Analysis of the electron and ion fluxes to the wall of a hot-filament discharge device

Abstract

The current densities of ions and electrons to the wall of a hot-filament discharge device are examined both experimentally and theoretically. The ion current to the wall as a function of neutral gas pressure is found theoretically from a model of the sheath and presheath that includes charge-exchange collisions of the ions with neutrals. The electron current is found from a model based upon the energy distributions of secondary electrons from ionization of the neutral gas and of secondary electrons from the wall. In a hot-filament discharge device with argon plasma (density (0.2–4.5)×109cm−3, electron temperature 0.14–0.21eV, and pressure 0.3–12mTorr), a gridded energy analyzer is placed behind a slit in the wall and the current collected is recorded as a function of the retarding potential. The dependence of the collector current on the grid bias potential identifies the electrons in the 10–65eV range as being mostly secondaries from ionization and those in the 0–10eV range as being mostly secondaries from the wall. Ions are collected at the most negative grid bias voltages. The measured ion currents are within about 40% of values calculated from a model with charge exchange collisions of ions, and at the highest pressure differ by about a factor of 3 from the collisionless value, indicating that the Bohm ion current should be corrected for collisions when the charge exchange mean free path is less than about 0.2 of the plasma radius.