Soft X-ray tomography on NSTX

Summary form only given. A set of poloidal ultrasoft X-ray photodiode arrays is operated by the Johns Hopkins group on the National Spherical Torus Experiment (NSTX), a toroidal magnetically confined plasma experiment. To enable internal plasma magnetohydrodynamic (MHD) mode identification and analysis, the McCormick-Granetz tomography code developed at MIT was adapted to the NSTX plasma equilibrium geometry. Tests of the code using synthetic data show that the present X-ray system is adequate for poloidal mode number m=1 tomography. In addition, we have found that spline basis functions may be better suited than Bessel functions for the reconstruction of radially localized phenomena in NSTX. The tomography code was also used to determine the necessary array expansion and optimal array placement for the characterization of higher m modes (m=2,3) in the future. Reconstruction of experimental soft X-ray data has been performed for m=1 internal modes, which are often encountered in high plasma pressure NSTX discharges. Higher order MHD modes may be identified using a maximum entropy analysis method to allow more freedom of the reconstruction parameters while reducing numerical image artifacts.