Influence Of Trapped Energetic Particles On Tearing Modes And Fishbone-Like Modes In Tokamak

Tearing mode(TM),a magnetohydrodynamic(MHD)instability driven by the radial gradient of the equilibrium toroidal current density,can break equilibrium magnetic field topology to form magnetic islands,which lead to great enhancement of radial transport in the magnetic island region.Once magnetic islands become large enough,their overlap can even cause a major plasma disruption.Thus,understanding the physical mechanism of TM is one of significant issues to achieve the steady-state operation of present and future tokamak devices.Moreover,energetic particles(EPs)are generated by deuterium-tritium fusion and auxiliary heating,which not only affects various MHD instabilities(such as TM),but also directly drives many instabilities like fishbone modes.These instabilities driven by EPs will in turn lead to a loss of EPs,thus a decrease of auxiliary heating efficiency and other negative impacts.Therefore,the study of EP physics is of importance for predicting the burning plasma in future devices such as International Thermonuclear Experimental Reactor.With the assumption of a circular cross section and large aspect ratio tokamak with low plasma ?,this thesis mainly studies the effect of trapped EPs on TM and the excitation of 2/1 fishbone-like modes by trapped EPs.The main results are as follows:Firstly,the effect of deeply trapped EPs on TM is investigated analytically.Under the consideration of the finite orbit width effect,the perturbed kinetic distribution function without resonance effect of deeply trapped EPs is analytically obatained using the characteristic line method under the condition that the magnetic island width is far smaller than the orbit width of EPs.Furthermore,a new stability criterion of TM is obtained deducing the outer region equation of TM modified by plasma pressure including both background particles and EPs' contributions.It is found that the direct coupling effect between the bad curvature and asymmetrical pressure of EPs leads to the modification of the Mercier indexd due to the limitation of deeply trapped EPs' orbits,and that coupling effect further destabilizes TM instability.Secondly,a physical mechanism is analytically proposed for understanding the formation of the 2/1 fishbone-like modes discovered in the HL-2A tokamak experiment.By neglecting finite orbit width effect,the dispersion relation of the 2/1 fishbone mode is obtained through deducing perturbed potential energy contributed by trapped EPs via energy principle method.The analytical expression of ? threshold and the critical real frequency of the 2/1 fishbonelike modes are provided.The effect of various parameters on the excitation of the 2/1 fishbonelike modes are analytically and numerically presented.It is found that,by analytically predicting ? threshold for the excitation of the 2/1 fishbone-like modes,the issue of unacquirable ? threshold due to the limitation of the experimental conditions has been solved.Both magnetic shear and plasma resistivity can stabilize the 2/1 fishbone-like modes.When?<1%,trapped EPs also destabilize TM,which is consistent with the results in last study.Thirdly,the influence of the critical energy on the excitation of the 2/1 fishbone-like modes is studied using a slowing down distribution function,which is closer to realistic cases.Besides,the effect of critical energy on the other branch,TM,is also studied during the excitation of fishbone-like modes process.It is found that the critical energy will reduce the ?threshold required for the excitation of the 2/1 fishbone-like modes.The growth rate and real frequency of TM are also affected.The impact of other parameters like magnetic shear on the excitation will also change accordingly.Finally,a brief summary of the finished work is given and a prospect of future work is brought up.