Comparison of basis functions in soft x-ray tomography and observation of poloidal asymmetries in impurity density

Natural basis functions (NBF), also known as natural pixels in the literatu re, have been applied in tomographic reconstructions of soft x-ray (SXR) me asurements in the JET (Joint European Torus) tokamak. The results are compa red with those obtained with local basis functions (LBF), and those obtaine d with a conventional constrained-optimization tomography method. Truncated singular value decomposition is used as the inversion method. Reconstructi ons without a priori information, such as the NBF reconstructions, are, as can be expected, less good than reconstructions in which a priori informati on is used, as in the conventional method, but the reconstructions are show n to be reliable by means of phantom simulations. Reconstructions with the same number of LBFs as NBFs without a priori information are comparable to NBF reconstructions, although the latter seem to be somewhat better. No sig nificant changes in results, apart from smaller reconstruction errors, are obtained if the measuring system has more regular or complete coverage than the JET SXR system. The various tomography methods are used to assess whet her a newly observed in-out asymmetry in the SXR emission during the inject ion of nickel into an RF-heated plasma, with the peak on the inboard side, is real. A possible explanation for the asymmetry in emissivity is an incre ased nickel density on the inboard side as a result of an RF-induced increa se of the hydrogen-minority density on the outboard side.