High beta n=2 stability and error field correction in DIII-D

DIII-D experiments show that the plasma angular momentum L can be maximized using a combination of toroidal mode number n = 1 and n = 2 applied fields, in a high β_N regime near the ideal MHD n = 2 no-wall limit. The n = 1 and n = 2 no-wall β_N limits of the experimental equilibria scale with 4.2ℓ_i and 4.7ℓ_i respectively, with 0.6 < ℓ_i < 0.8, and exhibit a clear oscillatory dependence on q_min. The optimal n = 1 amplitude and phase were determined using magnetic feedback, a standard DIII-D technique for optimizing L. In contrast, using n = 2 feedback yielded a lower L than the optimum inferred from open-loop applied field amplitude ramps at varying toroidal phases. The L optima will be compared with predictions from modelling incorporating the DIII-D error field source model and plasma response. The results affirm the presence of a significant, usually uncorrected n = 2 error field response at high β and motivate the continuing development of n > 1 magnetic feedback.