Rotation Profile Control Enabled by Multi-modal Response to 3D Fields*

Predictions of the Generalized Perturbed Equilibrium Code (GPEC) have been validated in DIII-D experiments testing the manipulation of the multi-modal plasma response for neoclassical toroidal viscosity (NTV) torque control. A multi-modal matrix formulation was employed to optimize spectra applied using multiple 3D field coil arrays to obtain desired NTV torque profiles. The new formulation [1] solves the anisotropic pressure perturbed equilibrium, representing the nonlinear torque as a “torque response matrix” that has been directly coupled to available experimental coils. The eigenmodes of such a torque response matrix then provide predicted optimal coil configurations for the maximum, minimum, core localized and edge localized NTV torque profiles. Each of these have been predicted for and applied in DIII-D experiments, where the multi-modal n=2 plasma response allows for significant manipulation of the NTV profile using the three 3D field coil arrays. The experiments validated the GPEC model in nonresonant field space, where it provides accurate predictions of quiescent braking profiles that could be used in rotation control algorithms with little impact on the particle or energy confinement.  Large edge resonant magnetic perturbations, however, caused large density pumpout not accounted for in the neoclassical model, significantly distorting the equilibrium from the perturbative model prediction and motivating the integration of 3D and 2D transport models. Within their regime of validity, the optimizations have demonstrated the clear ability to manipulate the NTV profile with existing coils and provide the best NTV spectra for the design of future 3D field coils.

[1] J.-K. Park and N.C. Logan, Phys. Plasmas 24, 32505 (2017)