Pulse Shortening in Recirculating Planar Magnetrons

Recirculating planar magnetron (RPM) experiments at the University of Michigan have utilized a 12-frame ultrafast intensified CCD camera to analyze the effect of plasma formation on microwave pulse duration. The RPM was driven by a -300-kV voltage pulse for 0.3-1.0 μs, with a 0.08-0.27-T axial magnetic field. The RPM has previously demonstrated peak microwave powers of over 150 MW, with typical microwave pulse durations of 50-150 ns, far shorter than the available voltage pulse. To investigate possible causes of pulse shortening, the RPM was imaged with both S-band (2 GHz) and L-band (1 GHz) anodes, as well as two different cathodes with substantially different anode- cathode (AK) gap widths and end caps. It was found that a smaller AK gap produced brighter plasma and allowed for improvements in temporal resolution of the imaging configuration at the expense of larger end-loss current. Many shots demonstrated a direct correlation between intervane anode plasma formation and a sharp reduction in RF power. Anode vane plasma is formed at the axial ends of the anode, indicating field enhancement effects. Contact resistance plasma was also observed at the back of anode vane cavities. Details of plasma formation are illustrated and methods for remediating these plasmas are proposed.