ION-BEAM DIAGNOSTICS BY MEANS OF AN ELECTRON-SATURATED PLANE LANGMUIR PROBE

A new sensitive method for the detection of low-energy ion beams in low-pressure nearly collisionless unmagnetized plasma is presented. The method makes use of a swept planar Langmuir probe in the electron saturation regime. An ion beam with direction perpendicular to the plane of the probe produces a double knee (beam edge") on the characteristic at a positive bias with respect to plasma potential. The magnitude of this bias is a measure of the directed energy of the beam. It seems to be possible to deduce the beam temperature from the form of the beam edge. The amplitude of the beam edge is proportional to the beam ion flux. The size of the effect is much greater than can be inferred from ion collection alone. An explanation is suggested based on the presence of a space-charge layer in front of the probe produced by the reflected beam and that this layer in turn causes enhanced electron collection by the probe."