Reaching and Investigating High Beta Plasmas in MAST-U
ORAL
Abstract
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, βN > 4 (near the n = 1 ideal MHD no-wall beta limit), encountering mode
locking and minor/major disruption. Sustained plasmas reached βN = 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 li > 1.0. MHD modes with n > 0 were
li 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.
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, βN > 4 (near the n = 1 ideal MHD no-wall beta limit), encountering mode
locking and minor/major disruption. Sustained plasmas reached βN = 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 li > 1.0. MHD modes with n > 0 were
li 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.
*Supported by US DOE Grant DE-SC0018623.
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Presenters
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Veronika Zamkovska
- Columbia University