Simulation Of Effects Of Energetic Particles On Low-frequency Magnetohydrodynamic Instabilities In Advanced Tokamaks

Sustainable and steady energy supply is the foundation of the existence and development of human society.Resource-exhaustion and pollution is faced by the conventional energy,so the nuclear energy becomes the important solution for energy problem.Nuclear fusion energy is tipped to the ultimate energy due to the abundance in source,safety,controllability and cleanliness.Tokamak is one of the most promising controllable fusion device.After decades of studies and development,many issues of engineering and physics have not been solved.The interaction between energetic particles and magnetohydrodynamic(MHD)instabilities is a crucial issue in tokamaks.On the one hand,the energetic particles can stabilize or destabilize some MHD instabilities.It can also excite some new instabilities.On the other hand,the loss and redistribution of energetic particles can be induced by MHD instabilities.Both the two aspects have an important effect on the operation of tokamaks.In advanced tokamaks,reversed magnetic shear configuration is employed to enhance the plasma confinement.The steady state scenario with reversed magnetic shear is one the two type of advanced operation scenarios of the International Thermonuclear Experimental Reactor(ITER).However,in the presence of reversed magnetic shear,there are adjacent rational surfaces on which the safety factor has the same value.The islands on the adjacent rational surfaces can drive each other.So the double tearing mode can be induced which has a serious damage to the plasma confinement.The previous works show that single tearing mode is stabilized by energetic particles in some condition.Motivated by this,the effects of energetic particles on the double tearing mode are investigated with MHD-kinetic hybrid code M3D-K.The energetic particle driven modes(EPMs)are also studied.The steady state operation is achieved with noninductive current in which the energetic particle current plays a significant role.The energetic particle effects is considered only in the momentum equation while the modification of Ohm's law caused by the energetic particle current is always neglected in the existing hybrid codes.In this work,based on the pressure coupling hybrid code M3D-K,the energetic particle current is calculated and is employed to modify Ohm,law.Some basics of tokamaks and the physics of MHD instabilities and energetic particles are sketched first in this thesis.Then the physical model and the numerical method of M3D-K are introduced.Next,the three main contents of this thesis are presented as follows:1.The effects of trapped energetic particles on the low frequency MHD instabilities with reversed magnetic shear profile are investigated.It is shown that the double tearing mode is destabilized by trapped energetic particles.When the energetic particle beta is larger than a threshold,an EPM,the fishbone-like mode,is excited.Namely,a mode transition from double tearing mode to fishbone-like mode is triggered.The threshold of the mode transition is enhanced by the resistivity.The effects of the energetic particles on double tearing mode and fishbone-like mode also depend on the gyro-radius and the injection velocity.For double tearing mode,as gyro-radius increases,the energetic particle effect is enhanced when the gyro-radius is smaller than the mode width,and is reduced when the gyro-radius is larger than the mode width.As injection velocity increases,the energetic particle effect is reduced.For fishbone-like mode,the energetic particle effect decreases with increasing gyro-radius and increases with increasing injection velocity.2.The effects of circulating energetic particles on the low frequency MHD instabilities in reversed shear configuration are studied.It is shown that the circulating energetic particles with low energetic particle beta almost have no effect on double tearing mode.For higher energetic particle beta,the effects of circulating energetic particles on double tearing mode depend on the orbit width.When the orbit width is small,the double tearing mode is destabilized by circulating energetic particles.The physics of the destabilizing effect is the same for both "co-" and "counter-" circulating particles.The double tearing mode is stabilized by the circulating energetic particles with large orbit.For the co-circulating energetic particles with large orbit,the double tearing mode is decoupled into two single tearing modes.For the counter-circulating energetic particles with large orbit,an EPM is excited.The mode transition from double tearing mode to the EPM can be triggered when the energetic particle beta is high enough.As the energetic particle beta increases,the dominant mode transits from double tearing mode to the EPM,and the two modes coexist in a wide range of the energetic particle beta.This mode transition occurs more easily with smaller resistivity and larger orbit.3.Based on the pressure coupling hybrid code M3D-K,the energetic particle current is calculated with newly developed module,and is coupled into Ohm's law.With this more complete hybrid code,the effects of energetic particles on tearing mode and double tearing mode are simulated.It is shown that when the energetic particle current modification of Ohm's law is considered,the linear growth rate of tearing mode and double tearing mode is changed by the fraction of the energetic particle current.