Diffusion cooling in neon, argon, and krypton afterglow plasmas

Abstract

Measurements are reported of ambipolar diffusion coefficients as determined from ion density loss rates, and of electron temperatures determined using the single Langmuir probe technique, in afterglow plasmas of spectroscopically pure neon, argon and krypton. In each gas there was a critical pressure,p 0 c , above which the product of the ambipolar diffusion coefficient,D a , and the reduced gas pressure,p 0, was pressure independent and above which the electron temperature was found to cool asymptotically to the gas temperature. For pressures belowp 0 c ,D a p 0 decreased with decreasing pressure, and the electron temperature cooled asymptotically to a steady equilibrium value,T eq, below the gas temperature, this equilibrium value itself decreasing with decreasing gas pressure. These results clearly show that diffusion cooling of electrons was taking place at these low gas pressures. A detailed study of the krypton afterglow showed that the values ofD a p 0 andT eq for a given gas pressure were related in a manner predicted by simple diffusion theory. During the course of these measurements values for the zero field mobilities of Ne+, Ar+ and Kr+ ions in their parent gases were obtained and are also reported.