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Description
Plasma rotation is a phenomenon of great interest in magnetic fusion research, since it is able, for example, to mitigate turbulence[1], contributing to the transition to the H-mode, and to shield magneto-hydro-dynamic (MHD) instabilities [2,3].
It has been observed in different devices [4-7] that plasma rotation can brake in the presence of magnetic islands or by externally applied magnetic fields, used for example during Edge Localized Modes (ELMs) control experiments [8], error field identification [4], magnetic island stabilization [9]. This phenomenon should be avoided, not to lose the positive effects of plasma rotation.
In this work, plasma rotation braking is investigated in the MAST-U device, during n = 2 error field identification studies, performed within the MU04 campaign. In these experiments, a growing magnetic field perturbation was applied by ramping up the current in the ELM coils [8], to induce a locked mode. Shots were realized with 750 kA plasma current, in Double Null Divertor configuration and in H-mode regime, thanks to 3.4 MW of NBI injection.
The experimental analysis has been performed on rotation data acquired by the Charge Exchange Recombination Spectroscopy, in combination with magnetic signals to study the MHD activity.
Plasma rotation has been observed to brake in the core, with the increasing amplitude of the n = 2 external field. Nevertheless, in these experiments also the plasma density was increasing, which is known to influence the core rotation profile [10], and another mode, with n = 1 toroidal mode number, was present, which could have contributed to the rotation braking.
To assess the cause behind the observed rotation reduction, simulations with the RFXlocking code [11], adapted to MAST-U, have been performed, which includes the effect of the electromagnetic torque and the neoclassical toroidal viscosity.
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[2] Strait E J 1995 Physical Review Letter 74 2483
[3] Buttery R J 2008 Physics of Plasmas
[4] Scoville J 1991 Nuclear Fusion 31 875
[5] Lazzaro E 2002 Physics of Plasmas 9 3906
[6] Kim K 2017 Nuclear Fusion 57 126035
[7] Li X Y 2019 Physics of Plasmas 26 052512
[8] Ryan D A 2024 Plasma Physics and Controlled Fusion 88 105003
[9] Huang W and Zhu P 2015 Physics of Plasmas 22 032502
[10] McDermott R M 2014 Nuclear Fusion 54 043009
[11] Zanca P 2009 Plasma Physics and Controlled Fusion 51 015006