Reaching and Investigating High Beta Plasmas in MAST-U

The spherical tokamak offers significant advantages as the core of a fusion power plant including

compact size and high magnetic field utilization at low aspect ratio and naturally high

elongation. Research on the MAST-U device has rapidly made key advancements in stability

performance and understanding aimed toward a high performance core plasma compatible with

the Super-X divertor exhaust solution. Attention is placed on analysis of a MAST-U campaign

thrust experiment aimed to develop, sustain, and study high performance plasmas at high ratios

of plasma to magnetic field energy (β). The plasmas transiently reached record device

normalized beta, β_N > 4 (near the n = 1 ideal MHD no-wall beta limit), encountering mode

locking and minor/major disruption. Sustained plasmas reached β_N = 3.5 avoiding terminations

due to mode locking. Stability investigations include elimination of internal reconnection events,

determination and avoidance of vertical stability limits, and significant alteration of rotating

MHD mode stability. Increased plasma elongation, κ, (more generally plasma shaping), high

poloidal beta (reaching 1.3, εβ<span style="font-size:10.8333px">p = 1) and plasma rotation profile variation was used to alter finite

toroidal mode number stability. Improvements in vertical control enabled operational states free

mitigated and mode locking was completely avoided at κ = 2.3. An observed natural decrease in

of vertical instability up to κ = 2.5 and internal inductance l_i > 1.0. MHD modes with n > 0 were

l_i and measured core q profile inversion observed with otherwise constant global parameters

indicates a core dynamo / flux pumping effect found to increase q₀ and improve stability. A new

high beta operational state with large Shafranov shift produces extreme outward shift and edge

shear of the plasma rotation profile that favorably alters n > 0 mode stability.