Abstract Details
Abstracts
Author: Koki Imada
Requested Type: Poster
Submitted: 2023-04-24 08:15:40
Co-authors: S. Gibson, A. Kirk, M. Knolker, T.H. Osborne, S. Saarelma, P.B. Snyder, H.R. Wilson and the MAST Upgrade Team
Contact Info:
York Plasma Institute, U. of York
University of York
Heslington, York YO10 5DD
United Kingdom
Abstract Text:
Modern tokamaks can operate in high-confinement (H-)modes, in which a steep edge plasma pressure gradient is established, creating a "pedestal". H-modes are, however, subject to a class of explosive edge localised modes (ELMs), which could cause serious damage to the vessel walls. Large ELMs, especially of Type-I kind, must therefore be mitigated or suppressed. According to the "peeling-ballooning theory" [1], stability of the pedestal region depends primarily on two parameters: normalised pedestal pressure gradient, α, and pedestal plasma current density, Jped. Too steep a pressure gradient tends to trigger high-n ideal ballooning modes (n is the toroidal mode number), which typically result in Type-I ELMs. On the other hand, too high a pedestal current density at lower α tends to make plasma unstable to low-n peeling modes. The challenge of improving confinement involves optimising the pedestal stability, in order to guide the plasma towards higher values of Jped and α.
The recent comparison between MAST Upgrade and its predecessor, MAST [2], has shown that their pedestal stability characteristics were radically different: MAST-U H-modes were closer to the peeling boundary, with a narrow region of stability extending to significantly higher values of Jped and α, indicative of weaker coupling between the peeling and ballooning modes.
This presentation reports on the progress of MAST-U pedestal stability analysis, surveying a range of H-mode discharges from the first and second campaigns. Our analysis shows a clearer pathway to improving pedestal performance, by steering plasma towards the peeling limit with a careful control of parameters, including the edge density and current profile. These results will make a valuable contribution towards the design of compact prototype reactors, such as STEP.
[1] J. W. Connor et al, Physics of Plasmas 5, 2687 (1998)
[2] K. Imada et al, Nuclear Fusion (in preparation)
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