Magnetized collisionless shocks are found where super-magnetosonic plasma flows interact in a pre-existing magnetic field, and are the most prevalent type of shocks in the universe, from planetary bow shocks and the heliopause to supernova remnants and galaxy clusters. These shocks are known to accelerate particles to extremely high energies, and are mediated by electromagnetic effects over characteristic length scales much shorter than the particle mean free path. Our understanding of these shocks has been traditionally based on spacecraft measurements and telescope observations, but these are limited to either the smallest or largest scales in uncontrolled plasma conditions. Consequently, many fundamental questions of shock physics remain unanswered, such as how energy is partitioned across a shock or how particles are accelerated by shocks.

By utilizing high-energy lasers and HED plasmas, these questions can be addressed in the lab, with the goal of complementing spacecraft and telescope data with well-controlled and diagnosed experiments.  The lasers ablate thin foils to drive supersonic piston plasmas, which sweep up ambient plasma and magnetic fields like a snowplow to drive a collisionless shock.  While these laboratory shocks occur over only a few millimeters and nanoseconds, orders of magnitude smaller than those seen in space, they match key dimensionless parameters like Mach number and scale size.  Combined with large particle-in-cell simulations, this allows direct comparisons between lab and astrophysical collisionless shocks.

We are currently performing experiments to study partilcle acceleration, shock heating, and shock structure on several of the world's largest HED facilities, including the Omega Laser Facility at the Laboratory for Laser Energetics (LLE), the National Ignition Facility (NIF) at Lawrence Livermore National Lab, and the Z Machine at Sandia National Labs.

Magnetic reconnection is a ubiquitous process that transforms magnetic topology, allowing the explosive release of stored magnetic energy, including in solar flares, substorms in the Earth’s magnetotail, pulsar wind nebula, and sawtooth crashes in tokamaks. Most astrophysical reconnection occurs in the large-scale, low-dissipation regime (characterized by a large dimensionless Lundquist number), in which the global reconnection current sheet is predicted to break up into a complex and turbulent region where reconnection occurs between many magnetic-island or “plasmoid” structures. Of particular interest are the mechanisms that drive reconnection in this regime and how efficiently particles are accelerated.

To study these questions in the lab, large system sizes and high-energy lasers are required, such as those on the National Ignition Facility (NIF) at Lawrence Livermore National Lab.  In these experiments, laser beams are focused onto two spots on a thin foil.  Each spot creates a large expanding plasma embedded with a self-generated magnetic field through the Biermann battery mechanism.  As the two plasmas collide, the oppositely-directed magnetic fields can reconnect, heating and energizing the plasma.  The field structures are measured with diagnostics such as proton deflectometry, and modeled with large 3D PIC simulations.