Consistency check of Zeff measurements in ergodic divertor plasmas on Tore Supra

On Tore Supra, the mean effective charge Zeff of the plasma is deduced routinely from line integrated measurements of visible Bremsstrahlung. In order to get a deeper understanding of this measurement, and to obtain high confidence in the data, we have compared the visible Bremsstrahlung data to line radiation of impurities present in the plasma, removing any error from the processing technique. The paper presents a systematic study of the correlation between the Zeff derived from the two methods, and the dependence on plasma parameters and geometrical factors. A good correlation is found, independent of the position of the outboard pump limiter (OPL). However, the fitting parameter of the empirical method is different for different plasma currents, due to the variation of the emission profiles. The agreement between the two methods is good, confirming the role of carbon and oxygen as the main impurities in the Tore Supra tokamak.