The Influence Of Geometry And Density Profile On The Toroidicity-induced Shear Alfvén Eigenmodes In Tokamaks

As the resource of the energy becomes exhausted, the peaceful use of fusion energy has got more and more attention. In order to realize the controllable fusion, high dense plasma with multi-million degrees in temperature should be confined for long enough in time. However, various instabilities in plasma can often destroy plasma confinement which causes particle and energy loss.Alfven waves are common Low-frequency electromagnetic waves in plasma. It is found that shear Alfven waves with the continuous spectrum in non-uniform plasma are in general stabilized; but in the toroidally confined device (e.g. tokamak), the continuous spectrum of the shear Alfven waves is broken down and gaps appear due to mode coupling induced by toroidicity and so on. Weakly damped discrete modes can exist in these gaps. These weakly damped modes can be driven to be unstable by energetic particles, which results in the particle loss. The toroidicity-induced shear Alfven eigenmode (TAE) caused by coupling between mth and m+lth modes is a prominent example.The existence of TAE that resulted from finite aspect ratio has been wellknown. But it's not very clear about the influences on TAE due to the shape geometry of device cross-section. In the thesis, NOVA code is used to calculate the frequencies of the continuous spectrum and TAE for different Tokamak geometries (different ellipticities and triangularities). It is found that for the condition with constant density and linear q-profile, the frequency of TAE-gap slightly increases with increase of the ellipticity, and decreases with increase of the triangularity. In the case of two nearest mode coupling only, the mode structure of TAE nearly remains unchanged while its frequency also increases with increasing of the ellipticity and decreases with increasing of the triangularity. With multimode coupling, a global TAE-gap rarely exists for the constant density and linear q-profile. The TAEs may interact with the continuous spectrum and experience strong damping. The frequencies of the continuous spectrum under different safety factors q and density profiles are also carried out for multimode coupling. It is found that global TAE-gaps may exist when the product between the density and the square of the safety factor changes weakly in the radial direction, or TAEs may be excited without continuum damping.