SELF-CLEANING LANGMUIR PROBE

A contamination-free Langmuir probe of novel design is described. Surf ace contaminants, which lead to erroneous evaluation of plasma paramet ers by distortion of the probe's current-voltage characteristic, are r emoved by the indirect heating of the probe tip with separate heating elements running parallel to the probe wire in multibore alumina tubin g. This configuration minimizes magnetic field perturbations, while ma intaining the compact profile and construction ease of an unheated pro be. The design and operating characteristics of such a probe are repor ted. Temperatures at the probe tip, as measured by a subminiature ther mocouple, can exceed 500-degrees-C. Experiments determining the effect of probe temperature, T(probe), on the characteristic have been perfo rmed 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 elect ron temperature is up to several times too large, indicating the prese nce 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.