Reversed Shear Alfven Eigenmodes Research Articles

Nonlinear gyrokinetic simulations of reversed shear Alfven eigenmodes in DIII-D tokamak

Nonlinear gyrokinetic simulations of reversed shear Alfven eigenmodes in DIII-D tokamak

Regulation of Alfvén Eigenmodes by Microturbulence in Fusion Plasmas.

Global gyrokinetic simulations of mesoscale reversed shear Alfven eigenmodes (RSAE) excited by energetic particles (EP) in fusion plasmas find that RSAE amplitude and EP transport are much higher than experimental levels at nonlinear saturation, but quickly diminish to very low levels after the saturation if background microturbulence is artificially suppressed. In contrast, in simulations coupling micro-meso scales, the RSAE amplitude and EP transport decrease drastically at the initial saturation but later increases to the experimental levels in the quasisteady state with bursty dynamics due to regulation by thermal ion temperature gradient (ITG) microturbulence. The quasisteady state EP transport is larger for a stronger microturbulence. The RSAE amplitude in the quasisteady state ITG-RSAE turbulence from gyrokinetic simulations, for the first time, agrees very well with experimental measurements.

Zonal structure effect on the nonlinear saturation of reverse shear Alfven eigenmodes

A gyrokinetic ion/fluid electron hybrid model is used to study the nonlinear evolution of reverse shear Alfven eigenmodes (RSAE) driven by energetic particles. When only the energetic particle nonlinear effects are included, the saturation amplitude of a single-n RSAE follows the trapping scaling, δB/B∼(γ/ω)2. When the nonlinear effects of thermal ions and electrons are included, zonal structures are force generated but do not affect the saturation amplitude for γ/ω≤0.03. No spontaneous generation of zonal structures is observed, in contrast to ion-temperature-gradient-driven modes. At larger energetic particle drive, the effects of zonal structures cause a significant reduction in the RSAE saturation amplitude. The reduction is not caused by the zonal flow shearing of the RSAE, but by the force-generated n = 0 component in the thermal ion distribution function and the electron density. These n = 0 perturbations lead to nonlinear evolution of the RSAE mode structure and enhance damping.

Numerical study of the existence criterion for the reversed shear Alfven eigenmode in the presence of a parallel equilibrium current

We have re-visited the existence criterion of the reverse shear Alfven eigenmodes (RSAEs) in the presence of the parallel equilibrium current by numerically solving the eigenvalue equation using a fast eigenvalue solver code KAES. The parallel equilibrium current can bring in the kink effect and is known to be strongly unfavorable for the RSAE. We have numerically estimated the critical value of the toroidicity factor Qtor in a circular tokamak plasma, above which RSAEs can exist, and compared it to the analytical one. The difference between the numerical and analytical critical values is small for low frequency RSAEs, but it increases as the frequency of the mode increases, becoming greater for higher poloidal harmonic modes.

Study of Reversed Shear Alfvén Eigenmodes with Down Sweeping Frequency in HL-2A Experiment atqmin∼ 1

The reserved shear Alfven eigenmodes (RSAEs) whose frequencies sweep down have been observed in HL-2A plasmas at qmin ∼ 1 during current ramp-up and NBI+ECRH recently. The kinetic Alfven eigenmode code (KAEC), which is a non-perturbative kinetic/magnetohydrodynamic eigenvalue code for calculating kinetic damping of shear Alfven eigenmodes, is used to investigate the modes according to the experimental data. It is found that the kinetic reversed shear Alfven eigenmodes (KRSAEs) with downsweeping frequency and localized radial structure would appear, which are consistent with the observations. The theoretical analysis for the existence of the mode and the scaling of the mode width are provided. It is shown that the mode width of KRSAEs is proportional to the square root of thermal ion Larmor radius.