Attenuation measurement of an atomic hydrogen beam as a diagnostic technique to determine the light impurity content of a plasma

The attenuation of a low intensity atomic hydrogen or deuterium beam, injected into the plasma of the ERASMUS Tokamak, was investigated during the whole duration of the discharge, as a function of the beam energy (3-15 keV) and of the radial position of the injection chord. The comparison with attenuations calculated from impurity models adapted to the ERASMUS plasma allows the evaluation of the content of oxygen assuming that this is the only light impurity. The sensitivity of the diagnostic was tested by injection of test impurities, such as oxygen and helium, and during RF heating in the ion-cyclotron range of frequencies.