Kinetic Study Of Impurity Effects On Microinstabilities And On Induced Turbulent Transport In Tokamaks

Tokamak is one of the most promising devices to realize controlled nuclear fusion.Maintaining steady-state operation of plasma with high temperature and density is essential for fusion research.Actually,the particle,momentum and energy transport observed in Tokamak experiments are often much higher than those predicted by classical and neoclassical theories.The anomalous transport induced by micro-instability will significantly affect the confinement efficiency and discharge quality of plasma.On the other hand,various non-hydrogenic impurity ions inevitably exist in tokamak plasma,which will not only lead to energy loss and fuel dilution,reduce fusion plasma temperature and fusion power,but also affect plasma transport behavior and confinement performance to a great extent.Therefore,considering the impurity effects on micro-instability and induced turbulent transport,improving the confinement and mitigating impurity accumulation are of great practical significance to understand the basic behavior of plasma and realize the steady-state safe operation.After decades of development,the first principle based gyrokinetic simulation has become a major tool to study the micro-instability and induced turbulent transport in tokamaks.In this thesis,the gyrokinetic theory and numerical code extended to the quasi-linear transport version are used to investigate micro-instabilities and turbulent transport behaviors in the presence of impurities plasmas,mainly focusing on the multiple ion temperature gradient(ITG)driven modes in transport barriers,ion temperature gradient(ITG)turbulence induced heat transport,interaction of ion temperature gradient(ITG)and trapped electron modes(TEM)induced heat transport.The specific contents of the thesis are given as follows:In chapter 1,the background and the aim of this thesis are presented;The overviews of the micro-turbulence and anomalous transport are given;The background of ion temperature gradient(ITG)mode,trapped electron mode(TEM)and impurity mode(IM)are briefly summarized.In chapter 2,based on gyrokinetic theory,a physical model describing the micro-instability and turbulent transport behavior in tokamak plasma is given,including electrostatic and electromagnetic versions;The gyrokinetic integral eigenvalue code HD7 is introduced,and extended to quasi-linear transport model and high electromagnetic version.In chapter 3,we investigate the impurity effects on multiple ion temperature gradient(ITG)driven instability in transport barriers.It is found that multiple ITG modes with conventional and unconventional ballooning mode structures can be excited simultaneously in TBs with steep gradients;The effect of impurity ions with outwardly peaked density profiles on the ITG mode depend on the competition between the destabilizing effect of the impurity density gradient and the stabilizing effect induced by dilution of the main ions when the ion temperature gradient parameter is fixed.For light impurities,the impurity ions with low(high)charge number and inwardly peaked density gradients have strong stabilizing effects on ITG modes in the strong(weak)ITG driving regions,while for heavy impurities,the impurity ions with high charge number have strong stabilizing effects in the whole temperature gradients regions.By comparing the mixing length estimation and quasi-linear particle flux methods,the turbulent transport induced by multiple ITG instabilities is analyzed.It is found that when the impurity density gradient is outwardly peaked,main ions transport outwards and the impurity ions accumulate inwardly;When the impurity density gradient is inwardly peaked,the result is just the opposite,that is,the main ions flow inwardly and impurity ions go outwards;The latter is conducive to main ion confinement and impurity decumulation.In addition,the particle flux induced by high-order ITG mode is equivalent to that of conventional mode,which demonstrates the importance of high-order instabilities in the transport barrier.In chapter 4,the impurity effects on quasi-linear heat transport induced by ion temperature gradient(ITG)turbulence are investigated.The simulation results show that heat fluxes depend not only on the saturation amplitude of the instability but also on the phase shift between perturbed temperature and velocity.The relationship of peaking factor of temperature/density profile(defined as the ratio of major radius to gradient scale length when the heat flux equals zero),and the stability threshold of ITG modes on the impurity parameters are also given.It is found that the main ion heat flux is dominated by contribution of off-diagonal(diagonal)diffusion fluxes for regions of weak(strong)ITG,respectively,related to ion density(temperature)gradients;The heat flux of impurity ions mainly depends on the contribution of off-diagonal diffusion and diagonal diffusion,and its direction is determined by the symbols of density gradient and temperature gradient,respectively.Generally speaking,the impurity ions with negative(positive)density gradients reduce(enhance)heat transport of main ions.However,interestingly,the effect of impurity ions with positive density gradients may transit from the enhancement to reduction of main ion heat flux for a steep ITG,corresponding to TBs.Such results may account for better energy confinement after impurity injection in experimental observations.In chapter 5,based on the experimental phenomenon that the ion temperature for the core and edge increases and the edge electron temperature remains unchanged after argon impurities seeded by the supersonic molecular bean injection(SMBI)on the HL-2A tokamak,the impurity effects on quasi-linear heat transport induced by interaction of TEM and ITG turbulence are studied.The results indicate that stability diagrams in terms of local parameters are divided into five regions: both ITG and TE modes stable,one of ITG modes or TEMs unstable,both unstable,hybrid ITG-TEMs unstable;The independent ITG modes and TEMs exist and even coexist in regions of weak electron density gradient,while hybrid ITG-TEMs arise after electron density gradient surpasses the threshold.Due to different instability types and dominant transport regions,distinct responses of ion and electron heat fluxes to impurity seeding are found.Therefore,the roles of impurity ions in heat transport are analyzed and compared in detail for the core and edge regimes,and it is found that the results are consistent with the experimental observations.Finally,the physical mechanism of energy pinch is systematically analyzed.Reversed magnetic shear regions as well as large temperature ratios of electrons to main ions substantially reduce heat fluxes and thus contribute to the formation and sustainment of energy pinch;The phase shift(larger than /2)is demonstrated to be a reasonable mechanism for the energy pinch phenomenon.Finally in chapter 6,summaries and innovation points of the thesis are presented,and the prospects for future work are given.