Abstract:
A soft x-ray generator includes a unique pulse trigger assembly which reliably and reproducibly provides a plasma to initiate the discharge between a cathode and an anode, and having a cone-shaped geometry. The soft x-ray generator of the present invention also includes a rotating anode which is generally disk-shaped with an outer circumferential edge which can be rotated with respect to the cathode to expose different sections of the anode to the plasma discharge, thereby reducing anode wear and providing longer term operation. Anode erosion is also reduced by the liquid cooling of the anode during use. The generator utilizes a relatively low capacitance for the cathode-to-anode discharge and a relatively high voltage and pulse repetition rate (frequency) to achieve continuous reproducible results.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. § 119(e) on U.S. Provisional Application No. 60/727,881, entitled S OFT  X-R AY  G ENERATOR , filed on Oct. 18, 2005, by Robert (NMI) Dotten, et al., the entire disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to improvements in a soft x-ray generator. U.S. Pat. No. 6,240,163 discloses a soft x-ray (also referred to as EUV) electromagnetic radiation source which provides improved short bursts of radiation in the about 75 ev to about 12 Kev range. These bursts of radiation have a maximum intensity for use in a variety of applications, including lithography, crystallography, and radiography, in the scientific, industrial, and medical fields. The disclosure of U.S. Pat. No. 6,240,163 is incorporated herein by reference. Although the system disclosed in the &#39;163 patent represents a vast improvement over prior art soft x-ray generators, there remains a need for a system which has a longer useful life under continuous high frequency operating conditions by preventing, for example, erosion of the anode as well as having a more predictable and reliable trigger operation for initiating the discharge between the anode and cathode for such continuous operation. 
   SUMMARY OF THE INVENTION 
   The soft x-ray generator of the present invention satisfies these needs and provides additional benefits by including a unique pulse trigger assembly which reliably and reproducibly provides a plasma and initiates the discharge between a cathode and an anode. The trigger assembly has a cone-shaped geometry which implements gas discharge to provide efficient and reliable operation of the trigger. In one embodiment, the soft x-ray generator of the present invention includes a rotating anode which is generally disk-shaped with an outer circumferential edge which is rotated with respect to the cathode to expose different sections of the anode to the vacuum spark discharge, which produces the plasma, thereby reducing anode wear and providing longer term operation. Anode erosion is also reduced by liquid cooling of the anode. The generator of this invention utilizes a relatively low capacitance for the cathode-to-anode discharge and a relatively high pulse repetition rate (frequency) to achieve its continuous reproducible results. 
   These and other features, objects and advantages of the present invention will become apparent upon reading the following description thereof together with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a soft x-ray generator embodying the present invention; 
       FIG. 2  is a perspective view, partly in phantom, of the soft x-ray generator housing and its components contained within the vacuum chamber shown in  FIG. 1 ; 
       FIG. 3  is a right elevational view of the x-ray generator shown in  FIG. 2 ; 
       FIG. 4  is a front elevational view of the x-ray generator shown in  FIGS. 2 and 3 ; 
       FIG. 5  is a cross-sectional view of the anode and trigger assembly shown in  FIGS. 2 and 4 ; 
       FIG. 6  is an exploded perspective view of the trigger assembly shown in  FIG. 5 ; 
       FIG. 7  is an enlarged cross-sectional view of the assembled trigger assembly shown in  FIG. 5 ; 
       FIG. 7A  is a front view of the cathode; 
       FIG. 8  is a perspective view of the anode assembly, the motor assembly, and the trigger assembly removed from the chamber; 
       FIG. 9  is a front elevational view of the anode and trigger assemblies; 
       FIG. 10  is a rear elevational view of the cathode assembly; 
       FIG. 11  is an exploded perspective view of the rotating anode assembly shown in  FIGS. 1 ,  2 ,  5 , and  8 ; 
       FIG. 12  is a perspective view of the anode shown in  FIG. 4 ; 
       FIG. 13  is a front elevational view of the rotating anode shown in  FIG. 12 ; 
       FIG. 14  is a cross-sectional view, taken along section lines XIV-XIV of  FIG. 13 , of the rotating anode shown in  FIGS. 1 ,  2 , and  13 ; 
       FIG. 15  is an exploded view of the drive and cooling system for the anode; 
       FIG. 16  is a vertical cross-sectional view of the assembled structure of  FIG. 15 ; and 
       FIG. 17  is an electrical circuit diagram, in schematic form, of a high voltage power supply used with the soft x-ray generator of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   It should be understood that the invention is not limited to the details of the particular arrangement shown and described since the invention is capable of other embodiments. Materials and the parameters used herein are for the purpose of description not of limitation. Referring initially to  FIG. 1 , there is shown an x-ray generator  10  of the present invention, which includes an outer chamber  12  through which various power supplies, cooling conduits, a gas supply conduit  27 , and a vacuum pump are sealably coupled in a conventional manner to supply operating voltage, an inert gas to the trigger electrode assembly via conduit  27 , different cooling fluids to both the trigger assembly and rotating anode, as well as a vacuum for the interior  11  of the sealed chamber  12 . The x-ray generator comprises in the preferred embodiment a trigger electrode assembly  100  and a rotating anode assembly  200 , which are shown in detail in the various drawing figures of this application. Both of these assemblies are contained within the vacuum chamber interior  11 , and one or more capacitor(s)  20  is/are coupled between the anode assembly  200  and trigger assembly  100  including cathode  106 . The capacitor  20  is charged by a pulsed power source  22  at from about −3 kv to about −20 kv, depending upon the desired operation characteristics. Power source  22  is shown and described in  FIG. 17 . The capacitor is charged/discharged at a repetition rate which corresponds to the frequency of trigger operation of from about 1 Hz to about 100 kHz. The trigger electrode  100  is coupled to a trigger power source  24 , which provides pulses at the same frequency with a voltage amplitude of from about −3.5 kv to about −30.0 kv. 
   The overall geometry of the x-ray generator  10  and the chamber  12  housing the component parts is shown in  FIGS. 2-4 . Chamber  12  comprises a generally cylindrical body  14  having an annular flange  18  at one end which includes a sealing O-ring  17  for sealing the open end when engaged by a hinged circular door  26 . In order to sealably clamp the door  26  in a closed, vacuum-tight seal with the chamber  12 , a locking clamp assembly  29  of conventional design and including a rotary locking knob, as seen in  FIGS. 3 and 4 , is employed. Door  26  is hinged to flange  18  by a hinge  28  and includes a sealed window  30  so the interior  11  of the chamber can be viewed, either through the door  26  or through a viewing port comprising a flange  32  extending from a conduit  34  from body  14 . The viewing port may include a pin diode  36  for observing the pulse discharge between the cathode and anode of the x-ray generator. Generator  10  also includes an exit port  40  (best seen in  FIG. 3 ) which includes a window  19  where an x-ray filter can be installed. When a beryllium window  19  is employed, it typically has a thickness of from about 3 to about 20 microns, and preferably of from about 8 to about 10 microns, and is fairly transparent to the soft x-rays generated in chamber  11 . The soft x-rays generated by the generator  10  pass through the window and can be applied to a device employing such x-rays through a mounting flange  42 . Such devices may include instruments used for lithography, crystallography, radiography, and other scientific or medical appliances which benefit from the soft x-rays generated by generator  10 . 
   Flange  42  and port  40  are also coupled to the cylindrical body  14  of chamber  12  through a cylindrical conduit which communicates with the cathode assembly as described below. Body  14  of chamber  12  is mounted by mounting brackets  38  and  39 , which are coupled to cylindrical body  14  and are mounted to a base  44  which is supported by a suitable cabinet which accommodates the remainder of the components, including the power supplies, control circuit, and fluid and liquid supplies and pumps for the generator  10 . 
   A relatively large cylindrical conduit  46  communicates with the interior space  11  of chamber  12  and is coupled to a high vacuum pump for evacuating the interior  11  of the chamber to achieve a vacuum of from about 10 −4  to about 10 −6  torr. Conduit  46  terminates in a flange  47  for coupling to the high vacuum pump (not shown). 
   Sealably coupled to the rear wall  13  ( FIG. 3 ) of housing body  14  by means of a flange  50  is a motor assembly  300 . Assembly  300  includes a vacuum bearing  360  in housing  310  ( FIGS. 15 and 16 ), a motor housing  320 , and a rotary cooling water supplying housing  330  described in greater detail below. Motor assembly  300  rotates a rotary drive shaft  312  coupled to the rotating anode  200  as well as provides cooling fluid, such as water, thereto. The components of the chamber  12  are suitably machined from stainless steel. 
   The trigger assembly  100  is shown in detail in  FIGS. 5-7  and is mounted in insulated relationship to chamber body  14  within space  11  ( FIG. 2 ) and adjacent rotating anode assembly  200  in the relationship also seen in  FIGS. 4 ,  5 , and  8 . Between the rotating anode  200  and the cathode  106  of trigger assembly  100 , there is mounted one or more capacitors  20  between spaced-apart mounting plates  21  and  23  for providing a capacitance of from about 1 nano-farad to about 1 micro-farad depending upon the desired operational characteristics of the system. Capacitor  20  comprises a pair of generally disk-shaped capacitors  20   a  and  20   b  coupled in parallel ( FIGS. 9 ,  10  and  17 ). 
   Trigger assembly  100  includes a generally annular trigger electrode  120 , which concentrically surrounds cone-shaped member  102  having a tip  103  engaging cathode  106 , as best seen in  FIG. 7 . Electrode  120  and cone-shaped member  102  define a cone-shaped gas-filled chamber  129 , as best seen in  FIGS. 5 and 7 . Nozzle  106  also includes a cone-shaped exit aperture  107 . The negative voltage from source  24  ( FIG. 1 ) is applied between the trigger electrode  120  and cone  102  and causes the ionization of the inert gas within the trigger chamber  129  between them and produces a plasma and free electrons. The gas typically can be an inert gas, such as argon or the like, or other kinds of gases, e.g. nitrogen or mixtures supplied to chamber  129 . When the gas discharge occurs in the chamber, the free electrons and the plasma diffuse through the holes  60  ( FIGS. 7 and 7A ) and the open tip  107  of the cathode nozzle  106  into the gap between the cathode and anode toward the grounded anode assembly  200 . The electrons are accelerated by the electric field provided by the applied high voltage between the cathode and anode from the pulsed power source  22 . When the electrons impinge upon the anode, an anode plasma is generated of the copper anode material (although other anode materials can be employed), which diffuses toward the cathode. Upon the meeting of both plasmas, the discharge of capacitor(s)  20  between the grounded anode and negative high voltage cathode is initiated. During the discharge process, electric current increases rapidly in nano seconds and, due to the current pinching effect, a plasma region of small size is formed between the cathode and anode where the plasma is increasing in temperature, and the copper ions and atoms in the plasma are multiple ionized. This results in the generation of soft x-rays of point source size which exit through exit port or window  105  ( FIGS. 5 and 7 ) of the trigger electrode assembly  100  into chamber  12 . Chamber  12  may include several radiation transparent windows, such as window  19 , to allow the soft x-rays generated between the cathode and rotating anode to be externally focused and employed for a variety of applications. 
   The relationship of the trigger assembly  100  to the anode assembly  200  is seen in  FIGS. 5 and 8 , with the cathode structure also shown in  FIG. 2 , being mounted within the chamber  12  by, in part, a phenolic insulator plate  16  which includes apertures  15  for mounting the cathode assembly in insulative relationship to rear wall  13  of chamber body  14 . A pair of brackets  33  and  35  ( FIG. 8 ) extend from insulator  16  and clamp the trigger housing  110  in place. Conductive plate  23  ( FIGS. 8-10 ) is coupled to capacitors  20   a ,  20   b  and is secured to the housing  110  of trigger assembly  100 , as seen in  FIG. 8 . As described below, the remaining conductive plate  21  is coupled to the opposite side of capacitors  20   a ,  20   b  and a conductive end bearing  228  of the rotary anode assembly  200 . Having briefly described the main components of the system and its operation, a detailed description of the geometry of the trigger assembly, and, subsequently, the rotating anode assembly is now provided in connection with  FIGS. 5-7 . 
   The trigger assembly is best seen in  FIGS. 5-7  and includes a fluid (such as oil) cooled trigger housing  110  which includes an annular, finned oil-cooling channel  126  formed therein surrounding and facing the trigger electrode  120 . The cooling channel is supplied with cooling fluid, such as oil, through an inlet  111  and an outlet  112  which provides a flow path through the sealed housing for the admission of cooling oil, which is cooled externally of chamber  11 . An oil inlet hose (not shown) supplies cooled oil to inlet  111  of housing  110  while an oil outlet hose (not shown) returns the heated oil from outlet  112  through a sealed coupling in the chamber conduit  46  to be cooled externally. 
   A trigger housing rear cover  116  is sealably mounted to trigger housing  110  by means of fasteners  119  and a sealing O-ring  114 . Cover  116  includes an inert gas inlet  118  for the admission of an inert gas into the interior of chamber  129  defined by the sealed assembly. The negative voltage applied to the trigger electrode  120  is applied through a conductor  117  in insulator  123 . Conductor  117  extends through trigger housing  110  ( FIG. 7 ) and is coupled to the ring-shaped trigger electrode  120 . The trigger voltage is applied through ( FIG. 7 ) which is extended to the ring-shaped trigger electrode  120  for applying a relative pulsed negative voltage between cathode  106  and electrode  120  of from about −0.5 kv to about −10 kv. 
   Trigger electrode  120  is insulated from the housing  110  by a rear insulator  122 , a front insulator  124 , and insulator  123 , all of which are mounted in sealable engagement within the trigger chamber  104  by a series of O-rings  130 ,  132 , and  134 . An O-ring  115  sealably couples the trigger front cover  128  to the trigger housing  110  and is held in place by suitable fasteners, such as fasteners  136  ( FIGS. 6 and 7 ). The cathode trigger nozzle  106  (forming the cathode) is secured to the trigger front cover  128  by means of a nozzle retaining disk  108  and suitable fastening screws  109  secured within threaded apertures in the front mounting plate  128 . A slight gap “g” of from about 0.010 inch to about 0.150 inch exists between the cone  102  and trigger electrode  120 . When the high voltage is applied via conductor  117  between trigger electrode  120  and cone  102 , a plasma is formed within chamber  129 , which communicates with the vacuum chamber  11  through one or more radially spaced apertures  60  in the cathode, as best seen in  FIG. 7A . The apertures  60 , as also shown in  FIG. 7 , communicate between chamber  129  and the opening  107  of the cathode nozzle  106 , which includes an annular mounting flange  64 , as seen in  FIG. 7A , to facilitate its mounting, as shown in  FIG. 7 , to the front mounting plate  128 . In one preferred embodiment of the invention, the apertures  60  were equally spaced at 120° intervals and had a diameter of approximately 0.030 inch with a nozzle opening  107  having an inner diameter of 0.040 inch. In some applications, a greater or fewer number of equally spaced apertures  60  may be provided to feed the plasma contained within conical chamber  129  to the exit aperture  107  of cathode  106 . When the plasma is formed within the chamber  129  of trigger electrode assembly  100 , therefore, the plasma is drawn by the negative pressure in chamber  11  outwardly through aperture  107  toward the rotating anode assembly  200  and to the edge  204  ( FIGS. 5 and 8 ) of the anode  202  (described below) with spacing “s” ( FIG. 5 ) from about 1 mm to about 6 mm between the tip of cathode  106  and the aligned edge  204  of anode  202 . 
   A perspective view of the assembled trigger assembly  100  is shown in  FIG. 8 , which illustrates the mounting of the capacitors  20   a  and  20   b  with one mounting plate  23  being secured by fasteners  127  ( FIG. 9 ) to the edge of trigger front cover  128  and the remaining mounting plate  21  supporting the rotating anode assembly  200  through a conductive bearing as described below. 
   The trigger assembly includes a second cone  102  spaced from the conical interior walls  121  ( FIG. 7 ) of the oil-cooled rear housing  116  and main housing  110 . Cone  102  also remains relatively hot and is not affected by the oil-cooled housing and further prevents debris from clogging the aperture in cone  102  adjacent cathode  106 . The beryllium window  19  typically has a thickness of from about 3 to about 20 microns and preferably of from about 8 to about 10 microns and is fairly transparent to the soft x-rays generated in chamber  11 . The window  105  is held in place on the open aperture of rear housing  116  by an annular mounting ring  125  which is held in place by threaded fasteners  101 . Having described the trigger assembly, a description of the rotating anode assembly is now provided in connection with  FIGS. 5 ,  8 , and  10 - 16 . Rotating anode assembly  200  comprises a generally circular disk-shaped anode  202  made of copper or other suitable metal, which, as best seen in  FIG. 14 , has an outer peripheral edge  204  formed at the flattened tip of the intersecting, converging walls  206  and  208  of the body of anode  202 . Anode  202  includes a concave depression  210  defined by a peripheral rim  215  on one side and a similar concave depression  212  defined by peripheral rim  219  on the opposite side separated by a central disk-shaped wall  211 . Anode  202  is sealably held between a cup-shaped first end wall  220  and a second cup-shaped end wall  230 , which extend within rims  219 ,  215  ( FIG. 16 ), respectively, and are sealed thereto by means of O-rings  222  and  232 , respectively. End wall  230  includes a threaded aperture  240  for receiving the threaded end  311  ( FIG. 15 ) of hollow drive shaft  312 . Cooling fluid, such as water, is introduced as described below through a supply conduit  332  concentric within hollow drive shaft  312  and can flow within the chamber defined by cavities  210 ,  212  between members  220  and  230  through central opening or aperture  213  in wall  211 . For such cooling, a plurality of spaced-apart apertures  214  ( FIGS. 11-14 ) are formed at angles which follow and are parallel to the converging walls  206 ,  208  and are spaced inwardly therefrom. The apertures  214  have an inner diameter of about 0.113 inch and their location therefor provides a flow path for coolant immediately adjacent edge  204 . the apertures  214  extend around the peripheral edge of wall  211  of anode  202  and extend toward the edge  204  to promote the flow of the water from cavity  212  under pressure through aperture  213  through apertures  214  into cavity  210  to maintain the anode at an operating temperature well below what normally would be encountered. The walls  206 ,  208  converge at about 45° forming an angle of about 90° at edge  204 . The thickness of the generally triangular peripheral edge where walls  206 ,  208  join the body  211  is about 0.5 inch. The anode is positioned, as best seen in  FIGS. 5 and 8 , with the edge  204  being spaced, as noted above, from about 1 mm to about 6 mm from the tip of cathode trigger nozzle  106 , such that the plasma drawn through aperture  107  bombards the highly charged edge  204  of the anode to form the metallic ions, which subsequently gather to form a pinch zone, and the generation of soft x-rays, as discussed above. In one embodiment, anode  202  had an outer diameter of about 4.9 inches, although other diameter anodes may be employed. 
   End walls  220 ,  230  of the sealed hollow anode assembly  200  are attached to the anode  202 , which includes three equally spaced recessed apertures  216  for receiving cap screws which thread into three equally spaced threaded apertures  217  ( FIG. 16 ) on the inner surface of end wall  230  to attach the anode to wall  230 . The anode, in turn, includes three equally spaced interspersed threaded apertures  218  which receive hex bolts  223  ( FIGS. 9 and 16 ) for attaching end wall  220  to anode  202  in sealed engagement therewith. End wall  220  includes a blind mounting boss  224  which is internally threaded to receive the threaded axle end  226  ( FIG. 16 ) of a conductive bearing  228  which is mounted to plate  21 . Bearing  228  includes an internally threaded stub axle  229  which receives and is secured to plate  21  by means of a threaded nut  233 . Thus, the axle  226  is allowed to rotate with respect to the fixed body  228  and stub axle  229  of the conductive bearing, as also illustrated in  FIG. 8 . Having described the anode assembly  200  and its rotatable mounting at one end to the stationary grounded plate  21 , a description of the rotating drive shaft  312  and its mounting to body  14  of the outer chamber  12  of x-ray generator  10  is now described in connection with  FIGS. 15 and 16 . 
   The drive system for the rotary drive shaft  312  coupled to the rotating anode assembly  200  is now described in connection with  FIGS. 15 and 16 . The assembly  300  is employed for not only rotating the anode  202  at a speed of up to about 1500 rpm but also to supply a cooling fluid, such as water, to the anode  202  through the hollow drive shaft  312 . The supply of fluid can be tap water and the return drained, or, in some applications, a loop of coolant can be employed and the heated coolant returned to a chiller for recirculation. This is accomplished by the structure shown in  FIGS. 15 and 16 , which includes the hollow drive shaft  312  having a central, longitudinally extending aperture  314  for receiving in spaced concentric relationship the cooling fluid supply tube  332 . Shaft  312  includes a threaded end  313  for receiving a rotatable fitting  336  of a water supply union  331  as shown in  FIG. 16 . The drive shaft  312  also is splined along the area  315  to center the motor rotor  322 , which includes a stater  324 . Rotor  322  is mounted between bearings  321  and  323  and affixed to shaft  312  in area  315  with epoxy within recess  333  of housing  310  at one end and to recess  334  in motor housing section  320 . Drive shaft  312  also includes circumferential snap-ring apertures  317  for receiving snap rings for securing bearings  321  and  323  in longitudinal alignment with shaft  312 . The diameter of shaft  312  increases toward end  311  as it extends through a ferrule fluidic bearing, which is commercially available, for example, from Ferrotec Corporation Model No. HS-1500-SLFBC. This vacuum bearing  360  is mounted within housing  310  and is held in place by a clamp  362 , which rotates with shaft  312 . Shaft  312  also includes circumferentially extending snap ring apertures  319  for holding bearing  321  in longitudinal alignment with shaft  312 . Stater  324  of the axially aligned motor  325  fits within a recess  339  of housing  310  and  337  of housing  320 , as seen in  FIG. 16 . 
   The coolant supply tube  332  has a first threaded end  335  which threadably extends into the union  331 , as illustrated in  FIG. 16 , and receives cooling fluid, such as water, through an inlet connector  340 . The stationary concentric tube  332  is held in longitudinally spaced alignment with an annular space between the outer diameter of tube  332  and the inner diameter of aperture  314  of rotary drive shaft  312  by its coupling to union  331  at one end and the extension through aperture  213  in anode  202 , which includes a conventional bushing  221  ( FIGS. 5 and 15 ) between the end  338  of supply tube  332  and aperture  213  as necessary. Union  331  is a commercially available rotating union manufactured, for example, by the Deublin Company of Northbrook, Ill. in their Model 57 Series. The flange  345  of housing  330  receives fasteners, such as bolts or cap screws  347  ( FIG. 16 ), which extend through housing  320  and into housing  310  in a conventional manner for securing housings  330 ,  320 , and  310  together, while other suitable fasteners, such as hex bolts or the like, extend through the apertures in flange  50  for securing the assembly  300  to the body  14  of chamber  12 . 
   The negative power supply  22  ( FIG. 1 ) for the capacitor(s)  20  charges the capacitor(s) at a rate at least as fast, if not slightly faster, than the pulse trigger voltage frequency. The power supply should have the same frequency as the trigger pulse rate. They are synchronized by the controller ( FIG. 17 ), such that there is a time delay between the two signals. The main pulse is first to charge the capacitor(s) to the given voltage, followed by the trigger pulse, such that, as soon as the trigger discharges the plasma within chamber  129  of the trigger assembly, the fully charged capacitor  20  discharges through the plasma in the gap “s” ( FIG. 5 ) between the anode edge  204  and cathode  106  effecting generation of soft x-rays within chamber  11 . The x-rays are then transmitted from the trigger assembly through the x-ray filters at window  19  or exit port  40  transparent to the x-ray radiations of the x-ray generator. Chamber  11  may have a plurality of similar windows, such as window  19  ( FIG. 1 ), at various locations around the housing  12 , for the exit of soft x-rays which can be focused external to generator  10  for subsequent use in lithography, crystallography, radiography, or the like, in a conventional manner. 
   In order to achieve a high repetition rate of discharge, fast charging of capacitors C 1  and  20  to about −3 kv to about −20 kv from a 500 volt DC ( FIG. 17 ) source is required. A two stage resonant charging circuit is employed for this, as schematically shown in  FIG. 17 . The first stage resonantly charges capacitor C 1  to 1000 volts through inductor L 1 , and the second stage resonantly charges capacitor  20  from capacitor C 1  through inductor L 1  using the step up transformer T 1 . The pulser is able to signal the controller (which includes a microprocessor) that a short circuit shunts  20  inside the vacuum chamber by sensing a negative voltage on C 1 . The controller stops to pulse the pulser to prevent potential damage of charging circuitry. To achieve high and controlled repetition rates of vacuum discharge for desired output power, a flexible design of controller and pulser module that can control and deliver high repetition rates up to 100 kHz is desirable. To obtain multiple charging rates from a single pulser circuit, multiplexing multiple pulsers are employed. 
   It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.