Ionic Analysis of Cataphoresis in He–Ne Mixtures

Abstract

the apparatus for the variable temperature measurements. J. R. Brews provided many answers to questions regarding radiative recombination. R. A. Logan and Mrs. H. V. Carlson made many suggestions useful in preparing the manuscript. We would especially like to thank T. N. Morgan for discussions and correspondence on the interpretation of his tunn'eIing analysis. One of us (D.J.S.) would like to thank M. DiDomenico for the opportunity to work on this problem during a summer at BTL.For studies of cataphoresis in He-Ne mixtures, an analytic expression is derived for the functional dependence of the neon atoms in the axial direction. The analysis, which involves the continuity equation, considers the production and loss mechanisms of the Ne+ ions. The present theory can be used to predict the degree of ionization of neon at a given pressure and current, when the appropriate rate coefficients are known. In addition, techniques for ion sampling from gaseous plasmas have been used in the present investigation. Measurements have been made in commercial helium-gas samples having a quoted neon concentration ::;0,001 % and in samples containing 0.05% neon. Studies were conducted over a pressure and current range of 7-30 Torr and 8-80 rnA, respectively. Depending on the experimental conditions, the following ions were observed: He2+, Ne+, Ne2+, and HeNe+. For the case of HeNe+, the present studies permit an estimate of the rate coefficient for the reaction Ne++2He->HeNe++He which is comparable to values obtained from afterglow studies. Measured values of the degree of ionization are in good agreement with values calculated using the present theory.

Cathaphoretic confinement of Zn evaporated into helium and neon discharges

“…However, at a certain distance from the metal source the metal density drops below a critical value, N crit , and its decrease changes from linear to exponential. This behaviour was theoretically predicted in [7] and to some extent experimentally proved for many other gas mixtures [8,9,10]. As can be expected, the characteristics of the Zn confinement are very similar to those of Cd because of the similar properties of these metals [11].…”

“…For the sake of simplicity we assume that the density of the neutral zinc atoms does not depend on the radial position. The same assumption was made in calculations of [2] and [9]. In fact, the density of metals evaporated into PC discharges has a minimum at the tube axis and gradually increases towards the walls [7,12].…”

“…The linear part of the axial metal density distribution develops in the case when the concentration of metal ions in the discharge is about equal to the electron concentration n e . Since the electrons carry most of the axial current in the PC the electron density (equal to [Zn + ]) can be obtained from the discharge current I and the electron drift velocity v d using the formula [9]:…”

“…The theory developed for He-Ne mixtures by Gaur and Chanin [9] is applied in the following to describe the exponentially decreasing part. Only a brief overview of the theory is given here, for more details we refer to [9]. All the equations are written for the case of zinc evaporated into helium.…”

“…Here is the flux of Zn + , K 1 and K 2 are the rate coefficients for reactions (7) and (8), v e and n e are the velocity and density of electrons, respectively. σ e v e is averaged over the electron energy distribution function (EEDF) and represents the rate of reaction (9). As far as the diffusion current of Zn + in the axial direction can be neglected compared to the ion-drift current the Zn + current density can be approximated by:…”