Integrated magnetic control to facilitate H-mode access and high performance in NSTX-U*

Integrated-control algorithms have been designed and tested within the GSEvolve simulation framework [1] to facilitate the sustainment of plasma equilibriums and shapes that enable H-mode access in NSTX-U. More specifically, vertical oscillations and dithering between upper and lower single null shapes at low frequency need to be avoided because they hinder access to H-mode and high performance in NSTX-U discharges [2, 3]. Due to the highly coupled nature of the magnetic-control problem in spherical tokamaks such as NSTX-U, the controllers developed here are integrated and use decoupling techniques to minimize the interaction between the ohmic and poloidal-field coils. Such control algorithms have been developed to avoid the aforementioned oscillations and enable H-mode access in highly nonlinear GSevolve simulations, which use the real NSTX-U Plasma Control System (PCS). It has been found that the strong dynamical coupling between plasma current, vertical plasma-position, horizontal plasma-position, and plasma shaping via the isoflux scheme [4] is key to understand the physics of this control problem and find successful algorithms. The overall goal of this work is to provide potential control solutions and initial control-development guidance for NSTX-U once it resumes operation.

 

[1] A. Welander et al, 2019 Fusion Engineering and Design 146 2361-2365

[2] M. D. Boyer et al, 2018 Nuclear Fusion 58 036016

[3] D. Battaglia et al, 2018 Nuclear Fusion 58 046010

[4] J.R.Ferron et.al., 1998 Nuclear Fusion 38