Computer simulation of ion trapping and detrapping in a PPM focused traveling wave tube

This paper presents results from a time-dependent, electrostatic electron gun simulation code, simulating ion trapping and detrapping in a traveling wave tube (TWT) focused by periodic permanent magnets (PPM). The simulations described indicate that ion loss is primarily radial through the beam tunnel walls, rather than axial through the gun or collector. The electrostatic potential well formed by the electron beam is constantly being filled by ionization of a background neutral gas. This effect constitutes the primary ion loss mechanism. Filling of the potential well is made possible by loss of the low energy electrons produced by ionization (secondary electrons) through periodic nulls in the magnetic field.