Megawatt, 330 Hz PRF tunable gyrotron experiments

Repetitively pulsed and cw gyrotrons have hitherto used thermionic cathodes, whereas cold cathode gyrotrons have normally operated as single shot devices. The novel results presented here show that cold cathode gyrotrons can be successfully pulsed repetitively. A tunable gyrotron with a pulse repetition frequency (PRF) of 150Hz is demonstrated. This system developed >4MW mm-wave output pulses at 100GHz. The gyrotron is based on a two-electrode configuration comprising a field-immersed, field emission, cold cathode and a shaped anode cavity. A superconducting magnet was used to produce the homogeneous intra-cavity magnetic field and a cable pulser was used to drive the electron beam. This pulser produced up to a (200±20)kV pulse with 10ns rise time, a 100ns flat top, a 10ns decay with a characteristic impedance of 200. The energy storage capacity of the cable pulser was 35J. The charging unit limited the maximum PRF to 330Hz. Due to spark gap switching limitations 330Hz was only obtainable in 5 to 10 pulse bursts. For substantial periods of the order of 30 seconds, 100Hz PRF was achieved over an oscillating range of 28 to 100GHz and 150Hz PRF was achieved at 80GHz. No degradation effects on the mm-wave output pulse was evident due to diode recovery time throughout this series of results. A subsequent conclusion is that the diode recovery time in our cold cathode gyrotron is less than 3ms.