Patent Publication Number: US-5834898-A

Title: High power current regulating switch tube with a hollow electron beam

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electron devices, and more particularly, to a switch tube adapted to rapidly change states between a high voltage non-conductive state and a high current conductive state. 
     2. Description of Related Art 
     High power switching devices are known in the art for switching between conductive and non-conductive states to provide short duration, high current pulses. A switching device must be capable of standing-off high voltages when in the non-conductive state, and rapidly switching to the high current conductive state with minimal voltage drop across the device. The high current pulses provided by a switching device have various applications in the art, such as plasma ion implantation, microwave tube current or voltage regulation, and the like. 
     Presently, there are two types of high power switching devices in common usage, and a third type disclosed in a previous patent by the inventor which has certain advantages over the two other types. The first type is the beam power tetrode switch tube generally comprised of a thoriated tungsten cathode wound into a cylindrical shape, a cylindrical control grid surrounding this, a screen grid, and finally a cylindrical anode outside the cylindrical screen grid. Usually, the control grid is run at an electric potential always negative with respect to cathode (if possible) to prevent interception of electrons on it and subsequent overheating. The control grid voltage is switched from a relatively high negative voltage in the beam off mode to a less negative voltage to switch the beam on. The screen grid is arranged to be in alignment with the control grid to shield it from electron interception. It is held at a potential that is positive with respect to cathode. Finally, the anode potential must be positive with respect to cathode in order to receive electrons emitted from the cathode. There are many drawbacks to this first type of tube, including mechanical fragility of the wires comprising the cathode and grids, very high required cathode heater power, difficulty in alignment of the grid wires which can lead to grid interception and either grid emission or grid burnout, and other cathode, thermal and mechanical issues which affect reliability and which can lead to life problems when these tubes are used in high power applications. 
     The second type of switch tube in common use is the magnetron injection gun (MIG) type. This tube comprises a cylindrical cathode disposed concentrically within a modulating anode structure with a space defined between the cathode and the modulating anode. A Faraday cage collector is disposed axially from the cathode and modulating anode to receive the cathode current while preventing secondary electron emission. An axial magnetic field provided by an externally disposed electromagnet has flux lines that extend through the space into the opening of the collector. To switch the MIG switch tube to the conductive state, an electric potential, positive with respect to the cathode, is applied to the modulating anode causing current to be emitted from the cathode. The axial magnetic field bends the beam, preventing it from reaching the modulating anode, and directing it into the collector. While this type of switch tube has proven to be very reliable and long-lived, it has a generally higher voltage drop between the cathode and collector than other types of switch tubes making it less electrically efficient. Further, it requires an electromagnet and corresponding electromagnet power supply, which adds weight, complexity, and cost to the device. 
     The third type of switch tube comprises a shadow gridded tetrode device constructed from a plurality of electron guns, each having a cathode and an anode. A series of aligned grids is disposed between each cathode and anode, including a shadow grid closest to the cathode, followed thereafter by a control grid, and a screen grid. The tube also includes a suppressor grid following the screen grid having an opening generally equal to that of the edge of the cathode. In this tube, the anode includes cavities that provide a set of Faraday cage collectors to receive the cathode current. In operation, the tetrode switch tube is switched between the conductive and non-conductive states by controlling the voltage potential applied to the control grid. An example of this type of switch tube is provided by U.S. Pat. No. 4,745,324 to True, for HIGH POWER SWITCH TUBE WITH FARADAY CAGE ANODE. While the shadow gridded tetrode switch tube overcomes major limitations of both beam power tetrode and MIG switch tubes, it possesses a degree of complexity that makes it more expensive than standard beam power tetrodes, and less reliable than MIG switch tubes. 
     Accordingly, it would be desirable to provide a switching device having a high degree of current regulation with the ability to switch high current levels, fast switch response time, high voltage standoff capability, high switch efficiency, and very high device reliability, while overcoming these and other drawbacks of the prior art devices. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a high power switching apparatus is provided. The switching apparatus can hold off high voltages with zero current flow, and rapidly switch to a high current conducting state by application of a voltage to a non-intercepting control element. The total voltage drop across the switching apparatus is kept low, which translates into high overall device efficiency. 
     The high power switching apparatus comprises an annular cathode having a surface capable of emitting a hollow electron beam therefrom and an anode cavity spaced from said cathode. The cavity has an annular opening smaller in dimension than a corresponding internal dimension that defines the cavity to provide a Faraday cage collector of the hollow electron beam. A control electrode is disposed between the cathode and the anode cavity in a non-intercepting intercepting position relative to the hollow electron beam. The control electrode further comprises a first electrode element disposed outside of the hollow electron beam and a second electrode element disposed inside of the hollow electron beam. A controlling electric field region is provided between the first and second control electrode elements for modulation of the hollow electron beam. An arc suppressing electrode is disposed between the control electrode and the anode. The arc suppressing electrode further comprises a first arc suppressing electrode disposed outside of the hollow beam and a second arc suppressing electrode disposed inside of the hollow beam. The arc suppressing electrodes are at the same electric potential as the cathode. A voltage, positive with respect to cathode, is applied to the control electrodes in order to draw the hollow electron beam from the emitting surface of the cathode. The potential of the anode cavity is generally positive with respect to the cathode in order for emitted electrons to reach it, however, it need not be at a potential as high as that of the control electrodes. 
     A more complete understanding of the high power current regulating switch tube will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side sectional view of a high power switch tube of the present invention; 
     FIG. 2 is an enlarged side sectional view of the cathode of the high power switch tube; 
     FIG. 3 is an end sectional view of the high power switch tube taken through the section 3--3 of FIG. 1; and 
     FIGS. 4A and 4B are computer simulations of the high power switch tube in non-conducting and conducting states, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention satisfies the need for a switching device having a high degree of current regulation with the ability to switch high current levels, fast switch response time, high voltage standoff capability, high switch efficiency, and very high device reliability. In the detailed description that follows, like element numerals are used to describe like elements illustrated in one or more of the figures. 
     Referring first to FIG. 1, a high power switch tube 10 of the present invention is illustrated. The switch tube 10 has two main portions defined relative to a centrally disposed mounting plate 12, including an electron emitting portion disposed to the left of the mounting plate as illustrated in FIG. 1, and an electron collecting portion disposed to the right of the mounting plate. It should be appreciated that the switch tube 10 would ordinarily be operated in a vertical configuration (rather than the horizontal configuration illustrated in FIG. 1), with the electron gun portion directed downward and the collector portion directed upward. The electron gun portion may be immersed in a fluid reservoir, such as a tank of oil, in order to prevent external high voltage arcing and disperse some of the heat generated during operation of the switch tube 10. When disposed in the vertical (i.e., operational) position, the mounting plate 12 provides a surface for fixedly mounting the switch tube 10 to the reservoir or other structural element. 
     The electron emitting portion of the switch tube 10 is provided with a rugged outer structure which is generally symmetrical around a central axis of the switch tube. The outer structure includes a first cylindrical housing segment 14 that engages a circular groove provided in a surface of the mounting plate 12. A transition adapter 16 is coupled to an end of the first housing segment 14 opposite from the mounting plate 12. A second cylindrical housing segment 18 extends from the transition adapter 16. The second housing segment 18 has an inside diameter slightly smaller than the inside diameter of the first housing segment 14, and the transition adapter 16 serves to transition between the two distinct housing segments. An outer end ring 17 mates with the second housing segment 18 to partially enclose an end of the switch tube 10 in conjunction with an intermediate end ring 15 and an inner end ring 13. The mounting plate 12, first housing segment 14, transition adapter 16, outer end ring 17, and intermediate end ring 15 may be comprised of a high strength, electrically conductive, non-corrosive material, such as stainless steel. The second housing segment 18 may be comprised of a thermally conductive, electrically insulating material, such as alumina ceramic. 
     The electron emitting portion of the switch tube 10 further includes a plurality of distinct electrodes that are electrically connected at the bottom end of the device (illustrated at the left side of FIG. 1). The electrical connections are provided as a series of concentric cylinders, including an outer arc suppression cylinder 21, a cathode heater cylinder 22, a cathode support and inner arc suppression cylinder 23, a control electrode support cylinder 24 and a control electrode cylinder 25. The control electrode support cylinder 24 and the control electrode cylinder 25 are terminated by an end cap 27 that is further joined to a control electrode terminal 26. An insulated plug 19 surrounds concentrically the electrode terminal 26, and is mechanically coupled to the control electrode support cylinder 24 for structural rigidity. 
     The intermediate end ring 15 is coupled to the outer end ring 17 which, in turn, is coupled to the inner end ring 13 which is coupled to the insulated plug 19. The cathode support and inner arc suppression cylinder 23 is coupled to the inner end ring 13. The cathode heater cylinder 22 is coupled to an electrical lead structure 29 that extends through an innermost portion of the inner end ring 13 and through the insulated plug 19 to provide a cathode heater terminal 28. The outer end ring 17, intermediate end ring 15, inner end ring 13, and insulated plug 19 collectively define the end of the switch tube 10. The electron emitting portion may further include one or more absorber buttons 36 affixed to a centrally disposed plate 35 coupled to the inner arc suppression cylinder 23. The absorber buttons 36 absorb undesired RF power within the switch tube 10, as known in the art. The absorber buttons may be comprised of silicon carbide-loaded beryllium oxide ceramic or other lossy material compatible with use in a vacuum. 
     In order to keep the control electrodes cool, the control electrode cylinder 25 must have high thermal conductivity and thus may be comprised of a highly conductive material, such as copper. Similarly, the control electrode terminal 26 and the cathode support and inner arc suppression cylinder 23 may be comprised of a refractory conductive material, such as molybdenum. The insulated plug 19 may be comprised of a thermally conductive, electrically insulating material, such as alumina ceramic. The inner surface of the insulated plug 19 facing the electrode terminal may be provided with a resistive metal layer 19a, such as molybdenum-manganese metallization or aquadag (carbon). The outer arc suppression cylinder 21 and the control electrode support cylinder 24 may be comprised of a high strength, electrically conductive, non-corrosive material, such as stainless steel. The cathode heater cylinder 22 may be comprised of an electrically conductive material, such as monel or kovar. 
     The electron collecting portion of the switch tube 10 includes a third cylindrical housing segment 42 that engages a circular groove provided in the surface of the mounting plate 12 opposite from the first housing segment 14. An annular-shaped double-walled Faraday cage collector 50 is coupled to the mounting plate 12 within the third housing segment 42, defining an annular-shaped electron receiving opening 51 formed by shoulders 54 disposed in the same plane as the mounting plate. As will be further described below, the electron receiving opening 51 provides an anode of the electron gun 40. A center plate 11 is coupled to the inner edge of the electron receiving opening, which is also disposed in the same plane as the mounting plate 12. The collector 50 includes an inner wall 52 that defines an inner dimension which is greater than the electron receiving opening, and an outer wall 56 having an inner dimension slightly larger than the inner wall 52 such that a coolant space is defined therebetween. As will be further described below, the electron receiving opening is disposed in substantial alignment with the electron gun 40 of the electron emitting portion described above. The inner wall 52, outer wall 56 and shoulders 54 may be comprised of a highly conductive material, such as copper. 
     The third housing segment 42 further includes a coolant flow inlet pipe 44 and a coolant flow outlet pipe 46. The coolant flow inlet and outlet pipes 44, 46 permit the attachment of the switch tube 10 to a coolant system which includes a coolant fluid reservoir (not shown). The coolant system provides a source of coolant fluid, such as water or alcohol, to the coolant flow inlet and outlet pipes 44, 46. A coolant flow path is defined through the electron collecting portion of the switch tube 10 between the coolant flow inlet and outlet pipes 44, 46, which includes the space defined between the inner and outer walls 52, 56 of the collector 50. The coolant flow path may further include heat radiating members, such as fins, to improve the heat conductance from the electron collecting portion to the coolant system. In addition, an ion pump 48 is provided at an end of the electron collecting portion adjacent to the coolant flown inlet and outlet pipes 44, 46. The ion pump 48 provides a vacuum within the switch tube 10, as known in the art. The third housing segment 42, the coolant flown inlet and outlet pipes 44, 46, and the center plate 11 may be comprised of a high strength, electrically conductive, non-corrosive material, such as stainless steel. 
     Referring now to FIG. 2, the electron gun 40 of the switch tube 10 is illustrated in greater detail. The electron gun 40 includes a cathode 66 having an electron emitting surface 67. A heater coil 69 is embedded within the cathode 66 and is electrically coupled via an electrical lead 68 to the cathode heater cylinder 22. The heater coil 69 is used to raise the temperature of the cathode 66 sufficiently to permit thermionic emissions of electrons from the electron emitting surface 67, as is known in the art. It should be appreciated that the cathode 66 and the electron emitting surface 67 have an annular shape due to the axial symmetry of the switch tube 10, as described above with respect to FIG. 1. The electron emitting surface 67 is slightly concave, which helps to prevent emitted electrons from striking the control electrode ends 62, 63 during operation of the switch tube 10, which will be discussed below. 
     The cathode 66 may be a tungsten matrix dispenser cathode as is known in the art. The surface 67 of cathode 66 may be coated with various elements or compounds such as osmium and ruthenium (providing a so-called M-type cathode) in order to lower the required cathode temperature for a given level of electron emission as is known in the art. Further, the heater coil 69 may be made from tungsten, molybdenum, or other refractory material, or combinations thereof, as is known in the art. The heater overwrap 68 may be a conductive refractory metal such as molybdenum. The heater coil 69 may be affixed within the cathode 66 by potting in alumina ceramic 69a as is known in the art. 
     The cathode 66 is mechanically supported within a conductive shell defined by an outer support member 64 and an inner support member 65. One or more heat shields 78 may be provided between the cathode 66 and the outer and inner support members 64, 65, to control heat radiation from the cathode. The outer and inner support members 64, 65 are generally cylindrical in shape, and are mechanically and electrically coupled together through a cross member 74, and to the outer and inner arc suppression cylinders 21, 23 through a cross member 73. The forward portions of the outer and inner support members 64, 65 adjacent to the electron emitting surface 67 include shoulders 71, 72, respectively. The shoulders 71, 72 provide a focusing electrode for the cathode 66 to define the shape of the electric field region formed between the cathode and the control electrode ends 62, 63. The outer and inner support members 64, 65 may be comprised of an electrically conductive refractory material, such as molybdenum. 
     Outer and inner control electrodes 38, 39 are spaced outwardly from the cathode 66 and outer and inner support members 64, 65, and are used to control electron flow from the cathode, as will be further described below. The outer and inner control electrodes 38, 39 are mechanically and electrically coupled together through a cross member 77 and to the control electrode cylinder 24. The outer and inner control electrodes 38, 39 are electrically isolated from the cathode 66. The forward portions of the outer and inner control electrodes 38, 39 adjacent to the electron emitting surface 67 and the shoulders 71, 72 have respective electrode ends 62, 63 with an opening defined therebetween. The electrode ends 62, 63 each have a hammerhead shape with rounded outer portions 82, 83, and tapered inner portions 84, 85, respectively. Between the outer portions 82, 83 and the inner portions 84, 85, the electrode ends 62, 63 have substantially parallel surfaces that contribute to the formation of a positive electric field region to choke off electron flow from the electron emitting surface 67, as will be described below. 
     Outer and inner arc suppression electrodes 33, 34 are spaced outwardly from the outer and inner control electrodes 38, 39 and are used to prevent arc current from flowing through the control electrode modulator power supply (V C ) and to reduce the Miller effect capacitance for faster switching speed. The outer and inner arc suppression electrodes 33, 34 are mechanically and electrically coupled together through a cross member 75. The outer arc suppression electrode 33 is further coupled through a flared coupler 32 to the outer arc suppression cylinder 21, and the inner arc suppression electrode 34 is further coupled to the inner cathode support and arc suppression cylinder 23. The outer and inner arc suppression electrodes 33, 34 are electrically isolated from the outer and inner control electrodes 38, 39, and are electrically coupled to the cathode 66 and to the outer and inner support members 64, 65. 
     FIG. 3 illustrates the symmetrical nature of the switch tube 10, in which the various electrodes appear as concentric cylinders. Particularly, from the exterior of the switch tube 10 inward, the concentric cylinders include the first housing segment 14, the outer arc suppression electrode 33, the outer control electrode 38, the inner control electrode 39, and the inner arc suppression electrode 34. The electron emitting surface 67 is also illustrated between the shoulders 71, 72. As best illustrated in FIG. 1, the electron emitting surface 67 is aligned with the space defined between the control electrode ends 62, 63, and the annular opening to the collector 50. 
     The operation of the switch tube 10 in its non-conductive and conductive states will be described with reference to FIGS. 4A and 4B, wherein the electron trajectories are shown as generally horizontal lines and the equipotential contours are shown as generally vertical lines in a computer plot. Referring first to FIG. 4A, the switch tube 10 is shown in a non-conductive state with the cathode 66 and the arc suppression electrodes 33, 34 connected to ground potential, or an electric potential of zero volts. The control electrodes 38, 39 are depressed to a potential below that of the cathode 55, such as -250 volts, by the control electrode modulator power supply (V C ). The anode 51 is connected to a voltage source (V A ) to apply a positive electric potential of greater than +100 kilovolts. In this condition, there is no current (I O ) flowing through the switch tube 10. 
     In FIG. 4B, the switch tube 10 is shown in a conductive state. As in the non-conductive state, the cathode 66 and the arc suppression electrodes 33, 34 are connected to ground potential, or zero volts. A voltage, positive with respect to the cathode 66, is applied to the control electrodes 38, 39 in order to draw the hollow electron beam from the emitting surface of the cathode to the anode 51. The potential of the anode 51 is generally positive with respect to the cathode 66, however, it need not be at a potential as high as that of the control electrodes 38, 39 especially when electrons are being drawn from the cathode. 
     In the exemplary embodiment, the potential on the control electrodes 38, 39 is increased from -250 volts to +25.2 kilovolts by the control electrode modulator power supply (V C ). The potential on the anode 51 drops to an electric potential of +7.7 kilovolts. With the switch tube 10 in the conductive state, a current carrying capacity of approximately 200 amps may be achieved. Thus, it can be seen in this embodiment of the invention that the control electrodes 38, 39 functions to turn on or off the beam current with a voltage change of roughly 25 kilovolts. While all the voltages have been expressed with respect to the cathode 66 which is at ground potential, it should be understood that the switch tube 10 could also be operated with the anode at ground potential and the cathode at a negative voltage. 
     The electrons of the beam pass the anode 51 into the collector 50, and are spread over the internal surface area of the collector. By spreading the electrons in this manner, there is more even heat transfer to the coolant flow which lowers the internal surface temperature of the collector, which, in turn, extends the life of the switch tube 10. The Faraday cage collector 50 also acts to prevent secondary emission of electrons from the collector. Moreover, the positive voltage on the control electrodes 38, 39 with respect to the cathode 66 forms an ion trap which prevents ions that may be created in the collector 50 from returning to the cathode. Ionic back-bombardment of the cathode is known to lead to reduced cathode life, and therefore its prevention is a desirable feature of the invention. 
     Having thus described a preferred embodiment of a high power current regulating switch tube, it should be apparent to those skilled in the art that certain advantages of the within system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.