托卡马克离子温度梯度湍流输运同位素定标修正中杂质的影响

托卡马克实验发现,在不同参数条件下,等离子体能量约束经验定标律会有或大或小的修正.为解释这种修正现象发生的原因,应用回旋动理学方法,对含重(钨)杂质等离子体离子温度梯度(ITG)(包括杂质模)湍流输运的同位素效应进行了数值研究.结果表明钨杂质效应极大地修改了同位素定标律和有效电荷效应.随着杂质离子电荷数Z和电荷集中度f_z的变化,同位素定标律在较大范围内变化. ITG模最大增长率定标大约为M_i~(-0.48→-0.12),杂质模的定标为M_i~(-0.46→-0.3),其中, M_i表示主离子质量数.在ITG模湍流中,有效电荷数越大,关于M_i的拟合指数偏离-0.5越远,表现为同位素质量依赖减弱.在两种模中,杂质电荷集中度越大,同位素质量依赖越弱.研究了杂质效应使定标关系发生偏离的原因,证实杂质种类、杂质电荷数和杂质浓度的不同,是引起同位素质量依赖发生改变的重要原因.结果证实并解释了不同参数条件下托卡马克同位素定标的差异性.研究成果可以为ITER实验安排及杂质相关输运实验中选择装置材料、工作气体和设置其他参数提供理论参考.

Role of impurities in modifying isotope scaling law of ion temperature gradient turbulence driven transport in tokamak

Tokamak experiments show that the plasma empirical energy confinement scaling law varies with plasma ion mass (A_i) in a certain range under conditions of different plasma parameters or different devices. In order to understand such a modification of the empirical energy confinement scaling law, the isotope mass dependence of ion temperature gradient (ITG, including impurity modes) turbulence driven transport in the presence of tungsten impurity ions in tokamak plasma is studied by employing the gyrokinetic theory. The effect of heavy (tungsten) impurity ions on ITG and impurity mode is revealed to modify significantly the isotope mass dependence and effective charge effect. As the charge number of impurity ions (Z) or impurity charge concentration (f_z) changes, the theoretical scaling law of ITG turbulence transport varies substantially in a relatively large range. The maximum growth rate of ITG mode scales as M_i^{-0.48→ -0.12}, whilst that of impurity mode scales as M_i^{-0.46→ -0.3}. Here, M_i is the mass number of primary ion in the plasma. In both cases the fitting index with M_i deviates further away from -0.5 when impurity charge concentration f_z increases. The isotope mass dependence of ITG turbulence gradually weakens when the effective charge number Z_eff increases. The isotope mass dependence of impurity mode turbulence also weakens with Z_eff increasing for the same impurity ion charge number (Z). In contrast, the isotope mass dependence gradually strengthens with effective charge number Z_eff increasing for the same impurity charge concentration (f_z). On average, the maximum growth rates of impurity mode scale roughly as γ_max~M_i^-0.35Z_eff^1.5 and γ_max~M_i^-0.4Z_eff¹, respectively, for Z_eff ≤ 3 and Z_eff > 3. The reason for the deviation of isotope scaling law from the normal case is investigated deliberately, and it is demonstrated that the isotope scaling index deviates from -0.5 more or less due to the fact that the impurity species, charge number and impurity concentrations vary in a certain range. These results demonstrate that it is impossible to deduce a unique isotope scaling law due to the variety of micro-instabilities and various plasma parameter regimes in tokamak plasma, which is consistent with the experimental observations. These results may contribute to the transport study involving heavy (tungsten) impurity ions in ITER discharge scenario investigation.