ON MEASUREMENTS OF THE ROTATION OF A MAGNETOPLASMA USING AN ION-ENERGY ANALYZER

This study is concerned with measurements of the anisotropic part of t he azimuthal component of the ion velocity distribution function by me ans of a singly gridded ion energy analyser. The experiments are perfo rmed at a magnetised plasma column in order to investigate the depende ncy of the E x B-rotation velocity of the plasma on the radius. The io n velocity distribution function is determined from the numerically di fferentiated current voltage characteristics of an ion energy analyser . The anisotropic part of the distribution function is found to be pro portional to the normalised difference of the derivatives of a pair of characteristics, recorded in up- and downstream direction with respec t to the local rotation velocity. The experimental data indicate an io n velocity distribution function, which closely resembles a Maxwellian in a frame moving with the plasma rotation velocity. The measured ani sotropic part of the ion velocity distribution function is evaluated a pplying 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 char acteristics. Furthermore it is shown that in the presence of a moving plasma only the difference of the derivatives of a pair of characteris tics indicates the plasma potential. Thus the local radial electric fi eld can be determined and indeed is used to calculate the dependency o f the E x B-rotation velocity on the plasma radius. These calculated E x B-rotation velocities agree with those measured by an ion energy an alyser within the range of experimental uncertainties, indicating mino r influence of the ion pressure gradient on the rotation of the plasma column studied.