Equilibrium reconstruction method for self-organized plasmas on reversed field pinches with polarimeter-interferometer

Abstract

In the reversed field pinch (RFP), plasmas exhibit various self-organized states. Among these, the three-dimensional (3D) helical state known as the “quasi-single-helical” (QSH) state enhances RFP confinement. However, accurately describing the equilibrium is challenging due to the presence of 3D structures, magnetic islands, and chaotic regions. It is difficult to obtain a balance between the available diagnostic and the real equilibrium structure. To address this issue, we introduce KTX3DFit, a new 3D equilibrium reconstruction code specifically designed for the Keda Torus eXperiment (KTX) RFP. KTX3DFit utilizes the stepped-pressure equilibrium code (SPEC) to compute 3D equilibria and uses polarimetric interferometer signals from experiments. KTX3DFit is able to reconstruct equilibria in various states, including axisymmetric, double-axis helical (DAx), and single-helical-axis (SHAx) states. Notably, this study marks the first integration of the SPEC code with internal magnetic field data for equilibrium reconstruction and could be used for other 3D configurations.