We have used the three‐dimensional, particle‐in‐cell codeQUICKSILVER[J. P. Quintenz,etal., Lasers and Particle Beams12, 283 (1994)] to simulate radial applied‐Bion diodes on the particle beam fusion accelerator II at Sandia National Laboratories. The simulations agree well with experiments early in the beam pulse, but differ substantially as the ion‐beam current increases. This is attributed to the oversimplified ion emission model. We see the same instabilities seen in earlier simulations with idealized diode geometries; Early in time there is a diocotron instability, followed by a transition to an ‘‘ion mode’’ instability at much lower frequency. The instability‐induced beam divergence for the ∼10 MeV beam during the diocotron phase is <10 mrad, significantly less than the total beam divergence in experiments early in the pulse, but increases to ≳25 mrad after the transition. The ion mode has a distinct harmonic structure along the applied field lines, making the instability transition sensitive to the diode geometry. The ion mode instability in our latest simulations is consistent with evidence of instabilities from recent experiments. ©1996 American Institute of Physics.