Inter-discharge optimization for fast, reliable access to ASDEX Upgrade AT scenario

Abstract

Rapid inter-discharge simulation and optimization using the RAPTOR code have allowed the development of a reliable and reproducible early heating strategy for an advanced tokamak (AT) scenario on ASDEX Upgrade. Solving for electron heat and current diffusion in RAPTOR with ad-hoc formulas for heat transport and EC current drive efficiency was found to robustly recover the coupled dynamics of Te and q, while maintaining model parameters fixed. The pedestal top boundary condition in pre-shot simulations is set by a newly derived scaling law for the electron pressure at ρ=0.8, using a data set of previous AT discharges. RAPTOR simulations have allowed to understand the onset of 3/2 tearing modes, which were observed to have a detrimental impact on confinement when low magnetic shear conditions are present at the rational surface during the high-β phase. Delaying the NBI heating, by a specific time interval found via simulations, has led to avoiding these modes. A non-linear optimization scheme has been applied to optimize the ECCD deposition radii to reach a stationary state with elevated qmin at the beginning of the flat-top phase, and has been successfully tested in experiment. However, further experiments, aiming for qmin > 1.5, also highlighted limitations of the present feedforward control approach in the presence of shot-to-shot variations that are not included in the applied model. Application of real-time model-based control is proposed to overcome model-reality mismatches in future work.