Electrostatic Probe Determination of Electron Density in Medium Pressure (0.2–28 Torr) Discharge Plasma of Argon

The validity of electron-density determinations using spherical probe in medium pressure discharge plasmas (rp/λe∼0.3 to 40, where rp=probe radius, λe=electron mean-free path) are examined for density range Ne=109−1011 cm−3. Various collision-dominated probe theories are used to obtain Ne from both electron and ion saturation currents (Ie and Ii). Ne determined from Ie coincide reasonably with those determined from microwave measurement, while Ne from Ii are always greater than those from microwave measurement by a factor of 2 to 100. The discrepancy increases with increasing gas pressure and total discharge current, and Ii can not be used for Ne determinations. There is ``break'' on the ion current characteristics which is the same as that reported in the measurement of very low-density flowing afterglow plasmas (Ne<108 cm−3). Explanation is offered in connection with the unusual increase of ion current caused by the release of electrons from the probe surface by collisions of metastables and the decrease of regular thermal-motion current because of the short mean-free path condition (λ6<rp).