Abstract
Collisionless shocks are ubiquitous in the Universe and are held responsible for the production of nonthermal particles and high-energy radiation. In the absence of particle collisions in the system, theory shows that the interaction of an expanding plasma with a pre-existing electromagnetic structure (as in our case) is able to induce energy dissipation and allow shock formation. Shock formation can alternatively take place when two plasmas interact, through microscopic instabilities inducing electromagnetic fields that are able in turn to mediate energy dissipation and shock formation. Using our platform in which we couple a rapidly expanding plasma induced by high-power lasers (JLF/Titan at LLNL and LULI2000) with high-strength magnetic fields, we have investigated the generation of a magnetized collisionless shock and the associated particle energization. We have characterized the shock as being collisionless and supercritical. We report here on measurements of the plasma density and temperature, the electromagnetic field structures, and the particle energization in the experiments, under various conditions of ambient plasma and magnetic field. We have also modeled the formation of the shocks using macroscopic hydrodynamic simulations and the associated particle acceleration using kinetic particle-in-cell simulations. As a companion paper to Yao et al. [Nat. Phys. 17, 1177–1182 (2021)], here we show additional results of the experiments and simulations, providing more information to allow their reproduction and to demonstrate the robustness of our interpretation of the proton energization mechanism as being shock surfing acceleration.
Highlights
The acceleration of energetic charged particles by a collisionless magnetized shock is a ubiquitous phenomenon in astrophysical environments, in which the most energetic particles are the ultrahigh-energy cosmic rays (UHECRs) accelerated in the interstellar medium (ISM).1,2 In these environments, the sources of collisionless dissipation are self-generated electromagnetic fields, resulting from kinetic instabilities such as the Weibel instability
Using our platform in which we couple a rapidly expanding plasma induced by high-power lasers (JLF/ Titan at LLNL and LULI2000) with high-strength magnetic fields, we have investigated the generation of a magnetized collisionless shock and the associated particle energization
The particle dynamics of a high-velocity shock and of the subsequent shock surfing proton energization are detailed in our previous paper,39 while here we focus on demonstrating the robustness of the shock surfing acceleration (SSA) mechanism that is at play in our experiment via 2D simulations, taking the nonstationarity65 into consideration
Summary
The acceleration of energetic charged particles by a collisionless magnetized shock is a ubiquitous phenomenon in astrophysical environments, in which the most energetic particles are the ultrahigh-energy cosmic rays (UHECRs) accelerated in the interstellar medium (ISM). In these environments, the sources of collisionless dissipation are self-generated electromagnetic fields, resulting from kinetic instabilities such as the Weibel instability. The acceleration of energetic charged particles by a collisionless magnetized shock is a ubiquitous phenomenon in astrophysical environments, in which the most energetic particles are the ultrahigh-energy cosmic rays (UHECRs) accelerated in the interstellar medium (ISM).1,2 In these environments, the sources of collisionless dissipation are self-generated electromagnetic fields, resulting from kinetic instabilities such as the Weibel instability. Calculated parameters: Ion collisional mean free path λmfp,i (mm) Ion larmor radius rL,i (mm) Ion collisionality λmfp,i/rL,i Plasma thermal beta βther Plasma dynamic beta βdyn Mach number M Alfvenic Mach number MA Magnetosonic Mach number Mms. In this paper, we will first show, in Sec. II, that laboratory experiments can be performed to generate and characterize globally mildly supercritical, quasi-perpendicular magnetized collisionless shocks, and detail their characteristics. IV, with the parameters characterized in the experiment, we will report the results of kinetic particlein-cell (PIC) simulations, which pinpoint that shock surfing acceleration (SSA) can be effective in energizing protons from the background plasma to 100 keV-level energies
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