Results of Investigations of Gas-Discharge Plasmas in Molecular Gases

Abstract

Since a probe located in a plasma perturbs the surrounding region, the range of conditions under which the probe method can be used is limited. The degree of perturbation caused by a probe is governed by the ratio of the probe radius to the mean free path of electrons in the plasma a/λ_e. The theory [38] demands that this ratio be small and this imposes an upper limit to the range of pressures in which the probe method can be used. If we assume that the probe radius a is the smallest realistic value of 0.02 mm and if we assume that the ratio a/λ_e = 0.1 is satisfactory, a pressure of 2 Torr is the upper limit in probe measurements. In molecular gas discharges the gas temperature is 1.5–3 times higher than the room value [74, 75]. Expulsion of the hot gas from the discharge tube reduces the number of molecules and increases the mean free path of electrons by a corresponding factor. Consequently, we can increase the upper limit of the pressures somewhat.