Observation of a new pedestal stability regime in MAST Upgrade H-mode plasmas

Imada, K.; Osborne, T.H.; Saarelma, S.; Clark, J.G.; Kirk, A.; Knolker, M.; Scannell, R.; Snyder, P.B.; Vincent, C.; Wilson, H.R.; ,

doi:10.1088/1741-4326/ad5219

Nucl. Fusion

2024

DOI: 10.1088/1741-4326/ad5219

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Observation of a new pedestal stability regime in MAST Upgrade H-mode plasmas

K. Imada,

T.H. Osborne,

S. Saarelma

et al.

Abstract: The first pedestal stability and structure analysis on the new MAST Upgrade (MAST-U) spherical tokamak H-mode plasmas is presented. Our results indicate that MAST-U pedestals are close to the low toroidal mode number (n) peeling branch of the peeling-ballooning instability, in contrast with MAST H-mode pedestals which were deeply in the high-n ballooning branch. This offers the possibility of reaching the ELM-free quiescent H-mode [1] or high-performance super H-mode [2,3] regimes. In addition, the coupling be… Show more

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2024

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Cited by 3 publications

References 42 publications

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ELM-free H-mode phase and decoupling of peeling–ballooning stability boundary in the MAST Upgrade tokamak

Imada,

Osborne,

Saarelma

et al. 2024

Plasma Phys. Control. Fusion

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The linear MHD peeling-ballooning stability analysis on the ELM-free phase of MAST Upgrade H-mode plasma is presented. In contrast to other similar discharges, #47018 is found to have significantly higher and wider pedestal during an ELM-free H-mode phase that lasts for approximately 80ms, which is made possible by the reduced core MHD mode activity on q=2 surface. During this period, there is sustained decoupling of peeling and ballooning branches of the stability boundary on J-α space, opening an access channel to the second stability regime with higher peaks in pedestal current density J N,ped and pressure gradient (α). Such decoupling of the stability boundary is not previously observed in MAST Upgrade H-modes, and if such condition can be readily reproduced, it opens a wide range of opportunity for MAST-U to explore low-collisionality peeling-limited pedestal regimes, as well as advanced scenarios such as quiescent H-modes that are relevant to future reactors, such as STEP and ITER.

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ELM-free H-mode phase and decoupling of peeling–ballooning stability boundary in the MAST Upgrade tokamak

Imada,

Osborne,

Saarelma

et al. 2024

Plasma Phys. Control. Fusion

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Overview of physics results from MAST upgrade towards core-pedestal-exhaust integration

Harrison,

Aboutaleb,

Ahmed

et al. 2024

Nucl. Fusion

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Recent results from MAST Upgrade are presented, emphasising understanding the capabilities of this new device and deepening understanding of key physics issues for the operation of ITER and the design of future fusion power plants. The impact of MHD instabilities on fast ion confinement have been studied, including the first observation of fast ion losses correlated with Compressional and Global Alfvén Eigenmodes. High-performance plasma scenarios have been developed by tailoring the early plasma current ramp phase to avoid internal reconnection events, resulting in a more monotonic q profile with low central shear. The impact of m/n = 3/2, 2/1 and 1/1 modes on thermal plasma confinement and rotation profiles has been quantified, and scenarios optimised to avoid them have transiently reached values of normalised beta approaching 4.2. In pedestal and ELM physics, a maximum pedestal top temperature of ∼350 eV has been achieved, exceeding the value achieved on MAST at similar heating power. Mitigation of type-I ELMs with n = 1 RMPs has been observed. Studies of plasma exhaust have concentrated on comparing conventional and Super-X divertor configurations, while X-point target, X-divertor and snowflake configurations have been developed and studied in parallel. In L-mode discharges, the separatrix density required to detach the outer divertors is approximately a factor 2 lower in the Super-X than the conventional configuration, in agreement with simulations. Detailed analysis of spectroscopy data from studies of the Super-X configuration reveal the importance of including plasma-molecule interactions and D2 Fulcher band emission to properly quantify the rates of ionisation, plasma-molecule interactions and volumetric recombination processes governing divertor detachment. In H-mode with conventional and Super-X configurations, the outer divertors are attached in the former and detached in the latter with no impact on core or pedestal confinement.

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Plasma burn—mind the gap

Meyer

2024

Phil. Trans. R. Soc. A.

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The programme to design plasma scenarios for the Spherical Tokamak for Energy Production (STEP), a reactor concept aiming at net electricity production, seeks to exploit the inherent advantages of the spherical tokamak (ST) while making conservative assumptions about plasma performance. This approach is motivated by the large gap between present-day STs and future burning plasmas based on this concept. It is concluded that plasma exhaust in such a device is most likely to be manageable in a double null (DN) configuration, and that high core performance is favoured by positive triangularity (PT) plasmas with an elevated central safety factor. Based on a full technical and physics assessment of external heating and current drive (CD) systems, it was decided that the external CD is provided most effectively by microwaves. Operation with active resistive wall mode (RWM) stabilization as well as high elongation is needed for the most compact solution. The gap between existing devices and STEP is most pronounced in the area of core transport, owing to high normalized plasma pressure in the latter which changes qualitatively the nature of the turbulence controlling transport. Plugging this gap will require dedicated experiments, particularly on high-performance STs, and the development of reduced models that faithfully represent turbulent transport at high normalized pressure. Plasma scenarios in STEP will also need to be such that edge localized modes (ELMs) either do not occur or are small enough to be compatible with material lifetime limits. The high current needed for a power plant-relevant plasma leads to the unavoidable generation of high runaway electron beam current during a disruption, where novel mitigation techniques may be needed. This article is part of the theme issue ‘Delivering Fusion Energy – The Spherical Tokamak for Energy Production (STEP)’.

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Observation of a new pedestal stability regime in MAST Upgrade H-mode plasmas

Cited by 3 publications

References 42 publications

ELM-free H-mode phase and decoupling of peeling–ballooning stability boundary in the MAST Upgrade tokamak

ELM-free H-mode phase and decoupling of peeling–ballooning stability boundary in the MAST Upgrade tokamak

Overview of physics results from MAST upgrade towards core-pedestal-exhaust integration

Plasma burn—mind the gap

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