The time evolution of beam divergence induced by electomagnetic instabilities in applied-B ion diodes is examined using the 3D particle-in-cell code quicksilver. The evolution is generally characterized by a high-frequency, low-divergence phase, associated with the diocotron mode, followed by a low-frequency, high-divergence phase, associated with the two-stream-like ion mode. Limiting the extent of the electron density profile evolution allows the diocotron phase to be sustained with a resulting divergence of ≊10 mrad.

• Received 3 July 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.3094