2013 1(12)

SPHERICALLY SYMMETRIC GAS DISCHARGE

Sakhapov S.Z., Zamchiy A.O., Serebryakova M.A.

Institute of Thermophysics SB RAS, Novosibirsk, Russia

Introduction

The stratification of the positive column of a glow discharge in tubes is a phenomenon well known.   Spherical striation was first observed in 1997, and then followed a number of publications devoted to the experimental and theoretical research of this phenomenon. Note the main differences between a tube discharge and a spherical discharge. Firstly, due to the spherical configuration of the discharge, all the plasma parameters depend on one coordinate only, which is the distance r from the anode. In order to describe the tube discharge, one needs to take into account processes not only along but also across the tube due to diffusion processes and the appearance of a transverse component of the electric field. Secondly, recombination processes in the spherical discharge are of volumetric character unlike those in the tube discharge, where recombination basically takes place on discharge tube walls. Thirdly, the current density in the spherical discharge is not constant but increases as the anode is approached, which is located in the center of the discharge chamber.

Purpose

The purpose of that work is experimental investigation of spherical gas discharge, both in a positive and a negative potential on the central electrodes.

Method

The experiments were carried out in a cylindrical vacuum chamber (60 cm in height and 50 cm in diameter) made of steel. It could be pumped up to 10-2 Pa and then filled with different gases. Pressure was controlled by MKS instruments pressure gauges. The experiments were carried out at the pressure of 13 Pa. Solid metallic electrode 15 cm in diameter or a hollow metallic electrode 5 cm in diameter were placed in the center of the chamber. The steel chamber walls served as a second electrode. The dc power supply (constant-current source), with maximum voltage of 3 kV allowed ranging discharge current within 0.05 – 150 mA. Volt-ampere characteristic of the discharge was measured by oscilloscope “Tektronix”. The vacuum chamber was equipped by a glass windows placed in the middle of the cylinder walls. They were used for visual observation and for taking pictures of the discharge lighting by photo camera. The movable electric probe (tungsten wire diameter of 0.26 mm and length of 3 mm) and electrostatic voltmeter were used to measure floating potential. Experiments have been carried out in argon and in acetone.

Results

Experiments have shown that the discharge was burning in several regimes at a positive potential of the central electrode: Townsend, stratified Townsend and glow discharge, which could be stratified by adding alcohol vapor. At a negative potential of the central electrode and the pressure of 13 Pa volt-ampere characteristics consistent with the abnormal glow discharge. When the pressure is reduced to 5 Pa, the VAH had no monotonic: for some value dropped sharply, by an amount of about 100 V, and in the center of the electrode arose plasma ball, which grew with the increase current. Probe measurements have shown that when a fireball floating potential within the electrode to become unstable. It was also found that changing the size of the central electrode is changed discharge characteristics - while increasing diameter stratification in a glow discharge occurs at higher currents.

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