Vacuum diodes with gap spacing of the order 1µm and gap voltages of the order 1V exhibit space-charge induced current modulation in the THz regime1. The frequency can be tuned by simply altering the applied DC vacuum electric field, and the frequency range corresponding to the range of the applied field can be set by selection of the size of the emitter area on the cathode2. The current in a microdiode of this type will typically range from tens to hundreds of microamperes, thus limiting the THz power that can be drawn from a single microdiode. Coupling between microdiodes is a problem of interest as it could offer an avenue to generate higher power levels by using an array of emitters. We present an overview of work done on vacuum microdiode oscillators, with some new results concerning beam quality in a single microdiode and interaction between beams from neighboring emitters.
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