Burst mode operation of >100 MW reflex triode vircator

摘要

Vircators are high power microwave sources which operate at frequencies approximately proportional to the plasma frequency of the electron beam as it passes through the anode. As a result, stable frequency operation requires flat top voltage pulses, and diodes which exhibit little variation in impedance over a given pulse duration. However, plasma formation on the anode and cathode followed by plasma expansion causes the effective A-K gap and thus the impedance of the diode to collapse with time. As the impedance collapses current density increases, possibly causing a shift to higher frequency operation. The vircator presently discussed exhibits frequency chirping from 4 to 5.5 GHz during a pulse length of 700 ns when operated with an 8 stage, 1.5 kJ Marx generator that has a pulse shape of an overdamped RLC discharge (assuming arc formation does not occur). However, stable frequency operation is observed with an 8 stage 80 J Marx, each stage constructed from pulse forming networks rather than discrete capacitors. The PFN Marx generator produces a flat top voltage for a 75 ns FWHM pulse; however the energy radiated from the vircator is much lower due to the reduced energy storage of the PFN Marx. Burst mode operation has the potential to improve frequency stability and energy radiated from the vircator. This contribution reports the experimental results of a compact sealed tube vircator operated in burst mode with a repetition rate of up to 100 Hz. The data show voltage and current waveforms during burst cycles as well as microwave waveforms with temporally resolved frequency spectra.