Magnetized collisionless shocks in the Universe and the laboratory

Magnetized collisionless shocks are ubiquitous in the Universe. High-power lasers enable the creation of these shocks in the laboratory. Shock dynamics depend on the angle θ_n between the background magnetic field and the shock propagation direction, with different physical processes active in quasi-perpendicular (θ_n > 45°) and quasi-parallel (θ_n < 45°) geometries.



Kinetic simulations have been used to model both (quasi-)perpendicular and (quasi-)parallel shocks. We have found that perpendicular shocks, which are readily achievable on kiloJoule, TeraWatt laser systems such as OMEGA EP, are mediated by a modified two-stream instability.



Quasi-parallel shocks are more difficult to form in the laboratory and to simulate because of their large spatial scales and long formation times. Our simulations show that the early stages of quasi-parallel shock formation are achievable in the ongoing Discovery Science experiments on the National Ignition Facility, and that particles accelerated by diffusive shock acceleration are expected to be observable in the experiments.



Energy partition between electrons and ions in collisionless shocks has long been an open question. In the shock simulations, significant energy exchange between ions and electrons is observed, which implies a collisionless electron heating mechanism. A multi-fluid dispersion relation indicates that resonances between electron whistler and ion magnetohydrodynamic waves provide the mechanism that accounts for the energy exchange between ions and electrons.