SECONDARY EMISSION CATHODE AND TUBE

Abstract: A crossed-field amplifier (10) has permanent magnets (22,23), pole pieces (20,21), a cylindrical anode (12), and a cathode (11) in the form of a water-cooled cylindrical heat sink (34) with radially extending portions (35) having outer faces coated with a P-N junction semiconductor (37) which is biased to the conductive state to cause the crossed-field amplifier (10) to amplify. The P and N regions (39,38) of the semiconductor (37) are connected to a cathode modulator (57) which is pulsed to produce conduction in the P-N junction (61) and thereby allow secondary emission from the cathode (11) into an interaction space (19) between the semiconductor (37) and an anode slow wave structure (14) that receives RF drive pulses from a source (58). Reverse bias voltage applied by the modulator (57) prevents secondary emission from the cathode (11). Only low voltages need be applied to the cathode P-N junction (61) to completely deactivate the crossed-field amplifier (10). Removal of the RF drive pulse applied to the anode-slow-wave circuit (14,15,16) and removal of the DC high voltage power supply (29) are not required.

Description: This invention relates to secondary-emission structure comprising a cathode comprising a semiconductor material having a P region and an N region separated by a P-N junction; and means for producing electrons impacting one of said regions with sufficient energy to produce secondary emission from said one of said regions. The invention relates further to crossed-field amplifier tubes comprising such a structure. Crossed-field amplifier tubes (CFAs) are usually used only in the one or two highest-power stages of an amplifier chain, where their efficiency is significant, and are usually preceded by a medium power travelling wave or klystron tube which provides most of the chain gain. CFAs may be of the re-entrant type where the DC electric field between the anode and cathode and the transverse DC magnetic field cause the electrons emitted from the cathode to be accelerated by the electric field and gain velocity, but the greater the velocity, the more the path of the electrons is bent by the magnetic field. As a result, in the absence of an RF field in the interaction space between the cathode and anode, the electrons are bent back to impinge upon the cathode without reaching the anode and, ideally, there is no current through the tube. An RF field of the correct frequency will interact with some of these electrons to extract energy from them and cause them to reach the anode and produce an anode current. In CFAs, it is possible in most cases to use a cold cathode. Even with a "good" vacuum inside the tube, there are sufficient gas molecules present so that some will be ionized when sufficient RF power is provided to the input of the CFA. Some of the free electrons thus produced will be driven back to the cathode. Alternatively, the high RF electric field at the cathode may produce these free electrons by field emission effects from the cathode. The returning electrons will initiate secondary emission from suitable cathode material and full cathode current will very rapidly build up. Under high average power condition, cooling of the cathode may be necessary to prevent overheating. Removal of the applied RF electric field does not cause the tube to immediately cease conducting thereby resulting is oscillation or noise output unless the DC electric field is also removed. In the prior art, cold secondary emitter cathodes of the non-semiconductor type were used in crossed-field amplifier tubes. These tubes may contain an auxiliary non-emitting control electrode located between the cathode and the anode of the amplifier. A tube operating directly from a DC supply may be turned off with a control voltage pulse (positive with respect to the cathode) applied to the control electrode at the time of removal of the RF drive pulse to collect electrons passing through the drift region and to cause the tube to turn off even though high voltage is still applied. The control electrode forms a segment within a drift region of the periphery of the cylindrical surface of the cathode but insulated from it. A disadvantage of the prior art pulsing technique is that the cut-off µ, the ratio of the anode voltage to cut-off voltage, is low, approximately 3.0. The cut-off current drawn by the control electrode is approximately 25% of the rated beam current. Cooling of the control electrode is difficult because it is electrically insulated from the cathode which is also cooled to prevent thermionic emission. Also, since a DC-operated control electrode CFA must withstand full DC voltage continuously without breakdown, its peak-power rating cannot be made as great as that of a comparable cathode-pulsed tube. US-A-3 646 388 describes a travelling wave crossed field device having a cathode structure in which spaced interdigitated emissive cathode members have alternately varying emissive properties and are spaced apart by approximately 1 electron cycloid trajectory throughout an interaction region. Alternate cathode members are mutually isolated. By biasing the lower secondary electron emissive material cathode members slightly positive with respect to the other cathode members, the emissive members, the device can be turned off relatively rapidly. A ratio, µ, of anode voltage to cut-off voltage of 30 may be achieved with this known device. Materials used for the two sets of cathode members are titanium and platinum. US-A-3 036 234 describes a magnetron having a cylindrical cathode comprising a body of semi-conductor material in which a central portion is P-type and a thin diffused peripheral region is N-type. The resultant P-N junction is reverse biased. The amount of secondary emission from the cathode surface is varied within limits by varing the junction bias voltage applied. To initiate operation by primary electron emission, a thermionic cathode, or a separate P-N junction portion of the semiconductor cathode provided with a caesium coating on the N-type surface with the P-N junction biased separately to avalanche breakdown point may be used, or the entire semiconductor cathode P-N junction may be raised momentarily to avalanche breakdown. It is stated that at a certain reverse bias voltage which is about one-half the avalanche breakdown voltage, carrier multiplication begins to occur which increases until the voltage reaches the avalanche breakdown. The reverse bias voltage is therefore to be varied in a range below but approaching the avalanche breakdown voltage so that the number of majority carriers in the thin conductivity-type region contiguous to the emitting surface likewise varies, and comprises the source of electrons available for secondary emission. According to the present invention, a secondary-emission structure of the type defined hereinbefore at the beginning is characterised by means for forward biasing said P-N junction into the conduction state and for reverse biasing into the non-conduction state; said one region producing secondary emission electrons when said P-N junction is forward biased into the conductive state and not producing secondary emission electrons from said one region when said P-N junction is reverse biased in the non-conduction state. A preferred embodiment of the invention comprises a crossed-field amplifier having a cathode in the form of a P-N junction semiconductor, with the P and N regions of the semiconductor connected to an energy source which is pulsed to produce conduction in the P-N junction and thereby allow secondary emission. A reverse bias voltage prevents secondary emission from the cathode. The amplifier requires only low voltages to be applied to the cathode solid state P-N junction and can be completely deactivated even without the removal of the RF drive pulse or the DC high voltage power supply, which need not be pulsed. The amplifier will typically have µ's of 100 and cut-off current of a fraction of a percent of the rated beam current. In addition, during the interpulse period, the amplifier remains completely passive. The aforementioned aspects and other features of the invention are explained in the following description taken in conjunction with the accompanying drawings wherein: FIG. 1 is a schematic view of a crossed-field amplifier incorporating the switched P-N junction cathode of the invention; FIG. 2 is a longitudinal cross-sectional view of a crossed-field amplifier having a switched P-N junction cathode and a slow-wave structure anode; FIG. 3 is a more detailed longitudinal sectional view of the cathode of FIG. 2 taken along section lines III-III of FIG. 4; FIG. 4 is an isometric view of the cathode in partial section taken along section lines IV-IV of FIG. 3; FIG. 5 is an axially transverse cross-sectional schematic view of a crossed-field amplifier tube having both cathode and anode slow-wave structures with the cathode being of the P-N junction semiconductor type; and FIG. 6 is an isometric view in partial cross-section of the cathode of FIG. 5 showing in more detail an interdigital slow-wave structure with P-N junction semiconductor emitting surfaces. Referring now to FIG. 1, there is shown a schematic view of an embodiment of the invention in the form of a crossed-field amplifier tube 1. Tube 1 comprises an anode 2, a cathode 3, and slow-wave structure(s) shown as the input loop 4 and output loop 5. The slow-wave structure(s) are integral with the anode alone or both the anode and cathode. The loops 4 and 5 also represent the input and output coupling to the slow-wave structure(s). The cathode 3 is a P-N junction 6 semiconductor where the semiconductor is selected to be a material whose secondary emission is greater than one. Suitable materials are gallium arsenide, cadmium sulfide and cadmium telluride. A pulsed bias voltage source 7 is connected across the P-N junction 6 of cathode 3 to bias the junction into conduction to allow secondary emission or to strongly back bias the junction to prevent secondary emission. The high DC voltage source 29 may be continuously applied to the tube 1. The microwave signal from source 58 applied to the slow-wave input 4 appears amplified at the load 59 of the output 5 of the slow-wave structure when the cathode junction 6 is in the conduction state. No amplification occurs when the junction is biased highly into cut-off. The secondary emitting surface of the cathode 3 may be either the P-type or the N-type material forming the semiconductor junction 6. In order to be useful as a cold cathode, the semiconductor material from which the cathode is made in the form of a P-N junction must have a secondary emission ratio greater than one. The secondary emission ratio is the ratio of the average number of electrons emitted from the surface of the cathode for each electron which strikes the cathode with a given amount of energy. In the preferred semiconductor materials, gallium arsenide, cadmium sulfide and cadmium telluride, the secondary emission is greater than one for electron energy levels corresponding to approximately 50-75 V (volts). Therefore, a tube made with such a cathode requires a relatively low RF input signal to provide a secondary emission and hence amplification. FIG. 2 shows a longitudinal cross-sectional view of a crossed-field amplifier tube 10 which incorporates the solid state switchable semiconductor cathode 11 of this invention. The tube 10 comprises in addition to the cathode 11 a conventional anode 12 comprising a cylindrical electrically conductive shell 13 to which radially extending vanes 14 are electrically attached. Alternate vanes 14 are electrically connected to straps 15, 16, respectively, to complete the slow-wave anode structure. The straps 15, 16 have a radial gap extending circumferentially over the spacing between two or more adjacent vanes 14. An input RF connector 17 is attached to one of the vanes 15, 16 on one side of the gap and an output RF connector 18 is connected to the other of the vanes 15, 16 on the other side of the gap. There is an interaction region 19 between the cathode 11 and the vanes 14. The tube 10 also comprises soft iron magnetic pole pieces 20, 21 which are brazed to the opposite ends of the cylindrical shell 13 of the anode. Magnets 22, 23, in contact with the pole pieces 20, 21, provide an axial field in the interaction space 19 between the anode 12 and cathode 11. The center hole 25 of the pole piece 21 is sealed with vacuum seal 26. The upper pole piece 20 supports the cathode 11 with a ceramic insulator 27 and a metallic spacer 28. The ceramic insulator 27 is capable of withstanding the high voltage provided by the high voltage DC supply 29 which is connected between the cathode and ground. The anode 12 is grounded. A cathode modulator 57 between the high voltage supply 29 and the cathode 11 provides a modulating pulse between the conductors 30, 31 of the cathode 11. Electrically non-conducting seal 33 (Fig 3) provides a vacuum seal between conductors 30, 31. The interior region 32 of tube 10 is a vacuum region enclosed by the cathode 11, insulator 27, pole pieces 20, 21, shell 13, RF connectors 17, 18, and vacuum seals 26, 33. Referring now to FIG. 3, there is shown in detail the structure of the cathode 11 of FIG. 2. Reference should also be made to the sectioned perspective view in FIG. 4 of the cathode taken along a section line corresponding to IV-IV of FIG. 3. The cathode 11 comprises a cylindrical heat sink 34 having radially extending portions 35. The number of radially extending portions and their circumferential extent should correspond with the number of vanes and their circumferential extent of the anode slow-wave structure in order to reduce spurious modes within the amplifier tube for reasons well known to those skilled in the art. The outermost surface 36 of the radially projecting portion 35 is electrically and mechanically bonded to a solid state semiconductor material 37 containing a PN junction with the N material 38 in contact with the portion 35 and the P region 39 facing the interaction region 19 of the tube. The most exterior portion of the regions 35 have axial grooves 40 and circumferential grooves 41 leaving square surface regions 36 to which the semiconductor material 37 is bonded. The resulting reduction in the area of contact of the radial projection 35 with the semiconductor material 37 reduces the mechanical stresses imposed upon the semiconductor material by the temperature change experienced by the cathode from ambient temperature to its normal operating temperature. Electrical connection to one terminal of the modulator 57 shown in FIG. 2, is through either or both cylindrical electrical conductors 31, 43. Electrical connection to the P region 39 is made by the electrical conductor 44 which is in the form of a rectangular grid which is in ohmic contact with the P region 39. Conductor 44 is formed in the shape of a grid in order to provide P region surfaces 45 which are in direct contact with impinging electrons so that the secondary emission therefrom is not impeded. Conductor 44 has a tab 47 which is in bonded electrical contact with the cathode end shield 46. End shield 46 is mechanically supported by a cylindrical electrical conductor 48 and a cylindrical ceramic insulator 49 which is bonded at its outer cylindrical surface 50 to the heat sink 34. Electrical conductor 30 is in electric contact with cylinder 48 and electrical connectors 31 and 43 are in electrical contact with heat sink 34 thereby providing through the cathode end shield 46 and gridded conductor 44 electrical connections to both sides of the PN junction 61 through the N region 38 and P region 39. Heat sink 34 is in electrical connection with upper cathode end shield 51 and the outer cylindrical electrical conductor 43. Cooling water flow, shown by direction arrows 52, provides cooling for the heat sink 34 which minimizes the thermal expansion of heat sink 34 and the consequent stresses induced in the semiconductor material cathode 37. Pipe 53 penetrates water seal 33 of electrically non-conductive material to provide water near the bottom of heat sink 34 which water flows through the cylindrical chamber formed by heat sink 34, conductor 31, and end shield 46, through the exit hole 55 into the cylindrical chamber formed by conductors 31, 43 from whence the water exits from pipe 56 through water seal 330. Referring back to FIG. 2, the conductors 30, 31, which are electrically connected to opposite sides of the PN semiconductor material 38, are connected to cathode modulator 57 which is serially connected to the continuous high voltage DC power supply 29. The input and output RF connectors 17, 18 are connected to microwave source 58 and load 59, respectively, Operation of the crossed-field amplifier tube 10 is obtained by providing a microwave frequency from source 58 through the input RF line 17 to one end of the anode slow-wave structure 60 with a load 59 connected to the output RF terminal 18. The cathode modulator 57 provides a voltage pulse of a polarity such that the PN junction of cathode 11 is forward biased. By forward biasing the PN junction, a high density of carriers is injected into the P-type region 39. These carriers diffuse across the P region to the secondary emission surfaces 45. The tube does not conduct anode current until the RF drive signal is applied by the microwave source 58. Thus, where the microwave source 58 is a continuous microwave source, pulsing of the modulator 57 to a forward bias condition of the PN junction 61 will cause the signal provided by source 58 to be amplified and to appear at the load 59 during the time that the cathode modulator 57 is providing forward biasing. Alternatively, the microwave source may be a pulsed microwave source which is of shorter duration but coincident with the pulse provided by the cathode modulator 57 in which case the amplified microwave energy appearing at the load 59 will be of the same pulse duration as the signal provided by the pulsed microwave source 58. The coincident application of the RF drive signal from source 58 and the forward biasing of the PN junction 61 initiates an electron multiplication process which, assisted by the reentrant electrons bombarding the surfaces 45 of the cathode 11, generates sufficient secondary emission electrons and hence anode current to allow the device to amplify. The process of obtaining the reentrant electrons in cold cathodes of crossed-field amplifier tubes is well known to those skilled in the art. The crossed-field amplifier tube 10 is turned off by reverse biasing the PN junction 61 by a negative voltage being applied to the N region 38 relative to the P region 39. When the semiconductor cathode 11 is reversed biased, the P material becomes a nonconductor. In the nonconducting state, charges build up on the P surfaces 45 of the cathode 11 reducing the secondary emission below a level necessary to sustain the anode current. Once the tube has shut off, no further anode current will flow and amplification of the signal provided by the microwave source will cease. The cut-off voltage required to reverse bias the PN junction is in the order of 100 volts. The distributed PN junction semiconductor cathode shown in FIGS. 2-4 has been fabricated in the form of a cathode slow-wave structure which couples strongly to the anode slow-wave structure 60. The resultant RF field produced in the interaction region 19, in conjunction with the DC field between the cathode 11 and the anode 12 and the magnetic field produced by magnets 22, 23, produce the initial electrons from the surfaces 45 of the cathode 11 which return to strike the cathode surface 45 to produce the secondary emission. It is well known by those skilled in the crossed-field amplifier art that the microwave signal to be amplified may be applied to an input of the slow-wave circuit of the cathode whose other end is terminated in a microwave load. The resultant RF field generated in the interaction region is amplified by the interaction of electrons with the DC voltage between the anode and the cathode and the crossed magnetic field. The switched semiconductor cathode of this invention may be substituted for the cold cathode material used in the prior art cathodes. FIG. 5 shows an axially transverse cross-sectional schematic view of a tube having a PN junction semiconductor cathode 80 and cathode slow-wave structure 62 suitable for excitation by a microwave RF source 58, for causing the PN junction to be conductive by modulation circuitry 57 and for connection to a high voltage DC source 29. The cathode slow-wave structure 62 is also connected by a line 38 to a matched termination 82. The details of an illustrative slow-wave line 62 and these connections are shown in FIG. 6. The anode slow-wave anode structure 60 of FIG. 5 may be the same type as that of FIG. 2, the load 59 being connected to the output line 18 and a matched termination 83 being connected to the input line 17. The electronic space charge 89 moves in the direction of direction arrows 90. The cathode and anode slow-waves move in the opposite direction of arrows 91, 92, respectively. Referring now to FIG. 6, it is seen how the cathode 11 of FIGS. 2-4 may be modified in order to allow the cathode to be used as a source of energy which is to be amplified in the crossed-field amplifier tube. FIG. 6 shows an isometric view of the cathode 80 slow-wave structure 62, the cathode 80 having a cylindrical form with the longitudinally centered portion of the structure comprising an interdigital slow-wave line cut through a cylindrical wall 84 to produce finger heat sinks 34 spaced by gaps 85. The slow-wave structure 62 will support a slow-wave propagating around the circumference of the structure. A slot 94 defines ends 95 of the interdigital line which are coupled respectively to the coaxial line 42 connected to an RF source 58 and the coaxial line 38 connected to the RF matched termination 82. The structure 62 is fabricated from an electrically conductive material such as copper. As can be seen in FIG. 6, a perforation at the ends of the gaps 85 progresses completely around the wall 84 to the slot 94 thereby dividing the wall 84 into upper and lower sections supported by electrical insulators 93 to the upper and lower cathode end shields 86, 87, respectively. The P regions 39 of the P-N junction semiconductor material 37 are connected via the grid wires 44, 47 to the cathode end shields 86, 87. The N regions 38 are electrically connected to the finger heat sinks 34. The end shields 86, 87 are electrically connected to each other and to one terminal 88 of the cathode modulator 57 and through the high voltage DC supply 29 to ground. The cathode end shields 86, 87 are electrically insulated from cylinder 84 by insulators 93. The heat sinks 34 are connected to the remaining terminal 88 of the modulator 57. Pulsing of the modulator 57 causes conduction of the PN junction of the semiconductor material 37 through grid wires 44 to the heat sinks 34. In operation, the RF source 58 may continuously supply RF energy to the cathode slow-wave circuit 81 and the high voltage DC source 29 may be continuously applied between the anode (not shown in FIG. 6) and the cathode 80 with RF energy appearing at the anode output 18 into load 59 only when the cathode modulator 57 applies a voltage to the PN junction semiconductor material 37 to cause the junction to become conducting. Although the illustrative embodiments of the invention have described the outermost surface of the cathode P-N junction as being of P-type material, the N-type material of the junction may alternatively be used as the outer layer with little change in performance of the tube. Having described a preferred embodiment of the invention, it will be apparent to one of skill in the art that other embodiments incorporating its concept may be used. It is felt, therefore, that this invention should not be limited to the disclosed embodiment but rather should be limited only by the scope of the appended claims.

Claims: A secondary-emission structure comprising a cathode (11) comprising a semiconductor material having a P region (39) and an N region (38) separated by a P-N junction (6, 61); and means (29, 20, 21, 22, 23) for producing electrons impacting one of said regions with sufficient energy to produce secondary emission from said one of said regions; characterised by means (7) for forward biasing said P-N junction into the conduction state and for reverse biasing into the non-conduction state; said one region producing secondary emission electrons when said P-N junction (61) is forward biased into the conductive state and not producing secondary emission electrons from said one region when said P-N junction (61) is reverse biased in the non-conduction state. A structure according to claim 1, characterised in that said means producing electrons impacting one of said regions comprises: means (12, 34) producing an electric field external to and in the vicinity of said one region. A structure according to claim 1, characterised in that said P- and N-type semiconductors regions (39, 38) are gallium arsenide. A structure according to claim 1, characterised in that said P- and N-type semiconductor regions (39, 38) are selected from the group of semiconductor materials consisting of gallium arsenide, cadmium sulfide, and cadmium telluride. Crossed-field amplifier tube (10) comprising a structure according to claim 1, as a cathode (11) and further comprising: an anode (12) with a slow-wave structure (14) adjacent said cathode (11) and forming an interaction region between said slow-wave structure (14) and said cathode (11); means (29) applying a DC electric field between said anode (12) and said cathode (11) in said interaction region; means (23, 20, 21, 22) applying a DC magnetic field transverse to said electric field; means (58) applying an RF signal to said anode slow-wave structure (14); means (59) terminating said anode slow-wave structure (14) in a load; the interaction of the DC magnetic field, the RF signal, and the DC electric field producing electron impact on said cathode (11); and said cathode (11) providing secondary emission electrons in the interaction region from electrons impacting on the cathode (11) and thereby producing amplification in said tube (1) when the cathode (11) is biased into the conduction state and no amplification when the cathode (11) is biased into the non-conduction state. A tube according to claim 5, characterised in that said P-N-type semiconductor regions are gallium arsenide. Crossed-field amplifier tube; comprising a structure according to claim 1, as a cathode (11) said cathode (80) having a slow-wave structure (62); an interaction region between an anode (12) and said cathode, said anode having a slow-wave structure (14); said cathode slow-wave structure (62) having a plurality of surfaces (45) nearest said interaction region; each one of said surfaces (45) comprising P- and N-type semiconductor material layers with a P-N junction, one of said layers being most adjacent said interaction region; said slow-wave structures (14, 62) having an input and an output; means (58) for applying a microwave signal to the input of said cathode slow-wave structure (62); means (82) for applying an RF matched termination to the output of said cathode slow-wave structure (62); means (83) for applying an RF matched termination to the input of said anode slow-wave structure (14); means (59) for applying a load to the output of said anode slow-wave structure (14); means (29) for applying a DC electric field between said anode (12) and said cathode (80) in said interaction space; and means (23, 30, 21, 22) for applying a DC magnetic field transverse to said electric field in said interaction space, whereby the application of a biasing voltage to produce P-N junction conduction results in secondary emission current by said electron impact from the cathode (80) with resultant amplification to the load of the input microwave signal, and the application of said biasing voltage to produce non-conduction of said P-N junction (61) results in no secondary emission current and no amplification of the input microwave signal. A tube according to claim 7, characterised in that said P- and N-type semiconductor material layers are selected from the group consisting of gallium arsenide, cadmium sulfide, and cadmium telluride. A structure or tube according to claim 1 or 5 or 7, characterised in that said means (7) biasing said P-N junction (61) into the conduction state and into the non-conduction state is a pulse means. A structure or tube according to claim 9, characterised in that said means (7) biasing said P-N junction provides a conduction voltage sufficient to provide conduction and a reverse voltage below the avalanche breakdown voltage of the P-N junction (61) to provide the non-conduction state.

Inventors: MACMASTER, GEORGE H., NICHOLS, LAWRENCE J.
Applicants: RAYTHEON COMPANY
Application year: 1986
Main classification: H01J23
IPC codes: H01J23, H01J25, H03F3, H01J1