Abstract:
A high voltage pulse generator provides a short, fast rise, high voltage pulse from a very low impedance suitable for initiating high energy electrical discharges in liquids and high pressure gases. Its low impedance allows extremely high currents from external energy storage capacitors to be conducted through the invention once the invention has initiated an arc. Its fast rise time is suitable for initiating multiple arcs or even sheet surface discharges in high pressure gasses under suitable conditions.

Description:
This application claims benefit of U.S. Provisional application No. 60/096,157 filed Aug. 11, 1998. 
    
    
     This invention was made with Government support under Contract DASG60-97-C-0003 awarded by the Ballistic Missile Defense Organization. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF INVENTION 
     This invention relates in general to a novel low impedance generator of short, fast rise, high voltage pulses. More specifically , the invention relates to a means of initiating an electrical arc in high pressure gasses and subsequently permitting the conduction of an extremely high electric current from an external, high energy, lower voltage source after the discharge arc has been established. 
     The use of a short high voltage pulse to trigger the discharge of electrical energy stored in capacitors is generally known in the art. However, a separate trigger electrode is required in devices such as electronic flash tubes, ignitrons, and high voltage spark gap switches. Such devices can also be triggered by momentarily placing a high voltage across those electrodes intended to conduct the primary discharge. This method of triggering discharges is not generally done because the trigger device would impede the heavy current flow of the primary discharge path. 
     A high voltage trigger pulse generator placed in series with the primary stored energy discharge path, however, would have to be capable of conducting the peak primary discharge current without adding a significant impedance. This requirement generally prohibits the use of a series trigger device. The discharge current of even the small electronic flash in a camera typically exceeds a hundred amperes while the primary discharge currents of some very high energy devices can exceed a million amperes. The inductance of the typical high voltage trigger transformer winding placed in series with the primary discharge path would severely limit the pulse current. 
     A reduced secondary inductance also reduces the leakage inductance as it appears in the secondary. The reduced secondary leakage inductance will decrease the rise time of the high voltage output pulse. This is yet another reason for designing a transformer with minimal inductance. 
     If a high voltage trigger transformer is designed for minimal secondary winding inductance, the low inductance of the primary winding then becomes a problem. The generation of a high voltage pulse with a transformer requires a high turns ratio. Typically, energy is stored at a relatively low voltage in a capacitor which is then dumped into the primary of the trigger transformer using a suitable switching device. If the inductance of the capacitor, switch, and connecting leads is significant compared to leakage inductance of the trigger transformer&#39;s primary winding there will be a significant drop in the peak voltage appearing across primary winding. Reducing the secondary winding&#39;s inductance to a tolerable value will often result in an intolerably low leakage inductance appearing in the primary. 
     The ultimate low inductance pulse transformer will have but a single turn on an air core as the primary. This single turn would be in the form of a cylindrical sheet conductor with the secondary wound directly over or directly under the sheet single turn. The primary winding leakage inductance of such an arrangement can be extremely low. This inductance can be estimated by counting the number of square flux tubes that are enclosed in the space between the primary and secondary windings. Each square flux tube can be considered to represent an inductance of 1.26 uH per meter of length. The flux tubes represent inductances in parallel so the total is the inductance of a single flux tube divided by the total number of parallel flux tubes. A 6 inch diameter, 12 inch long cylindrical sheet primary, spaced 0.25 inches from the secondary, for example, would have a leakage inductance of approximately 0.013 uH. It would be difficult to hold the stray primary circuit inductances to a value insignificant compared to 0.013 uH. In reality, the stray circuit inductances would probably be several times that of the transformer primary allowing only a small fraction of the capacitor voltage to appear across the transformer input. 
     A means of overcoming the problems associated with a series triggering device just described, however, could be used with high pressure capillary discharge devices where tensile strength requirements preclude the use of electrical insulators as the supporting walls of a pressure vessel. A trigger electrode is generally placed in the center of a capillary discharge device such as an electronic flash. A high pressure capillary device, however, can require trigger voltages that exceed 50,000 volts and generate pressures above 10,000 psi. The insulation required around the conductor used to make the connection to the trigger electrode through the capillary wall would unacceptably weaken the capillary structure. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a new and novel means of generating high voltage pulses from an extremely low impedance source allowing high currents to be delivered to low impedance loads. 
     It is a further object of the invention to provide a high voltage pulse source with the capability of conducting an extremely high current from an external source into a common load. 
     It is a further object of the invention to provide a means of initiating high current plasma discharges in liquids and high pressure gasses. 
     It is a further object of the invention to provide high voltage, high current pulses with very short rise times. 
     It is a further object of the invention to provide high voltage, low impedance, fast rise pulses suitable for initiating arc discharges in liquids and high pressure gasses and multiple arc or sheet surface discharges on a dielectric material in high pressure gasses. 
     Briefly, the foregoing and additional objects are accomplished by a device consisting of an energy storage capacitor formed by thin stack of two or more conductive plates that also serve as the single turn primary winding of a pulse transformer. Each plate is separated from the adjacent plate by a layer of dielectric material. Alternate conductive plates protrude from the dielectric sheets on opposing edges of the stack allowing the plates to be interconnected so as to form a single capacitor with the terminals on opposite edges of the stack. The stack is formed around a cylinder with the capacitor terminals close together but held sufficiently distant from each other so as to provide a gap with the desired dielectric breakdown strength. If this capacitor, when charged, is suddenly discharged by short circuiting the gap, the discharge current path is the equivalent of a single turn sheet cylindrical coil that can be used as the primary of a pulse transformer. The addition of a secondary winding placed inside or wound around the outside of the hollow cylindrical capacitor will provide the high voltage output. This arrangement totally eliminates any stray inductances due to the interconnects between a separate energy storage capacitor and transformer primary winding. 
     The foregoing and additional objects, features, and advantages of the present invention will be apparent to those skilled in the art from the following detailed description of a preferred embodiment, taken with the accompanying claims and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified cross sectional view of the invention. 
     FIG. 2 is detailed cross sectional view of a preferred embodiment of the simplified cross sectional view shown in FIG.  1 . 
     FIG. 3 is a sectional view of the device taken along line  3 — 3  of FIG.  2 . 
     FIG. 4 is a schematic representation of the present invention. 
     FIG. 5 is a schematic representation of a simple circuit used to aid in the explanation of the invention&#39;s operating principles. 
     FIG. 6 is a schematic representation of a typical application of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to a more detailed consideration of the invention, reference is now made to FIG. 1, which is a simplified cross sectional view of a cylindrical structure intended to illustrate the concept of using two or more conductive plates  7 ,  8  and  9  to form both a capacitor and a single turn inductor. Each plate is insulated from the adjacent plates with a suitable dielectric material  4 . The plates are electrically connected to a pair of terminals  5  and  6  with each plate connected to the opposite terminal as is its adjacent plate. This arrangement results in a fixed capacitance appearing between the terminals  5  and  6  which can be easily calculated, by those skilled in the art, knowing the area of the plates and the dielectric constant and thickness of the dielectric material. 
     If the capacitance is charged by momentarily connecting the terminals to a voltage source, then abruptly discharged by momentarily short circuiting the terminals, the discharge current flowing in the plates will form a single turn sheet current loop. This current will rapidly increase at a rate determined by the initial charge voltage and the inductance of the sheet current loop. 
     A simple method of providing the momentary short circuit is to charge the capacitance until the air in the gap between terminals  5  and  6  breaks down resulting in an arc. The breakdown process is very fast and the inductance of the sheet current loop is very low resulting in an extremely high rate of magnetic flux change within the boundary of the sheet current loop. One or more turns of another conductor sharing this magnetic flux will have a voltage induced in it. Voltages exceeding ten kilovolts per turn are easily obtainable, thus providing the means of generating short, fast rise, high voltage pulses from a very low impedance. 
     While the inductance of the sheet current loop is a function of the circumference and length of the sheet current loop, it is also a function of the geometry of the short circuit. Terminals  5  and  6  can be the full length of the cylindrical structure along the edge of the conductive plates. A short circuit applied simultaneously along the entire edge will result in a lower inductance than a short circuit applied at opposing points somewhere along the edge. 
     The inductance of the sheet current loop, while difficult to calculate, when current distributions are not uniform, is easy to measure. The inductance and capacitance of the plates form a resonant circuit and the sudden discharge will result in a damped sinusoidal current waveform. Since the capacitance is easily determined by measurement or calculation, the inductance can be determined indirectly by measuring the frequency of the discharge waveform. The frequency can be measured using an oscilloscope to display the voltage waveform induced in a small loop of wire placed near or within the sheet current loop. 
     It also should be pointed out that the discharge current sheet is uniform around the circumference of the device when the entire length of the gap is shorted. Each capacitor plate will have a maximum current density at the end which is connected to a terminal. The current density will begin decreasing linearly at the point it encounters an adjacent plate connected to the opposite terminal, decreasing to zero at its far end. Since the current gradient is in opposite directions in adjacent plates the net result is that the total current is uniform around the circumference of the device. 
     FIGS. 2 and 3 illustrate the structural features of a preferred embodiment of the high voltage pulse generator. The main supporting element is a dielectric tube  10  upon which capacitor/coil stack  16 , comprised of alternate layers of conducting foil  7 ,  8 , and  38 , and the dielectric material  4 , are located. The dielectric tube also serves as a support for a helical secondary winding  11  and an insulating barrier between the primary capacitor/coil stack  16 . A terminal  13  at each end of the helical secondary winding provides a means of making electrical connection to the high voltage output. Typically an odd number of conducting foil layers is used so that both the outer  7  and inner  8  foil layers are connected to the same terminal  5  placing both the inner and outer foils at the same potential. This is useful in certain applications where terminal  5  can be at ground potential. In other applications, however, it would make no difference whether an odd or even number of foil layers are used. Any number of intermediate layers  38 ,  39  and  49  can be used to obtain the desired total capacitance. 
     The layers of foil are secured to the terminals by sandwiching them between a terminal clamping device  14  and  15  and the terminal bases  35  and  36 . An adjustable spark gap  12  is used to control the voltage at which the discharge occurs. This preferred embodiment uses the simple spark gap illustrated, because adequate performance for the intended application was obtained by this means. The output impedance can be further lowered and the output rise time further shortened by using the terminals  35  and  36  as a rail gap switch and triggering the discharge with a third trigger electrode as is done in a rail gap switch. This invention, with the simple spark gap shown in this preferred embodiment, would be an ideal device to trigger a rail gap switch used in a much larger version of the invention. In FIGS. 3 and 4 terminals  5  and  6  may be rods extending along the tube  10 . 
     FIG. 4 is a diagrammatic representation of the preferred embodiment illustrated in FIG.  2  and FIG.  3 . The switching device is depicted as the simple spark gap  12  used in the preferred embodiment while the foil and dielectric stack  16  is depicted as two closely spaced but electrically isolated semicircles representing the single turn sheet loop that serves as the primary of a transformer. Terminals  35  and  36  receive the input power. The transformer&#39;s secondary winding  11  is shown connected to an inductor  17  and a capacitor  18  as well as the secondary terminals  13 . The inductor  17  represents the transformer&#39;s leakage inductance as it appears to the secondary while the capacitor  18  represents the effective secondary winding capacitance. It is important to determine the values of these stray reactances when designing any embodiment of the invention because of their influence on the invention&#39;s performance characteristics. The rise time characteristics of the output pulse is a function of the value of these stray components. Additionally, there is an optimum total secondary capacitance that results in the maximum transfer of energy between the primary and secondary. 
     FIG. 5 depicts a simple circuit that can be used to illustrate the transfer of energy between two capacitors  20  and  21  connected through an inductor  22  and a switch  23 . If capacitor  20  is initially charged to some voltage and capacitor  21  is completely discharged, the closing of the switch  23  will cause the charge on the initially charged capacitor  20  to begin to charge the initially discharged capacitor  21 . The current through the inductor  22  will continue to increase until the voltage on the two capacitors is equal and the current reaches a maximum. Subsequently, the energy stored in the inductor will cause the current to continue flowing until the inductive energy decreases to zero. If the switch is opened at the instant the current reaches zero the energy represented by the initial charge will now be distributed between the two capacitors in a manner determined by their relative values. If the capacitors are of equal value all of the energy will now appear in the initially discharged capacitor  21  while the initially charged capacitor  20  will be completely discharged. If, however, the initially discharged capacitor  21  is smaller than the initially charged capacitor  20 , the initially charged capacitor  20  will not completely discharge before the current flow stops. Conversely, if the initially discharged capacitor  21  is larger than the initially charged capacitor  20 , the current flow will not stop when the initially charged capacitor  20  has completely discharged but will begin charging this capacitor in the opposite polarity until the current flow stops. This happens because at the instant the energy in the initially charged capacitor  20  is zero there is energy stored in the inductor  22  which is subsequently added to both capacitors. The reverse charge represents energy in the initially charged capacitor  20  that could not be transferred to the originally discharged capacitor  21 . Only in the case where the capacitors are of equal value will all of the initial energy be transferred to the opposite capacitor. 
     In the disclosed invention, however, the energy transfer occurs across a transformer. Energy initially stored in the capacitor/coil stack  16  is transferred to the stray secondary capacitance  18  and to any load connected to the secondary terminals  13 . In this case the effective turns ratio between the primary and secondary must be considered. The value of the stray secondary capacitance is transformed by the square of the effective turns ratio into a larger capacitance. If, for example, the effective turns ratio is ten, then the stray secondary capacitance and any additional capacitance in an external load would appear to be one hundred times greater than it is. 
     It is important to consider these capacitances in the design of any embodiment since the capacitance of the primary capacitor/coil stack would generally be matched to the apparent value of the secondary capacitance considering the effective turns ratio of the transformer. The effective turns ratio is not precisely equal to the physical turns ratio since a significant portion of the total magnetic flux is leakage flux - flux not shared by both windings. The effective turns ratio will always be somewhat less than the physical turns ratio because the primary and secondary cannot occupy the same space. 
     The determination of the effective stray secondary capacitance is not as straightforward as it may first appear. Most of this capacitance is due to the capacitance between the secondary winding and the primary capacitor/coil stack. This capacitance must be charged when a voltage is induced in the secondary winding but this capacitance is distributed along the secondary winding in a way that charges each point to a different voltage. Consequently, each point along the secondary winding appears to have a different turns ratio relating it to the primary. The effective capacitance is not the same as the value measured between the secondary winding and the capacitor/coil stack but it can be approximately determined from that value. If it is assumed that both the winding capacitance and voltage generated along the helical secondary winding are a linear function of distance along the helix, the energy stored can be related to the energy stored if the entire helix were at the potential existing at the end of the helix. Energy stored in a capacitor is a function of the square of the voltage. If the length of the conductor forming the helix is considered unity, and x represents a position along the conductor length the energy stored in a small increment dx relative to the energy existing in dx when x=1 is: 
     Relative Energy dx =x 2 dx 
     and the total energy stored in the helix capacitance relative to the energy stored if all of the helix were at the same potential is:          Relative                 Energy     =       ∫   0   1            x   2             x                                
     and,          ∫       x   2             x         =       x   3     3                            
     therefore:          Relative                 Energy     =           1   3     3     -       0   3     3       =     1   3                              
     The energy stored in the capacitance between the helical secondary and the capacitor/coil stack is one third the energy that would exist if the entire helical secondary winding were at its output potential. The distributed capacitance can therefore be represented by a capacitance at the output of the secondary that is one third the value measured between the helical secondary winding and the capacitor/coil stack. However, this only applies to situations where one end of the secondary winding is grounded or held at some fixed potential which will usually be the case. 
     Once the capacitor/coil stack has discharged its energy and the spark gap&#39;s arc has extinguished, the helical secondary winding will behave as a simple inductor with an inductance equal to that calculated for the helical secondary alone. A typical application for the invention is to trigger the discharge of high energy storage capacitor banks into a plasma that has been formed by the high voltage trigger pulse in a gas or liquid. These energy storage banks typically use a pulse forming network to a shape high energy discharge waveforms. The helical secondary winding can be designed to provide the inductance requirements of a component in the pulse forming network thus serving two purposes - triggering the discharge and shaping the high energy pulse. 
     FIG. 6 shows a diagrammatic representation of the invention  23  used in a typical application, the triggering of the discharge of a high energy pulse forming network  27  into a load  26 . The charging supply  28  is used to store electrical energy in the capacitors  29  of a pulse forming network (PFN)  27 . A spark gap  25  can be added to the secondary circuit  30  as shown if the pulse power load  26  is not an open circuit prior to the application of a high voltage trigger pulse. The spark gap  25  is adjusted to withstand the peak voltage used to initially charge the PFN  27 . Once the PFN is fully charged, a high voltage trigger generator driver  24  is used to charge the capacitor/coil stack of the invention until its spark gap  12  breaks down. This breakdown produces a short high voltage pulse at the output  30  of the invention causing the breakdown of the spark gap  25  if one is used, or the breakdown of pulsed power load  26  itself. Once an arc is established, it can be maintained with a much lower voltage than that required to initially cause the breakdown. Subsequently, the electrical energy stored in the PFN  27  will be dumped into the load  26 . In this manner, a trigger energy of a few joules or less can initiate the discharge of energy from a PFN storing many kilojoules or even megajoules of electrical energy. 
     Although the invention has been shown and described in terms of a single preferred embodiment, variations and modifications will be apparent to those skilled in the art. It is, therefore, intended that the invention not be limited to the disclosed embodiment, the true spirit and scope thereof being set forth in the following claims.