Abstract:
A plasma gun with two gap electrodes on opposite ends of a chamber of ablative material such as an ablative polymer. The gun ejects an ablative plasma at supersonic speed. A divergent nozzle spreads the plasma jet to fill a gap between electrodes of a main arc device, such as an arc crowbar or a high voltage power switch. The plasma triggers the main arc device by lowering the impedance of the main arc gap via the ablative plasma to provide a conductive path between the main electrodes. This provides faster triggering and requires less trigger energy than previous arc triggers. It also provides a more conductive initial main arc than previously possible. The initial properties of the main arc are controllable by the plasma properties, which are in turn controllable by design parameters of the ablative plasma gun.

Description:
BACKGROUND 
       [0001]    The present invention generally relates to plasma guns, particularly to ablative plasma guns, and also relates to triggers for electric arc devices. 
         [0002]    Electric arc devices are used in a variety of applications, including series capacitor protection as described in U.S. Pat. No. 4,259,704 of the present assignee, high power switches, acoustic generators, shock wave generators, and pulsed plasma thrusters. Such devices have two or more electrodes separated by a gap of air or another gas. A bias voltage is applied to the electrodes across the gap. A triggering device in the gap ionizes a portion of the gas in the gap, providing a conductive path that initiates arcing between the electrodes. 
         [0003]    Conventional spark gap triggering involves application of high voltage pulses to a trigger pin. The trigger pulse magnitude depends largely on the bias voltage across the spark gap. Although such pulse triggering is widely used, the cost of the trigger source and its electronics is several times higher than the cost of the main spark gap itself. For example, in a 600V system the required trigger voltage is at least 250 KV for a gap of 20 mm. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    An aspect of the invention resides in a plasma gun with two gap electrodes in diagonally opposite ends of an open-ended chamber of ablative material such as an ablative polymer. A divergent nozzle ejects and spreads an ablative plasma at supersonic speed. 
         [0005]    Another aspect of the invention resides in using the ablative plasma to trigger a main arc device, such as an arc crowbar or a high power switch, faster and with less trigger energy than existing triggers. 
         [0006]    Another aspect of the invention resides in controlling the initial properties of a triggered arc in a main arc device via properties of an ablative plasma, which are in turn controllable by design parameters of an ablative plasma gun. 
         [0007]    Another aspect of the invention resides in reducing cost for triggering arc devices by means of inexpensive ablative plasma gun designs and by the reduced triggering energy and related trigger circuit requirements. 
     
    
     
       DRAWINGS 
         [0008]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0009]      FIG. 1  is a sectional view of an ablative plasma gun according to aspects of the invention. 
           [0010]      FIG. 2  is a general circuit diagram of an ablative plasma gun used to trigger an electric arc device. 
           [0011]      FIG. 3  is an exemplary circuit diagram of an ablative plasma gun trigger of an electric arc device. 
           [0012]      FIG. 4  is a sectional view of an ablative plasma gun triggering an arc crowbar. 
           [0013]      FIG. 5  is a perspective view of an ablative plasma gun triggering an arc crowbar. 
           [0014]      FIG. 6  shows an embodiment of an ablative plasma gun molded of a single material in a single mold. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  is a sectional view of a plasma gun  20  with first and second electrodes  22 ,  24 , a cup of ablative material  26  and a divergent nozzle  30 . A pulse of electrical potential applied between the electrodes  22 ,  24  creates an arc  32  that heats and ablates some of the cup material  26  to create a highly conductive plasma  34  at high pressure. The plasma exits the nozzle  30  in a spreading pattern at supersonic speed. 
         [0016]    Characteristics of the plasma jet  34  such as velocity, ion concentration, and spread, may be controlled by the electrode dimensions and separation, the dimensions of the interior chamber  28  of the cup  26 , the type of ablative material, the trigger pulse shape and energy, and the nozzle shape. The cup material may be Polytetrafluoroethylene, Polyoxymethylene Polyamide, Poly-methyle methacralate (PMMA), other ablative polymers, or various mixtures of these materials. The chamber  28  may be generally elongated and cylindrical with a closed end, to minimize trigger pulse energy, ablation response time, and ejection time, and maximize plasma production, or it may be another shape. 
         [0017]    The plasma gun may have a base  36  for supporting the electrodes  22 ,  24  and the cup  26  as shown. A cover  38  may enclose the other elements and provide the nozzle  30 . The cup  26  may be retained between the base  36  and the cover  38  as shown. The base  36  and the cover  38  may be made of the same material as the cup or of different materials, such as a refractory or ceramic material. Each electrode  22 ,  24  has a respective distal end  23 ,  25  that enters the chamber  28  through the cup  26  walls. The electrodes  22 ,  24  may be formed as wires as shown to minimize expense, or they may have other known forms. The distal ends of the electrodes  23 ,  25  may be diagonally opposed across the chamber  28  and separated along the length of the chamber  28  as shown to provide a gap for the gun arc  32 . The material of the electrodes, or at least the distal ends of the electrodes, may be tungsten steel, tungsten, other high temperature refractory metals/alloys, carbon/graphite, or other suitable arc electrode materials. 
         [0018]    The inventors have innovatively recognized that an ablative plasma gun embodying aspects of the present invention provides a more efficient arc gap trigger than conventional triggering methods mentioned above.  FIG. 2  is a general schematic diagram of an ablative plasma gun  20  that may be used as a trigger in a main gap  58  of a main arc device  50 . In the context of the foregoing sentence, the term “main” is used to distinguish elements of a larger arc-based device from corresponding elements of the present plasma gun (e.g., used as a trigger), since the plasma gun also constitutes an arc-based device. The main arc device may be for example an arc crowbar, a series capacitor protective bypass, a high power switch, an acoustic generator, a shock wave generator, a pulsed plasma thruster, or other known arc devices. 
         [0019]    For readers desirous of general background information in connection with an example main arc device, reference is made to U.S. patent application Ser. No. 11/693,849, filed Mar. 30, 2007 by the assignee of the present invention, titled “Arc Flash Elimination Apparatus And Method”, and herein incorporated by reference in its entirety. This application describes an innovative arc crowbar that may be triggered by an ablative plasma gun embodying aspects of the present invention. The arc crowbar has two or more main electrodes separated by a gap of air or another gas in a pressure-tolerant case. Each electrode is connected to an electrically different portion of a power circuit. An ablative plasma gun is mounted in the gap. When an arc flash is detected on the power circuit, the arc crowbar is triggered by a voltage or current pulse to the plasma gun. The gun injects ablative plasma into the crowbar gap, reducing the gap impedance sufficiently to initiate a protective arc between the main electrodes that quickly absorbs energy from the arc flash and opens a circuit breaker. This quickly stops the arc flash and protects the power circuit. 
         [0020]    Generally, a main arc device  50  has two or more main electrodes  52 ,  54  separated by a gap  58  of air or another gas. Each electrode  52 ,  54  is connected to an electrically different portion  60 ,  62  of a circuit, for example different phases, neutral, or ground. This provides a bias voltage  61  across the arc gap  58 . A trigger circuit  64  provides a trigger pulse to the ablative plasma gun  20 , causing it to eject ablative plasma  34  into the gap  58 , lowering the gap impedance to initiate an arc  59  between the electrodes  52 ,  54 . 
         [0021]      FIG. 3  shows an example of a circuit used in testing an arc crowbar  70 . An arc flash  63  on the circuit  60 ,  62  is shown reducing the bias voltage  61  available across the gap  58 . The impedance of the main electrode gap  58  may be designed for a given voltage by the size and spacing of the main electrodes  52 ,  54 , so as not to allow arcing until triggering. Characteristics of the plasma  34  may be determined by the spacing of the gun electrodes  22 ,  24 , the ablative chamber  28  dimensions, the trigger pulse shape and energy, the material of the chamber  28 , and the dimensions and placement of the nozzle  30 . Thus the impedance of the main gap  58  upon triggering can be designed to produce a relatively fast and robust main arc. 
         [0022]      FIGS. 4 and 5  show the ablative plasma gun  20  as may be configured in one example embodiment to trigger an arc crowbar  70  in a pressure-tolerant case  72 , as described in the foregoing patent application. Upon receiving a trigger signal  74 , the trigger circuit  64  sends a trigger pulse to the ablative plasma gun  20 , causing it to inject an ablative plasma  34  into the gap  58  between main electrodes  52 ,  54 ,  56  of the crowbar to initiate a protective arc  59 . The case  72  may be constructed to be tolerant of explosive pressure caused by the protective arc, and may include vents  73  for controlled pressure release. 
         [0023]    The arc crowbar electrode gap  58  should be triggered as soon as an arc flash is detected on a protected circuit. One or more suitable sensors may be arranged to detect an arc flash and provide the trigger signal  74  as detailed in the related patent application. In the case of a 600V system, during arc flash the voltage across the gap  58  is normally less than 250 volts, which may not be enough to initiate the arc  59 . The ablative plasma  34  bridges the gap  58  in less than about a millisecond to enable a protective short circuit via the arc  59  to extinguish the arc flash before damage is done. 
         [0024]    In a series of successful tests of an arc crowbar  70 , the crowbar electrodes  52 ,  54 ,  56  were about 40 mm diameter spheres, each spaced about 25 mm from the adjacent sphere, with sphere centers located at a radius of about 37.52 mm from a common center point. The trigger was an ablative plasma gun  20  with a cup  26  made of Polyoxymethylene with a chamber  28  diameter of about 3 mm and chamber length of about 8 mm. The nozzle  30  was located about 25 mm below the plane of the electrode  53 ,  54 ,  46  sphere centers. 
         [0025]    Gap bias voltages ranging from about 120V to about 600V were triggered in testing by the ablative plasma gun using a triggering pulse 8/20 (e.g., a pulse with a rise time of about 8 microseconds and a fall time of about 20 microseconds) with respective current and voltage ranges from about 20 kA to about 5 kA and from about 40 kV to about 5 kV. For example, a gap bias voltage of about 150V was triggered by a trigger pulse of about 20 kV/5 kA. In contrast, a conventional trigger pin would require a trigger pulse of about 250 kV for this same bias voltage, making the conventional trigger pin and its circuitry several times more expensive than the main electrodes. 
         [0026]      FIG. 6  shows an embodiment  20 B of the plasma gun molded of a single ablative material in a single mold. This would provide an incremental cost reduction in production in view of the relatively low cost and favorable molding properties of polymers such as Poly-oxymethylene. Such construction and low cost can make the plasma gun easily replaceable and disposable. Electrode lead pins  40 ,  42  may be provided for quick connection of the plasma gun to a female connector (not shown) on the main arc device, with appropriate locking and polarity keying as known in connector arts. Alternately (not shown), the cup  26  of  FIG. 1  can be made replaceable by providing it with lead pins for a female connector in the base  36 , and threading the cover  38  onto the base  36 . 
         [0027]    It will be appreciated that an ablative plasma gun embodying aspects of the present invention may be used as both a main arc device, and as a trigger. For example, an ablative plasma gun may be provided as a main arc device in an acoustic generator, a shock wave generator, or a pulsed plasma thruster, and may be triggered by a smaller ablative plasma gun as described herein. 
         [0028]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.