EXPERIMENTAL TESTS OF LANGMUIR PROBE THEORY FOR STRONG MAGNETIC-FIELDS

The suppression of electron current to a single Langmuir probe immersed in a strongly magnetized plasma has been the subject of theoretical analysis since the original work of BOHM et al. ]1949, in Characterictics of Electrical Discharges in Magnetic Fields (Edited by A. GUTHRIE and R. K. WAKERLING). McGraw-Hll, New York]. Experimentally, studies of the phenomenon have been restricted to a comparison of measured data with analytic predictions of the I-V characteristic. Both theory and experiment show that the simple exponential law for electron collection assumed in the derivation of T(e) from the characteristic no longer holds for probe potentials above the floating potential. It is assumed, however, that the portion of the characteristic below floating potential can be used in driving T(e). By monitoring the potential of a small, electrically floating pin as a function of the voltage applied to a much larger plate located just behind it and interpreting the pin voltage as a direct measure of the plasma potential near the plate, the validity of this important assumption has been experimentally confirmed for the first time. Certain theoretical aspects of net electron collection have also been tested and have found to be in reasonable agreement with experiment, indicating that more effort may be justified in attempting to use this region of the characteristic as a diagnostic tool.