Realistic multi-machine tokamak profile simulations and numerical ramp-down optimization using the RAPTOR code

Predictive modelling of plasma profiles is an essential part of ongoing research in tokamak plasmas, required for a successful realization of future fusion reactors. In this research we focus on upgrading the RAPTOR code to extend the area of its applicability for plasma modelling and scenario development and also demonstrate new strategy for ramp-down optimization. The RAPTOR transport model has been extended to take into account the influence of the time-varying plasma equilibrium geometry and background kinetic profiles on the evolution of the predicted plasma profiles. Also transport equations for the ion temperature and plasma particles (electrons and ions) have been implemented in the code. Benchmarks have been performed with more sophisticated transport ASTRA and CRONOS codes and with prescribed data for the particle transport in ITER. A new ad-hoc transport model based on constant gradients for core and pedestal regions, that is suitable for simulations of transition between H and L modes, has been implemented into RAPTOR. This model assumes ``stiffness'' of the plasma profiles in the core region, reflecting their relatively weak reaction to changes in the heat flux. We demonstrate the capabilities of RAPTOR for realistic predictions of plasma state over the entire plasma discharges, i.e. from ramp-up to ramp-down, for TCV, ASDEX Upgrade and JET plasmas.
Automatic optimization algorithms can be applied for searching the optimal ramp-down trajectory. Here we define the goal of the optimization as ramping down the plasma current as fast as possible while avoiding any disruptions caused by reaching physical or technical limits. Physical constraints are relevant for most tokamaks, others are technical and related to the specific tokamaks. A proper plasma shaping during the current ramp-down can reduce significantly the plasma internal inductance, improving its vertical stability. Results of numerical and experimental ramp-down studies for TCV, AUG and JET plasmas are presented.