Direct Observations of Particle Dynamics in Magnetized Collisionless Shock Precursors in Laser-Produced Plasmas

Collisionless shocks are ubiquitous throughout the heliosphere, but despite decades of observations and numerical simulations, there remains no clear understanding on how energy is partitioned between electrons and ions across a shock. Laboratory experiments can help address this question by providing direct observations of particle dynamics and plasma heating in magnetized collisionless shocks under well-controlled and reproducible plasma conditions. In this work, we present laboratory observations of time-resolved electron and ion velocity distributions in magnetized collisionless shock precursors. Thomson scattering of a probe laser beam was used to observe the interaction of a laser-driven, supersonic piston plasma expanding through an ambient plasma in an external magnetic field. From the Thomson-scattered spectra we measure time-resolved profiles of electron density, temperature, and ion flow speed, as well as spatially-resolved magnetic fields from proton radiography. We observe direct evidence of the coupling between piston and ambient plasmas, including the acceleration of ambient ions driven by magnetic and pressure gradient electric fields, and deformation of the piston ion flow, key steps in the formation of magnetized collisionless shocks. Even before a shock has fully formed, we observe strong density compressions and electron heating associated with the pile up of piston ions. The results demonstrate that laboratory experiments can probe particle velocity distributions relevant to collisionless shocks, and can complement, and in some cases overcome, the limitations of similar measurements undertaken by spacecraft missions.