Das Neutralgasprofil und die Elektronentemperatur in der Freifallsäule bei hohem Ionisationsgrad

Publisher Website
Google Scholar
Abstract
The neutral gas flux from the wall into the column ‐ connected with the ion flux to the wall ‐ and the anisotropy of the velocity distribution of the neutral gas are included into the theory of the steady‐state positive column. d/ln and vn0/v0 occur as characteristic parameters. d denotes the radius of the column, ln the mean free path of the neutral atoms for ionization, vn0 the mean velocity of the neutral atoms re‐emitted from the wall, v0 = (2kTe/M)1/2, M the ion mass, Te the electron temperature. On the axis the neutral gas density Nn is decreasing, if d/ln is rising. At the wall Nn is increasing for vn0 ≪ v0, but it is almost constant or decreasing for vn0 ≫ v0 at the same time. In the plasma the total number of the atoms and the ions is taken as constant. In the case of small vn0 the degree of ionization is high only for d ≫ ln. However, it is already high for d ≈ ln in the case of high vn0. Therefore, the radial profiles of the neutral gas densities of different gases in a column can differ from each other. Almost full ionization can be reached near the axis. These results hold, too, if the initial velocity of the ions and a magnetic field are taken into account. d/ln, the degree of ionization, and the electron temperature are given as functions of the electric power input and of the total number of the neutrals and the ions. The velocities of the re‐emitted atoms depend on the accomodation coefficient of the ions recombining at the wall. With rising d/ln an increasing number of the neutral atoms is re‐emitted with superthermal velocities and the total number of the neutrals and the ions can be decreased. The anisotropy of the velocity distribution of the neutral atoms and the ions must be taken into account for the interpretations of spectroscopical measurements.