Plasma Potential Measurements by Electron Emissive Probes

Several methods of using hot wire, electron emissive probes for potential measurements in quiescent plasmas are described. A plot of the probe emission characteristic gives a precise dc determination; the floating probe rapidly follows potential fluctuations of several volts magnitude; the cutoff slope detects small signals up to tens of megacycles per second. Calculations are discussed which describe the potential distributions around an emissive wire immersed in plasma, with and without the formation of a virtual cathode. Sheath dimensions are ∼10 wire diameters at 1010 ions/cm3; orders of magnitude larger at 106 ions/cm3. Emission levels appropriate to given plasma densities are shown. The range of application has a low density limit of ∼104 ions/cm3 because of minimum useful emitted current, and a high density limit of ∼1012 ions/cm3 because of electron collection from the plasma.