Current Profile Optimization for Tearing Mode Avoidance in DIII-D Steady-State Scenarios*

DIII-D discharges with a relatively high value of internal inductance (l_i) or with a high minimum value of q (q_min) have improved confinement and stability that is beneficial for steady-state operation at high normalized pressure β_N. In these high β_N discharges, the duration of the high-performance phase can be limited by the occurrence of tearing modes (TMs). Tearing mode onset is addressed with theory-based integrated modeling using IPS-FASTRAN. The model is validated against the experimental profiles from the magnetic axis to divertor/wall for DIII-D high β_N discharges by iterating core transport, edge pedestal, equilibrium, stability, heating, and current drive self-consistently. Resistive MHD simulations of TMs performed with accurate equilibrium reconstruction and well-measured plasma profiles determine the onset dependency on the tearing stability index Δ′ for the purpose of predicting TM onset and evolution in the experiment. Based on IPS-FASTRAN coupled with the TM predictor, the current and pressure profiles are optimized simultaneously for high-performance steady state operation without deleterious TMs.