Investigation of Alfvén eigenmodes and Energetic Particle Modes in EAST with neutral beam injection

Abstract

Alfvén eigenmodes (AEs) and energetic particle modes (EPMs) have been observed in EAST neutral beam injection plasma with the presence of heavy tungsten impurity in the core region. The AEs are identified as toroidal Alfvén eigenmodes (TAEs) with roughly constant frequency f TAE ∼ 100 kHz and a special set of AEs with frequency chirping up from frequency f AE ∼ 70 kHz to f AE ∼ 100 kHz. The frequency of EPMs is around f EPM ∼ 60 kHz, exhibiting with frequency chirping down feature. A transition from EPMs to AEs is found, and the location and mode numbers are measured by experimental diagnostics. Hybrid simulations with the global kinetic-magnetohydrodynamic code M3D-K have been carried out to investigate the observed AEs and EPMs, and linear simulations find the EPM with toroidal mode number n = 2 and poloidal mode number m = 3. Nonlinear simulations show that the EPM frequency chirps down nonlinearly, and then two modes emerge with frequencies around 72.7 kHz and 88.0 kHz: the mode with higher frequency is a TAE, and the other mode is an EPM. Afterward, the TAE disappears quickly with the frequency almost unchanged, and the EPM with lower frequency chirps down continuously. The mode frequencies and locations of the simulated EPM and TAE, as well as the chirping down feature of the EPM frequency, are consistent with the experimental measurements. The transition from EPM to TAE is found to be a self-consistent dynamic of nonlinear evolution with the expel of energetic ions due to EPM.

Analysis of beam ion driven Alfvén eigenmode stability induced by Tungsten contamination in EAST

“…The analysis also explains the destabilization of AEs after the Tungsten contamination and identifies optimization trends with respect to the AE stability required to improve the heating efficiency of the EAST plasma heated by tangential NBIs. FAR3d simulations results show a reasonable agreement with the calculations performed using M3D-K/GTAW code [16]. It should be noted that the optimization is based on the weakening or stabilization of the AE activity.…”

“…LHW power is 1 MW. There is a Tungsten contamination caused by a small piece of tile at t = 6.0 s of the discharge time once several AEs are excited [16]. The analysis is performed at the discharge times t = 4.5 and 6.5 s, before the Tungsten contamination and after the Tungsten contamination once the maximum AE activity is observed.…”

“…Consequently, AEs are triggered in discrete frequency ranges and radial locations [13,14]. Different AE families exists with respect to the frequency gaps such as the toroidicity induced AE (TAE) [3,[15][16][17][18] coupling two adjacent poloidal modes; reverse shear AE (RSAE) [19][20][21] triggered at the minimum of non-monotonic q profiles; beta induced AE (BAE) [22][23][24] induced by finite compressibility effects. Besides AEs, there is another instability triggered by EPs called energetic particle mode (EPM) [1,2,14] destabilized inside the continuum if the EP driving is large enough to overcome the continuum damping effect.…”

Coupling between the Alfvén eigenmode and edge-coherent mode on the experimental advanced superconducting tokamak

“…Toroidal AEs (TAEs) and ellipticity-induced AEs (EAEs) excited by energetic particles in the Experimental Advanced Superconducting Tokamak (EAST) have been investigated extensively [30,31,[34][35][36][37][38][39]. TAEs, which are driven by fast ions, have been observed in EAST NBI high β N plasma [37,38]. These TAEs are located in the core region with toroidal numbers n = 2-4.…”

Effect of the neutral beam injector operational regime on the Alfven eigenmode saturation phase in DIII-D plasma

“…EPs and alpha particles can destabilize Alfvén Eigenmodes (AEs) if their transit, bounce or drift frequencies resonate with AE mode frequencies [15,16], triggered in the spectral gaps of the shear Alfvén continua [17,18]. In addition, if the EP drive is strong enough to exceed the continuum damping effect, energetic particle modes (EPMs) can be triggered inside the continuum [19,20].…”