Electron beam transport in a weak plasma

The propagation of an electron beam through a dilute plasma is studied experimentally. A beam is injected into a low pressure gas (P≃10−6 Torr) resulting in sufficient ions to neutralize the beam so that space charge blow-up does not occur directly. However, a low frequency beam-drift mode is unstable, which results in a radial displacement of the beam. This can produce sufficient space charge to stop beam propagation. The experimentally observed threshold is in agreement with linear theory, and the limiting current agrees with an estimate of the space charge limit for this geometry.