Development of a tomographic reconstruction method and test for soft x-ray array diagnostics for KSTAR plasmas

Abstract

Summary form only given, as follows. Tomographic reconstruction combined with soft x-ray array diagnostics can give valuable information about MHD activities and transport in high temperature plasmas. Although several different mathematical algorithms have been developed for tomographic reconstruction including Cormack method (analytical method), maximum entropy method (MEM), modified back-projection method (MBP) etc., each of these algorithms has its pros and cons depending on the shape of the plasma cross-section and emissivity profile. In this work, Cormack method, MEM, and MBP were compared in order to find out a proper reconstruction algorithm for accommodating various situations of KSTAR plasmas. With the Cormack's analytical method, expansion coefficients of reconstructed emissivity function depend on mode number and basis functions. The expansion coefficients were optimized by changing basis functions and constraints. The MEM using Lagrange's multipliers revealed that it is able to handle global MHD properties of the expected KSTAR plasma. Fast calculation of Lagrange's multipliers from nonlinear equations enables MEM to achieve fast reconstruction results. The MBP method is an alternative method for the conventional back-projection method which blurs reconstruction images. The modified backprojection technique was tested to a hollow emissivity profile to give good local property of a soft X-ray image with reducing a blurring. Based on the results, the detailed design of the soft x-ray array diagnostics will be performed for KSTAR plasmas.