Simulations of magnetically insulated multistage ion diodes

Abstract

An analytic theory for magnetically insulated multistage acceleration of high intensity ion beams has been presented [J. Appl. Phys. 67, 6705 (1990)]. This theory predicts an operating behavior that is strongly dependent on the electron density profile. A numerical investigation, using both two-dimensional (2-D) and three-dimensional (3-D) particle-in-cell codes, of multistage diode operating behavior is presented in this paper. The 2-D results are consistent with the analytic results based on a very thin electron sheath. In contrast, the 3-D simulations are consistent with the analytic theory based on a thick electron sheath. The different results are due to the growth of electromagnetic instabilities in the 3-D simulations, which generate fluctuations that broaden the electron sheath. The 2-D simulations did not properly model these instabilities because they propagate in the direction that was ignored. In addition to these results, the 3-D code was used to study the generation of ion divergence due to the instability induced fluctuations. These simulations show a positive correlation between the ion current density (normalized for space-charge effects) and the growth of transverse ion velocities during acceleration. It is found that, at low beam current densities, ion divergence can be reduced significantly by postacceleration.