[Paper Review] Detailed characterization of a laboratory magnetized supercritical collisionless shock and of the associated proton energization

W. Yao, A. Fazzini+20 more|arXiv (Cornell University)|Apr 25, 2021

Laser-Plasma Interactions and DiagnosticsPhysics and Astronomy61 references15 citations

TL;DR

This study presents a detailed experimental and simulation-based characterization of a laboratory-generated, magnetized, super-critical collisionless shock using high-power lasers and strong magnetic fields. It demonstrates that proton energization occurs via shock surfing acceleration (SSA), with kinetic particle-in-cell simulations confirming the robustness of this mechanism under varying plasma and magnetic field conditions.

ABSTRACT

Collisionless shocks are ubiquitous in the Universe and are held responsible for the production of non-thermal particles and high-energy radiation. In the absence of particle collisions in the system, theoretical works show 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 for shock formation. Shock formation can alternatively take place when two plasmas interact, through microscopic instabilities inducing electromagnetic fields which are able in turn to mediate energy dissipation and shock formation. Using our platform where we couple a fast-expanding plasma induced by high-power lasers (JLF/Titan at LLNL and LULI2000) with high-strength magnetic fields, we have investigated the generation of magnetized collisionless shock and the associated particle energization. We have characterized the shock to be collisionless and super-critical. We report here on measurements of the plasma density, temperature, the electromagnetic field structures, and particle energization in the experiments, under various conditions of ambient plasma and B-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 of \citet{yao2020laboratory}, here we show additional results of the experiments and simulations, providing more information to reproduce them and demonstrating the robustness of our interpreted proton energization mechanism to be shock surfing acceleration.

Motivation & Objective

• To experimentally generate and characterize a magnetized, super-critical collisionless shock in a controlled laboratory environment.
• To investigate the mechanisms of particle energization, particularly proton acceleration, in such shocks.
• To validate the shock surfing acceleration (SSA) mechanism through multi-diagnostic measurements and kinetic simulations.
• To assess the robustness of SSA across varying ambient plasma and magnetic field conditions.
• To provide a reproducible, high-resolution dataset for benchmarking astrophysical shock models.

Proposed method

• Utilized high-power laser facilities (LULI2000 and JLF/Titan) to create a fast-expanding plasma with ion temperatures exceeding 100 eV.
• Applied strong, pre-existing magnetic fields (up to ~100 T) to magnetize the plasma and induce a collisionless shock via magnetic piston dynamics.
• Employed multiple diagnostics: proton radiography for magnetic field mapping, optical Thomson scattering for electron and ion temperature and density, and electric field probes for electromagnetic structures.
• Conducted macroscopic hydrodynamic simulations to model shock formation and kinetic particle-in-cell (PIC) simulations to model particle acceleration mechanisms.
• Varied ambient plasma density and magnetic field strength to probe the robustness of the acceleration mechanism.
• Compared experimental measurements with PIC simulation outputs to validate the SSA mechanism and rule out alternative models such as shock drift acceleration (SDA).

Experimental results

Research questions

• RQ1 What are the key plasma and electromagnetic parameters (density, temperature, B-field, E-field) that characterize a magnetized, super-critical collisionless shock in the laboratory?
• RQ2 Which particle acceleration mechanism—shock surfing acceleration (SSA) or shock drift acceleration (SDA)—dominates proton energization in this magnetized shock environment?
• RQ3 How robust is the SSA mechanism across different ambient plasma and magnetic field conditions?
• RQ4 To what extent do kinetic PIC simulations reproduce the observed proton energy spectra and shock structure?
• RQ5 Can the laboratory shock replicate key features of astrophysical collisionless shocks, such as non-thermal particle populations?

Key findings

• The shock formed is confirmed as collisionless and super-critical, with a magnetosonic Mach number Mms ≳ 2.7, indicating it is not sustained by classical dissipation alone.
• Proton energy spectra exhibit a non-thermal tail extending up to ~10 MeV, consistent with shock surfing acceleration (SSA) rather than thermal or diffusive processes.
• Kinetic PIC simulations reproduce the observed proton energy distribution and show that particles gain energy by surfing on the cross-shock electric field (E = −v × B) at the shock front.
• The shock width is found to be much smaller than the proton Larmor radius, a key condition for SSA to dominate, supporting the SSA interpretation over SDA.
• Experimental measurements of magnetic field and electric field structures show good agreement with PIC simulation predictions, validating the model's fidelity.
• The proton energization mechanism is robust across varying ambient plasma densities and magnetic field strengths, indicating the generality of the SSA mechanism in such magnetized shock systems.