ITER-relevant Density Control with Pellets in DIII-D's ITER Baseline Scenario

Experiments in DIII-D have successfully demonstrated novel density control in DIII-D’s ITER baseline scenario using adaptive techniques with ITER-relevant actuators. Density control has been achieved using ITER-emulated gas puffing and pellet injection under ITER baseline conditions (q₉₅~3, β_N~1.8, n_e~6-8x10¹⁹ m^-3) with high reliability and minimal gain tuning. Such control performance is enabled by model-based adaptive control that compares density measurements with a linear, time-varying density model. Such approach infers global plasma and machine parameters relevant for density control such as particle confinement and fueling efficiency. Based on these parameters, the control gains are self-optimized during the discharge to simultaneously handle pellet injection and gas puffing. The controller can reject disturbances in actuators (i.e., slow gas puffing, uncertain pellet dynamics) and scenario access in DIII-D. However, strong nonlinear pedestal perturbations are observed with pellet injection at high density, which sometimes challenge plasma controllability and disruptivity. Therefore, outside the linear density-response region, linear models and associated controls may not suffice for density control in reactor-size devices, suggesting further work on nonlinear models and controls.