an application Ser. No. 10/187,336, filed on Jun. 28, 2002, and United States Published Patent Application No. 2002/0191654A1, entitled LASER LITHOGRAPHY LIGHT SOURCE WITH BEAM DELIVERY, with inventor(s) Klene et al., published on Dec. 19, 2002, based on an application Ser. No. 10/141,216, filed on May 7, 2002, the disclosure of each of which is hereby incorporated by reference.  
         [0004]     The present application is also related to U.S. Pat. Nos. 6,625,191, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, issued to Knowles, et al. on Sep. 23, 2003, and 6,549,551, entitled INJECTION SEEDED LASER WITH PRECISE TIMING CONTROL issued to Ness, et al. on Apr. 15, 2003, and 6,567,450, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, issued to Myers, et al. on May 20, 2003, the disclosures of each of which is hereby incorporated by reference.  
       SUMMARY OF THE INVENTION  
       [0005]     A method and apparatus for producing a very high repetition rate gas discharge laser system in a MOPA configuration is disclosed which may comprise a master oscillator gas discharge layer system producing a beam of oscillator laser output light pulses at a very high pulse repetition rate; at least two power amplification gas discharge laser systems receiving laser output light pulses from the master oscillator gas discharge laser system and each of the at least two power amplification gas discharge laser systems amplifying some of the received laser output light pulses at a pulse repetition that is a fraction of the very high pulse repetition rate equal to one over the number of the at least two power amplification gas discharge laser systems to form an amplified output laser light pulse beam at the very high pulse repetition rate. The at least two power amplification gas discharge laser systems may comprise two power amplification gas discharge laser systems which may be positioned in series with respect to the oscillator laser output light pulse beam. The apparatus and method may further comprise a beam delivery unit connected to the laser light output of the power amplification laser system and directing to output of the power amplification laser system to an input of a light utilization tool and providing at least beam pointing and direction control. The apparatus and method may be a very high repetition rate gas discharge laser system in a MOPO configuration which may comprise: a first line narrowed gas discharge laser system producing a first laser output light pulse beam at a pulse repetition rate of ≧2000 Hz; a second line narrowed gas discharge laser system producing a second laser output light pulse beam at a pulse repetition rate of ≧2000 Hz; a beam combiner combining the first and second output light pulse beams into a combined laser output light pulse beam with a ≧4000 Hz pulse repetition rate. The apparatus and method may comprise a compression head comprising a compression head charge storage device being charged at x times per second; a gas discharge chamber comprising at least two sets of paired gas discharge electrodes; at least two magnetically saturable switches, respectively connected between the compression head charge storage device and one of the at least two sets of paired electrodes and comprising first and second opposite biasing windings having a first biasing current for the first biasing winding and a second biasing current for the second biasing winding and comprising a switching circuit to switch the biasing current from the first biasing current to the second biasing current such that only one of the at least two switches receives the first biasing current at a repetition rate equal to x divided by the number of the at least two sets of paired electrodes while the remainder of the at least two magnetically saturable switches receives the second biasing current. The apparatus and method may be utilized as a lithography tool or for producing laser produced plasma EUV light. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  shows a schematic view of a very high repetition rate laser system according to aspects of an embodiment of the present invention delivering light to a lithography tool;  
         [0007]      FIGS. 2A and 2B , respectively show a schematic side view and plan view of aspects of an embodiment of the present invention;  
         [0008]     FIGS.  3 A-C show schematically alternative embodiments of a solid state pulse power system module according to aspects of an embodiment of the present invention; and,  
         [0009]      FIG. 4  shows a timing diagram illustrative of a timing of firing between an oscillator laser and an amplifier laser according to aspects of an embodiment of the present invention;  
         [0010]      FIG. 5  shows partly schematically aspects of an embodiment of the present invention utilizing two parallel gas discharge regions;  
         [0011]      FIG. 6  shows schematically a compression head portion of a pulse power system according to aspects of an embodiment of the present invention useable with the embodiment of  FIG. 5 ; and,  
         [0012]      FIG. 7  shows schematically aspects of an embodiment of an optical system useable with the embodiment of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0013]     Turning now to  FIG. 1  there is shown a schematic view of a very high repetition rate laser system  10 . The laser system  10  may delivery light, e.g., DUV light, to a lithography tool, e.g., a scanner or stepper/scanner  12 . The light, e.g., DUV light, source may comprise, e.g., a two chamber laser system comprising, e.g., a master oscillator laser system  18 , the output of which is a narrow band laser output pulse beam  14 A. The master oscillator  18  system may comprise a master oscillator laser gas discharge chamber  18   c , an output coupler  18   a  and a line narrowing module  18 B together forming the oscillator cavity for the master oscillator laser system  18 .  
         [0014]     The system  10  may also comprise, e.g., a power amplification system  20 , which may comprise, e.g., a pair of power amplification laser chambers  20 A,  20 A 1  and  20 A 2 , which may, e.g., be in series with each other, such that the master oscillator laser system  18  output light pulse beam passes first through chamber  20 A 1  and then through chamber  20 A 2  (both of which could be formed into a single chamber  20 A) and to a beam reflector  20 B creating a second pass of the beam  14 A through the chamber(s)  20 A 1  and  20  A 2  in reverse order of the first pass to form power amplification system  20  output laser light pulse beam  14 B.  
         [0015]     The output beam  14 A may pass from the output coupler  18   a  of the master oscillator laser system  18  through a line center analysis module  27  that, e.g., measures the center wavelength of the narrow band light output of the master oscillator and then through a master oscillator wavefront engineering box, which may incorporate, e.g., relay optics or portions thereof to relay the output beam  14 A to a power amplification wavefront engineering box  26  that redirects the beam  14 A into the power amplification laser system  20  as explained in more detail below.  
         [0016]     The output of the power amplification laser system  20  may then pas through a spectral analysis module that, e.g., measures the bandwidth of the output beam  14 B and through a pulse stretcher  22 , comprising, e.g., multiple reflecting mirrors  22   a -D that may, e.g., increase the total integrated spectrum (“TIS”) of the output beam  14 B to form an output beam  14 C that may be, e.g., delivered to the lithography tool  12  through, e.g., a beam delivery unit  40 . The beam delivery unit  40  may comprise, e.g., mirrors  40 A and B at least one of which may be a fast acting beam directing mirror to modify, e.g., the beam direction and pointing of the output beam  14 C as it enters the lithography tool. A beam analysis module  38  may be positioned, e.g., essentially at the input of the light to the lithography tool  12 , e.g., measuring beam intensity, direction and pointing as it enters the lithography tool  12 .  
         [0017]     The lithography tool may have, e.g., beam intensity and quality detectors  44 ,  46 , that may, e.g., provide feedback to the laser system  10  controller (not shown) Similarly outputs from the LAM  27 , SAM  29  and BAM  38  may be used by the laser system control for such things as controlling charging voltage and/or firing timing between the MO and PA systems and gas injection into either or both of the MO and PA systems. The laser system may also include a purge gas system to purge one or more elements in the LAM  27 , SAM  28 , MOWEB  24 , PA WEB  26 , pulse stretcher  22  and/or beam delivery unit  40 .  
         [0018]     As shown schematically in  FIG. 2   a , the output beam  14 A from the MO  18  may pass through the output coupler  18 A and be reflected by an essentially totally reflecting mirror  24 A in the MO WEB  24  to another essentially totally reflecting mirror  26 B in the PA WEB  26 . It will be understood that the beam detector  16  in the PA WEB  26  is shown schematically out of place in the optical path of the output beam  14 B of the PA system  20  for clarity sake. Turning to  FIG. 2B  there is shown schematically the fact that in a top plan view, the mirror  26 B is slightly out of the optical axis of the PA output beam  14 B and reflects the output beam  14 A from the MO system  18  through the PA system  20  at a slight angle to the optical and discharge longitudinal centerline axis of the PA. In the embodiment shown illustratively, where the PA laser system may be in two chambers or a single chamber, the tilted path may intersect the longitudinal centerline optical and discharge axes of a pair of electrode pairs  90 A,  92 A and  90 B,  92 B, and then be reflected by, e.g., two essentially totally reflecting mirrors  20 B 1  and  20 B 2  in the beam reflecting module  20 B back through the PA system  20  chambers  20 A 2  and  20 A 1  in that order, essentially along the longitudinal centerline optical and gas discharge axis of the electrodes  90 A,  92 A and  90 B,  92 B. This may simplify the optics utilized and at the same time optimize the utilization of the amplification occurring in the discharge regions between the electrode pairs,  90 A,  92 A and  90 B,  92 B respectively. It will be understood by those skilled in the art that the respective MO chamber and PA chamber(s) are not drawn in this schematic view to any kind of scale, e.g., in longitudinal length.  
         [0019]     Turning now to  FIG. 3A  there is shown a solid state pulse power module  60  according to aspects of an embodiment of the present invention which may incorporate, e.g., a charging capacitor C 0    70  that is the input, through a solid state switch S 1  to a first stage of a commutator module  80 . Upon the closing of switch S 1  once the charging capacitor C 0  is fully charge, by a resonant charger (not shown) the second stage capacitor C 1  is charged through a magnetic saturable reactor L 0 , which compresses the pulse. When the charge on second stage capacitor C 1  is sufficient to close a second magnetically saturable reactor switch L 1 , by saturating the switch magnetically, the charge on the second stage capacitor C 1  in the commutator section  80  is stepped up in one of a pair of fractional winding step up transformers  78 A,  78 B, e.g., containing N (or M) single winding primary coils in parallel and a single winding secondary, such that the voltage output is stepped up N (or M) times, where N may equal M. The transformers  78 A,  78 B may be, e.g., connected in parallel to the output of the second compression stage of the commutator section  80 , i.e., the output of L 1 .  
         [0020]     The stepped-up voltage output of the transformer  78 A may be, e.g., connected to the input of a compression head stage comprising, e.g., a capacitor C 2A  and a magnetically saturable reactor switch L 2A , the output of which may be connected to a peaking capacitor C P , which may be, e.g., connected across the electrodes of the MO System  18 ,  90 A and  92 A. The stepped-up voltage output of the transformer  78 B may, e.g., be connected in parallel to a compression head  82  and a compression head  84 , each of which may also comprise, e.g., a capacitor C 2B  and C 2C  a magnetically saturable reactor switch L 2B  and L 2C , respectively and a respective peaking capacitor C PB  and C PC . The respective peaking capacitors C PB  and C PC  may be connected to respective PA chamber(s) electrodes  90 B,  92 B and  90 C,  92 C. Which of the electrode pairs  90 B,  92 B or  90 C,  92 C will receive the output of the respective compression head  82 ,  84  each time the electrodes  90 A,  92 A of the MO system  18  receive an electric pulse from C PA  may be determined, e.g., by solid state switches S 3  and S 4 .  
         [0021]     In this way, the PA chamber(s) with their respective electrode pairs  90 B,  92 B and  90 C,  92 C may be alternatively selected for producing a gas discharge for a given MO laser output pulse  14 A.  
         [0022]     It will be understood by those skilled in the art that by the arrangement according to aspects of an embodiment of the present invention, the MO may be optimized for line narrowing as is well understood in the art of molecular fluorine or excimer gas discharge MOPA laser configurations and the PA chamber(s) may be optimized for current state of the art pulse repetition operation, e.g., around 4 KHz or so, allowing for the overall system  10  to achieve very high repetition rates of, e.g., 8 KHz and above without exceeding critical performance parameters which currently prevent a single chamber PA system from operating at any anywhere near, e.g., 8 KHz, e.g., fan speed, fan temperature, fan vibration, etc. necessary for operating at around 8 KHz with a single set of PA electrodes. It will also be understood, that the relatively low power MO operation may relatively easily be brought up to pulse repetition rates of around, e.g., 8 KHz and still output a line narrowed relatively low power output beam  14 A at such very high pulse repetition rates.  
         [0023]     Turning now to  FIG. 3C  there is shown another embodiment of a pulse power system  60  wherein there are three parallel circuits, each with a C 0 , C 0A , C 0B , and C 0C , and with three step up transformers  78 A,  78 B and  78 C and three compression heads  76 A,  76 B and  76 C. In such an embodiment, e.g., the timing of the closing of switch S 1 , which may be to the compression head  76 A for the MO chamber and may be closed in time to discharge the electrodes in the MO chamber, e.g., at 8 KHz for the and the switches S 2  and S 3  may be closed alternately at rates of, e.g., 4 KHz to alternately fire the electrodes  90 B,  92 B and  90 C,  92 C in the two PA sections, e.g.,  20 A 1  and  20 A 2 .  
         [0024]     It will further be understood that the arrangement according to aspects of embodiments of the present invention may be configured as noted above and in other manners, e.g., the magnetic switching circuits may be employed in conjunction with a single compression head being charge at a rate of 8 KHz, the same as a corresponding compression head for the MO chamber, to switch, downstream of the step-up transformer  78 , i.e., on the very high voltage side of the step-up transformer, to charge respective peaking capacitors on the PA module, e.g., for the electrodes  90 B,  92 B and  90 C,  92 C alternately at rates of, e.g., 4 KHz.  
         [0025]     In operation therefore, the laser system according to aspects of an embodiment of the present invention may take advantage of the relative simplicity of running, e.g., a MO chamber at, e.g., 8 KHz+ while still being able to take advantage of a PA configuration, i.e., e.g., the wider discharge for multiple passes for amplification and not suffer the consequences of, among other things, trying to clear the wider discharge electrode discharge region pulse to pulse as rates of higher than about 4 KHz.  
         [0026]      FIG. 4  shows a timing diagram for the firing of an MO chamber gas discharge and a PA gas discharge, for a single pair of electrodes in the PA, with the only difference being according to an aspect of an embodiment of the present invention being that the PA electric discharge at τ 1PA  plus τ 2PA  will occur alternatively between electrodes  90 B,  92 B and  90 C,  92 C, with perhaps a slight adjustment to τ 1PA  to account for the delay in the beam  14 A passing through electrodes  90 B,  92 B to reach electrodes  90 C,  92 C when the discharge is to be between electrodes  90 C,  92 C according to aspects of an embodiment of the present invention.  
         [0027]     It will also be understood by those skilled in the art that there may be applications for the present invention in which line narrowing is not crucial, but high power output at very high repetition rates, even up to 10 KHz and above may be required, e.g., for the driving laser of an LPP EUV light source. In this event, e.g., the beam delivery unit  40  discussed above may not deliver the laser beam  14 C to a lithography tool per se, but to an EUV light source that in turn delivers EUV light to a lithography tool. In that event, e.g., the line narrowing module  18 B may not be required according to aspects of an embodiment of the present invention and, e.g., also the Sam  29  may not be required to measure, e.g., the bandwidth of the beam  14 B, and only, e.g., beam direction and pointing need be controlled, e.g., in the BDU  40 .  
         [0028]     According to aspects of an embodiment of the present invention if the MO beam were made, e.g., roughly half as wide as the PA discharge(s), then a double pass of the PA chamber(s) electrodes,  90 B,  92 B and  90 C,  92 C can be performed to essentially entirely sweep the gain in the PA chamber(s). As noted above, this effectively separates high repetition rate problems in reaching, e.g., 8-10 KHz from high power problems.  
         [0029]     Another possibility according to aspects of an embodiment of the present invention may be, e.g., to use a single PA chamber  20  with a single set of paired electrodes, e.g.,  90 B,  92 B also configured as a line narrowed oscillator, i.e., having a LNM (not shown) and alternately firing the laser chamber electrodes in an inter-digitated fashion (“tic-toc” fashion) to achieve a narrow band output at very high repetition rates, e.g., 10-16 KHz. This would sacrifice pulse power in each pulse, but could achieve very very high pulse repetition rates, e.g., using a combiner, e.g., a polarizing combiner (not shown) to recombine the two narrow band output beams (not shown) from the two oscillators into a single output beam.  
         [0030]     It will also be understood by those skilled in the art that aspects of an embodiment of the present invention may be used, e.g., to achieve a pulse repetition rate of, e.g., about 6 KHz, e.g., using an MO firing at 6 KHz and two PA, each firing at 3 KHz, or other possible combinations for pulse repetition rates o, e.g., greater than 4 KHz.  
         [0031]     Turning now to  FIG. 5  there is shown schematically an alternative embodiment according to aspects of an embodiment of the present invention. In  FIG. 5  three is shown and embodiment of a dual electrode system  100 , which may comprise, e.g. a first cathode  102  and a second cathode  104  which may be positioned, e.g., in a single chamber each with a respective main insulator  106 ,  108 . The two electrodes along with a single anode  110 , having appropriately formed anode discharge regions opposite the respective cathode  102 ,  104  form elongated electrode pairs within the chamber and define elongated discharge regions  120 ,  122  (into the plane of the paper). The anode  110  may be positioned on an anode support  112 . The cathode and single anode may be formed, with or without insulation, e.g., a ceramic insulator, between discharge regions. The cathodes  102 ,  104  may be separated by an elongated converter, e.g., a catalytic converter  130  for transforming, e.g., F into F 2  between the discharge  120  and the discharge  122 . Laser gas may be circulated between the electrodes  120 ,  110  and  122 , 110  and the respective discharge regions  120   122  by a fan  140 .  
         [0032]     An electric discharge may be created alternatively between the electrodes  120 ,  110  and  122 ,  110  respectively creating gas discharges in the discharge regions  120 ,  122  by a power supply system  150 , e.g., as shown in  FIG. 6 , which is a modification of the system shown, e.g., in  FIG. 3A , wherein a single compression head capacitor C 2  may be charged at a rate of, e.g., 8 Khz and the circuit  150  provide alternating electric discharge voltages on respective peaking capacitors C PA  and C PB  through respective magnetically saturable reactor switches L 2A  and L 2B . The switches L 2A  and L 2B  may be switched between oppositely directed biasing currents from bias current sources I B1  and I B2 , e.g., at 8 KHz, utilizing a suitable switching circuit (not shown) to cause the charge on C 2  alternatively to be dumped on C PA  and C PB  at the desired, e.g., 8 KHz.  
         [0033]     Turning now to  FIG. 7  there is shown schematically aspects of an embodiment of the present invention shown in  FIGS. 5 and 6  wherein, e.g., only one line narrowing package  160  is needed. As shown in  FIG. 7 , the first discharge light, indicated by single arrows, may pass, e.g., through a rear window  152  in, e.g., an oscillating cavity, which may be oriented according to the polarization of the light desired to pass through that window,  152 , e.g., a first polarization direction and into and through a polarizing beam splitter that is essentially transparent to light of the first polarization direction. The light from the discharge  120  may then pass into a line narrowing package  160  configured for operation with light of the first polarization direction through a half wave plate  158  or other polarizing mechanism that, e.g., may be a rotating half wave plate  158  that is rotated at the pulse repetition rage of the laser system  100 , such that when the light from the discharge  120  is traversing from and to the line narrowing package, the half wave plate  158  is not in the optical path. It will be understood that the polarizing mechanism may also be, e.g., an electrically or magnetically or mechanically or otherwise actuated optical element, that can be, e.g., periodically switched (actuated) to pass light of one polarizing direction, e.g., the first polarizing direction, or another, e.g., the second polarizing direction.  
         [0034]     Similarly, the laser light pulses produced in the discharge  122  in laser system  100  may be passed through, e.g., a rear window  180  that may be, e.g., oriented to pass light of a different polarization direction, e.g., a second polarization direction, indicated by double arrows, which may then be reflected by a mirror  182  that is essentially totally reflective of the light of the second polarization direction and onto the polarizing beam splitter that is essentially totally reflective of the light of the second polarization direction and then through the polarizing mechanism  158 , e.g., the half wave plate, which in the case of the light from the discharge region  122  may convert the light from the second polarization direction to the first polarization direction for line narrowing in the line narrowing package  160 . Upon return from the line narrowing package  160 , this light from the discharge region  122  may again pass through the polarizing mechanism, e.g., half wave plate  158  and be again converted back to the second polarization direction for passage pack through the resonance cavity of the discharge  122 , e.g., through a front window  184  oriented for the second polarization direction and the reflecting mirror  190  essentially totally reflective for light of the second polarization direction and not to, e.g., a polarizing beam splitter  174  that is essentially totally transparent to the light of the first polarization direction exiting the output couple of the cavity of discharge region  120  and totally reflective of the light of the second polarization direction exiting the output coupler  186  of the resonance cavity of the discharge region  122 . Another polarizing mechanism  176 , similar to that referenced above in regard to polarizing mechanism  158 , may intermittently also change the polarization of either the light of the first polarization direction from the resonance cavity of the discharge region  120  to the second polarization direction of the light of the discharge region  122 , to produce an output of a selected polarization direction, e.g., the first polarization direction.  
         [0035]     In operation according to aspects of an embodiment of the present invention there is provided a method and apparatus for the delivery of pulsed energy to the two sets of paired gas discharges, e.g., in two PA sections that may comprise a compression head (capacitive storage with electrical pulse-compression utilizing a saturable reactor magnetic switch. Between the peaking capacitors (final stage a across the electrodes) and the compression head each of the paired discharges may have a separate saturable magnetic switch, which may be biased in such an opposite fashion as to have each of the paired discharge electrodes operate at, e.g., half of the total output repetition rate that the compression head (and the MO chamber) experiences. The biasing power requirements for a biasing power supply can be used to switch many (multiple) discharge regions. The discharges, e.g., in the PA sections may be in a single chamber or more than one chamber and the same resonance charger may drive both the MO chamber discharges and the PA chamber(s) discharge at 8 KHz (C 0  charging), while the PA electrodes are alternately fired at, e.g., 4 KHz.  
         [0036]     It will be understood by those skilled in the art that modification of the polarization of the output of the laser system  100  may occur, e.g., in the BDU  40 , or may occur downstream even of the BDU, e.g., inside of a lithography tool. It will also be understood that the laser system  100  could be configured, e.g., along with a single or multiple, e.g., double chambered (double discharge region) power amplifier or even power oscillator to produce MOPA and/or MOPO configurations and/or that the system  100  could be a PO in a MOPO, e.g., receiving MO output pulses at the ultimate output pulse repetition rate of the entire MOPO system and interdigitated between the discharge region  120  and the discharge region  122  each operating at one half the ultimate output pulse repetition rate of the, e.g., MOPO system. Further such a configuration could easily be modified to operate as a very high repetition rate POPO system.