Destabilization of reversed shear Alfvén eigenmodes driven by energetic ions during NBI in HL-2A plasmas withq_min∼ 1

Abstract

Two groups of frequency sweeping modes are observed and interpreted in the HL-2 A plasmas with qmin ∼ 1. The tokamak simulation code calculations indicate the presence of a reversed shear q-profile during the existence of these modes. The mode frequencies lie in between TAE and BAE frequencies, i.e. ωBAE < ω < ωTAE, and these modes are highly localized near qmin, i.e. r/a ∼ 0.25. A group of modes characterized by down-sweeping frequency with qmin decrease due to qmin > 1 and nqmin − m > 0, and another group of modes characterized by up-sweeping frequency with qmin drop, owing to qmin < 1 and nqmin − m < 0 before sawtooth crash. The kinetic Alfvén eigenmode code analysis supports that the down-sweeping modes are kinetic reverse shear Alfvén eigenmodes (KRSAEs), and the up-sweeping modes are RSAEs, which exist in the ideal or kinetic MHD limit. In addition, the down- and up-sweeping RSAEs both have fast nonlinear frequency behaviour in the process of slow frequency sweeping, i.e. producing pitch-fork phenomena. These studies provide valuable constraint conditions for the q-profile measurements.

Simulation of fast-ion-driven Alfvén eigenmodes on the Experimental Advanced Superconducting Tokamak

“…Plenty of modes in the Alfv en frequency range are found in the simulation. The frequency and radial width of the modes observed in the simulations are compared with the Alfv en continua to categorize the modes, which indicates the modes include the toroidicity-induced Alfv en eigenmodes (TAEs), 1,3,4,19,22 reversed shear Alfv en eigenmodes (RSAEs), 7,8,[23][24][25][26][27][28][29][30][31] and energetic-particle continuum modes (EPMs). 5,20,32,33 The simulation results are also compared with the results of an MHD eigenvalue code, 34 which shows agreement with respect to the frequency, dominant poloidal mode number, and radial location of the modes.…”

Parallel equilibrium current effect on existence of reversed shear Alfvén eigenmodes

“…8 However, the KAEC code 10 seems to have found an eigenmode as in the experiment by excluding the kink term or including kinetic effects. 8,11 The existence of RSAEs has also been studied intensively theoretically, including the effects of energetic particle, 12 toroidicity, 13 finite plasma pressure, [14][15][16] pressure gradient, 17,18 density gradient, 19 as well as some kinetic effects. 10,20 The effect of the parallel equilibrium current corresponding to the kink term has also been studied recently by Deng et al 21,22 using the GTC code and an analytical model for the dispersion relation, where the toroidal coupling is ignored.…”

“…8 Under that RSAE experimental parameters, NOVA 9 cannot find the RSAE mode. 8 However, the KAEC code 10 seems to have found an eigenmode as in the experiment by excluding the kink term or including kinetic effects.…”

“…8 Under that RSAE experimental parameters, NOVA 9 cannot find the RSAE mode. 8 However, the KAEC code 10 seems to have found an eigenmode as in the experiment by excluding the kink term or including kinetic effects. 8,11 The existence of RSAEs has also been studied intensively theoretically, including the effects of energetic particle, 12 toroidicity, 13 finite plasma pressure, [14][15][16] pressure gradient, 17,18 density gradient, 19 as well as some kinetic effects.…”

Progress of HL-2A physical experiments

“…(a)-(c) Counts of hard-x-ray (HXR) photons at different energy ranges; (d) central line-average density; (e) magnetic probe signal; (f) the spectrogram of magnetic probe signal [30].图 24 (网络版彩图)HL-2A装置上典型的RSAE现象, 两炮 磁涨落的功率谱[Color online) Typical RSAE phenomena in HL-2A. Spectrograms of magnetic fluctuations for two shots[32].…”