Experiments to Study the Microphysics of Collisionless Plasma Flows in an External Magnetic Field*

Fast, low-density astrophysical plasma flows are described as “collisionless” when the mean free path between Coulomb collisions is large compared to the system size. When these flows encounter a magnetic field they can compress the field, causing a density increase in the flow and, in some cases, generating shocks. We explored these dynamics in a set of experiments at the OMEGA-EP laser facility. In these experiments an external field was generated using the MIFEDS instrument, which magnetized a low-density background plasma. A high-intensity laser was used to drive a quasi-collisionless plasma flow perpendicular to the imposed magnetic field, and the interaction region was diagnosed with proton radiography and optical (4ω) diagnostics. The compressed magnetic field deflected the probe protons, and the particle density increase deflected the 4ω probe. We measured the shock speed from the proton images and inferred the magnitude of the compressed magnetic field by comparing the images to particle-in-cell (PIC) simulations.