Kinetic Study Of Microinstabilities And Turbulent Impurity Transport In Transport Barriers Of Tokamak Plasmas

Magnetic confinement fusion energy is one of the most promising energy sources.Effective confinement of plasma with high temperature and density in fusion devices like tokamaks is essential for fusion reactions.The anomalous transport induced by micro-instabilities will lead to the energy and particle loss of plasma and significantly reduce the core plasma confinement,which has been one of the key problems to the fusion community.Considering the high level of anomalous transport measured in experiments,reducing the anomalous transport and improving the confinement are quite crucial to sustain self-heated burning plasmas.The high-confinement(H-mode)and improved confinement(I-mode)discharge experiments show that the transport is significantly reduced in transport barriers(TBs)and the impurity ions contribute to this decrease as well.Moreover,improved confinement is achieved via the edge peaking radiating impurities in radiation improved confinement(RI-mode)discharges,which also benefits for mitigating plasma-wall interactions.Although the transport level is significantly reduced in TBs,the steep temperature gradients arised from the TBs conversely provide plenty of free energy for the micro-instabilities.Whereas,the formation and sustainment of TBs are incompletely understood yet,and the underlying physical mechanism for sustaining the edge peaking impurity ion density profile still remains an unsolved challenge.In this thesis,a domestic gyrokinetic code HD7 containing complete kinetic effects,is used to investigate microinstabilities and turbulent impurity transport in TBs.The correlated experimental observations in HL-2A and EAST tokamaks are also analysed.The thesis contains seven chapters.Chapter 1 briefly introduces the background and motivation,including a review on the experiments of transport and confinement as well as the transport models.Chapter 2 reviews the transport theory,physical model of HD7 and basic theory of impurity mode(IM).The following four chapters are the main contents of this thesis.Conclusions,innovations and a prospect are summarized in chapter 7.The specific contents from chapter 3 to 6 are as follows:Chapter 3 presents the turbulent transport in TBs from the aspects of gyrokinetic simulations as well as the integrated analysis of theory and I-mode experiment in EAST.It is found that multiple ITG modes with conventional and unconventional ballooning mode structures can be excited simultaneously in TBs with steep temperature and density.Unconventional modes with large mode-number dominate in large keps region,and the radial transport coefficients of these modes dominate in the parameter region with large temperature and medium density gradients.This suggests the high-order ITG modes are non-ignorable in TB region,especially for I-mode.On the other hand,the transition between the ITG mode and the TEM turbulences consists well with the experimental identification of edge temperature ring oscillation and alternating turbulence transitions for sustaining the stationary I-mode in EAST.The following three chapters will discuss the issue of turbulent impurity transport owing to its significance.In chapter 4,it is found that kinetic effects of impurity ions,which can not be completely described if treated as a test particle,strongly influence the background ITG and TEM turbulences.The impurity transport is dominated by the diffusion(diagonal)transport and sub-dominated by the thermal-diffusion(off-diagonal)part of particle transport.Instead of ITG mode and TEM,impurity mode(IM)is another important microinstability,especially in pedestal region.The following chapters will investigate the turbulent transport of IM and its relevant experiments.Chapter 5 mainly shows the turbulent transport of IM.The fundamental theory of IM indicates it is driven by impurity ion density gradient opposite to that of electrons.The IM can induce heat transport even when the temperature profiles are flat.This indicates the convection terms of heat flux play the key role when the diagonal term equals zero.Besides,the trapped electron effects mainly affect the particle transport while the impurity effects influence the particle as well as the heat transports,which suggests particle and heat transport are decoupled.To sustain the edge peaking impurity ion density profiles,chapter 6 particularly focuses on the particle transport of IMs.Main ITG and electron temperature gradient(ETG)are found to reduce influx of impurity ions significantly,denoting turbulent temperature screening effect.The simulation results such as peaking factor of impurity ion density profile and the turbulent temperature screening effects are found in coincidence with the argon seeding experiment in HL-2A.Thus,the IM turbulence is demonstrated to be a plausible mechanism for sustaining the edge peaking impurity ions density profiles.In summary,the micro-instabilities excited in TBs and turbulent impurity transport are investigated in this thesis.The theoretical and numerical findings successfully explained the mechanism of sustaining I-mode in EAST experiments,which is due to the alternating transition between the ITG mode and TEM.The investigations of impurity transport reveals that a large temperature gradient and a low electron density gradient are expected to be beneficial for sustaining the edge peaking impurity ion density profiles and reducing the impurity accumulation in core plasma.Such kind of new discharge regimes are believed to improve the plasma confinement in consideration of advantages of I-mode and RI-mode.