Abstract
A new tomographic scheme is proposed for reconstructing three dimensional (3D) impurity emission distributions from two dimensional (2D) measurements with a single field-of-view in the magnetized edge plasma in a Large Helical Device (LHD). The 2D image is obtained with a multi-channel fiber array spectrometer, which views the entire region of the edge stochastic magnetic layer of LHD, including divertor plates, divertor legs, the stochastic layer, and the last closed flux surface. The scheme introduces new regularization terms in the Lagrangian function, based on the transport feature in magnetized plasma that the transport parallel to the magnetic field lines is much faster than the transport across the magnetic field, thus assuming smooth distribution in the parallel direction. The scheme is benchmarked with the test data of 3D distribution in the measurement volume, where the effectiveness of the various regularization terms is surveyed and feasibility of the scheme is confirmed. The new scheme is applied to the experimental data in LHD for carbon impurity emissions of C1+ and C3+, where the obtained distributions are discussed taking into account the plasma wall interaction and charge dependence of ionization potentials.
Highlights
In magnetically confined fusion reactors, understanding of impurity transport in edge region is a critical issue to control impurity radiation in divertor region as well as impurity influx into the confinement region
We propose a new tomographic scheme to reconstruct three dimensional (3D) impurity emission distribution from single field of view 2D measurement in magnetized plasma by redistributing the signal along the line of sight
A single field-of-view tomography method has been proposed for inverting the impurity emission distributions measured in magnetized edge plasma
Summary
The relation between the viewing area of the spectrometer and the LHD torus is shown in Fig.. The open field lines in the edge region go around torus along helical coils until they are cut by the divertor plates. In order to obtain good focus of the fiber image in wide area on the CCD detector, an astigmatism is suppressed by introducing toroidal and spherical mirrors in the spectrometer. Exposure time and cycle time are set to 0.09-0.19 sec and 0.2-0.3 sec, respectively These settings were changed depending on the gratings and the plasma parameters
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