Time: 2012-09-28 14:00:00
Speaker: Ilija Chavdarovski [IFTS, postdoc]
Place: Rm. 413, Bld. 11#, Yuquan Campus
Abstract: TRAPPED PARTICLE CONTRIBUTION TO THE LOW FREQUENCY ALFVèN WAVES DISPERSION RELATION
The structure of the low-frequency shear Alfvén continuous spectrum in large aspect ratio (1/ ϵ = R0 / a >> 1) tokamak equilibria is analytically derived considering the wave-particle interactions with magnetically trapped thermal ions. We focus on the linear kinetic derivation of the so-called generalized inertia term , which can later be used to investigate the generation and the stability of the modes inside the β-induced gap, following the general approach of the fishbone-like dispersion relation i=Wf+Wk.
A simplified model that treats all the trapped particles as deeply trapped and all the circulating as well circulating will be explained. Even with this assumption it is shown that both types of particles can be incorporated in the same equations preserving all the relevant physics. Once the appropriate modification to the well circulating model allowing circulating particles with small v|| are applied, the derived equations can be used throughout the entire low frequency range 0 < ω < ωBAE. The resulting theoretical description asymptotically recovers known results in the relevant limits of both high and low frequencies. The low frequency limit confirms that in this range the trapped particle dynamics becomes dominant, bringing an O (ϵ−1/2 ) contribution to the low frequency inertia enhancement of the plasma. In the fluid limit the O (ϵ1/2), terms of circulating and trapped particles cancel out, proving that the GAM frequency correction due to the finite aspect ratio is of O (ϵ).
The importance of the geometric effects in non-collisional burning plasmas is once again confirmed, showing that the most important wave particle interaction comes from the coupling of the particle magnetic drift motion with the geodesic curvature. Numerical study of the inertial term show significant contribution of the trapped particles dynamics in this frequency range. Finally, comment will be made on the characteristics of the so called Beta-induced Alfvén-acoustic Eigenmodes (BAAE).