Title: "Nonlinear damping of Reverse Shear Alfven Eigenmodes induced by electron streaming along the perturbed field line"
Speaker: Prof. Yang Chen, University of Colorado at Boulder
Location: Rm. 413, Bld. 11#, Yuquan Campus
Time: April 28 (Sunday), 2 pm
Alfven eigenmodes can be excited by energetic particles (EP) and cause anomalous EP transport. Predicting the amplitude of these modes is a great challenge for direct gyrokinetic simulation.
For near marginal instabilities, the dominant saturation mechanism is the trapping of resonant particles in the wave field.
In this case reduced models can be used for computational efficiency.
As the growth rate increases, thermal species nonlinearity leads to the generation of zonal structures and change the nonlinear behavior.
GEM simulations of the Reverse Shear Alfven Eigenmodes (RSAE) with fluid electrons show that zonal structures are force generated, and significantly reduce the saturation amplitude. This strong nonlinear effect occurs at moderate mode amplitude, delta B/B<0.001.
The reduction in the saturation amplitude is not caused by the zonal flow shearing of the RSAE, but by the force-generated n=0 component in the distribution function. These findings are verified with kinetic electrons. It is found that the strong nonlinear effect of the background plasma comes from the electron magnetic fluttering term in the electron drift-kinetic equation. This nonlinear term leads to the self-coupling of an n=4 RSAE and generates n=(0 ,8) components in the electron distribution. These n=(0, 8) components have short radial structures and readily modifies the RSAE mode structure, leads to nonlinear damping.