Studying Quasi-Parallel Collisionless Shocks in the Laboratory

Quasi-parallel collisionless shocks are common in heliospheric and astrophysical systems. Measurements show that quasi-parallel shocks are capable of accelerating particles (including ions) to high energies through diffusive shock acceleration. This process intrinsically depends on the structure of quasi-parallel shocks, which are more turbulent, unstable, and spatially extended than quasi-perpendicular shocks. We review recent laboratory experiments combining a large, moderate-density ambient plasma and a laser-produced plasma to study some of the beam instabilities involved in quasi-parallel shock formation. These experiments reproduced waves observed by spacecraft upstream of the Earth's quasi-parallel bow shock. However, to date, no experiment has reproduced a full quasi-parallel shock or observed diffusive ion acceleration. We describe scaling relations and simulations that suggest that smaller-scale, higher-density experiments may provide a path to creating quasi-parallel collisionless shocks in the laboratory.