ION-BEAM DIVERGENCE FROM UNSTABLE FLUCTUATIONS IN APPLIED-B DIODES

An electron plasma oscillation driven unstable by ion streaming is ide ntified with the low-frequency mode observed in QUICKSILVER [Computati onal Physics, edited by A. Tenner (World Scientific, Singapore, 1991), pp. 475-4821 numerical simulations. This mode heats the electrons alo ng the magnetic field and is ultimately stabilized by the thermal spre ad. A quasilinear theory determines the saturation level of the fluctu ations, the ion divergence, and the ion energy and momentum spread as they exit the diode. The ion divergence is predicted to be independent of the ion mass for fixed diode voltage and scales as the product of the effective gap and the ion beam enhancement factor over Child-Langm uir current.