Abstract:
A low-inductance, air-insulated gas switch uses a de-enhanced annular trigger ring disposed between two opposing high voltage electrodes. The switch is DC chargeable to 200 kilovolts or more, triggerable, has low jitter (5 ns or less), has pre-fire and no-fire rates of no more than one in 10,000 shots, and has a lifetime of greater than 100,000 shots. Importantly, the switch also has a low inductance (less than 60 nH) and the ability to conduct currents with less than 100 ns rise times. The switch can be used with linear transformer drives or other pulsed-power systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/752,259, filed Jan. 14, 2013, which is incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U. S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to pulsed-power technology and, in particular, to a high-voltage, low-inductance gas switch that can be used in linear-transformer-driver cavities. 
     BACKGROUND OF THE INVENTION 
     Linear transformer drivers (LTDs) are a relatively new technology being developed because of their potential to provide high performance and greater versatility of applications at a significantly reduced cost. LTDs can potentially achieve compactness and low cost by going directly from DC-charged capacitors to a 100-ns pulse propagating on a vacuum or water transmission line, without any of the pulse-compression sections normally associated with large pulsed-power drivers. See E. A. Weinbrecht et al.,  Proc.  15 th    IEEE Pulsed Power Conf.,  170 (2005); D. Johnson et al.,  Proc.  15 th    IEEE Pulsed Power Conf ., 314 (2005); I. D. Smith et al.,  IEEE Trans. Plasma Sci . 28, 1653 (2000); J. J. Ramirez et al.,  Proc.  7 th    IEEE Pulsed Power Conf ., 26 (1989); K. R. Lechien et al.,  Phys. Rev. ST Accel. Beams  11, 060401 (2009); J. R. Woodworth et al.,  IEEE Trans. Plasma Sci . 32, 1778 (2004); and P. Sincerny et al.,  Proc.  11 th    IEEE Pulsed Power Conf ., 698 (1997). However, in order to induce a voltage pulse in a magnetically insulated vacuum transmission line with a ˜70 ns or less rise time, the inductance of the capacitors, switch, and the transmission line leading to the vacuum section of the LTD must be minimized. 
     Therefore, the switches required to power LTDs must be reliable and perform within precise parameters that include low inductance, low jitter, and high longevity. In particular, LTDs require gas switches that can be DC charged to ˜200 kV, triggered with ˜5-10 ns one sigma jitter, be low inductance, have very low prefire and no-fire rates, and have lifetimes of at least several thousand shots. See J. R. Woodworth et al.,  Phys. Rev. ST Accel. Beams  12, 060401 (2009), which is incorporated herein by reference.  FIG. 1  shows a cross-sectional side view illustration of a prior gas switch  10  designed by Kinetech, LLC and described in Woodworth et al. This switch  10  is axially symmetric about a centerline. The distance between electrodes  11  and  12  can be varied by screwing the insertable end pieces  13  and  14  in and out along the threads in the main outer “barrel” housing  15 . The switch with the threads screwed in as far as possible provides the lowest possible inductance configuration. In this configuration, the switch has a diameter of 7.5 cm and is 12 cm long. The end caps  16  and  17  of the switch are clamped in split rings that can bolt directly to the ends of the LTD capacitors. This switch has two hemispherical electrodes  11  and  12  with a 0.95-cm spacing therebetween. Four trigger pins  18  are used instead of one to minimize erosion of the trigger pins that can otherwise limit the lifetime of the switch. The four 0.3-cm diameter trigger pins  18  extend inwardly from the midplane of the housing  15  towards the center of the switch. The spark gap between each trigger pin  18  and the main electrodes  11  and  12  is 0.5 cm. The electrodes  11  and  12  are a copper-tungsten alloy, the trigger pins  18  are pure tungsten, and the end caps  16  and  17  are an aluminum alloy. The gas inlet and outlet ports are also on the midplane of the switch. The outer insulators  13  and  14  and housing  15  are a glass-fiber loaded ULTEM™ plastic and the inner liner  19  is Kel-FTM plastic. The entire switch  10  can be submerged in an insulating fluid during operation. 
     Since the discharge normally occurs from one electrode to a trigger pin and then from the trigger pin to the other electrode, this switch effectively has two 0.5-cm spark gaps.  FIG. 2A  shows an electrostatic field plot of the prior switch with 200 kV between the electrodes. For this two-dimensional simulation, the trigger pins were approximated as an annular sheet on the switch midplane. The maximum electric field at the surface of the electrodes was 270 kV/cm.  FIG. 2B  shows an electrostatic field plot of this switch with the trigger pin at +100 kV as the trigger pulse arrives at the switch. The peak electric field in the switch when the trigger pulse arrives was slightly over 500 kV/cm. This prior switch was very robustly designed, relatively simple, and triggered with less than 5 ns jitter. However, it operated at relatively high pressure (e.g., 242 psia) which can pose problems in some applications. Further, the switch&#39;s inductance was surprisingly high (e.g., 100 nH) for a switch this small. 
     Therefore, a need remains for a high-voltage, low-inductance gas switch that can be used with LTDs and other pulsed-power systems. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a high-voltage, low-inductance gas switch, comprising a hollow housing pressurized with a gas; a pair of axially symmetric opposing high voltage electrodes spaced apart and located on a centerline within the pressurized housing; and an annular trigger ring disposed on a midplane between the opposing high voltage electrodes, wherein the surfaces of the trigger ring and the opposing high voltage electrodes are shaped to provide a uniform electric field in the gap formed therebetween. The surfaces of the high voltage electrodes can have a modified Rogowski profile and the annular trigger ring can have a double concave (de-enhanced) inner surface to provide the uniform electric field. For example, the pressurized gas can comprise dry air, nitrogen, hydrogen, or sulfur hexafluoride. The housing is preferably cylindrically symmetric about the centerline with internal threads on each end and each high voltage electrode can be mounted on an externally threaded insulating end piece that can be rotatably inserted into opposing ends of the housing, thereby enabling adjustment of the spacing of the electrodes therebetween. The annular trigger ring can be mounted on the inside wall of an insulating tubular liner and the outside wall of the tubular liner can be compressibly sealed to the inner wall of the housing when the housing is pressurized. At least one spark plug can be provided at the midplane to provide a spark to pre-ionize the pressurized gas. 
     The gas switch can be air-insulated, DC chargeable to 200 kilovolts or more, triggerable, have a low jitter (5 ns or less), have pre-fire and no-fire rates of no more than one in 10,000 shots, and have a lifetime of greater than 100,000 shots. Importantly, the switch also has a low inductance (less than 60 nH) and the ability to conduct currents with less than 100 ns rise times. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers. 
         FIG. 1  is a cross-sectional side view schematic illustration of a prior gas switch. 
         FIGS. 2A and 2B  are plots of electric field intensities inside the prior switch when charged to +1-100 kV.  FIG. 2A  is a plot of DC charge with the trigger pins at ground.  FIG. 2B  is a plot of the fields in the switch when the trigger pins are at +100 kV during the triggering process. 
         FIG. 3A  is a cross-sectional side view schematic illustration of an exemplary gas switch of the present invention.  FIG. 3B  is a cutaway view schematic illustration of the gas switch.  FIG. 3C  is an exploded view illustration of the gas switch. 
         FIG. 4  is an electric field plot inside the gas switch, showing a more uniform field after profile adjustment. 
         FIG. 5  is a cross-sectional side view schematic illustration another gas switch embodiment of the present invention. 
         FIG. 6  is a cross-sectional side view schematic illustration another gas switch embodiment of the present invention. 
         FIG. 7  is a schematic illustration of an LTD brick electrical circuit for switch testing. 
         FIG. 8  is a graph of the self-break voltage as a function of pressure for the gas switch. 
         FIG. 9A  is a graph of typical trigger voltage and switch current waveforms. 
         FIG. 9B  is a graph of a typical switch power waveform. 
         FIG. 10  is a graph of trigger delay after 25,000 shots. 
         FIG. 11  is a graph of trigger jitter (RMS) after 25,000 shots. 
         FIG. 12  is a graph of peak current after 25,000 shots. 
         FIG. 13  is a graph of peak current jitter (RMS) after 25,000 shots. 
         FIG. 14  is a graph of peak current after 50,000 shots. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3A  is a cross-sectional side view illustration of an exemplary high-voltage, low-inductance gas switch  20  of the present invention.  FIG. 3B  is a cutaway perspective view illustration of the gas switch  20 .  FIG. 3C  is an exploded view illustration of the gas switch  20  which shows the dissembled switch. The gas switch  20  is preferably axially symmetric about a centerline. The switch  20  comprises an insulating hollow housing  25  that can be internally threaded to contain a high pressure gas. For example, the housing  25  can comprise a reinforced plastic (e.g., glass-reinforced polyetherimide or Nylon 66) or other suitable high-strength insulating material. Importantly, the switch  20  comprises a midplane annular trigger ring  28  having a double concave inner profile that wraps completely around the ends of opposing high-voltage electrodes  21  and  22 . The inner profile of the trigger ring  28  and the ends of opposing high voltage electrodes  21  and  22  can be shaped to provide a uniform electric field for the axially symmetric system. For example, the surfaces of the electrodes  21  and  22  can be shaped to have a modified Rogowski profile. The electrodes  21  and  22  and trigger ring  28  can be made of an ablation-resistant, electrically conductive, and easy-to-machine material, such as a copper-tungsten alloy. The trigger ring  28  can have enough mass and surface area to reliably survive 100,000 shots. The opposing electrodes  21  and  22  are spaced apart and the anode-cathode (A-K) gap between them can be adjustable (e.g., from 0.404″ to 1.154″), enabling the switch to operate at lower pressures, or voltages above 200 kV. An insertable, externally threaded insulating end piece  23  or  24  can be captured between each electrode  21  or  22  and an electrically conducting end cap  26  or  27 . Therefore, the switch gap can be adjusted by simply rotating the end caps  26  and  27  without having to remove the entire switch from the insulating fluid for adjustment. The adjustment is possible because of internal Whitworth threads  34  that reduce the stress in the outer housing  25 , and sliding O-rings  35  on the outer wall of an insulating tubular liner  29  which are compressed by the internal pressure of the switch. Therefore, the pressurized gas is contained by the internal Whitworth threads, rather than by longitudinal external rods as with some prior gas switches. The liner  29  can be one piece and can be made of polychlorotrifluoroethylene (PTCFE) or other suitable insulator material. In addition to providing sealing for the gas envelope, the liner  29  also provides UV protection to the housing  25  and end pieces  23  and  24 . The inner wall of the liner  29  can be shaped to accept the trigger ring  28  and can be undercut behind the trigger ring  28  to prevent the shockwave of the arc from damaging the insulator and causing pre-fires. Gas inlet and outlet ports  33  with cooling passages for purged dry air or other insulating gas can also be on the midplane of the switch. The trigger ring  28  can further comprise modified spark plugs  36  centrally located on the midplane of the trigger ring to provide a spark for pre-ionizing the insulating gas. The end caps  26  and  27  can be clamped in split rings  31  and  32  that can bolt directly to the ends of the LTD capacitors. The entire switch and LTD capacitors can be submerged in an insulating fluid (e.g., oil) during operation. As shown in  FIG. 3C , the housing  25  can further comprise external ribs  37  to inhibit flashover in the insulating fluid. The end caps  26  and  27  can be DC charged oppositely to high voltage and induce a voltage pulse on the opposite ends of the LTD capacitors when triggered. 
     For reliable triggering and self-break voltage scalability, a de-enhanced (concave) gap can be used on the annular trigger ring in which a uniform field can be created to lower the total inductance. See M. E. Savage and B. S. Stoltzfus,  Phys. Rev. ST Accel. Beams  12, 080401 (2009), which is incorporated herein by reference. The switch can also be made to eliminate electric field hot spots, thereby enabling it to perform better and prevent tracking of the insulator. The electric field intensity projected for the switch can be iterated to optimize the trigger ring profile for the electric fields. For example,  FIG. 4  shows electric field plots of the trigger ring—electrode gap before and after optimization of the trigger ring profile. The optimized geometry can be translated to CAD files and used in the fabrication of the parts. The unique electric field shaping between the de-enhanced (concave) annular trigger ring and the modified Rogowski electrodes causes the performance of the gas switch to be dependent on its geometry, unlike prior gas switches that use hemispherical electrodes and simple trigger pins. In particular, the de-enhanced annular trigger ring and uniquely defined geometry significantly reduces inductance compared to the prior switches that use highly enhanced trigger pins. 
       FIGS. 5 and 6  shows cross-sectional side view illustrations of other exemplary embodiments of the present invention.  FIG. 5  shows an exemplary switch  40  that has reprofiled electrodes  41  and  42 , a larger wrap-around midplane trigger ring  48 , and a centrally located pre-ionizing spark plug  46 . The pre-ionizing spark can reduce jitter.  FIG. 6  shows a similar exemplary switch  50  that further comprises an insulating inner liner  59 . 
     Testing of the Gas Switch 
     A switch test apparatus was constructed that uses the capacitors and brick design of an LTD.  FIG. 7  shows the electric circuit for the test apparatus, including the LTD brick. Table 1 shows the specifications for the test apparatus. The test brick shown comprises two capacitors, a triggered gas switch  20 , and a flowing resistive load. The two 60 nF capacitors are DC charged to opposite polarities with the gas switch between their high-voltage outputs. In an LTD system, the other ends of the capacitors would normally be coupled to a vacuum transmission line cavity. However, in the test apparatus the other ends of the capacitors are simply connected to each other through the liquid resistive load. A current-viewing resistor (CVR) is placed between the two halves of the flowing load to monitor the current in the circuit. The trigger system has a coaxial arrangement that has a fast rise time (1 ns) at 40 kV. The entire test apparatus is submerged in a tank filled with insulating fluid. When the switch is triggered and conducts current, a voltage pulse is induced on the opposite ends of the capacitors, driving a current through the flowing load resistor. See J. R. Woodworth et al.,  Phys. Rev. ST Accel. Beams  12, 060401 (2009); and I. A. Smith,  Phys. Rev. ST Accel. Beams  7, 064801 (2004). The test apparatus could be charged to relatively low voltages and fired with a small gas switch to determine the inductance of the brick assembly with the CVR and the flowing load resistor. A Swagelok filter was added to the test apparatus to remove dust and particles, preventing them from flowing through the switch when it was purged. The particle remover rate was &gt;99.9999999% at 0.003 microns, and the maximum flow rate of the filter was 225 liters/minute. This filter was installed in the purge air supply line immediately before the switch. This was done to remove any possibility of prefires due to particles which may have entered the switch. The oil was evacuated and degassed before it was pumped into the tank. There is a pump filter to keep the oil clean and circulated and an oil cooler to control the temperature 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Test apparatus specifications. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Power supplies 
                 ±100 kV 2.5 mA Glassman High  
               
               
                   
                 Voltage Power Supplies 
               
               
                 Trigger generator 
                 1 ns rise time 40 kV coaxial ASR 
               
               
                   
                 Corporation/Kinetech Trigger  
               
               
                   
                 Generator (Positive Pulse) 
               
               
                 Automated control system 
                 ASR Corporation controls and  
               
               
                   
                 software 
               
               
                 Insulating fluid 
                 Shell Dial-AX degassed, filtered,  
               
               
                   
                 and cooled 
               
               
                 Test tank 
                 24″ H × 24″ W × 24″ D Polyethylene 
               
               
                 Gas switch insulating gas 
                 Ultra Zero Air Grade 0.1 Certified  
               
               
                   
                 &lt;2 ppm H 2 0 
               
               
                 Purge air filter in line before  
                 Swagelok 0.003 micron 225 liters/ 
               
               
                 gas switch 
                 minute high purity filter 
               
               
                 CVR 
                 0.000983 Ω 
               
               
                 Capacitors (2 in series) 
                 General Atomics 60 nF double ended 
               
               
                   
                 0.580 J total energy stored 
               
               
                 System inductance (calculated) 
                 149.9 nH 
               
               
                 Measured load resistance 
                 0.926 Ω 
               
               
                   
               
             
          
         
       
     
     An exemplary gas switch was tested using the test apparatus. The specifications for the gas switch tested are shown in Table 2. The tested gas switch was designed to use different gasses including hydrogen, which operates at much higher pressures. Because of this, the switch is very robust and can survive arcs in any area surrounding the switch including the failure of capacitors. Finite element analysis determined that the minimum safety factor at 250 psi was 5.43. The switch was operated at 200 psi for these tests. No leaks or mechanical failures occurred during testing. 
     
       
         
               
             
               
               
             
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Test specifications for first exemplary switch. 
               
               
                   
               
             
             
               
                 Design Specifications 
               
             
          
           
               
                 Initial DC voltage across  
                 200 kV in a balanced ±100 kV 
               
               
                 the switch 
                 configuration 
               
               
                 Size 
                 2.75″ diameter with a length of 4.75″ 
               
               
                 Operating pressure range 
                 1 atm to 1,080 psi 
               
               
                 Current capacity 
                 Tested to 80 kA, designed to 120 kA max 
               
               
                 Gasses 
                 Zero air, N 2 , SF 6 , and H 2  (for high rep- 
               
               
                   
                 rates) 
               
               
                 Nominal electrode diameter 
                 1″ 
               
               
                 Anode-Cathode (A-K) gap  
                 0.404″ The switch only fires across this 
               
               
                 at the center of the switch 
                 gap in self-break mode. 
               
               
                 Uniformity of the electric  
                 See field plot in FIG. 4 
               
               
                 field across the electrodes 
                   
               
               
                 Minimum safety factor  
                 5.43 
               
               
                 at 250 psi 
                   
               
             
          
           
               
                 Materials Specifications 
               
             
          
           
               
                 Housing 
                 Glass-reinforced polyetherimide (PEI) or 
               
               
                   
                 glass-reinforced Nylon 66 
               
               
                 Insulator 
                 Polychlorotrifluoroethylene (PCTFE) 
               
               
                 Electrodes 
                 25% copper, 75% tungsten 
               
               
                 Trigger ring 
                 Brass, or CuW75 
               
               
                 O-rings 
                 Viton 
               
               
                 Gas lines 
                 1/8″ OD PEEK 
               
               
                 End caps 
                 Aluminum 
               
               
                   
               
             
          
         
       
     
       FIG. 8  shows the self-break voltage as a function of dry air pressure for the switch after the initial 1,000 shots. The self-breakdown voltage increases nearly linearly to +1-100 kV at 130 psi. 
       FIG. 9A  shows typical voltage and current waveforms after the initial 1,000 shots. The 60 nF capacitors were charged to +1-100 kV. The flowing resistive load was adjusted so that the system was at a near matched condition load with only a small current reversal. The trigger voltage was near 100 kV with a 10-90% rise time of 48 ns.  FIG. 9B  is a graph of the switch power waveform. 
       FIG. 10  shows the trigger delay for 1000 shots after an additional 24,000 shots had been performed.  FIG. 11  shows the trigger jitter (RMS) for 1000 shots after an additional 24,000 shots had been performed. The switch had an average trigger delay of about 56 ns with a 1-σ jitter of +1-1.2 ns. 
       FIG. 12  shows the peak current for 1000 shots after an additional 24,000 shots had been performed. The peak load current was about 58.7 kA.  FIG. 13  shows the peak current jitter (RMS) for 1000 shots after an additional 24,000 shots had been performed. 
     The above data was typical up until after 50,000 shots at which point the switch started to prefire.  FIG. 14  shows two prefires during the 1000 shots after 50,000 shots. 
     A summary of the test results is shown in Table 3. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Performance of first exemplary gas switch. 
               
               
                 Switch Performance 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Switch inductance 
                 35 nH 
               
               
                   
                 Switch jitter (one  
                 1.2 ns variation over 
               
               
                   
                 sigma) 
                 1000 shot test 
               
               
                   
                 Operational 
                 190 to 200 psi Ultra Zero 
               
               
                   
                 environment 
                 Air 
               
               
                   
                 10-90% rise time 
                 48 ns 
               
               
                   
                 90-10% fall time 
                 90 ns 
               
               
                   
                 Rep rate 
                 0.125 Hz with 1 second 
               
               
                   
                   
                 purge at 200 psi 
               
               
                   
                 FWHM of current 
                 127 ns 
               
               
                   
                 Peak load current 
                 58.7 kA with 60 nF Caps 
               
               
                   
                 Lifetime 
                 57,200 
               
               
                   
                   
               
             
          
         
       
     
     The present invention has been described as a high-voltage, low-inductance gas switch. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.