Custom Spindt Cathode for Diamond Based BWO

A diamond-based sub-mm backward-wave oscillator (BWO) presently under development by Genvac Aerospace places daunting requirements on the device\´s cathode. Because of the high interaction impedance of the bi-planar, inter-digital, slow-wave circuit used by Genvac; these BWOs operate with beam currents in the range of 1.5 to 2.0 mA. However, due to the very small dimensions required for sub mm BWO operation, these currents translate into cathode current densities of several hundred amps/cm . This, along with the very small physical dimensions, makes the use of a thermionic cathode unrealistic. However a microfabricated cold field emitter array cathode of the kind developed by SRI can produce the required current and current density with an average emission of 15 muA per tip, which is well within the envelope of demonstrated performance for the these cathodes. The need to operate the BWO over a wide frequency range requires a wide range of operating voltages while maintaining constant current. This is most conveniently realized by designing a dual anode electron gun. The first anode is held to a voltage of 1.8 kV that is compatible with reliable cathode operation, while the second anode is adjusted to provide the desired electron velocity for operation over range of frequencies. Typically SRI cathodes have been packaged and operated in TO-5 headers for convenience in handling and making electrical connections. However, for this application the TO-5 header is much too large. It will be necessary to fabricate the emitter array on a "mesa" as schematically shown in this paper. The cathode "mesa" will then be inserted into a concave lithographically formed structure for precise alignment with the BWO body. The fabrication of a cathode in this format is a significant departure from previous SRI practice. Details of the design and emission performance for the sub-mm BWO cathode were given