Retarding Field Energy Analysis for Ion Temperature Measurement in the Tokamak Edge

At the energies characteristic of the tokamak edge plasma, the Retarding Field Analyser (RFA) constitutes probably the simplest method of directly measuring the ion velocity distribution parallel to the magnetic field and the magnitude of the sheath potential fall. For the case of Maxwellian velocity distributions, the technique provides an estimate of the edge ion temperature. Despite their relative simplicity, care must be taken both to operate these devices correctly in the rather unique tokamak edge environment and to consider carefully the interpretation of data thus obtained. This contribution addresses some aspects of both of these issues using the results of RFA experiments conducted on the DITE tokamak.