Collisionless Weibel shocks: full formation mechanism and timing

TL;DR

This paper elucidates the physical mechanism behind collisionless Weibel shock formation in unmagnetized plasmas, demonstrating that such shocks always form when relativistic plasma shells collide, with formation times validated by simulations.

Contribution

It provides a detailed physical explanation for collisionless shock formation via the Weibel instability and predicts the shock formation time, aiding experimental design and understanding in plasma physics.

Findings

Weibel shocks always form in relativistic, unmagnetized plasma collisions.

The predicted shock formation time matches simulation results.

The work enables optimization of experimental setups for shock production.

Abstract

Collisionless shocks in plasmas play an important role in space physics (Earth's bow shock) and astrophysics (supernova remnants, relativistic jets, gamma-ray bursts, high energy cosmic rays). While the formation of a fluid shock through the steepening of a large amplitude sound wave has been understood for long, there is currently no detailed picture of the mechanism responsible for the formation of a collisionless shock. We unravel the physical mechanism at work and show that an electromagnetic Weibel shock always forms when two relativistic collisionless, initially unmagnetized, plasma shells encounter. The predicted shock formation time is in good agreement with 2D and 3D particle-in-cell simulations of counterstreaming pair plasmas. By predicting the shock formation time, experimental setups aiming at producing such shocks can be optimised to favourable conditions.