两种径向稀疏波在非磁化环境等离子体中的碰撞：环境等离子体密度的影响
Collision of two radial rarefaction waves in unmagnetized ambient plasma: effects of the ambient plasma density
AI总结：
研究了两种径向稀疏波在非磁化环境等离子体中的碰撞，发现环境等离子体密度影响离子堆积和反向激波形成。
摘要：
通过粒子-网格模拟研究了两个圆形稀疏波在真空或薄层环境等离子体中的扩展情况，该模拟能够解析两个空间维度。在无环境等离子体的模拟中，稀疏波在两个稀疏波中心对称线附近相互穿透。稀疏波的密度随距离呈指数衰减，这意味着它们的密度总和在对称线附近不会形成密度最大值。没有密度最大值，这将导致流入稀疏波离子在对称线附近产生排斥的电势，而离子束的高速相互穿透导致在重叠层中产生离子-离子不稳定性，而不是激波。在有环境等离子体的模拟中，稀疏波将环境等离子体的离子堆积在对称线附近。形成一个局部的热环境离子活塞。如果其密度足够大，其热电场允许反向激波在稀疏波中增长。这些反向激波在模拟框架中缓慢移动，并包围下游等离子体的板状区域。稀疏波离子在激波上游的速率随时间减小，导致其坍缩。
英文摘要：
The expansion of two circular rarefaction waves in vacuum or in a thin ambient plasma is examined with particle-in-cell simulations that resolve two spatial dimensions. In the simulation with no ambient plasma, the rarefaction waves interpenetrate near the symmetry line between both rarefaction wave centers. The exponential density decrease of rarefaction waves with distance implies that the sum of their density does not lead to a density maximum near the symmetry line. The absence of a density maximum, which would yield a repelling electric potential for the inflowing rarefaction wave ions near the symmetry line, and the high interpenetration speed of the ion beams lead to ion-ion instabilities rather than shocks in the overlap layer. The simulations with ambient plasma show that the rarefaction waves pile up the ions of the ambient plasma near the symmetry line. A localized piston of hot ambient ions forms. If its density is large enough, its thermoelectric field allows reverse shocks to grow in the rarefaction waves. These reverse shocks move slowly in the simulation frame and enclose a slab of downstream plasma. A decrease in the speed of the rarefaction wave ions upstream of the shocks with time leads to their collapse.
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