Ion beam diagnostics by means of plane Langmuir probes

An ion beam normally incident on a plane Langmuir probe in a plasma gives rise to an extra knee in the electron saturation part of the current-voltage characteristics of the probe. Findings from a numerical investigation, together with some experimental results, are presented, confirming that the effect arises from a modulation of the electron collection current because of space-charge effects from beam ions reflected in front of the probe. It is verified that beam energy, temperature, and, in principle, beam density can be derived from the position, relative shape, and size of the extra ion-beam-induced knee of the Langmuir characteristics.