Interplay between beam-driven chirping modes and plasma confinement transitions in spherical tokamak ST40

Journal Article · Wed Dec 14 00:00:00 EST 2022 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/aca54d· OSTI ID:1914089

Bland, J.  ^[1]; Varje, J. ^[1]; Gorelenkov, N. N.  ^[2]; Gryaznevich, M. P. ^[1]; Sharapov, S. E.  ^[3]; Wood, J. ^[1]

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1. Tokamak Energy Ltd, Milton Park, Oxfordshire (United Kingdom)
2. Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)
3. Culham Science Centre, Abingdon (United Kingdom). Culham Centre for Fusion Energy (CCFE), EURATOM/UKAEA Fusion Association

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.

• Research Organization: Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)
• Sponsoring Organization: USDOE Office of Science (SC); Engineering and Physical Sciences Research Council (EPSRC)
• Grant/Contract Number: AC02-09CH11466; EP/W006839/1
• OSTI ID: 1914089
• Journal Information: Nuclear Fusion, Vol. 63, Issue 1; ISSN 0029-5515
• Publisher: IOP Science
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
fast particle effects in plasma
plasma–beam interactions
spherical tokamak
magnetic confinement fusion
H-mode
chirping modes