Maximum entropy based reconstruction of soft X ray emissivity profiles in W7-AS

The reconstruction of 2-D emissivity profiles from soft. X ray tomography measurements constitutes a highly underdetermined and ill-posed inversion problem, because of the restricted viewing access, the number of chords and the increased noise level in most plasma devices. An unbiased and consistent probabilistic approach within the framework of Bayesian inference is provided by the maximum entropy method, which is independent of model assumptions, but allows any prior knowledge available to be incorporated. The formalism is applied to the reconstruction of emissivity profiles in an NBI heated plasma discharge to determine the dependence of the Shafranov shift on beta, the reduction of which was a particular objective in designing the advanced W7-AS stellarator.