Simulations reveal how plasma collisions create shocks—key to understanding cosmic phenomena
Physicists used computational models to watch what happens when two clouds of charged particles collide head-on, finding that instabilities trigger shock formation through a predictable sequence. The findings could improve predictions of high-energy events in space and inform the design of fusion energy systems, where controlling plasma behavior remains a critical engineering challenge.
Originaltitel: Simulation study of the formation of a non-relativistic pair shock
<p>We examine with a particle-in-cell (PIC) simulation the collision of two equally dense clouds of cold pair plasma. The clouds interpenetrate until instabilities set in, which heat up the plasma and trigger the formation of a pair of shocks. The fastest-growing waves at the collision speed <img src="https://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20170124095833254-0163:S0022377816001288:S0022377816001288_inline1.gif" data-mimesubtype="gif" data-type="simple" />$c/5$, where <img src="https://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20170124095833254-0163:S0022377816001288:S0022377816001288_inline2.gif" data-mimesubtype="gif" data-type="simple" />$c$ is the speed of light in vacuum, and low temperature are the electrostatic two-stream mode and the quasi-electrostatic oblique mode. Both waves grow and saturate via the formation of phase space vortices. The strong electric fields of these nonlinear plasma structures provide an efficient means of heating up and compressing the inflowing upstream leptons. The interaction of the hot leptons, which leak back into the upstream region, with the inflowing cool upstream leptons continuously drives electrostatic waves that mediate the shock. These waves heat up the inflowing upstream leptons primarily along the shock normal, which results in an anisotropic velocity distribution in the post-shock region. This distribution gives rise to the Weibel instability. Our simulation shows that even if the shock is mediated by quasi-electrostatic waves, strong magnetowaves will still develop in its downstream region.</p>