The Influence Of Q-Profile Flattening On Fishbone Instabilities In Tokamak Plasmas

In last two decades,the problems of fast ion-induced instability have been studied widely in theory and experiment.Particularly,the energetic particle mode with the low mode number not only has a global displacement but also has a large growth rate,which takes a critical limit for plasma confinement.These modes may play an important influence on the future ITER advanced hybrid scenarios operation.Therefore,fishbone mode as one basic magnetohydrodynamics(MHD)instability in burning plasmas,has been taken wide attention in past decades.Due to the complexity of the nonlinear dynamics,such basic physics mechanisms remain to be solved as an crucial problem in future.In this paper,we study the properties of one fast-particle-driven instability,i.e.fishbone mode,with a local flattened_q(28)1 surface.The linear dispersion relation of the mode is modified due to the q-profile flattening;the effect of q-profile flatness is discussed in detail.The parameter dependences of fast particle beta?_h,potential energy of background plasmas,and precession frequency on the instability of the fishbone mode is given;the physical mechanism of fish bone mode instability is clarified.At the same time,the eigenvalue method is used to calculate the effects of different conditions on the fishbone mode structure.The relations of mode structure,frequency and growth rate with fast particle beta,flatness of q-profile is shown in this thesis.The main conclusions of this thesis are as follows:1.We find that the general dispersion relation of the mode is invalid when the safety factor profile becomes flattening at q=1 rational surface.The modified dispersion relation of the fishbone mode is derived using the variation principle theory.The physical mechanism of fishbone mode instability is studied numerically with experimental parameters.2.The strong dependences of mode structure and mode frequency on the different q-profiles are analyzed by the solution of dispersion relation.3.The result also indicates that the mode saturation amplitude is dominated by the layer nonlinearity rather than the linear driving when the q-profile is flattened around q=1.