Abstract:
A crossed-field device such as a crossed-field amplifier or magnetron has a generally peripheral cathode body portion and an anode which cooperates with a crossed magnetic field to maintain emitted electrons on cycloidal and amplify an rf input signal as it travels to an rf outlet. A control electrode positioned generally at a drift region away from the crossed-field amplification region interrupts the sustained electron emission to shut down the device between working cycles, and an auxiliary electrode positioned internally of the cathode diverts electrons into a gap proximally of the control electrode to reduce the control electrode energy requirements. The cathode is carried by a support structure, and the auxiliary electrode may be a rod axially extending in a counter-bore in the support structure. The auxiliary electrode is dc biased and may advantageously operate at anode potential, thereby obviating the need for any additional power source for the auxiliary electrode. Preferably the auxiliary electrode forms an inverted magnetron arrangement with the cathode counterbore and diverts electrons from the active region without creating unwanted rf output signal components.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to provisional patent application Ser. No. 60/101,469 filed on Sep. 23, 1998. The benefit of that provisional filing is hereby claimed. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to pulsed or intermittently operated crossed-field devices such as magnetrons and crossed field amplifiers (CFAs). By way of example, U.S. Pat. No. 3,255,422 shows one such CFA device wherein a microwave entry waveguide provides radio frequency energy to an entry port for a slow wave propagating structure on an anode. A cathode is opposed to this anode across a gap. A solenoid maintains a magnetic field perpendicular to the applied electric field. The cathode is formed of a material having a secondary emission ratio greater than unity so that electrons emitted from the cathode due to the electric field follow re-entrant trajectories in the magnetic field and bombard the cathode to cause further electron emission. Energy exchange between the emitted electrons and the rf field results in amplification of the input signal, which is then coupled out at a microwave outlet port as an amplified signal. 
     Because the cathode is formed of a material selected to copiously emit secondary electrons, such devices, if not provided with a means for shutting down the electron emission, could continue to run spontaneously even when the input rf is removed. Thus, as set forth in the aforesaid U.S. Pat. No. 3,255,422, it is customary to provide a control electrode which during a turn-off phase is pulsed near anode potential to capture electrons and end the secondary electron re-emission. However, the control electrode requires a relatively high-powered pulse and high potential to dependably quench the electron flux. This may result in an inflexibility of operating characteristics, so that the device does not work dependably with power supplies or drivers having slightly different characteristics, or fails to shut down after hot, relatively long, operating cycles. Thus, for example, when used to amplify or supply high energy radar pulse sequences, shut down may become erratic when used with different models of power supply or pulse timing units. 
     Accordingly, it would be desirable to provide a crossed-field device having improved shut-down characteristics. 
     It would further be desirable to provide such a device wherein the control electrode operates at a lower potential or energy. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes deficiencies of known devices by providing a crossed-field device such as a crossed-field amplifier or magnetron wherein a distributed cathode body is spaced from an anode to provide an electric field in a traveling wave region between an rf inlet and an rf outlet. The traveling wave region is arranged to have a magnetic field oriented perpendicular to the electric field, so that some electrons emitted by the cathode cycle back to the cathode. A control electrode is positioned to interrupt the operation of the device by collecting some of the circulating electron flux in the gap, and by diverting the remainder of the electron flux to an auxiliary electrode. The diversion of electron flux to an auxiliary electrode reduces the energy requirements on the control electrode. In a preferred embodiment, the cathode extends along a cylindrical arc, and the control electrode occupies a segment extending along a minor portion of the periphery and spaced from, but along a generally continuous contour with, the cathode. The auxiliary electrode is positioned in a gap, and preferably behind the cathode syrface, so that electrons traveling along the amplification path are diverted away from the cathode and the traveling wave path before reaching the control electrode. 
     Preferably, the cathode is dimensioned appropriately for the frequency of the input rf drive, with the dimensions of height and diameter under several inches each for the case of L-band operation. The cathode is carried by an insulating support structure and the auxiliary electrode may be a rod which extends axially parallel to but spaced from the cathode, in a gap between the cathode and control electrode. The auxiliary electrode may be positioned in a counterbore in the cathode support structure, where it operates as a second anode in the crossed field to capture electrons from the gap. Preferably the auxiliary electrode is operated above cut-off so that electrons are rapidly and completely collected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the invention will be understood from the description below taken together with the drawings wherein: 
     FIGS. 1 and 1A illustrate a crossed field amplifier (CFA) device of the prior art; 
     FIG. 2 illustrates a perspective view of electrode elements of one embodiment of a device of the present invention; 
     FIG. 2A shows a top view thereof; and 
     FIG. 3 illustrates operation of the device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a perspective view of a representative cross-field amplifier device  1  of the prior art. As shown, the cross-field amplifier device includes an inlet port  3  for providing input rf microwave energy into a body  2 , where it is amplified by the crossed-field interaction as it travels to an outlet  4  which carries the amplified rf energy away. A crossed-field amplifier (CFA) tube can be described as part magnetron and part traveling wave tube. Like a magnetron it utilizes crossed electric and magnetic fields to produce rf energy from emitted electrons. Like a traveling wave tube (TWT) the electronic interaction is with a traveling wave, and the device is an amplifier. Power is generated with high efficiency for the same reasons that a magnetron operates efficiently; power is also generated at voltage levels similar to those of a magnetron, i.e., many kilovolts. As shown in FIG. 1, a CFA may look quite like a magnetron, with the same form factor but with the addition of an input port. 
     FIG. 1A schematically represents the elements and operation of a CFA. The CFA device includes a slow wave circuit, an input/output system, and an electronics system. The slow wave circuit, or delay line as it sometimes called, is a periodic structure which has the circuit characteristics of a band-pass filter. It propagates rf energy over the frequency range of interest while providing fringing electric field lines with which electrons may interact. These fields must have a phase velocity approximately equal to the velocity of the electron stream. The input/output system provides an input and output impedance transformation between the rf transmission line system external to the amplifier, and the slow wave circuit itself. These impedance transformations or circuit matches may determine the useful bandwidth of the CFA itself. The electronics system generates electrons, and confines them to an interaction area, i.e., to the slow wave circuit, where they give up energy to the rf field and thus “amplify” the input energy. The electronics system also collects electrons when they are spent. Some CFAs have a relatively large cathode which extends the entire length of the slow wave circuit. In these CFAs, electrons are generated along the entire length of the cathode, giving rise to the name “distributed emission amplifier”. The cathode is also called the sole, from which the name “emitting sole amplifier” has arisen. In the device of FIG. 1A, a control electrode is positioned at the drift region, and a crossed magnetic field is applied between the cathode and the anode. 
     The distributed emission amplifier can be arranged in a number of ways. It can be made in either a linear or a circular architecture. Amplifiers made with the circular format may collect electrons at one end of the circuit, or the input and output sections may be brought close enough together so that the electrons from the output are permitted to continue along and re-enter the interaction area at the input. Re-entrancy is employed in many amplifiers to enhance efficiency. When re-entrance is employed, however, it is possible that re-entering electrons may be modulated with information which will subsequently be amplified. This is equivalent to providing an rf feedback, and this feedback must be considered in determining the behavior of the amplifier. It is also possible to obtain re-entrance after demodulating the electron stream to eliminate such rf feedback. 
     CFAs are mostly used for high-power applications, as opposed to small signal use, and the slow wave circuit must be capable of dissipating the collected beam and transferring that energy to a heat sink. A typical use, for example, is as a broad band phase stable microwave amplifier for a coherent radar chain, to efficiently generate very high peak output power from a relatively low input voltage which can be either applied to the cathode or to an electrode similar to a TWT cathode, or may operate by grid pulsing. Such CFAs may be produced in small lightweight packages. The invention will be described below with respect to an essentially cylindrical arrangement of opposed cathode and anode elements in which the traveling wave or interaction region occupies a major portion of the circumference, between an rf inlet port and an rf outlet port. However, the construction of the invention may also be implemented in other distributed emission devices. 
     FIG. 2 shows an illustrative embodiment of the invention as a generally cylindrical CFA  10 , in a view showing the interior electrode structure, with the anode and input/output rf matching elements removed. CFA  10  has a generally cylindrical cathode  14 , control electrode  17  and auxiliary electrode  25 . The control electrode  17  is supported on an insulating ceramic block  18 , and is separated by spaces or gaps  26 ,  27  from the adjacent cathode  14 . 
     FIG. 2A shows a top view of the device of FIG. 2, with the anode and input/output elements illustrated schematically in a section perpendicular to the cylinder axis. As shown, a central support  12  carries a cylindrical cathode  14  which is spaced across a gap  15  from the inner diameter wall of the anode structure  16 . The cathode is preferably made of a cold secondary emission type material such as beryllium or platinum. For a typical L-band amplifier, the device may, have an outer diameter of approximate ten centimeters, with a cathode outer diameter of about seven centimeters and a height of approximately four centimeters. Typically the cathode is carried by a ceramic support structure, with supporting conductors of copper or other suitable metal. As further shown in FIG. 2A, an inlet  23  provides an rf input signal into the cavity or traveling wave space between the cathode surface and the anode surface, and the rf signal then travels along the peripheral gap to the rf outlet  24 . 
     Typically the outer structure, anode  16  is maintained at ground potential, while the cathode is typically ten to twelve kilovolts negative, so that electrons are drawn from the surface of the cathode into the gap  15 . The entire assembly is maintained in a permanent magnet package or solenoid (not shown) which provides a strong magnetic field in the gap  15  with lines perpendicular to the electric field. 
     Electrons emitted form the cathode  14  are accelerated radially outward because of the voltage potential between the cathode  14  and anode  16 . If there is no rf drive power, the perpendicular (axial) magnetic field will cause the electrons to cycloid back to the cathode surface since the interaction space  15  is normally operated at a voltage below cut-off. However, when rf drive power is present, electrons emitted from the cathode  14  are sorted into two groups. The first group of electrons, known as the favorable phase electrons, give up dc potential energy to the rf wave. These electrons are collected at the anode  16 . The second group of electrons, the unfavorable phase electrons, absorb some energy from the rf wave on the anode  16  circuit. With this additional energy, these electrons are driven back into the surface of cathode  14  with several hundred electron volts of energy. As a result of this electron bombardment, the cathode  14  emits new electrons with a yield δ by a process known as secondary emission. If δ is greater than one, a dense region of electrons will be maintained near the surface of the cathode  14 ; this dense region of electrons is usually referred to as the hub. Typically, the material of the cathode  14  has a secondary yield greater than two so that maintaining the hub is not a problem. The amount of dc energy given to the rf wave on the anode circuit by the favorable phase electrons more than offsets the amount of energy absorbed by unfavorable phase electrons, and hence there is a net rf amplification of the rf circuit wave on the anode  16 . 
     In this manner the device amplifies the input rf drive power, so the outlet  24  receives a greatly increased rf power. 
     As further shown in FIG. 2A, the control electrode  17  occupies a partial circumference of the cylinder, which, in this embodiment is located in drift region D the region of the inlet and outlet rf ports, and away from the traveling wave interaction area which makes up the major portion of the circumference of the device. The control electrode  17  is supported on a ceramic support  18  which may for example be formed of beryllia ceramic. The control electrode need not itself, and preferably does not, emit electrons, and therefore is formed of copper or other robust conductor. As is understood by those skilled in the art, the control electrode operates to control the emission of electrons and to abruptly stop the high power operation of the amplifier, typically by being pulsed to near anode potential to collect electrons in the drift region D away from the interaction area so that no electrons re-enter the downstream edge of the sole after the rf input pulse has terminated. In a typical mode of operation, the cathode is maintained at negative 10.5 kV with respect to ground (anode) potential, and the control electrode is pulsed toward anode potential during the turn-off phase, i.e., at the trailing edge of the rf input pulse. 
     In accordance with a principle aspect of the present invention, there is also provided an auxiliary electrode  25  across a gap from the cathode  14 , e.g., at the gap  26  between the cathode and the control electrode  17 . As illustrated, the auxiliary electrode  25  in this embodiment is a rod extending parallel to the gap  26  and within a cavity formed in the support  12  so that it is spaced across from the cathode. Here, the auxiliary electrode is positioned on the opposite side of the cathode from the anode  16  and in a sequestered space of the insulating support. It is thus positioned to operate as a second anode to trap electrons proceeding along their cyclotron trajectories around the edge of the cathode, before they can approach the control electrode  17 . For such operation, the auxiliary electrode is preferably operated from dc voltage which may advantageously be anode voltage so that no additional power source is required. 
     FIG. 3 illustrates operation of the auxiliary electrode  25  and its effect on electron paths when so energized. As shown, during early phase turn-off a substantial proportion of the electron flux at the output end of the cathode/anode gap is forced by its cyclotron trajectories to crawl around the overhung cathode edge  14   a  into a sequestered space  30  formed by the auxiliary electrode bore and control electrode gap. 
     The gap around the auxiliary electrode is an inverted magnetron gap, e.g., the outer electrode is at cathode potential and the auxiliary (inner) electrode is at anode potential. This gap is designed so that it is “cut-off” in order to more effectively obtain electron collection on the auxiliary electrode. The equation for cut-off V co  voltage in a magnetron gap is:          V   co     =       1   2            e   o       m   o                    B   2          (       r   c     -     r   a       )       2          [         r   c     +     r   a         2        r   a         ]       2                              
     where e o =1.6021×10 −19  C, m o =9.1091×10 −  kg, B is the axial (z directed) magnetic field in Tesla, r c  is the counterbore radius around the auxiliary electrode in meters, and r a  is the radius of the auxiliary electrode in meters. The radii r a  and r c  are thus chosen so that the operating voltage between the auxiliary electrode and the cathode, V, is greater than V co . 
     The auxiliary electrode need not be round as shown, but may have another shape, with the gap arranged accordingly for the magnetic field so the gap between the auxiliary electrode and the cathode is cut-off. For example, in a planar gap the cut-off voltage is given by:          V   co     =         e   o       2        m   o              B   2          d   2                              
     where d is the separation between the auxiliary electrode and the cathode, and the other parameters are as described above. 
     The preferred location of the auxiliary electrode is inside the cathode outer diameter and generally behind the space  26  between the control electrode  17  and the cathode  14  on the end associated with the rf output waveguide. Some interaction space electrons move between this gap and migrate into the auxiliary electrode between the outer anode structure  16  and the support ceramic  18  of the control electrode as illustrated by the typical trajectories of the FIG. 3, which shows Brillouin hub electrons during the early phase of turn-off when the control electrode may be, for example, about 1 kV above the cathode potential. 
     The location of auxiliary electrode may be at various positions inside the outer diameter of the cathode and a cavity  30  ahead of the control electrode. The concept also extends to the case wherein additional holes or slots are formed in the control electrode to allow more electron flow through for collection by an auxiliary electrode extending behind the holes. 
     Measurements were made on a prior art CFA device, a model VXL-1169, to determine the voltage and current required for reliable turn-off control when operating at full duty and at rated rf output power. Similar measurements were then made on a VXL-1169 modified to include an auxiliary electrode as illustrated in FIGS. 2 and 2A above. 
     The unmodified CFA device required a peak control electrode voltage and current of 12 kV and 9.5 A for dependable turn-off control, whereas the device modified with the auxiliary electrode of this invention required only 7.5 kV and 5 A to operate dependably. Thus, the pulse energy requirements of the control electrode  17  were greatly relaxed. As noted above, similar advantages are expected from a variety of other auxiliary electrode embodiments in which the inverted magnetron arrangement traps electrons before the control electrode structure or away from the traveling wave space. Advantageously, the auxiliary electrode may be positioned around an edge of the cathode to create a diverting electron-trapping field. In yet other embodiments the control electrode may be provided with through-openings through which the electrons pass to the auxiliary electrode. 
     The invention being thus disclosed and representative embodiments thereof described, further variations and modifications will occur to those skilled in the art, and all such variations and modifications are considered to be within the spirit and scope of the invention set forth in the claims appended hereto and equivalents thereof.