Pulse width of a reflex triode virtual cathode oscillator

Experiments were carried out to generate intense relativistic electron beams and High Power Microwaves (HPM) with reflex triode geometry from 1 kJ pulse power system. The 1 kJ pulse power system is a Marx generator based repetitive system capable of operation at 10 Hz. The typical electron beam parameters were 200 kV, 4 kA, and 300 ns, with a current density of a few hundreds of amperes per square centimeter. The cathode used was planar graphite and the anode is a woven stainless steel mesh of 70% transparency. Beam generation studies were conducted for diode gaps from 8-20 mm. Cathode plasma expansion velocity has been calculated from the time dependent diode perveance and was found to be 1.7 cm/μs. It was also found that the microwave (base not FWHM) pulse width increased from ~ 150 ns to ~ 700 ns as the diode gap is increased from 8 mm to 20 mm. A large pulse width is obtained because of longer time of plasma motion across the gap and reduced rate of the effective gap change. Measured frequency was 2.2 GHz at 15 mm anode-cathode gap and decreases with the increased gap.