Development of a virtual Z(eff) diagnostic for the W7-X stellarator

For the W7-X stellarator a diagnostic system for measurement of local Z(eff) values from the visible bremsstrahlung continuum is foreseen. The method is based on passive, absolute measurement of the bremsstrahlung intensity along several lines of sight. In order to eliminate the spurious impact of other radiation sources of different spectral intensity distribution than the bremsstrahlung, like the line radiation, it is intended to spectrally resolve the detected radiation, e.g., by use of microspectrometers. The visible bremsstrahlung background can be extracted by making use of the 1/lambda(2) dependence of its intensity (expressed in unit Wm(3) sr nm) in the high temperature plasmas by using Bayesian data analysis techniques. In a second step, the local values of Z(eff) as a function of the effective plasma radius are derived by inversion, using different regularization methods, of the line-integrated bremsstrahlung signals with the knowledge of the magnetic flux surfaces. Inversion of the full model based on statistical methods allows taking into account all conceivable uncertainties accompanying Z(eff) measurement and provides uncertainties of the local Z(eff) values and valuable information on other uncertain parameters of the model. In this paper we show the first steps in developing a virtual Z(eff) diagnostic for W7-X which allows the optimization of the statistical model as well as of the future diagnostic setup.