Book/Report

FZJ-2018-03516

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Über die räumliche Verteilung der Magnesium-Atome und -Ionen in einer Helium-Glimmentladung mit Magnesium-Kathode: eine Untersuchung mit Laser-induzierter Fluoreszenz

Changlong, N. (Corresponding author)FZJ*

1991
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich

Please use a persistent id in citations: http://hdl.handle.net/2128/18909

Report No.: Juel-2484

Abstract: In this work, a low current DC-glow discharge in helium with a plane magnesium cathode, which is in a parallel configuration with the anode, has been investigated. The density distributions of sputtered magnesium atoms and ions in the discharge, especially in the cathode call region and in the negative glow, have been measured by means of laserinduced fluorescence (LIF) with high spatial resolution (2 mm) and sensitivity ($\geq$ 10$^{4}$ atoms/cm$^{3}$). The discharge with a newly polished cathode starts from a stable mode (the so-called "soft" discharge) and reaches another stable mode ("hard" discharge) after sufficient glowing with high power (> 0.2 W/cm$^{2}$). In the latter case, the discharge has a (2-6 times) higher cathode potential and a much higher (30-300 times) density of sputtered magnesium atoms and ions than that in the former case. The big differences between the two modes can be explained by the oxidization of the cathode surface during the polishing process and the deoxidization by sputtering during the high power discharge. A model based on the energy distribution of helium ions at the cathode surface and an empirical function of the sputtering yield near the sputtering threshold can explain the dependence of the maximum density of sputtered atoms on the current. The density distributions of magnesium atoms and ions on the axis of the discharge have been measured in both cases in a wide range of pressures and currents (0.5 - 2.0 mbar; 0.25 - 10.0 mA, cathode with 6 cm diameter). The density profiles of the atoms show a maximum (10$^{5}$-10$^{8}$ atoms/cm$^{3}$) near the cathode (4 mm distance at 1 mbar), the position of which depends on the values of pressure and current. The profiles can not be described by a model which takes only diffusion processes into account, but ionization has to be considered as well. The profiles of the ions on the axis show a maximum (10$^{4}$-10$^{6}$ ions/cm$^{3}$) in the negative glow region (30 mm from the cathode at 1 mbar), which also depends on the pressure and on the current. The degree of ionization of the magnesium atoms in this region reaches values of more than 50% ; it leads to the conclusion thatPenning ionization is the dominant process for the ionization of magnesium atoms. The density distributions of magnesium atoms and ions in the radial direction of the discharge have also been measured. From these measurements, the good homogeneity of the current density at the cathode can be seen and estimates about the relative importance of the axial and radial transport can be made. The drift velocities of the magnesium ions have been obtained by measuring the Dopplershift of a resonance line under laser excitation. Velocities of about 9 $\cdot$ 10$^{4}$ cm/s have been observed at the transition between the negative glow and the cathode fall region for a pressure of 0.5 mbar and a current density of 33 $\mu$A/cm$^{2}$. This, together with the known mobility of magnesium ions in helium, gives the electrical field strength at the point of measurement. A simple model, considering the fluxes of the atoms and ions along the axis, gives the strength of the electric field on the axis further away from the cathode; it shows fields below 0.4 V/cm in the negative glow and a sharp increase at the border between the negative glow and the cathode fall region.

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Contributing Institute(s):

1. Publikationen vor 2000 (PRE-2000)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

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