Main ions influence impurity dynamics through a variety of mechanisms; in particular, via impurity-ion collisions. To lowest order in an expansion in the main ion mass over the impurity mass, the impurity-ion collision operator only depends on the component of the main ion distribution that is odd in the parallel velocity. These lowest order terms give the parallel friction of the impurities with the main ions, which is typically assumed to be the main cause of collisional impurity transport. Next-order terms in the mass ratio expansion of the impurity-ion collision operator, proportional to the component of the main ion distribution that is even in the parallel velocity, are usually neglected. However, in stellarators, the even component of the main ion distribution can be very large. In this article, such next-order terms in the mass ratio expansion of the impurity-ion collision operator are retained, and analytical expressions for the neoclassical radial flux of trace impurities are calculated in the Pfirsch-Schlüter, plateau and $1/ν$ regimes. The new terms provide a drive for impurity transport that is physically very different from parallel friction: they are associated to anisotropy in the pressure of the main ions, which translates into impurity pressure anisotropy. It is argued that main ion pressure anisotropy must be taken into account for a correct description of impurity transport in certain realistic stellarator plasmas. Examples are given by numerically evaluating the analytical expressions for the impurity flux.
Enhancement of Mid-/High-Z Impurity Transport by Continuous Li-granule Dropping in a Stellarator Plasma
Laboratorio Nacional de Fusión, CIEMAT（国家聚变实验室，CIEMAT）；Max-Planck Institute for Plasma Physics（马克斯·普朗克等离子体物理研究所）；Princeton Plasma Physics Laboratory（普林斯顿等离子体物理实验室）；National Institute for Fusion Science, National Institutes of Natural Science（国立研究开发法人自然科学研究机构 国立融合科学研究所）；The Graduate University of Advanced Studies, SOKENDAI（综合研究大学院大学，SOKENDAI）；Interdisciplinary Graduate School of Engineering Sciences, Kyushu University（九州大学理工学研究院）；Department of Electrical Engineering, Royal Institute of Technology（皇家理工学院电气工程系）；National Institutes for Quantum Science and Technology, Naka Institute for Fusion Science and Technology（国立研究开发法人量子科学技术研究开发机构 那珂融合科学与技术研究所以）
Solving the problem of overdetermination of quasisymmetric equilbrium solutions by near-axis expansions. II. Circular axis stellarators
我们将近轴展开法应用于具有各向异性压力的准对称磁场（在配套论文第I部分[E.罗德里格斯，阿。Bhattacharjee，arXiv：2008.04715（2020）]）来构造圆轴仿星器的数值解。该解在距离轴的距离上是二阶的，这在标准Garren-Boozer构造中是不可能的[D. A. Garren和A. H. Boozer，Physics of Fluids B：等离子体物理，2822（1991）]，其假设具有各向同性压力的静磁平衡。在零各向异性的限制，它表明，可以选择一个子集的系数，以避免超定问题。
A Variational Framework for Guiding-Center Kinetics, Anisotropic Equilibria, and Quasisymmetry in Stellarators
Courant Institute School of Mathematics, Computing, and Data Science, New York University（纽约大学库朗数学科学研究所）；Department of Astrophysical Sciences, Princeton University（普林斯顿大学天体物理科学系）
