Numerical study on a novel method for the plasma vertical position control using non-axisymmetric coils in JT-60SA

A numerical investigation explored the feasibility of controlling the plasma vertical position using non-axisymmetric coils in JT-60SA. Non- axisymmetric coils offer an alternative to conventional axisymmetric in-vessel coils, addressing challenges in installation and maintenance, particularly in the context of the Japanese DEMO reactor. However, the main concern using non-axisymmetric coils is the potential generation of locked modes, and the shortage of control margin for such as edge localized mode suppression. In this sense, we developed control logic to maximize accessible elongation while simultaneously minimizing coil currents. To assess controllability, we employed the MHD equilibrium control simulator and vacuum magnetic field analysis. We demonstrated that with the application of non-axisymmetric coils, the plasma elongation was extended from 1.75 which was achieved with superconducting poloidal field (SCPF) coils to 1.95, even in the presence of a minor Ip collapse. The impact of non-axisymmetric magnetic fields on locked modes was evaluated using a three-mode error index (BTMEI), revealing that n = 1 perturbations remained within acceptable limits (BTMEI≤ 0.05 mT) in most scenarios.