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 literature, have been applied to tomographic reconstructions of soft x-ray (SXR) measurements in the JET (Joint European Torus) tokamak. The results are compared with those obtained with local basis functions (LBF), and those obtained with a conventional constrained optimization (CO) tomography method. Truncated singular value decomposition is used as the inversion method. Reconstructions without a priori information, such as NBF reconstructions, are, as can be expected, less good than reconstructions in which a priori information is used, as in the conventional method, but the reconstructions are shown 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 significant 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 whether a newly observed in-out asymmetry in the SXR emission during the injection of nickel into a RF-heated plasma, with the peak on the inboard side, is real. A possible explanation for the asymmetry is emissivity is an increased nickel density on the inboard side as a result of an RF-induced increase of the hydrogen-minority density on the outboard side.
Bibliographic Reference: Report: JET-P(99)47
Availability: Available from JET Joint Undertaking, Abingdon, Oxfordshire (GB)
Record Number: 200012766 / Last updated on: 2000-09-20
Original language: en
Available languages: en