Study Of Influences Of Energetic Particles On Ion Temperature Gradient Modes And Internal Transport Barriers In Tokamak

Improving the plasma confinement is beneficial for designing the future nuclear fusion devices and optimizing the performance of present devices.An important limiting factor of the plasma confinement in fusion devices is microturbulence.As a significant driver of plasma turbulence,the ion temperature gradient(ITG)instability is principally responsible for the degradation of the ion energy confinement.Therefore,it is greatly valuable to study the mechanisms which limit its development.Energetic particles are mainly generated by fusion reactions,as well as auxiliary heating systems,such as neutral beam injection(NBI)and ion cyclotron resonance heating(ICRH).Understanding the behaviour of energetic particles is important since they are an essential component of the fusion plasma and play a major role in sustaining fusion-relevant bulk temperatures.In addition,energetic particles carry the large power which implies that even small energetic particle losses can damage the first wall of fusion device.Energetic particles can interact with plasma waves and instabilities,which not ony can enhance(or excite)plasma instabilities,such as fishbone instability,but also can stabilize plasma instabilities.In some numerical and experimental works,it was shown that energetic particles can stabilize the ITG instability,which is beneficial to the formation of the internal transport barrier(ITB).The confinement performance has been improved in tokamak devices by forming a transport barrier in the core region termed the internal transport barrier.In EAST experiments,it is found that the formation of the ITB is associated with the fishbone instability.Fishbone instability can lead to the redistribution of energetic particles and strengthen the stabilization of ITG instability.When the turbulent transport is improved appropriately,an ITB can be built.In this work,the influence of energetic particles on ITG modes and ITB are studied.The main contents are as follows:Firstly,based on both analytical and numerical calculations,the effects of trapped energetic particles on ITG modes have been studied and the relevant physics mechanisms have been explained.It is found that energetic particles can strongly affect ITG mode through a wave-particle resonance mechanism when the precession frequency of trapped energetic particles is close enough to the frequency of ITG mode.The energetic particle stabilizing effect depends on density,temperature,the density and temperature gradients of energetic particles.Energetic particle resonance destabilizes ITG mode at very low temperature,but is stabilizing as soon as energetic particle temperature exceeds a certain value.By investigating the effect of the energetic particle temperature in more detail,it is found that the effect of adiabatic part and energy gradient driven term of energetic particles almost cancel each other.So the effect of energetic particles mainly results from the space gradient driven term.The space gradient driven term is from density and temperature gradients.The density gradient term of energetic particles plays a destabilizing role on ITG mode.Moreover,one of temperature gradient driven term((?)ηf term)is destabilizing on ITG mode,but another part(-3ηf/2 term)plays a stable role on ITG.When the threshold condition (?)0r<3/2 is satisfied.stabilization of ITG mode by energetic particles can be found.In addition,both increasingηf and Lne/Lnf can strengthen the energetic particle resonant stabilization effect.Secondly,based on the formation of the ITB observed in the experiment of EAST tokamak,a possible theoretical interpretation of the ITB formation during the fishbone instability on EAST tokamak is offered.The fishbone instability induces the redistribution of energetic particles and leads to the accumulation of energetic particles in a local region near the ITB,where the ITG mode exists.Energetic particles can interact with the ITG mode through the dilution,α and wave-particle resonance mechanisms.It is found that energetic particles can stabilize the ITG mode where the stabilizing effects are determined mainly by the density,temperature and their gradients of energetic particles.The density of trapped energetic particles near the ITB is increased after the fishbone instability appears meanwhile the temperature of the expelled energetic particles caused by the fishbone instability is lower than the energetic particle temperature before the fishbone instability appears.The higher density of energetic particles leads to a stronger stabilizing effect on ITG mode.The lower energetic particle temperature leads to a stronger resonance stabilizing effect but a reduced α stabilizing effect on ITG mode.In addition,energetic particles have the enhanced density gradient and temperature gradient near the ITB after the fishbone instability appears.Both the bigger density and temperature gradients have a stronger a stabilizing effect on ITG mode.The resonance effect changes little with the enhanced density gradient but the resonance stabilizing effect is enhanced by the enhanced temperature gradient.Compared with the effect before the fishbone instability appears,the total stabilizing effect on ITG mode is enhanced after the fishbone instability appears,which is beneficial to the formation of the ITB.When the transport of the background thermal ions is reduced adequately,an ITB can be built.