Nonlinear Simulation Study Of Energetic Particle Physics In Tokamaks

In future reactor scale Tokamaks,e.g.,ITER(International Thermonuclear Experimental Reactor),the desired outcome is to produce a confined plasma achieving a self-sustaining state of Deuterium-Tritium(D-T)fusion,a so-called burning plasma.In this burning state,the Energetic Particles(EPs),mainly composed of 3.5MeV α particles through the D-T fusion process,play the dominant role in heating the background plasma and sustaining the continuous burning plasma system.During this burning process,Shear Alfven Wave(SAW)instabilities are easily destabilized by EPs via wave-particle resonance.Also,EPs greatly influence evolutions of MagnetoHydroDynamic(MHD)instabilities.Therefore,studying interactions between EPs and both SAW and MHD instabilities is important for understanding EP physics in fusion plasmas.The work presented in this doctoral thesis reports on key numerical results based on simulations produced with the MHD-kinetic hybrid code,CLT-K.Firstly,equivalences of two different coupling schemes,the original current coupling scheme and the newly adopted pressure coupling scheme,are strictly verified under different approximations.Secondly,the influences of EPs on the linear stability of the m/n=2/1 tearing mode(where m and n represent the poloidal and toroidal mode numbers,respectively)are discussed systematically.Results show that both co-passing and trapped EPs have a stabilization effect on the tearing mode,while counter-passing EPs exhibit a destabilization effect.EPs with varying distribution functions can excite an m/n=2/1 Energetic Particle Mode(EPM).In phase space,the resonance conditions of EPM with co-passing,counterpassing,and trapped EPs are different.The frequency of EPM is determined by characteristic orbit frequencies of EPs.In addition,the nonlinear interactions of the m/n=2/1 tearing mode and n=1 Toroidal Alfven Eigenmode(TAE)in the presence of EPs are also investigated.During this process,the n=0 zonal flow component exhibits two nonlinear growth stages.Firstly,the zonal flow grows exponentially over time,with the growth rate twice that of the TAE,and the saturation level of TAE is greatly influenced by the zonal flow.Secondly,during the saturation stage of the tearing mode,the zonal flow shows a secondary nonlinear growth and eventually becomes the dominant mode in the system.This dominant zonal flow has a high saturation level and a wide radial distribution.The mode structure and the final saturation level of TAE are modulated by the zonal flow.Through phase space analysis,it is found that the redistribution of EPs by tearing mode results in a continuous drive of the background plasma and ultimately produces the dominant zonal flow nonlinearly.Furthermore,it is noted that the tearing mode can also nonlinearly generate a localized zonal flow component,which is a prerequisite for the nonlinear saturation of the tearing mode.Finally,the tearing mode is found to modulate the EP distribution,leading to an enhanced eruption of the originally unstable TAE and destabilization of the marginally stable TAE.In summary,a set of comprehensive numerical simulations for a nonlinear Tokamak plasma system is carried out,including EPs,SAW instabilities,MHD instabilities,and zonal flow.This work may improve understanding of the physical mechanisms behind wave-particle and wavewave nonlinear interactions in Tokamaks.In addition,the varying physical schemes and the numerical algorithms of the MHD code,CLT,and the hybrid code,CLT-K,have been significantly upgraded and improved,and should remain very useful tools for simulating more complex burning plasma physics of Tokamaks in future research.