Laboratory study of the initial stages of quasi-parallel collisionless shocks at high Alfvén Mach number*

Collisionless shocks are ubiquitous in astrophysics and a possible source of the highest-energy cosmic rays in our universe. Recent experimental and numerical efforts have shown that ion-Weibel instability (IWI) is a leading candidate mechanism for collisionless shock formation in unmagnetized astrophysical objects. In a magnetized environment, ion beam instabilities, such as the right-hand instability or the non-resonant instability (NRI), can dominate the dynamics and mediate the development of collisionless shocks. This mediation occurs in a quasi-parallel configuration, meaning that the plasma flow is parallel to the ambient magnetic field.

We present an experimental and simulations investigation of the formation stage of a quasi-parallel collisionless shock at a high-Alfvén Mach number, M_A ~ 200. In experiments, a laser-driven super-Alfvénic plasma flow interpenetrates a premagnetized background plasma at a high velocity, v_flow ~ 1800 km/s. The background magnetic field (10 T) is aligned with the bulk ion velocity. As these kinetic ions interact with the background plasma and magnetic field, streaming instabilities develop and produce magnetic fields, which can eventually mediate collisionless shocks. Proton deflectometry was used to visualize the electromagnetic fluctuations at different times during the evolution, allowing the determination of the streaming instabilities involved in collisionless dissipative processes. A thorough analysis of the proton images indicates the development of the IWI followed by the non-resonant parallel streaming instability, also known as the Bell instability, after an ion gyroperiod.  3D hybrid PIC simulations were performed and support the growth of the non-resonant instability in the experimental conditions.  Simulations suggest that NRI provides an efficient source of dissipation for the formation of a shock, especially in the non-linear regime reached after eight ion gyroperiods.