Experimental validation of plasma tomography algorithms at ISTTOK

Abstract

Plasma tomography is an ill-conditioned inversion problem. Algorithms based on Tikhonov regularization and minimum Fisher information try to overcome this issue by introducing regularization parameters that require empirical tuning. In general, to validate the implementation of these algorithms, either artificial phantoms are used, or one must rely on information provided by other diagnostics. In this work, an experimental setup was developed that allows the use of physical phantoms to tune and validate the reconstruction algorithms. The setup was assembled in a replica of the ISTTOK tokamak with a cylindrical cold cathode lamp placed perpendicular to the poloidal plane at multiple radial and angular positions. It is assumed that this physical phantom acts approximately as a point source in the emissivity profile. Knowing these profiles a priori, it is possible to compare them with the reconstructions produced by the algorithms, in order to tune and optimize the reconstruction parameters. After validation with these physical phantoms, several reconstructions were successfully computed for some tokamak discharges.