Equilibrium operational space and stability limits in early operation of MAST-U

The MAST-U spherical tokamak has recently completed its first campaign of physics operation. MAST-U has operated with Ohmic, or one or two neutral beams for heating, at 400-800 kA plasma current, in conventional or “SuperX” divertor configurations. Equilibrium reconstructions with magnetics only or additionally with motional Stark effect diagnostic measurement of the magnetic pitch angle provide key plasma physics parameters vs. time for each discharge. The disruption event characterization and forecasting (DECAF) code produces diagrams which show where the prevalence of operation occurred as well as the limits in various operational spaces. When compared to stability limits, the operation of MAST-U so far has generally stayed out of low q, low density instability regions, and below the high density Greenwald limit, high beta global stability limits, and high elongation vertical stability limit. MAST-U has reached β_N/l_i = 3.3 so far, well below the ideal no-wall limit, which was projected to be about β_N/l_i = 7 [1]. MAST-U still has the potential to reach higher elongation, which could benefit the plasma performance. [1] Berkery J W et al 2020 Plasma Phys. Control. Fusion 62 085007.