Recent MAST Upgrade results to advance the physics basis for spherical tokamaks
ORAL · Invited
Abstract
Recent results from MAST Upgrade are making key advances in understanding the physics issues governing the operation of future devices. The operating space has expanded due to new capabilities; an active lower divertor cryopump and advanced real-time equilibrium shape control. These developments have allowed access to higher elongation (κ ≈ 1.8 → 2.5) and a broader range of triangularity (δ ≈ -0.15 → 0.5).
A cryopump reduces the lower divertor neutral pressure by ~30-50%, enabling operations with attached Super-X divertor legs. The main chamber and upper divertor pressures are weakly affected by the cryopump, enabling the detachment state of the upper and lower divertors to be controlled independently. In the Super-X divertor configuration, buffering of transient heat loads (E ≤ 1 kJ) with D2 fuelling was observed. First studies of power exhaust in H-mode in the X-point target configuration show detached outer divertors with evidence of stronger plasma-neutral interactions.
On- and off-axis Neutral Beam Injectors (NBI) enable studies of super-Alfvénic fast ions mimicking fusion products. The first measurements of fusion protons have been made with a diamond detector, more resilient than conventional silicon detectors. Fast ion loss measurements using a Fast Ion Loss Detector match ASCOT simulations, showing that type-III ELMs increase fast ion losses by 25-30%.
Operation at κ ≈ 2.5 with early divertor formation and NBI injection enables sustainment of q0 > 2 for ~ 300 ms, avoiding low-n mode activity and enabling plasma studies of q profiles closer to those envisaged in future STs. The onset of the performance limiting 2/1 tearing mode has been postponed by modifying the edge pedestal profiles via fuelling and modifications to the plasma edge shape. The highest transiently achieved βN is ~ 4.2, near the n = 1 ideal MHD no-wall beta limit. At higher beta and q0 > 1, no sawteeth have been observed for several confinement times, indicating flux pumping in STs for the first time.
First studies of NBI heated negative triangularity plasmas have been performed in a low aspect ratio device, suggesting a transition from ELMing to ELM-free regimes at δ < -0.06. In positive triangularity, a high-density stationary ELM-free pedestal regime has been seen at high squareness, in agreement with improved peeling-ballooning stability.
A cryopump reduces the lower divertor neutral pressure by ~30-50%, enabling operations with attached Super-X divertor legs. The main chamber and upper divertor pressures are weakly affected by the cryopump, enabling the detachment state of the upper and lower divertors to be controlled independently. In the Super-X divertor configuration, buffering of transient heat loads (E ≤ 1 kJ) with D2 fuelling was observed. First studies of power exhaust in H-mode in the X-point target configuration show detached outer divertors with evidence of stronger plasma-neutral interactions.
On- and off-axis Neutral Beam Injectors (NBI) enable studies of super-Alfvénic fast ions mimicking fusion products. The first measurements of fusion protons have been made with a diamond detector, more resilient than conventional silicon detectors. Fast ion loss measurements using a Fast Ion Loss Detector match ASCOT simulations, showing that type-III ELMs increase fast ion losses by 25-30%.
Operation at κ ≈ 2.5 with early divertor formation and NBI injection enables sustainment of q0 > 2 for ~ 300 ms, avoiding low-n mode activity and enabling plasma studies of q profiles closer to those envisaged in future STs. The onset of the performance limiting 2/1 tearing mode has been postponed by modifying the edge pedestal profiles via fuelling and modifications to the plasma edge shape. The highest transiently achieved βN is ~ 4.2, near the n = 1 ideal MHD no-wall beta limit. At higher beta and q0 > 1, no sawteeth have been observed for several confinement times, indicating flux pumping in STs for the first time.
First studies of NBI heated negative triangularity plasmas have been performed in a low aspect ratio device, suggesting a transition from ELMing to ELM-free regimes at δ < -0.06. In positive triangularity, a high-density stationary ELM-free pedestal regime has been seen at high squareness, in agreement with improved peeling-ballooning stability.
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Presenters
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Sarah Elmore
UKAEA - United Kingdom Atomic Energy Authority
Authors
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Sarah Elmore
UKAEA - United Kingdom Atomic Energy Authority