[en] An electron plasma oscillation driven unstable by ion streaming is identified with the low-frequency mode observed in QUICKSILVER [Computational Physics, edited by A. Tenner (World Scientific, Singapore, 1991), pp. 475--482] numerical simulations. This mode heats the electrons along the magnetic field and is ultimately stabilized by the thermal spread. A quasilinear theory determines the saturation level of the fluctuations, 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--Langmuir current