Momentum Transfer to Cathode Surfaces by Impinging Positive Ions in a Helium Arc

Abstract

The momentum transfer to an auxiliary cathode has been studied in the positive column of a low-voltage helium arc. The auxiliary cathode was a flat molybdenum plate insulated on one side by glass and suspended in such a way that its deflection gave a measure of the pressure against it. The measured pressure on the cathode is believed to be due to two phenomena. The first is the recoil of those ions which retain some of their kinetic energy after neutralization. The second is a radiometer effect due to the heating of the cathode by positive ion bombardment. On the basis of these assumptions it was possible to calculate from the experimental data an accommodation coefficient for helium positive ions and the fraction of the measured current carried by electrons. Although the accuracy of the experiment is not high, the values of the accommodation coefficient are in qualitative agreement with those obtained by Compton and Van Voorhis. The fraction of the current carried by electrons agrees fairly well with the results of Harrington.