Regulation of the central safety factor and normalized beta under low NBI torque in DIII-D

A. Pajares, E. Schuster, W. P. Wehner, K. H. Burrel, J. R. Ferron, M. L. Walker, D. A. Humphreys

Fusion Engineering and Design 187 (2023) 113363

Abstract

An algorithm has been designed to simultaneously control the central safety factor (𝑞0) and normalized beta (𝛽𝑁) while ensuring near-zero torque from neutral beam injection in DIII-D. Feedback control of 𝑞0 and 𝛽𝑁 in tokamaks can be beneficial due to the close relationship that these variables have with plasma performance and magneto-hydrodynamic stability. In addition, low neutral-beam-torque conditions are of special interest in present devices because future burning-plasma tokamaks such as ITER will most likely operate at very low plasma rotation. The control synthesis of the algorithm presented in this work is based on a linearized, one-dimensional (1D) model of the current-profile dynamics coupled with a zero-dimensional (0D) plasma-energy balance. The actuators considered are neutral beam injection and electron-cyclotron heating and current drive, and discrete logic determines the neutral-beam injection powers that deliver near-zero torque. The algorithm has been tested in nonlinear, 1D simulations using COTSIM (Control-Oriented Transport SIMulator) and in DIII-D experiments, demonstrating satisfactory performance.