Abstract:
A spark gap switch, including a first planar electrode including a discharge portion and a support portion. The spark gap also includes a second planar electrode parallel to and spaced apart from the first electrode and includes a discharge portion and a support portion. The discharge portions are mutually opposite, and the support portions are mutually staggered.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to high voltage switches and, more particularly, to a multistage spark gap switch that is more compact than those presently known. 
     A spark gap switch is a high voltage closing switch that is used in pulsed power systems and for protection from transients. A basic spark gap switch consists of two electrodes separated by an insulating medium that can be vacuum or a fluid (gas or liquid). The switch is initially open. It closes upon the formation of a conductive plasma channel (spark) in the insulating medium between the electrodes when a sufficiently high voltage difference is imposed on the electrodes. The conductive channel is formed by a breakdown mechanism that can be driven in one of two ways. The first way (self-breakdown) involves the application of a voltage difference across the electrodes that is higher than the voltage breakdown threshold of the switch, i.e., the voltage at which the electric field in the gap between the electrodes exceeds the electric strength of the fluid, or induces sufficient electron emission from the surfaces of the electrodes into a vacuum. The second way is to induce breakdown at a voltage difference across the electrodes that is below the voltage breakdown threshold. This is done by using a third, trigger electrode to briefly raise the electric field in the gap between the electrodes, or by means such as radiation or a change in insulator pressure that induce degradation of the electric strength of the insulating medium. The simple and robust structure of spark gap switches, and their ability to self-close and to float to high voltages, makes them popular components of devices such as Marx generators. 
     The repetition rate of the operation of a spark gap switch is limited by the time required for the plasma to recombine and for the heat associated with the discharge to be dissipated so that the insulator returns to its initial electric strength. Therefore, high repetition rate spark gap switches commonly use a fluid (gas or liquid) insulator that flows through the interelectrode discharge gap. Nevertheless, the repetition rate of these spark gap switches usually is only a few tens of hertz. In addition, the high flow rates required by some applications tend to degrade switching reproducibility and introduce complications in overall system design. 
     FIG. 1A shows a multistage spark gap switch, which is essentially a series of two-electrode spark gap switches connected back to back. Electrodes  10  are held apart by insulating spacers  12  to define discharge gaps  14 . The total switch voltage is divided capacitively among discharge gaps  14 , allowing discharge gaps  14  to be very small. This gives the multistage structure fast recovery times, enabling operation at repetition rates upwards of several kilohertz. If the insulating medium is a gas, the pressure of the gas can be atmospheric, simplifying the mechanical and operational complexity of the switch. Fluid flow rate can be very low, or fluid flow may not be required at all. The small discharge gap and low pressure allow the switch to operate in a less violent discharge mode, which considerably increases the lifetime of the electrodes and hence of the switch as a whole. 
     Historically, the multistage spark gap switch, then called a “quenched spark gap”, was first used in the 1920s in sparking transmitters because of its fast recovery time and its high repetition rate. Newer transmitter technologies rendered the multistage spark gap switch obsolete in this application, and it has found little application since then. Until recently, high energy, high voltage pulsed power applications required only a low repetition rate, for which a single stage spark gap switch is adequate. The higher repetition rates of the newest high voltage pulsed power generators requires a different switch technology. In principle, the multistage spark gap switch of FIG. 1A is appropriate for these high repetition rates. In practice, however, the length of a typical multistage spark gap switch gives it an undesirably long closing time and an undesirably large inductance in the conducting phase. The extra length of a multistage spark gap switch, compared with an equivalent single stage spark gap switch, also complicates the layout of a generator with many such switches and may increase the size of the generator, thereby degrading its performance in some applications because of the increased weight, larger inductance and longer rise time associated with the larger size. 
     Thus there is a widely recognized need for, and it would be highly advantageous to have, a multistage spark gap switch design that is shorter than those presently known. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a spark gap switch, including: (a) a first substantially planar electrode including a discharge portion and a support portion; and (b) a second substantially planar electrode parallel to and spaced apart from the first electrode and including a discharge portion and a support portion, the discharge portions being mutually opposite, and the support portions being mutually staggered. 
     According to the present invention there is provided a spark gap switch including: (a) a first stack of at least two substantially planar, mutually parallel electrodes, each of the electrodes including: (i) a discharge portion, and (ii) a support portion; the discharge portions of adjacent electrodes being spaced apart and mutually opposite, the support portions of adjacent electrodes being mutually staggered. 
     According to the present invention there is provided a spark gap switch including: a first stack of at least two substantially planar, mutually parallel electrodes, each of the electrodes including a discharge portion, the discharge portions of adjacent electrodes being mutually opposite and spaced apart by at most about one millimeter. 
     In the prior art spark gap switch of FIG. 1A, spacers  12  must have a certain minimum length to ensure that the spark is confined to discharge gaps  14  and does not propagate from one electrode  10  to the next along the outer surface of an intervening spacer  12 . In practice, this length is several (typically three) times the width of any one discharge gap  14 . Electrodes  10  are nonplanar, so that when electrodes  10  are stacked as shown, peripheral gaps  16  that are wider than discharge gaps  14  accommodate spacers  12 . The main contribution to the length of these prior art spark gap switches is the width of peripheral gaps  16 . 
     FIG. 1B shows an alternative prior art design of a multistage spark gap switch in which planar electrodes  10 ′ are separated by insulating spacers  12 ′. In this design, discharge volumes  14 ′ are not well-defined and may overlap onto spacers  12 ′. Therefore, plasma that is produced in discharge volumes  14 ′ attacks spacers  12 ′. This leads to frequent surface breakdowns on spacers  12 ′ that result in irregular operation and short lifetime. 
     As noted above, to eliminate surface breakdowns along the spacers, the potential surface discharge path along the spacers should be several times longer than the path length of the volume discharge. In the design of FIG. 1B, these path lengths are equal. An improved design in this respect, but still lacking well-defined discharge regions, is shown in FIG.  1 C. Planar electrodes  10 ″ are separated by insulating, spacers  12 ″ that have corrugated outer surfaces. The corrugations increase the lengths of the spark propagation paths along the outer surfaces of spacers  12 ″, but in practice, in such a design, the threshold voltage for surface breakdown can not exceed the threshold voltage for volume breakdown. Therefore, even in the design of FIG. 1C, undesired surface discharges occur quite often. 
     Another disadvantage of the designs of FIGS. 1B and 1C is that it is impractical to produce spacers  12 ′ for gaps of about 1 millimeter or less, or spacers  12 ″ for gaps of a few millimeters or less, for three reasons. First, spacers  12 ′ and  12 ″ generally are made of ceramic materials, which are too fragile to withstand the mechanical shocks associated with repeated switch discharge. Second, the corrugations of spacers  12 ″ are less effective in preventing a surface breakdown on such a small scale. Third, small insulators are more sensitive than large insulators to local imperfections such as impurities in the ceramic. 
     FIG. 2 is a partial perspective view of a simple multistage spark gap switch of the present invention. In FIG. 2 are shown three planar electrodes  20   a ,  20   b  and  20   c , representative of a stack of parallel planar electrodes  20 . Each electrode  20  has a discharge portion  24  and a support portion  26 . Discharge portions  24  are positioned opposite each other to define discharge gaps  28  therebetween. Spacers  22 , of which only one is shown in FIG. 2, are placed between support portions  26 , as in the prior art multistage spark gap switches, to separate electrodes  20 . Support portions  26  also serve to conduct heat away from discharge portions  24 . Unlike the prior art multistage spark gap switch, support portions  26  are staggered so that spacers  22  separate nonadjacent electrodes  20 . So, for example, in FIG. 2, support portion  26   b  is staggered with respect to support portions  26   a  and  26   c . As a result, the length of the multistage spark gap switch of FIG. 2 is determined only by the thickness of electrodes  20  and the width of discharge gaps  28 . 
     A. Anvari and O. Steinvall, in “Study of a 40 kV multistage spark gap operated in air at atmospheric pressure”,  Journal of Physics E , vol. 6 (1973) pp. 1113-1115, presented a multistage spark gap with planar, disc-shaped electrodes separated by annular separators. Their design resembles the design of FIG.  1 B. However, to confine the discharge portions of the electrodes to the vicinity of the electrode centers, and in particular to avoid spark propagation along the sides of the spacers, the electrodes were provided with small central holes, and a trigatron arrangement was used to trigger the switch. The electrodes were spaced 4 mm apart. This design is unsuitable for electrodes spaced about one millimeter or less apart because the separators would be too fragile to withstand the shocks associated with repeated discharge. Another disadvantage of this design is its lack of a proper self-closing capability, due to the equal lengths of the potential surface and volume discharge paths, which result in frequent surface discharges along the surfaces of the separators. 
     In a multistage spark gap switch the present invention, planar electrodes  20  have relatively large, well-defined discharge portions  24  and are spaced relatively close to each other, as compared to prior art multistage spark gap switches. This gives the multistage spark gap switch of the present invention more compactness, a longer lifetime and better reproducibility than the prior art multistage spark gap switches, as well as self-closing capability. 
     The embodiment of FIG. 2 is not a preferred embodiment of the present invention. It is presented herein only to illustrate the principle of the present invention. Preferred embodiments of the present invention are presented below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIGS. 1A,  1 B and  1 C are schematic depictions of prior art multistage spark gap switches; 
     FIG. 2 is a partial perspective view of a simple embodiment of the present invention; 
     FIG. 3 shows a preferred embodiment of a planar electrode; 
     FIGS. 4A and 4B show two designs for spacers; 
     FIG. 5A is a perspective view of a stack of the electrodes of FIG. 3, in two interleaved substacks, separated by the spacers of FIG. 4A; 
     FIG. 5B is a schematic perspective illustration of a stack of the electrodes of FIG. 3 in three interleaved substacks; 
     FIG. 6 shows an end cap; 
     FIG. 7 shows an end flange; 
     FIG. 8 shows a ceramic sleeve; 
     FIG. 9 illustrates three different trigger schemes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is of a multistage spark gap switch that is more compact than those known heretofore. 
     The principles and operation of a multistage spark gap switch according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Referring now to the drawings, FIGS. 3A (top view) and  3 B (side view) show a preferred planar electrode  30  of the present invention. Electrode  30  includes a central discharge portion  32  from which project three lobes  36  that constitute the support portion of electrode  30 . Discharge portion  32  is annular and has a central hole  34 . Each lobe  36  includes a hole  38  that accommodates a spacer. 
     FIGS. 4A and 4B show two designs  40  and  42  for spacers. Spacer  42  is drawn partly in section to show socket  44  that accommodates tip  46  of a preceding spacer  42 . FIG. 5A shows an assembled stack of  20  parallel electrodes  30 , separated by spacers  40 . Note that lobes  36  of successive electrodes  30  are staggered, so that there are two interleaved substacks of  10  electrodes  30  each, with electrodes  30  of each substack separated by spacers  40 . Holes  34  define a channel  48  for axial fluid flow. Channel  48  also enhances the coupling between discharge gaps by optical mechanisms, mainly UV photons. 
     Preferably, electrodes  30  are made of aluminum and spacers  40  or  42  are made of alumina. Aluminum has a relatively low sputtering coefficient, which prolongs the lifetime of electrodes  30 , and a relatively high heat conductivity, which shortens recovery time and also prolongs the lifetime of electrodes  30 . If the insulating medium is air, the main sputtering product is alumina, the same material as spacers  40  or  42 . Other electrode materials, such as brass, tend to produce a conductive coating on spacers  40  or  42 . 
     In general, an electrode stack of the present invention includes N independent interleaved substacks, staggered with respect to each other. The electrodes of any particular substack are N discharge gaps apart. FIG. 5A illustrates the usually preferred case of N=2. N&gt;2, with the concomitant longer spacers  40  or  42 , and longer and narrower lobes  36 , is useful if electrodes  30  are particularly thin or if the gaps between electrodes  30  are large. FIG. 5B illustrates the case of N=3, with three interleaved substacks of electrodes  30 ′, each substack with its own set of spacers  40 ′. 
     Let g represent the axial gap width between two electrodes and let d represent the thickness of an electrode. As a rule, the axial length S=N(g+d)−d of the portion of spacers  40  or  42  that separates two electrodes  30  of a particular substack should be at least 3 times the corresponding cumulative axial gap width G=Ng. For example, if electrodes  30  are 2 mm thick and the gaps between electrodes  30  are 0.5 mm wide, then with N=2 (cumulative gap width of 1 mm), the spacers are 3 mm long, and with N=3 (cumulative gap width of 1.5 mm), the spacers are 5.5 mm long, so either N=2 or N=3 is a satisfactory design. If electrodes  30  are 3 mm thick and the gaps between electrodes  30  are 1 mm wide, then N=3 (cumulative gap width of 3 mm) gives a spacer length of 9 mm, which is satisfactory, but N=2 (cumulative gap width of 2 mm) gives a spacer length of 5 mm, which is unsatisfactory. 
     The rule of thumb S≧3G gives the following relationship for the ratio R between electrode thickness d and gap width g: 
     
       
           R ≧2 N /( N −1) 
       
     
     So N=2 requires R≧4, N=3 requires R≧3, and as N grows large, the minimum value of R approaches 2. Using corrugated spacers, as in the prior art design of FIG. 1C, the rule of thumb can be relaxed, and electrodes  30  can be made thinner for a given gap width g, producing an even more compact switch. 
     The complete multistage spark gap switch of the present invention includes appropriate packaging for the stack of electrodes. Separate components of packaging for the electrode stack of FIG. 5A are shown in FIGS. 6,  7  and  8 . 
     FIGS. 6A (perspective axial section) and  6 B (top view) show an end cap  50 , two of which are mounted on the outermost electrodes  30  of the electrode stack of FIG.  5 A. End cap  50  includes three pins  52  that fit into holes  38  of the adjacent outermost electrode  30  and three sockets  54  that accommodate the outermost spacers  40 . End cap  50  also includes a central hole  56  that helps to define channel  48 . 
     FIGS. 7A (axial section),  7 B (transverse section) and  7 C (perspective axial section) show an end flange  60 . The electrode stack of FIG. 5A is mounted between two such end flanges  60 , with the outermost electrodes  30  pressed against end caps  50  and against end flanges  60  to form an ohmic contact. A stepped center depression  62  in one face of end flange  60  accommodates end cap  50  in step  70 . Mounting socket  64  in the other face of end flange  70  is used for mounting the multistage spark gap switch. Sockets  66  are provided for the necessary HV connections. Channel  68  is an interface for the insulating fluid: in one end flange  60 , channel  68  is a fluid inlet; in the other end flange  60 , channel  68  is a fluid outlet. The multistage spark gap switch is sealed using a cylindrical ceramic sleeve  80 , shown in axial section in FIG.  8 A and in perspective in FIG. 8B, and two o-rings (not shown). Step  74  of center depression  62  accommodates one o-ring. The electrode stack is positioned inside sleeve  80  and the two end flanges  60  are pressed onto the two ends of sleeve  80 , so that the two ends of sleeve  80  are accommodated in step  72  of center depression  62  and the o-rings form a seal against sleeve  80 . Holes  76  admit insulating rods (not shown) whose ends are threaded to receive nuts (also not shown); these rods and nuts hold the assembled multistage spark gap switch together. 
     In a variant on the design illustrated in FIGS. 3-8, electrodes  30  lack central holes  34  and end caps  50  lack central holes  56 . The insulating fluid flows through this variant around the peripheries of electrodes  30 . 
     Two different variants of this multistage spark gap switch correspond to three different trigger schemes, illustrated in FIG.  9 . The principle of all three schemes is to induce a voltage breakdown by momentarily raising the voltage in the discharge gaps above the self-breakdown voltage. In FIG. 9, the multistage spark gap switches are represented schematically by stacks of electrodes  30  defining channel  48  and flanked by end flanges  60 . Also shown schematically in FIG. 9 are sources  84  and sinks  86  of insulating fluid. 
     In the first trigger scheme, illustrated in FIG. 9A, two multistage spark gap switches are connected back to back, and the HV trigger pulse is introduced to the two center end flanges  60 . The trigger generator is capacitively isolated from the spark gap switches. 
     In the second trigger scheme, illustrated in FIG. 9B, the two center end flanges  60  are combined to form a single center flange  61 . This scheme uses a direct ohmic contact to the center electrodes  30 . Center flange  61  has sockets analogous to sockets  66  and two center depressions analogous to center depression  62 . Instead of a channel  68 , center flange  61  has a central circular hole (not shown) that helps to define fluid channel  48 . The mechanical advantage of this variant over a simple multistage switch with the same total number of stages is that in this variant, cumulative departures from design criteria such as strict parallelism of electrodes  30  and uniform interelectrode gap widths are accumulated over only half as many stages. 
     The third trigger scheme, illustrated in FIG. 9C, uses a capacitive trigger. A circumferential conductor, such as a conducting ribbon  82 , encircles ceramic sleeve  80  at the middle of the stack. The trigger pulse is introduced to ribbon  82  and the pulse voltage is capacitively divided between the ribbon-stack capacitance (high) and the half-stack capacitance (low). Thus, the trigger voltage is transferred to the middle region of the electrode stack, and the process proceeds as in the second trigger scheme. The electrical advantage of the third scheme is its inherent capacitive isolation of the trigger generator and its consequent low trigger current/energy consumption. 
     As noted above, one of the advantages of multistage spark gap switches generally is that they can be operated at atmospheric pressure. This is true, of course, for the switch of the present invention. Nevertheless, using air as the insulating medium, it has been found advantageous to operate the switch of the present invention at pressures from 0 to 12 psi above atmospheric, with the preferred range being from 3 to 5 psi above atmospheric pressure. Because the breakdown voltage increases monotonically with the pressure of the gaseous insulating medium, using pressures several psi above atmospheric increases the dynamic range of the switch. 
     Substituting inert gases such as nitrogen or noble gases such as helium or xenon, or mixtures thereof, for air as the insulating medium has the advantage of increasing switch lifetime. 
     Preferred structural and operational parameters for an N=2 multistage spark gap switch of the present invention are as follows: 
     gap g=0.5 mm 
     insulating medium: air at between 3 psi and 5 psi above atmospheric pressure 
     voltage drop per gap: between 1 kV and 2 kV 
     current: hundreds of amperes to several kiloamperes 
     The total holdoff voltage is the product of the voltage drop per gap and the number of gaps. In a typical 39-stage switch of the present invention, this holdoff voltage is about 40 kV. 
     while the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.