ELECTRON-ENERGY DISTRIBUTION FUNCTION MEASUREMENTS IN A MAGNETIC MULTIPOLE PLASMA

Langmuir probes have been used to measure the electron energy distribution function (EEDF) in a magnetic multipole hydrogen plasma operating with H//2 gas. The EEDFs are obtained from the second derivative of the current-voltage characteristic of the probe. The measurements confirm the presence of substantial densities of fast electrons. The use of movable Langmuir probes establishes the spatial variation of the EEDF, shows the confinement of the fast electrons by the magnetic fields and allows the effective plasma production volume to be determined. The measurements also show the effectiveness of a virtual magnetic filter in producing a fast-electron-free plasma.