Observation of tungsten impurities induced 2/1 snake fluctuations and their interactions with discrete double BAE pairs on EAST

Abstract

The snake perturbations around q = 2 surface with strong poloidal asymmetry are frequently observed on EAST after the injection of high-Z impurities. A new pair of Beta-induced Alfvén eigenmodes (BAEs) with lower frequencies also come along with the 2/1 snake fluctuations, in addition to the conventional BAE pair that accompany the pure 2/1 tearing modes (TMs) on EAST, which have already been investigated (Xu et al 2013 Plasma Phys. Control. Fusion 55 065002). The radial structures of the intensities of the snake perturbations and the two BAE pairs are mapped out for the first time, indicating that these BAE quartet are discrete modes over the radial extension and are situated slightly outside the annulus of TM activities but centered at slightly different positions. The constant frequency spectra of the BAEs are explained as the eigen-solutions of the radial differential equation for the global modes, and the rich multiplicity of BAEs is attributed to various levels of BAE continuum splittings when more detailed physics are taken into consideration.

Influence of low-Z impurity on the stabilization of m/n = 2/1 tearing/locked modes in EAST

“…Assessment of the impurity radiation in the core and divertor plasmas for ITER and DEMO has been performed in [4]. However, a series of tearing mode (TMs) instabilities are triggered by the concentration of impurities [5][6][7][8][9][10], and the structures of the magnetic islands are formed accordingly, as in [11], where the structure of double tearing modes is also observed for the central concentration of high-Z impurity in EAST [12]. The interaction between the impurity density and TM instability is more dangerous for the plasma confinement, e.g.…”

Experimental observation of the localized coupling between geodesic acoustic mode and magnetic islands in tokamak plasmas

“…The twin BAEs are counter-propagating waves, and form standing wave structure in the island rest frame. The features of twin BAEs have been reported on various devices [10,[18][19][20][21][22][23][24][25], and two theoretical models have been proposed for the excitation of BAEs, i.e. the subtleties of the Alfvén continuum inside magnetic islands [26][27][28] and the magnetic islands induced free energy [29,30].…”

Beta-induced Alfvén eigenmodes destabilized by resonant magnetic perturbations in the J-TEXT tokamak

“…The mode structure of BAEs is mainly distributed around the rational surface [6,7], therefore, the interaction between BAEs and the magnetic island is ubiquitous in experimental observation. Specifically, m-BAE coupling with tearing mode (TM) are observed in FTU [26,27] and HL-2A [17] Ohmic plasmas, EAST with lower hybrid wave (LHW) plasmas [28] and energetic ions plasmas [29], TJ-II stellarator with energetic ions plasmas [30], TEXTOR [31] and J-TEXT low-density Ohmic plasmas [22]. Results are in common that m-BAEs are a pair of counter-propagating waves, form a standingwave structure in the island rest frame and only appear above a threshold of island width.…”