Predict-first startup scenario modelling supporting MAST-U commissioning*

A reduced semi-empirical model for breakdown and the initial I_p ramp-up in spherical tokamaks (STs) was recently developed to support the preparation and execution of plasma startup in MAST-U.  The reduced model uses vacuum field calculations in order to achieve rapid solutions suitable for predict-first and between-shot analysis applications. The model was developed using a large database of startup discharges from NSTX, NSTX-U and MAST that use induction and different approaches to pre-ionization.  One key finding is that STs achieve a rapid breakdown (several milliseconds) by accessing a large E/P regime where ionized electrons are continually accelerating. Another finding is that the timescale of the initial increase in I_p is reproduced only if a mechanism for a self-generated electric field is included, similar to conventional-A tokamaks. The first plasma experiments on MAST-U are in good agreement with the model predictions and identified additional constraints that improve the predictive model.