Encapsulated cathode-heater package

A novel heater-package consists of one or more thin layers of chemically machined metal compressed under temperature into a solid block of high purity alumina ceramic. High power level tubes, especially of small size and high efficiency, require cathodes capable of high emission densities. Tungsten matrix cathodes are often used. These cathodes are capable of up to 5 amperes per square centimeter of cathode surface area. Unfortunately, these cathodes operate between 1100 to 1200 C. The heaters for these cathodes operate at even higher temperatures. It is not unusual to see 1500 C or 1600 C heater temperatures. The heater life is seriously diminished, due to sag, evaporation, embrittlement and electrochemical effects. When the new heater is used in heater-cathode packages, the heater temperature is typically less than 1350 C for cathode temperatures above 1100 C. The photo-etched or chemically machined pattern is usually bifilar, the precise geometry and cancelling action result in minimum stray magnetic field on the cathode surface. The low heater temperature and turn to turn voltages minimize the electrolytic action. The voids characteristic of presently used potted heaters are absent, making this heater suitable for D.C. operation. The time to reach full operating temperature after application of the heater voltage is minimized allowing "quick start" operation. The heater-ceramic package can be potted into a groove machined in the cathode matrix, located so as to obtain the uniform cathode temperature. D.C. operation, heater cycling, efficiency and other tests, including life, will be discussed. Comparison with known heater cathode packages will be made.