Ion Current to a Langmuir Probe in Very Low Density Flowing Plasmas for Electron-Density Determinations

The mechanism for ion-current collection by Langmuir probes and the validity of electron-density determinations using ion-current characteristics are examined for low-density (Ne=109−105 cm−3) flowing plasmas in various gases (He, Ar, N2, and air) for the pressure range 0.05–1 Torr. When h≫rp (where h=electron Debye length, and rp=probe radius), both cylindrical and ring probes show spherical-probe behavior; the experimental ion current Ii varies almost linearly with the probe voltage Vp. The results are compared with Su and Lam's collision-dominated theory. Experimental ratios of the electron to the ion current Ie/Ii agree with those predicted by the plasma ``sheath criterion'' for Ne>108 cm−3. But for Ne<106 cm−3, Ii increases so that Ie/Ii is low by an order of magnitude, and Ii cannot be used for Ne determinations. Explanations are offered in terms of the method used to evaluate the ion current, interference of the discharge-tube wall, and, at higher pressure, the release of electrons from the probe surface by collisions of metastables (or photons).