Evolution of modes in magnetically insulated crossed-field diodes

Abstract

Listen

Translate article

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. The transient behavior of the system is examined in detail and is divided into three separate stages: cycloidal flow, collapse of cycloidal flow and sheared (near-Brillouin) flow. It has been 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. This observed cycloidal flow instability is a kinetic mode, not a fluid mode such as the magnetron or diocotron instability. The growth of the kinetic mode is faster than that of either of the above mentioned fluid instabilities. After the kinetic mode saturates, the fastest growing fluid mode grows to dominate the system. A slow wave structure (SWS) is added to the anode that matches the wavelength and frequency of the fastest growing fluid instability. 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 is to add a circuit to the anode that does not affect the DC electric fields, the second is to add the SWS by placing a thin dielectric (with and unphysical large dielectric constant), 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 simulations are done with a Cartesian mesh and cylindrical mesh and the time-to-solution and accuracy are compared