Collision of two radial rarefaction waves in unmagnetized ambient plasma: effects of the ambient plasma density

TL;DR

This study uses particle-in-cell simulations to explore how two radial rarefaction waves interact in unmagnetized plasma, revealing effects of ambient plasma density on wave dynamics, ion pile-up, and shock formation.

Contribution

It provides new insights into the collision dynamics of rarefaction waves in plasma, especially how ambient density influences shock development and wave behavior.

Findings

Rarefaction waves interpenetrate without forming a density maximum.

Ambient plasma causes ion pile-up and piston formation.

Reverse shocks can develop with sufficient ambient plasma density.

Abstract

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.