Investigation into ion edge temperature behaviour using CER spectroscopy at DIII-D

Measurements with high spatial resolution in the neighbourhood of the separatrix show edge ion temperatures in the 1-2 keV range, exceeding the electron temperature substantially, in neutral beam heated H mode discharges on DIII-D. Charge exchange recombination (CER) spectroscopy on light ions such as He²⁺ and C⁶⁺ is used to measure ion temperature, ion density and poloidal and toroidal plasma rotation profiles. Near the separatrix the spatial resolution of the measurements is a few millimetres. The scale length for the edge ion temperature gradient is found to be about one third of the poloidal gyroradius, i.e. a factor of 2 to 3 longer than the neoclassical scale length. The electron temperatures at the plasma edge are substantially ( approximately=1 keV) lower than the ion temperatures. Outside the separatrix, T₁ is also found to be higher than T_e. We investigate the power balance globally and locally at the plasma edge. It is found that the high heat transfer from the ions to the electrons, just inside the separatrix, can be balanced by local ion heating due to neutral beam absorption.