O ct 2 00 4 Reconstruction of Z eff profiles at TEXTOR through Bayesian source separation

We describe a work in progress on the reconstruction of radial profiles for the ion effective charge Z eff on the TEXTOR tokamak, using statistical data analysis techniques. We introduce our diagnostic for the measurement of bremsstrahlung emissivity signals. Z eff profiles can be determined by Abel-inversion of line-integrated measurements of the bremsstrahlung emissivity (ǫ ff) from the plasma and the plasma electron density (ne) and temperature (Te). However, at the plasma edge only estimated values are routinely used for ne and Te, which are moreover determined at different toroidal locations. These various uncertainties hinder the interpretation of a Z eff profile outside the central plasma. In order to circumvent this problem, we propose several scenarios meant to allow the extraction by (Bayesian) Blind Source Separation techniques of either (line-integrated) Z eff waveshapes or absolutely calibrated signals from (line-integrated) emissivity signals, using also density and temperature signals, as required. 1. The role of impurities and Z eff in a tokamak plasma The behavior of plasma impurities in a tokamak plasma is a critical issue [1]. On the one hand, these impurities increase the fuel dilution and, at reactor parameters, they can be responsible for a considerable power loss from the plasma core by bremsstrahlung. On the other hand, line radiation by low-Z impurities at the plasma edge can be advantageous for the reduction of the plasma-wall interaction. To study the behavior of impurities in the plasma, one needs a local measure for impurity concentration. A parameter of great interest is the so-called ion effective charge of the plasma, Z eff , which is averaged over all species. Once the plasma electron density (n e) and temperature (T e) are known, it is proportional to the bremsstrahlung emissivity (ǫ ff) from the plasma [2], which originates mainly from the acceleration of electrons in the field of ions: