Laser-Produced Plasmas as Drivers of Quasi-Parallel Collisionless Shock Formation in the Laboratory

Quasi-parallel collisionless shocks are objects of considerable interest in space and astrophysics, most notably as possible sites of cosmic ray acceleration. Such shocks occur naturally in systems such as supernova remnants and planetary bow shocks, where the complex and turbulent structures they form are commonly observed by spacecraft. However, \textit{in situ} spacecraft measurements have some inherent limitations, such as a moving reference frame and non-repeatable measurements. Generating a quasi-parallel collisionless shock in a repeatable, well-diagnosed laboratory environment could therefore improve our understanding of their formation and structure. 

The quasi-parallel collisionless shocks observed in space and astrophysics are far too large to fit in a laboratory, but scaled versions of these systems can be created using smaller, denser plasmas with similar dimensionless parameters. However, quasi-parallel collisionless shocks are particularly challenging to scale to a feasible experiment. The shock formation process is mediated by several electromagnetic ion/ion beam instabilities which require long length scales ($>500$ ion-inertial lengths) to grow, so an experiment must include a long magnetized background plasma. This background plasma must be overlapped over the same length by a highly super-Alfv\'enic beam plasma. Matching the dimensionless parameters of the shocks observed in space sets demanding requirements on the densities of both plasmas as well as the background magnetic field strength. Laser-produced plasmas (LPPs) provide a promising beam plasma source (a ``driver'') for such experiments.

A recent experimental campaign has been conducted at UCLA to investigate the potential of LPPs as drivers of quasi-parallel collisionless shocks. These experiments combine one of two high-energy lasers with the magnetized background plasma of the Large Plasma Device (LAPD) to drive the electromagnetic ion/ion beam instabilities responsible for shock formation. The experiments have observed electromagnetic waves consistent with the very early stages of quasi-parallel shock formation. These waves are similar to the ultra-low frequency (ULF) waves observed by spacecraft upstream of the Earth's quasi-parallel bow shock. At present, the amplitudes of the waves generated by these experiments are too low ($dB/B_0 \sim 0.01$) to fully form a quasi-parallel shock.

The wave amplitudes observed in these experiments are low because the conditions for beam instability growth are only met in a small region near the laser target. Outside of this region, decreasing LPP density due to velocity dispersion and cross-field transport terminates the wave growth and consequently the shock formation process. Future experiments will require technical innovations to expand this growth region in order to produce larger-amplitude waves. Promising approaches including trains of laser pulses and heating electrons in the background plasma to reduce collisional cross-field transport. Along with comparisons to analytic theory and simulations, the results of the current experiments can inform the design of future laboratory quasi-parallel shock experiments.