Cylindrical Langmuir probes beyond the orbital-motion-limited regime

Abstract

The current I to a cylindrical probe at rest in an unmagnetized plasma, with probe bias highly positive, is determined. The way I lags behind the orbital-motion-limited (OML) current, IOML∝R, as the radius R exceeds the maximum radius for the OML regime to hold, is of interest for space-tether applications. The ratio I/IOML is roughly a decreasing function of R/λDe−Rmax/λDe, which is independent of bias, with λDe the electron Debye length and Rmax/λDe roughly an increasing function of the temperature ratio, Ti/Te. The dependence of current on ion energy is used to discuss the effect of probe motion through the plasma, a case applying to tethers in low orbit.