非均匀磁场尘埃等离子体中颗粒的复杂运动

Complex motions of grains in dusty plasma with nonuniform magnetic field

本文研究了在非均匀磁场尘埃等离子体中不规则尘埃颗粒的复杂运动,包括圆滚运动、尖头圆滚运动、圆周运动以及波浪运动等.放置在电极上的圆柱形磁铁的主要作用是改变鞘层的径向分布,进而对颗粒产生径向约束,使尘埃颗粒悬浮于圆柱形磁铁周围,其磁场并不足以磁化颗粒使其做圆滚运动.通过与球形尘埃颗粒的对比实验发现,圆滚运动是不规则尘埃颗粒在等离子体中特有的一种运动.我们提出了一种新的机理:由于不规则颗粒的自旋而引起的横向反Magnus力对颗粒的圆滚运动起了重要的作用.文中通过受力分析定性地对实验中观察到的非球形颗粒的各种运动给出了合理的解释.

We have studied various complex motions of the irregular dust grains immersed in non-uniformly magnetized plasma. The cylindrical magnet that we used for experiments significantly alters the radial distribution of the sheath potential which confines the negatively charged grains. Grains are horizontally illuminated by a 50 mW, 532 nm laser sheet and imaged by a CCD camera from the upper transparent electrode. Hypocycloid and epicycloid motions of grains are observed for the first time as far as we know. Cuspate cycloid motions, circle motion, wave motion, and stationary grains are also observed. Their trajectories can be obtained by using long-time exposure, and the characteristic parameters of the grain movement are measured by using the image processing with MATLAB. Though the dust grains can move around the magnet steadily in various trajectories, the induced magnetic field is too weak to give rise to cycloid motions of grains. Then we propose a new mechanism that an inverse Magnus force induced by the spin of the irregular grains plays an important role in their cycloid motions. The pollen pini we used for experiment is not a regular microsphere, there is a symmetry in the shape. On the basis of Bernoulli principle, the pressure difference between the left and right side of the forward moving grains produces the inverse Magnus effect. Additional comparison experiments with regular microspheres are also performed to confirm that the cycloid motions are distinctive features of an irregular dust grain immersed in the plasma. The periodical change of the cyclotron radius as the grain travels would result in the （cuspate） cycloid motions, and the maximal value of angular velocity of spin is about 105 rad/s. Our experimental observations can be well explained based on the force analysis in 2D horizontal plane.