Exploration of plasma vertical position control with three dimensional non-axisymmetric coils

A novel plasma vertical position control method was numerically explored with 3D non-axisymmetric coils. In DEMO reactors, due to the difficulties in machine maintenance and the critical damages caused by neutron flux, it is expected to be challenging to introduce axisymmetric coils near the plasma for fast control of the n=0 mode vertical instability. Additionally, introducing such coils outside the vacuum vessel will be suffer from poor controllability due to the thick conducting shell in turn. Alternatively, the conceptual design of installing 3D non-axisymmetric coils for ELM control together with blanket modules closer to the plasma is considered as the JA DEMO design. In this study, we investigated the controllability of the n=0 mode using 3D coils by utilizing the MHD equilibrium control simulator and vacuum magnetic field analysis. The verification of controllability was conducted with JT-60SA simulation model. It was shown that n=0 mode control was achieved with the elongation of 1.9 using 3D coils, while the control failed when using only poloidal field coils. During the n = 0 mode control, we could avoid critical error fields that could generate locked modes during vertical position control by applying a damper to the control logic. It was found that the maximum error field was B_{r, err} = 6.88×10⁻⁷ T, which is sufficiently smaller than the locked mode error field threshold B_{r, err} ≤ ∼1×10⁻⁴ T.