FAST WARM-UP CATHODE ARRANGEMENT

Abstract: A dish-shaped dispenser cathode (28) is mounted at one end of a cathode housing (10), with a heater wire coil (42) mounted behind the cathode (28). A warm-up power supply (70) electrically connected between the central region and the periphery of the cathode (28) causes heating current to flow through the cathode (28) during a short period of time at the start of cathode operation to provide rapid warm-up of the cathode (28). A sustaining power supply (76) electrically connected between the respective ends of the heater wire coil (42) causes sustaining current to flow through the heater wire coil (42) to provide sustaining heat to the cathode (28) by radiation from the heater coil (42) to maintain the cathode (28) at its desired operating temperature.

Description: FAST WARM-UP CATHODE ARRANGEMENT TECHNICAL FIELD This invention relates generally to thermionic cathodes, and more particularly relates to a fast warm-up cathode arrangement especially suitable for use in traveling-wave tubes. BACKGROUND OF THE INVENTION In traveling-wave tubes an electron beam is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic wave. Certain applications of traveling-wave tubes require that operational conditions be established in a very short time, such as about one second, after the tube is initially turned on. Thus, it is necessary that the cathode employed to emit the beam electrons be capable of rapid warm-up. In the past warm-up of directly heated cathodes has been achieved by applying a high potential, or overvoltage, to the cathode at turn-on, and timing devices were utilized to switch the high potential on and off. Such overvoltage techniques not only could result in overheating of the cathode, but complex mechanical and electrical arrangements were required to enable switching of the overvoltage on and off. Even then, warm-up times were excessive for some applications. A quick-heating cathode for an electron tube is described in U.S. Patent 3,299,317 to J. W. Kendall, Jr. In this cathode a wire braid is connected in series with the cathode cylinder. The braid has a high electrical resistance when hot and a low electrical resistance when cold, thus permitting large amounts of current to initially surge through the braid to heat the cathode directly at turn-on. After the initial high current surge, the braid becomes hot and its electrical resistance becomes high. When the braid is hot, less current passes through it for direct heating of the cathode; however, at this time the braid also heats the cathode indirectly due to its high electrical resistance. A further fast-heating cathode for an electron tube is disclosed in U.S. Patent 2,996,643 to F. C. Johnstone et al. In this cathode arrangement a first voltage is initially applied across a filament spaced from the back surface of the cathode, causing the filament to emit thermionic electrons. A second voltage applied between the filament and the cathode accelerates the emitted electrons to the back surface of the cathode, These electrons bombard the back surface of the cathode to produce rapid heating of the cathode. After the cathode reaches electron emission temperature, the voltage between the cathode and filament is removed, and thermal radiation from the filament maintains the cathode at its operating temperature. SUMMARY OF THE INVENTION It is an object of the present invention to provide a simple, reliable, and inexpensive cathode arrangement capable of rapidly reaching its desired operating temperature without any tendency for overheating. It is a further object of the invention to provide a fast warm-up cathode arrangement which does not require any electron bombardment source or special variable resistance materials. It is still another object of the invention to provide a long-life cathode arrangement capable of reaching its desired operating temperature in less than one second. A cathode arrangement according to the invention includes a cathode member and a heater wire coil disposed adjacent to the cathode member. Heating current is caused to flow through a first current path including at least a portion of the cathode member during a short period of time to provide rapid warm-up of the cathode member to a desired operating temperature. Sustaining current is caused to flow through a second current path including the heater wire coil to provide sustaining heat to the cathode member by radiation from the heater wire coil to maintain the desired operating temperature. Additional objects, advantages, and characteristic features of the present invention will become readily apparent from the following detailed description of a preferred embodiment of the invention when considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a longitudinal sectional view illustrating a cathode arrangement according to the invention; FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1 and partly broken away; FIG. 3 is a schematic circuit diagram of the cathode arrangement of FIG. 1; and FIG. 4 is a plot showing current as a function of time flowing through portions of the arrangement of FIG. 1. DETAILED DESCRIPTION OF THE INVENTION Referring to FIGS. 1 and 2 with greater particularity, a cathode arrangement according to the invention may be seen to include a tubular housing 10 which may be of a metal such as molybdenum or a metal alloy such as Kovar, for example. Attached to one end of the housing 10 is an annular plate 12, which may be of molybdenum, for example, defining a central circular aperture 14. The back surface of the plate 12 is provided with an annular mounting flange 16 which projects into the interior of the housing 10. Attached to the end surface of the mounting flange 16 is a cathode support member 18, which may be of molybdenum or a rhenium alloy, for example, having a central dish-shaped portion 20 and a plurality of legs 22, 24, and 26 that are attached to the mounting flange 16. In the specific exemplary embodiment illustrated in FIGS. 1 and 2, three attachment legs 22, 24, and 26 are shown for illustrative purposes, it being understood that other numbers and configurations of supports alternatively may be employed. A dispenser cathode 28 is mounted on the central portion 20 of the support member 18. Cathode 28 may comprise a sintered tungsten member which has been impregnated with suitable electron emissive material such as barium aluminate, for example, although it should be understood that other cathode materials may be used instead. Cathode 28 is configured as a dish- shaped member of a diameter slightly less than that of aperture 14 in plate 12, with typical diameters ranging from about 0.1 to about 0.3 inch. As a specific example for illustrative purposes, in a preferred embodiment of the invention, the cathode member 28 may have a diameter of 0.15 inch and thickness of 0.008 inch. Cathode member 28 is preferably made with a slightly concave configuration to aid in focusing the emitted electrons into a beam. The central portion 20 of the cathode support member 18 is configured similarly and prevents emissive material from evaporating out of the back of the cathode member 28, thereby extending the life of the cathode. As shown in FIG. 2, the cathode 28 and the cathode support portion 20 are both provided with a plurality of aligned arc-shaped slots 30 disposed along each of a plurality of concentric circles in order to reduce the initial surge of current during the direct heating phase of operation of the arrangement. By employing adjacent to the cathode a shadow grid having arc-shaped segments aligned with and matching the slots 30, proper focusing of the generated electron beam may be maintained. A cylindrical header 32 of electrically insulating material such as alumina ceramic is mounted within the housing 10 behind and slightly spaced from the cathode support member 18. Header 32 defines a central cylindrical portion 34 projecting toward the cathode support member 18 and to which is attached a heat shield 36 which may be of molybdenum, for example. Heat shield 36 has a tubular portion 38 extending from the header 32 toward the cathode 28 and a dish-shaped portion 40 mounted slightly behind the cathode support portion 20 and configured similarly to the portion 20. Mounted on the heat shield 36 between the portion 40 and the cathode support portion 20 is a heater wire coil 42 which supplies sustaining heater current for the cathode 28. The coil 42 may be of a tungsten-rhenium alloy (typically 97% tungsten and 3% rhenium), for example. Heater wire portion 44 emerging from one end of the coil 42 extends through a hole in header 32 and is electrically connected to an eyelet 46 mounted on the opposite side of the header 32 from the heater coil 42. Eyelet 46, in turn, is electrically connected to a sustaining power supply lead 48. Heater wire portion 52 emerging from the other end of the coil 42 is connected to the heat shield 36 which, in turn, is connected via an electrically conductive ribbon 54 to the housing 10. A lead 56 connects the housing 10 to an appropriate level of reference potential such as cathode potential. In order to provide a direct electrically conductive path to the central region of the cathode 28, an electrically conductive rod 58, which may be of nickel, for example, is attached to the cathode support portion 20. As shown in FIG. 1, the rod 58 preferably extends through an axial aperture in the support portion 20 into direct contact with the cathode 28. Rod 58 is coaxially mounted within housing 10 and extends through axial apertures in heat shield portion 40 and header 32. An electrically conductive ribbon 60 electrically connects the end of rod 58 remote from the cathode 28 to an eyelet 62 mounted on the side of the header 32 away from the cathode 28. Eyelet 62, in turn, is electrically connected to a warm-up power supply lead 64. Referring to FIG. 3, in order to furnish heating current to the cathode 28 a fast warm-up power supply 70 is provided having output terminals 72 and 74. Terminal 72 is connected to lead 64 which applies the warm-up power supply voltage to the central region of cathode 28, while terminal 74 is connected to the aforementioned level of reference potential. As a specific example for illustrative purposes, power supply 70 may provide a voltage of about 1 volt at a current of around 15 amps. In order to furnish sustaining current to the heater coil 42 a sustaining power supply 76 is provided having output terminals 78 and 80. Terminal 78 is connected to lead 48 which applies the sustaining power supply voltage to one end of heater coil 42, the other end of coil 42 being connected to power supply terminals 74 and 80. As a specific example for illustrative purposes, power supply 76 may provide a voltage of about 6 volts at a current of around 1.5 amps. The operation of a cathode arrangement according to the invention will now be described with reference to FIG. 4. At a time t]_ when it is desired to commence operation of the cathode arrangement, warm-up power supply 70 and sustaining power supply 76 are both turned on. Current flows from warm-up power supply terminal .72 through lead 64 to the central region of the cathode 28, radially outwardly through the cathode 28, and through lead 56 to power supply terminal 74. As the current flows through the cathode 28 it produces direct and rapid heating of the cathode 28. As shown by waveform 90 of FIG. 4, an initial surge of current occurs at time t^, after which the magnitude of the current decreases somewhat as the cathode 28 warms up. Since the sustaining power supply 76 is also turned on at time t^, current also flows from power supply terminal 78, through lead 48, heater coil 42, and lead 56 to power supply terminal 80. As current flows through the heater coil 42, some indirect heating of the cathode 28 occurs due to radiation from the coil 42, although the indirect heating effect is small relative to that caused by direct current flow through the cathode 28. As shown by waveform 92 of FIG. 2, the current flow through the heater coil 42 also has an initial surge at time t- and decreases somewhat thereafter as the coil 42 warms up, although this decrease is smaller than that for the current flowing through the cathode 28. By a time t2, which is less than one second after time ti, the cathode 28 has reached its desired operating temperature which typically is around 1100°C. Thus, at time t2, the warm-up power supply 70 is turned off, and direct current flow through the cathode 28 ceases. The sustaining power supply 76 remains on, however, and current flow through the heater coil 42 continues as shown by waveform 94 of FIG 4. The resulting radiant heat from the coil 42 produces sufficient indirect heating of the cathode 28 to maintain the cathode 28 at the desired operating temperature. It may be seen that the present invention provides a simple r reliable, and inexpensive arrangement for heating a cathode to its desired operating temperature in less than one second. No complex or expensive equipment such as mechanical switches, electron bombardment sources, or special variable resistance materials is required. Moreover, long cathode life in excess of 800 hours may be achieved. Although the present invention has been shown and described with reference to a particular embodiment, nevertheless various changes and modifications which are obvious to a person skilled in the art to which the invention pertains are deemed to lie within the spirit, scope, and contemplation of the invention.

Claims: CLAIMS What is Claimed is: 1. A cathode arrangement comprising: a cathode member; a heater wire coil disposed adjacent to said cathode member; means for causing heating current to flow through a first current path including at least a portion of said cathode member during a short period of time to provide rapid warm-up of said cathode member to a desired operating temperature; and means for causing sustaining current to flow through a second current path including said heater wire coil to provide sustaining heat to said cathode member by radiation from said heater wire coil to maintain said desired operating temperature. 2. A cathode arrangement according to Claim 1 wherein said sustaining current flow is commenced at the same time as said heating current flow and continues during and subsequent to said short period of time. 3. A cathode arrangement according to Claims 1 or 2 wherein said means for causing heating current flow includes a first power supply electrically coupled between spaced locations on said cathode, and said means for causing sustaining current flow includes a second power supply electrically coupled between the respective ends of said heater wire coil. 4. A cathode arrangement according to Claim 3 wherein said first power supply provides a substantially larger current and a substantially smaller voltage than said second power supply. 5. A cathode arrangement according to Claims 3 or 4 wherein said cathode member is substantially dish- shaped, and said first power supply is electrically coupled between a first location substantially at the center of said cathode member and a second location substantially at the periphery of said cathode member. 6. A cathode arrangement according to Claim 5 and further comprising: a substantially tubular electrically conductive housing; said cathode being a substantially dish-shaped member coaxially mounted within said housing and having a concave outwardly facing front surface; a heat shield mounted in said housing behind the back surface of said cathode member; said heater wire coil being mounted in said housing between said heat shield and the back surface of said cathode member; an electrically conductive rod disposed along the axis of said housing and electrically connected to the central portion of the back surface of said cathode member; and means for supporting said cathode member within said housing and electrically connecting said housing with a peripheral portion of said cathode member. 7. A cathode arrangement according to Claim 6 wherein said means for supporting said cathode member includes a dish-shaped element of a configuration conforming to that of said cathode member attached to the back surface of said cathode member and extending along most of the back surface of said cathode member. 8. A cathode arrangement according to Claim 7 wherein both said cathode member and said dish-shaped element define a plurality of aligned arc-shaped slots located along each of a plurality of concentric circles.

Inventors: MANOLY ARTHUR E, MANOLY, ARTHUR, E.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1987
Main classification: H01J23
IPC codes: H01J1, H01J23