Effects of periodic DC and AC electric fields on spoke formation in a crossedfield diode

Abstract

Summary form only given. The time-dependent behavior of electron sheaths in a magnetically insulated (B>BHull) anode-cathode gap with crossed electric and magnetic fields is studied. The crossed field, space-charge limited diode is modeled for various magnetic fields by means of multidimensional (1d and 2d), self consistent, electromagnetic, particle-in-cell (PIC) simulations in both cylindrical and planar geometries. It is shown in 1d planar geometry that the cycloidal flows collapse into a steady, near-Brillouin flow. Our 2d electromagnetic PIC simulations (both planar and cylindrical) show that cycloidal flow also collapses into a perturbed flow that is dominated by the E cross B drift, but is neither steady nor stable. A slow wave structure (SWS) is added to the anode that matches the wavelength and frequency of the fastest growing fluid instability in the smooth-bore case. The SWS is added by three different methods to separate the RF effects from the DC electric field effects created by the SWS. The first method to add the SWS is to add a thin material with a large permeability to the anode that does not affect the DC electric fields; the second is to add a thin dielectric (with and unphysical large dielectric constant) which does affect the DC electric fields, and last is to add the geometric SWS. The SWS is then perturbed so that wavelength and/or frequency does not match the smooth bore diode growth rate and the region of `lock-in' to the SWS is found. The same set of simulations are performed again, this time changing the cathode to limit the emission area (representing a PAL cathode), adding perturbations to the cathode (representing a `shaped' cathode), and adding an emitting rod above the surface (representing a transparent cathode). These simulations focus on explaining the qualitative change in flow for these different cathodes under current development