Probe measurements of electron energy distributions in a strongly magnetized low-pressure helium plasma

Electron energy distributions (EED) in strongly magnetized (similar to 0.3 T), low-pressure (similar to 0.2 Pa) helium plasmas of the toroidal device "Blaamann'' [K. Rypdal et al., Plasma Phys. Controlled Fusion 36, 1099 (199 4)] have been measured. In the analysis of measurements, one applies a simp lified expression for the limit of a strongly magnetized plasma relating th e electron energy distribution to the first derivative of electron probe cu rrent with respect to the probe potential. It is shown that for the conditi ons investigated this approximation gives the same electron densities and s lightly lower temperatures (up to 10%) as the kinetic theory for arbitrary magnetic field strength. Cylindrical probes, which are oriented along and p erpendicular to the magnetic field, are used in the measurements. It is sho wn that these probes give nearly identical results. However, the probe that is oriented perpendicular to the magnetic field can give the electron ener gy distribution in a wider energy range and with better accuracy. The cross -field diffusion coefficient of electrons near the probe is estimated and s hown to be classical. (C) 1999 American Institute of Physics. [S1070-664X(9 9)01201-X].