SELF-CLEANING LANGMUIR PROBE

A contamination-free Langmuir probe of novel design is described. Surface contaminants, which lead to erroneous evaluation of plasma parameters by distortion of the probe's current-voltage characteristic, are removed by the indirect heating of the probe tip with separate heating elements running parallel to the probe wire in multibore alumina tubing. This configuration minimizes magnetic field perturbations, while maintaining the compact profile and construction ease of an unheated probe. The design and operating characteristics of such a probe are reported. Temperatures at the probe tip, as measured by a subminiature thermocouple, can exceed 500-degrees-C. Experiments determining the effect of probe temperature, T(probe), on the characteristic have been performed in a Q-machine plasma (T(i) almost-equal-to T(e) almost-equal-to 0.2 eV). We find that for T(probe) < 250-degrees-C, the measured electron temperature is up to several times too large, indicating the presence of a eontamination layer. This contamination layer is removed for T(probe) greater-than-or-equal-to 300-degrees-C, and accurate electron temperatures are recovered.