Towards reactor-relevant electromagnetic turbulence in the MAST-U spherical tokamak
Abstract
We investigate electromagnetic ion-scale turbulence in a high-performing H-mode plasma of the Mega Ampere Spherical Tokamak Upgrade (MAST-U) with local gyrokinetic simulations, asking whether it approaches the non-zonal transition: the nonlinear transition to a high-flux state above a critical value of q2βe found in simulations of STEP, where q is the safety factor and βe the ratio of electron thermal pressure to magnetic pressure. We examine two flux surfaces: one at t = 531 ms, close to peak normalised pressure βN, at normalised poloidal flux ψN = 0.50; and one at t = 591 ms and ψN = 0.60, the ideal-ballooning-stable surface of largest q2βe inside ψN = 0.8 at that time. On both surfaces the dominant ion-scale instabilities are microtearing and electromagnetic ballooning-like modes, as in STEP. Without equilibrium flow shear, at least one of the surfaces lies above the large flux threshold with every set of kinetic species we simulate, with ion-scale heat fluxes far exceeding power balance, in turbulence characterised by a highly stochastic magnetic field. Strong flow shear driven by neutral-beam injection in the real experiment suppresses this turbulence and holds the plasma below the transition. When the carbon impurity is included in the simulation, the ion-scale flux is then close to power balance on both surfaces. Without rotation, gyrokinetics suggests the transition would limit the accessible profiles. This could degrade MAST-U plasma performance using heating schemes that lack a momentum source, and this will be tested in forthcoming electron Bernstein wave heating experiments.
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BibTeX
@misc{kennedy2026towards,
title = {{Towards reactor-relevant electromagnetic turbulence in the MAST-U spherical tokamak}},
author = {Daniel Kennedy and Yujia Zhang and Bhavin S Patel and Sam Blackmore and Charles Vincent and Colin M Roach},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.00218},
eprint = {2611.00218},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.00218},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Towards reactor-relevant electromagnetic turbulence in the MAST-U spherical tokamak
AU - Daniel Kennedy
AU - Yujia Zhang
AU - Bhavin S Patel
AU - Sam Blackmore
AU - Charles Vincent
AU - Colin M Roach
PY - 2026
DA - 2026/11/01
PB - arXiv
DO - 10.48550/arxiv.2611.00218
UR - https://arxiv.org/abs/2611.00218
AB - We investigate electromagnetic ion-scale turbulence in a high-performing H-mode plasma of the Mega Ampere Spherical Tokamak Upgrade (MAST-U) with local gyrokinetic simulations, asking whether it approaches the non-zonal transition: the nonlinear transition to a high-flux state above a critical value of q^2β_e found in simulations of STEP, where q is the safety factor and β_e the ratio of electron thermal pressure to magnetic pressure. We examine two flux surfaces: one at t = 531 ms, close to peak normalised pressure β_N, at normalised poloidal flux ψ_N = 0.50; and one at t = 591 ms and ψ_N = 0.60, the ideal-ballooning-stable surface of largest q^2β_e inside ψ_N = 0.8 at that time. On both surfaces the dominant ion-scale instabilities are microtearing and electromagnetic ballooning-like modes, as in STEP. Without equilibrium flow shear, at least one of the surfaces lies above the large flux threshold with every set of kinetic species we simulate, with ion-scale heat fluxes far exceeding power balance, in turbulence characterised by a highly stochastic magnetic field. Strong flow shear driven by neutral-beam injection in the real experiment suppresses this turbulence and holds the plasma below the transition. When the carbon impurity is included in the simulation, the ion-scale flux is then close to power balance on both surfaces. Without rotation, gyrokinetics suggests the transition would limit the accessible profiles. This could degrade MAST-U plasma performance using heating schemes that lack a momentum source, and this will be tested in forthcoming electron Bernstein wave heating experiments.
ER -