MAST Upgrade Results Towards Integrating High Core Confinement and Divertor Power Dissipation
A key objective of fusion research is to develop pathways to integrate good confinement in the core and pedestal with strong dissipation of power and particles in the divertor through a combination of experimental demonstration and modelling. The MAST Upgrade spherical tokamak has unique capabilities to find solutions to this integration challenge. To optimize core and pedestal confinement, on- and off-axis neutral beam heating and current drive has been used to robustly achieve H-mode and to tailor the equilibrium q profile. The large number of poloidal field coils affords significant flexibility to vary the core plasma shape, which in turn modifies the pedestal stability. Excellent plasma exhaust performance has been demonstrated in up-down symmetric, tightly baffled divertors with Super-X capability. In the first experimental campaign of MAST Upgrade, high performance H-mode scenarios were sustained for 1s with 3.2 MW of NBI heating and an energy confinement time of 30-50 ms, corresponding to H98y,2 ~1. The highest achieved βN was ~3 (Te0 ~1keV, ne0 ~ 6.0×1019 m-3), limited by the growth of core m/n = 1/1 and 2/1 modes, leading to core temperature profile flattening and a transition from H-mode to L-mode. Good pedestal confinement was observed, with pedestal top temperatures exceeding 200eV. Initial studies of the Super-X divertor configuration in lower power Ohmic heated L-mode conditions indicated the outer mid-plane separatrix density required to reach the detached divertor regime is reduced by 50% compared with a conventional divertor and at lower density, in attached conditions, the outer divertor peak heat flux is reduced by at least a factor of 10. Detailed spectroscopic measurements and SOLPS-ITER simulations have yielded new insights into the role of atomic and molecular mechanisms governing detachment and identified possible improvements in predictive modelling codes. Recent advances in improving and understanding core confinement, pedestal stability and power exhaust in MAST Upgrade will be presented, including the benefits of combining a high performance ELMy H-mode core and pedestal with the Super-X divertor configuration. A central aspect of this scenario integration is that the improved power exhaust in the Super-X configuration enables operation with tolerable power exhaust over a wider operating space, including lower density and higher temperatures in the core plasma that are beneficial for confinement.