Interplay between beam-driven chirping modes and plasma confinement transitions in spherical tokamak ST40
Autor: | J. Bland, J. Varje, N.N. Gorelenkov, M.P. Gryaznevich, S.E. Sharapov, J. Wood, Team The ST |
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Jazyk: | angličtina |
Rok vydání: | 2022 |
Předmět: | |
Zdroj: | Nuclear Fusion, Vol 63, Iss 1, p 016024 (2022) |
Druh dokumentu: | article |
ISSN: | 1741-4326 0029-5515 |
DOI: | 10.1088/1741-4326/aca54d |
Popis: | Experiments on the high field spherical tokamak ST40 have led to the recent observation of interplay between beam-driven modes of sweeping frequency (chirping modes) and transitions to the enhanced global confinement regime (H-mode) and back to the low confinement regime (L-mode). The H-modes of plasma confinement are identified from decreased intensity of D _α signal and from clear distinctions in the edge gradients of the visible plasma boundary (observed as a sharp plasma edge in camera images). The beam-driven chirping modes, identified as ideal magnetohydrodynamics beta-induced Alfvén acoustic eigenmodes modes, are observed in Mirnov coil signals, interferometry, and soft x-ray diagnostics. A moderate amplitude ‘primer’ chirping mode usually precedes an H–L transition. This is followed by a ‘dominant’ chirping mode with higher amplitude during the L-mode. The L–H transition back to the improved confinement occurs on a longer time scale of tens of ms, consistent with the slowing down time scale of fast beam ions. A dramatic decrease in toroidal plasma rotation is systematically observed associated with chirping modes sweeping down to zero frequency. Resonance maps built for the beam-driven chirping modes with the ASCOT (accelerated simulation of charged particle orbits in torodoial devices) code show that the resonant beam ions have orbits near the trapped-passing boundary. The ASCOT modelling assesses how losses of the resonant fast ions caused by the chirping modes with high enough amplitude modify the torque, potentially affecting the plasma rotation. |
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