Fast warm-up cathode for high power vacuum tubes

Abstract: In order to bring a high power vacuum tube to full power in a few seconds, it is necessary to heat the cathode quickly to 1100°C. In large tubes, prior art structures cannot be simply enlarged. A novel cathode structure (10) in which the heater element (14) is anisotropic pyrolytic graphite coated with anisotropic pyrolytic boron nitride for insulation and then sintered to the cathode avoids these problems.

Description: This invention pertains to a fast warm-up heater for use in a high power vacuum tube and methods of forming same. In vacuum tubes for high power transmitters, it is often desirable to be able to switch the tube on to full power rapidly. Tubes, however, employ electron emitting cathodes which must be heated before they emit. The problem of switching the tube to full power rapidly then hinges on the ability to heat the cathode rapidly. State-of-the-art fast warm-up cathodes are formed by sintering a heater to a low-mass cathode. The heater is made with cataphoretically coated tungsten insulated with an Al₂O₃ ceramic material. The sintering is usually done at 1300°C using a mixture of 95% tungsten with 5% nickel. For small tubes, this approach is workable since the cathodes are small, usually 6,35 mm (0.25 inches) diameter or less. Sometimes a mix of molybdenum and ruthenium is used instead of W-Ni. The sintering temperature is then approximately 1600°C. For larger cathodes, 12mm diameter or higher, this approach becomes less workable. The problems become unacceptable for cathodes of greater than 25mm diameter. The problems can be illustrated by considering a requirement for a 1 megawatt klystrode tube with a 10 second warm-up time. The cathode would have to be about 63mm diameter. The heater would have to heat the cathode itself, the heater wire, the insulating coating, the sintering material and the cathode support. To heat such a large cathode to operating temperature would require 15,000 joules. This amount of energy requires high currents and high voltages. The voltage across the Al₂O₃ would exceed the breakdown voltage of the material. In addition, currents of the order of 100 amperes have to be delivered to the active heater area. The connections then would have to be substantial conductors which would carry away heat and increase the current requirement further to compensate for the heat loss. An additional problem in the prior art heaters if the great differences between the coefficients of expansion of the tungsten and the Al₂O₃. The different rates of expansion cause stress during heating which results in fatigue and failure. To reduce the requirements for energy in a fast warm-up cathode, as illustrated in the example above, a bombarder heater is often used. An example is shown in US-A-4675573. The bombarder is a heated emitting structure placed behind the cathode. There is a significant electric field between the bombarder and the cathode. Electrons emitted from the bombarder are accelerated into the back of the cathode to heat the cathode. A quick-heating cathode for an electron tube is described in US-A-3299317. 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 US-A-2996643. 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 temperatures US-A-4675573 assigned in common with the present patent, discloses a fast warm-up cathode arrangement in which the cathode is directly heated with a burst of current through the cathode and then heated from behind by a heater coil. It is therefore a primary objective of the present invention to provide a heater for a cathode assembly which avoids the use of Al₂O₃ as an insulator, and uses a more modest current than tungsten wires. It is a further object of the invention to devise a structure in which the coefficients of thermal expansion of the materials match in order to prolong the life of the heater. These objects of the invention and other objects, features and advantages to become apparent as the specification progresses are accomplished by the invention according to which, briefly stated, an anisotropic pyrolitic graphite heater coated with a layer of anisotropic pyrolitic boron nitride is used for the heater. A suitable method of applying the heater to the cathode is by sintering to the back of the cathode body. An important advantage of the present invention is that the breakdown voltage of the heater insulation at elevated temperatures is approximately two orders of magnitude better than for Al₂O₃ ceramic. Another advantage is that the surge current required is an order of magnitude less than for the design of the prior art. Still another advantage of the invention is that the coefficients of expansion of the heater and the insulator are closely matched. These and further objectives, constructional and operational characteristics, and advantages of the invention will no doubt be more evident to those skilled in the art from the detailed description given hereinafter with reference to the figures of the accompanying drawings which illustrate a preferred embodiment by way of non-limiting example. FIG. 1 shows a sectional view of the structure according to the invention mounted in one end of a vacuum tube with bombarder included. FIG. 2 shows a first method for forming the structure of the invention. FIG. 3 shows a second method of forming the structure of the invention. FIG. 4 shows a third method of forming the structure of the invention. FIG. 5 shows a fourth method of forming the structure of the invention. Referring now to the drawings wherein reference numerals are used to designate parts throughout the various figures thereof, there is shown in FIG. 1 a sectional view of the structure according to the invention. A cathode assembly 10 has a cathode 12 preferably of tungsten to which is sintered on the backside the heater 14 according to the invention. Behind the heater 14 there is shown an optional bombarder heater 16 for large diameter tubes. A lead 18 at the central axis of the tube leads to the center of the heater 14. The return path for the heater current is a common ground from the outer perimeter of the heater. In a klystrode, a grid 20 is placed in front of the cathode. Various vacuum seals and insulators used to seal the structure to the tube and electrically insulate the elements from each other are well known tot hose skilled in the art. The device according to the invention can be formed in several alternate methods. The first method is shown in FIG. 2. At the top of the figure in step a, an anisotropic pyrolytic boron nitride form 100 is made to the desired shape to conform to the cathode. In step b, the form is coated with a layer of anisotropic pyrolytic graphite 102. In step c, the heater pattern 104 is milled through the anisotropic pyrolytic graphite into the anisotropic pyrolytic boron nitride form. In step d, the milled heater is coated with a layer of anisotropic pyrolytic boron nitride 106. In step e, a laser cutter is used to separate adjacent parts of the heater pattern 104. In step f, the device is first coated with a thin layer of anisotropic pyrolytic graphite and then with a thin layer of tungsten 108. In step g, the device is sintered to the cathode 110 using a W-Ni mix 112 at about 1300°C. An alternate method shown in FIG. 3, begins by forming a blank of anisotropic pyrolytic graphite 200 in a shape fitting to the shape of the cathode in step h. In step i, the heater pattern 202 is laser cut into the anisotropic pyrolytic graphite. Then in step j, the heater is coated with anisotropic pyrolytic boron nitride 204 all around. In step k, the heater is coated with a thin layer of anisotropic pyrolytic graphite and then with a thin layer of tungsten 206 all around. In step l, the heater is sintered to the cathode 208 at about 1300°C using a W-Ni mix 210. In the third method shown in FIG. 4, an anisotropic pyrolytic boron nitride form 100 is shaped to conform to the cathode in step m. The form is coated with anisotropic pyrolytic graphite 102 in step n. The heater pattern 104 is milled through the anisotropic pyrolytic graphite in step o. The pattern is coated with anisotropic pyrolytic boron nitride 106 in step p. The device is coated with anisotropic pyrolytic graphite and then with tungsten 108 in step q. The device is sintered to the cathode 110 in step r using a W-Ni mix 112 at about 1300°C. In another alternate method shown in FIG. 5, a workpiece of anisotropic pyrolytic boron nitride 302 coated on both sides with anisotropic pyrolytic graphite 304, 306 is preformed in step s either concave to fit the back of the cathode or flat in the case of very small cathodes. In step t, the heater pattern 308 is formed in the backside coating of anisotropic pyrolytic graphite 306. In step u, the workpiece is then sintered to the back of the tungsten cathode 310 with a tungsten-nickel sintering mix 312. The entire structure is mounted on a molybdenum holder 314. One can purchase certain of these workpieces made to order and then form them into heaters. Such adaptations are cost effective, but do increase the heating time by 10 to 20%. The voltage for breadown of the anisotropic pyrolytic boron nitride at elevated temperature is approximately two orders of magnitude better than for Al₂O₃ ceramic. The voltage breakdown for the anisotropic pyrolytic boron nitride at 1200°C is approximately 20,000 volts/cm as compared to 200 volts/cm for the Al₂O₃ ceramic at the same temperature. The coefficients of expansion for the heater conductor and insulator are much more closely matched for the heater of the invention than for tungsten with Al₂O₃, thereby reducing stress while heating. Also, the hot-to-cold resistance ratio of tungsten wire is approximately 5:1 as compared to 1:2 for anisotropic pyrolytic graphite. This makes it easier to maintain the temperature at a lower current with the invention after the fast warm-up. In summary, the novel fast warm-up heater for a cathode assembly according to the invention is uniquely suited for large diameter cathodes such as those used in klystrode tubes. In addition, the reduction in heater current and the excellent voltage breakdown characteristics of the anisotropic pyrolitic boron nitride insulation makes this design a good candidate for super fast applications where the bombarder approach was the only available solution in the prior art.

Claims: A method for forming a fast warm-up heater (14) for a cathode assembly (10) in a vacuum tube, comprising the steps of: (a) obtaining a workpiece (100, 200) of anisotropic pyrolytic boron nitride which is coated on the back with a layer (102) of anisotropic pyrolytic graphite on the side opposite to that which will contact the back of the cathode; and (b) forming a heater pattern (104, 202) through the layer of anisotropic pyrolytic graphite on the workpiece. The method of Claim 1 wherein the step of forming a heater pattern is accomplished by milling. The method of Claim 1 wherein the step of forming the heater pattern is followed by the step of: (c) coating the heater pattern with a layer (106, 204) of anisotropic pyrolytic boron nitride. The method of Claim 3 wherein the step of coating the heater pattern with a layer of anisotropic pyrolytic boron nitride is followed by the step of: (d) cutting apart adjacent elements of the heater pattern on the workpiece. The method of Claim 1 in which the step of obtaining a workpiece includes obtaining a workpiece (302) of anisotropic pyrolytic boron nitride coated on both sides with anisotropic pyrolytic graphite (304, 306) A method of Claim 1 or Claim 2 wherein the step of forming the heater pattern is followed by the step of coating the workpiece with anisotropic pyrolytic boron nitride (204) all round. The method of Claim 6 wherein the step of coating the workpiece with anisotropic pyrolitic boron nitride all round is followed by the step of coating the workpiece with anisotropic pyrolitic graphite. The method as claimed in Claim 7 when the step of coating the workpiece with anisotropic pyrolitic graphite is followed by the step of coating the workpiece with a layer of tungsten (206). A method as claimed in any one of claims 3 to 8 wherein the step of coating the heater pattern is followed by the step of sintering the workpiece to the back of the cathode. A heater for a cathode in an electronic device comprising: a substrate layer (100,200) of anisotropic pyrolytic boron nitride; a heater element (104, 202) of anisotropic pyrolytic graphite formed as integral coating on said substrate. The heater-cathode structure of Claim 10 wherein the heater is sintered to the cathode with a sintering compound comprising tungsten and nickel.

Inventors: MIRAM GEORGE VALENTINE, MIZUHARA YOSUKE MAXWELL, MIRAM, GEORGE VALENTINE, MIZUHARA, YOSUKE MAXWELL
Applicants: VARIAN ASSOCIATES, VARIAN ASSOCIATES, INC.
Application year: 1990
Main classification: H01J9
IPC codes: H01J1, H01J23, H01J9, H01J19