We have investigated the propagation of a magnetic dipole assuming a simple model of forward and return flow of fast electrons under a condition of plasma-density inhomogeneity by a particle-in-cell simulation. An exact propagating depiction of the dipolar structure is given under the framework of a simplified 'electron magnetohydrodynamic' fluid model (Yadav et al 2008 Phys. Plasmas15 062308; Yadav et al 2009 Phys. Plasmas16 040701; Yadav and Das 2010 Phys. Plasmas17 052306) in a dense plasma. We reproduce this structure in our kinetic calculations. The results indicate that, with a steep plasma density gradient, the structure evolves rapidly toward plasma in a process involving shock formation and rapid dissipation of beam energy, which is consistent with the fluid simulations. In addition, new features are also reported, such as the pinching of the two dipole lobes to form a very strong shear layer, which develops into a Kelvin–Helmholtz instability. The magnetic energy is rapidly converted to kinetic energy of electrons leading to additional plasma heating in inhomogeneous regions, such as the core region in an imploded plasma.
我研究了磁偶极子在等离子体密度不均匀条件下，采用快电子前向和返回流的简单模型，通过粒子网格模拟对其传播进行了研究。在简化的“电子磁流体力学”流体模型框架下，给出了偶极结构的精确传播描述（Yadav等，2008年《物理学报》15卷062308；Yadav等，2009年《物理学报》16卷040701；Yadav和Das，2010年《物理学报》17卷052306），该模型适用于稠密等离子体。我们在动力学计算中重现了这一结构。结果表明，在陡峭的等离子体密度梯度下，该结构迅速演化为等离子体，涉及激波形成和束能量快速耗散，这与流体模拟结果一致。此外，还报道了新的特征，例如两个偶极瓣发生箍缩，形成极强的剪切层，进而发展为开尔文-亥姆霍兹不稳定性。磁能迅速转化为电子的动能，导致在非均匀区域（如内爆等离子体的核心区域）产生额外的等离子体加热。