Experiments on the Behavior of an Ionized Gas in a Magnetic Field

Abstract

Measurements with probes in a plasma of helium ions and electrons in the afterglow of an induction-excited discharge in a toroidal tube with a toroidal magnetic field reveal an oscillatory probe current which is presumably indicative of fluctuations in the ion density. The space distribution of these fluctuations and the dependency of their amplitude and frequency on the pressure and magnetic field are consistent with the assumption that plasma waves, similar to the magnetohydrodynamic type described by Alfvèn, exist in this toroid. Measurements of electron density by a microwave method in another toroidal tube, which is in the form of a microwave resonant cavity, show that in this tube the degree of diffusion control in helium at low pressures is apparently orders of magnitude less than one would calculate from the classical theory of ambipolar diffusion of ions and electrons in a magnetic field. The measurements show a marked minimum in the diffusion coefficient as a function of magnetic field at about 600 gauss. An attempt is made to explain this minimum and the lack of diffusion control in terms of plasma waves. Experimental evidence that plasma waves exist in this toroid in a magnetic field has been seen in oscillations which appear on the rf envelope of the exciting pulse from the magnetron.