On measurements of the rotation of a magnetoplasma using an ion-energy analyser

Abstract

This study is concerned with measurements of the anisotropic part of the azimuthal component of the ion velocity distribution function by means of a singly gridded ion energy analyser. The experiments are performed at a magnetised plasma column in order to investigate the dependency of the E × B-rotation velocity of the plasma on the radius. The ion velocity distribution function is determined from the numerically differentiated current voltage characteristics of an ion energy analyser. The anisotropic part of the distribution function is found to be proportional to the normalised difference of the derivatives of a pair of characteristics, recorded in up- and downstream direction with respect to the local rotation velocity. The experimental data indicate an ion velocity distribution function, which closely resembles a Maxwellian in a frame moving with the plasma rotation velocity. The measured anisotropic part of the ion velocity distribution function is evaluated applying a procedure that treats the thermal and rotation velocities of the ions as free parameters of a (non-linear) model function and fits them to the experimental data in the ion retarding regime of the characteristics. Furthermore it is shown that in the presence of a moving plasma only the difference of the derivatives of a pair of characteristics indicates the plasma potential. Thus the local radial electric field can be determined and indeed is used to calculate the dependency of the E × B-rotation velocity on the plasma radius. These calculated E × B-rotation velocities agree with those measured by an ion energy analyser within the range of experimental uncertainties, indicating minor influence of the ion pressure gradient on the rotation of the plasma column studied.