Both ion and electron temperatures in the scrape-off layer (SOL) of the Tore Supra tokamak were measured by a retarding field analyzer during an ohmic density scan. SOL T i was found to be higher than T e by a factor of 4-7. The ion-to-electron temperature ratio τ decreases with increasing density. Core T i and T e measurements in Tore Supra combined with a multi-machine database of τ measurements show that τ > 1 also in the edge of the confined plasma and increases with radius.

The retarding field analyzer (RFA) is one of the only widely accepted diagnostics for measurements of ion temperature Ti in the tokamak scrape-off layer. In this paper we analyze some instrumental effects of the RFA and their influence on Ti measurements. It is shown that selective ion transmission through the RFA slit is responsible for an overestimation of Ti by less than 14%, even for a relatively thick slit plate. Therefore, thicker slit plates are preferable, since they reduce, e.g., the risk of melting during off-normal events, and the effect of positive space charge inside the cavity. The influence of the electron repelling grid, as well as misalignment of the slit with respect to the magnetic field on Ti measurements are negligible.

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