Emissive probe measurements in sheath, presheath, and virtual cathode

Weakly collisional argon plasma was studied in a multi-dipole device operated at 0.5 mTorr. Plasma was generated by energetic electrons emitted from filaments biased at V­­_f = -60V. Plasma potential V_p was measured in the sheath and presheath near a plate biased at -90V with disc Langmuir probes (LPs) and emissive probes (EPs). V_p was given by the inflection point of the cold LP I-V characteristic and by extrapolating the inflection point to the limit of zero emission of the heated filament EP. The EPs and LPs showed similar positive potential profiles outside the sheath. Within the sheath EPs still worked but LPs did not give plausible results. However, different EP heating procedures yielded different results. A series of I-V traces were recorded with the heating voltage changed from trace to trace. These were taken from hot to cold or from cold to hot. These traces did not agree with each other. In addition, the time between traces was varied. The resulting traces again did not agree with each other. When the neutral pressure was 0.1 mTorr, V_f = -22V, and the plate was grounded, V_p in the bulk plasma was negative and a virtual cathode was found with EPs near the plate. When V_f = -20V, the virtual cathode disappeared.