Ion Temperature-Measurements in Tokamak Plasmas by Rutherford Scattering

A Rutherford scattering diagnostic has been applied at the TEXTOR tokamak to obtain spatially and temporally resolved information on the temperature of the bulk ions in the plasma. In the experimental setup, a helium atomic beam (30-keV, 12-mA equivalent current) passes vertically through the plasma core. A small part of the injected atoms is scattered elastically by the thermally moving plasma ions. The ion temperature in the scattering volume can be determined from the broadening of the energy spectrum of the scattered particles. Energy analysis of the scattered atoms is performed by a mass-selective time-of-flight analyzer detecting the particles at an observation angle which is selectable between 3-degrees and 8-degrees. Coincidence techniques have been successfully applied in this detector for rejection of background events triggered by detections of neutrons and gamma radiation. Ion temperature profiles were measured on a shot-to-shot basis by shifting the cross section of the diagnostic beam and the observational volume of the analyzer through the plasma. The ion temperatures measured in ohmic deuterium plasmas were found to be in reasonable agreement with those obtained from passive neutral particle analysis. Up to now, ion temperatures have been measured throughout the complete discharge with an accuracy of 8% and a time and space resolution of 100 ms and 0.10 m at a scattering angle of 7-degrees. Deuteron density profiles could be deduced from the scattering yield measured at different radial positions in the plasma. The ratio of the isotopes, hydrogen and deuterium, was determined from their separate contributions to the spectrum of helium particles scattered on hydrogen and deuterium. Although theoretical predictions showed that the majority of the probing helium atoms loses one of its electrons during the elastic scattering process on multiply charged carbon and oxygen ions, contributions from impurities to the observed experimental spectrum are shown to appear dominant for impure plasmas.