2-D electron dynamics in a magnetically immersed foilless diode

Theory and simulation of the 2-D electron dynamics in a magnetically immersed foilless diode are presented. The diode consists of a cylindrical needle point cathode and flat anode, all immersed in a large (∼20 T) solenoidal magnetic field. High-brightness electron beam experiments utilizing such diodes have been conducted at Sandia National Laboratories on the Hermes III accelerator. Power is delivered to the diode load via a magnetically-insulated-transmission-line (MITL). The nominal e-beam parameters in these experiments are 10 MeV, 40 kA, 1 mm radius, with pulselengths of order 30 ns. The electron flow in and near the entrance to the diode is complicated by the presence of the applied field. An effective potential Veff is defined which describes the available electron phase-space as well as identifies the fixed points of the dynamics. Veff is a function of both the azimuthal component of the vector potential and voltage and is, therefore, dependent on the diode geometry. Results from simulations with the TWOQUICK PIC code demonstrate that contours of Veff appear to represent the streamlines of the electron flow for electrons which are magnetically insulated. This is similar to the well known para-potential flow of electrons in MITL's. The presence of long-lifetime electrons trapped near the fixed points of Veff is discussed, as well as the influence of the diode geometry on diode performance and beam quality, including beam halo and transverse energy