Cataphoresis and Nitrogen-Removal Processes for dc Discharges in He–N2 Mixtures

Abstract
Ion-sampling measurements have been made from the positive column of dc discharges in mixtures of 0.05%, 0.10%, and 0.28% nitrogen in helium. The major ions observed were He+, He2+, N+, and N2+. Traces of Ne+, Ar+, N3+, N4+, O+, O2+, NO+, and N2O+ were also detected. The variation of N+ and N2+ with distance from the cathode was measured as a function of gas pressure and discharge current. Using the axial variations, estimates have been made of the average degree of ionization of nitrogen. Measurements using certain types of cathodes for various discharge conditions show the volume removal of nitrogen from the mixture due to processes occurring at the cathode. Conditions which produce the removal of nitrogen are discussed. Considerations for the purification of noble gases contaminated by small concentrations of nitrogen are also mentioned. The result emphasize the difficulty of conducting dc gas-discharge studies, on known small concentrations of a molecular gas such as nitrogen.