Bin Ahn, Yegeon Lim, Hoiyun Jeong, Hae June Lee, Gyung Jin Choi, Y.-C. Ghim
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引用次数: 0
Abstract
Collisionless charged particle motion and its transport in the two-wire model (TWM) with no axial magnetic fields is investigated numerically. The TWM configuration contains a magnetic X-point, and single particle motions in such a field have two conserved quantities: the total kinetic energy and the base field line value which is a quantity derived from the axial canonical momentum. As gyrating particles travel along the field lines, they may reach near the X-point region where the magnetic moment, the first adiabatic invariant, can be occasionally shifted due to a large gradient of the field. When the magnetic moment becomes large, resulting in a large Larmor radius, particles probabilistically cross the X-point to migrate to the opposite side of the TWM configuration. These phenomena are investigated with single particle simulations. We find that the statistical behaviour of the seemingly chaotic magnetic moment shifts are completely determined by the two aforementioned conserved quantities, and also that there exists a threshold energy, determined by the base field line value, allowing only particles with a higher energy to cross the separatrix and migrate. It is found that the crossing time is distributed exponentially, and that the migration confinement time, which is the average crossing time, is shorter for particles with a base field line closer to the separatrix and a higher energy. We provide an empirical expression, derived with the simulations, for estimating the collisionless migration confinement time.
对无轴向磁场的双线模型(TWM)中的无碰撞带电粒子运动及其传输进行了数值研究。TWM 构造包含一个磁 X 点,单粒子运动在这样的磁场中有两个守恒量:总动能和基磁场线值,基磁场线值是从轴向典型动量导出的量。当回旋粒子沿着场线运动时,它们可能会到达 X 点区域附近,在该区域,磁矩(第一个绝热不变量)会由于场的巨大梯度而偶尔发生偏移。当磁矩变大,导致拉莫尔半径变大时,粒子很可能会穿过 X 点,迁移到 TWM 配置的另一侧。我们通过单粒子模拟研究了这些现象。我们发现,看似混乱的磁矩移动的统计行为完全由上述两个守恒量决定,而且存在一个由基本场线值决定的阈值能量,只有能量较高的粒子才能穿过分离矩阵并迁移。研究发现,穿越时间呈指数分布,对于基场线更靠近分离矩阵且能量更高的粒子,迁移限制时间(即平均穿越时间)更短。我们提供了根据模拟推导出的经验表达式,用于估算无碰撞迁移约束时间。
期刊介绍:
JPP aspires to be the intellectual home of those who think of plasma physics as a fundamental discipline. The journal focuses on publishing research on laboratory plasmas (including magnetically confined and inertial fusion plasmas), space physics and plasma astrophysics that takes advantage of the rapid ongoing progress in instrumentation and computing to advance fundamental understanding of multiscale plasma physics. The Journal welcomes submissions of analytical, numerical, observational and experimental work: both original research and tutorial- or review-style papers, as well as proposals for its Lecture Notes series.