Ions and their Impact on Gap-Closure and Pulse-Shortening in a High-Power Crossed-Field Diode

Summary form only given. The presence of plasma has long been suspected to be a major cause of gap closure in high power crossed-field diodes and HPM sources, even under the condition of magnetic insulation as in relativistic magnetrons. Our recent work [Lau et al., 2007] suggested that the presence of positive charge, anywhere in the diode gap, always increases the extent of the electron hub height. Thus, the rate at which positive charge is introduced into the gap can determine the rate at which electrons migrate toward the anode, mimicking gap closure, even in the absence of motion of the positive charge. This effect is shown to be more pronounced if the ions are generated near the cathode region. Thus, this effect is (a) not related to ambipolar diffusion, and (b) can rapidly detune magnetrons from Buneman-Hartree resonance, leading to reduced efficiency and even pulse shortening in a relativistic magnetron. Since the rate of plasma formation is of critical importance to this phenomenon, we investigate various mechanisms for ion production in relativistic magnetrons, including volumetric ionization of neutral gas by the electron flow, thermal and stimulated desorption of neutrals and ions from the anode and the subsequent acceleration across the gap, and finally explosive emission of ions and neutrals from the cathode.