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

Nuclear Fushion

article

J. Bland¹  J. Varje¹  N.N. Gorelenkov²  M.P. Gryaznevich¹  S.E. Sharapov³  J. Wood¹  Team The ST40¹ 

[1] Tokamak Energy Ltd;PPPL, 100 Stellarator Rd, Princeton;CCFE, Culham Science Centre

关键词: fast particle effects in plasma;    plasma–beam interactions;    spherical tokamak;    magnetic confinement fusion;    H-mode;    chirping modes;   

DOI  :  10.1088/1741-4326/aca54d

来源: Institute of Physics Publishing Ltd.

【 摘 要 】

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 ofD_αsignal 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.

【 授权许可】

Unknown