Model based optimization of Advanced Tokamak plasma start-up

AT scenarios offer improved stability, confinement and pulse length compared to standard scenarios due to an increase of the plasma’s bootstrap current j_bs∝q∇p. They are accessed by externally manipulating the safety factor profile q, which can be applied during the current ramp-up, or after an equilibrium is reached. The former allows for a longer discharge and more unconventional current distributions, but due to the volatility of the early plasma, creating such a scenario experimentally, with feed-forward control usually takes a lot of trial and error.

To combat this, a model has been developed in the transport code ASTRA, capable of accurately predicting the plasma response to actuator changes with a run-time of only a few minutes. This allows to quickly test large amounts of possible actuator configurations. The model includes Gyro-Bohm based core transport, edge transport according to a recently developed scaling law as well as the L/H-transition based on the heating power at the separatrix. Good agreement between simulation and experiment has been achieved.

The suitability of this model to design such a scenario has successfully been tested at ASDEX-U. Further improvement of the physics models and assumptions towards predictive capability for future machines is planned.