Plasma Potential Measurements by Electron Emissive Probes

TLDR

Hot‑wire electron emissive probes are employed to measure plasma potential in quiescent plasmas, with sheath dimensions of roughly ten wire diameters at 10¹⁰ ions cm⁻³ and expanding to orders of magnitude larger at 10⁶ ions cm⁻³. The probe’s emission characteristic yields precise dc potential determination, its floating mode tracks several‑volt fluctuations, the cutoff slope detects signals up to tens of megacycles per second, and calculations model the potential distribution around the emissive wire with or without a virtual cathode. Emission levels appropriate for specific plasma densities are demonstrated, indicating an application range from about 10⁴ to 10¹² ions cm⁻³ limited by minimum useful emitted current and electron collection, respectively.

Abstract

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.