[Paper Review] Laboratory evidence for proton energization by collisionless shock surfing

W. Yao, A. Fazzini+20 more|arXiv (Cornell University)|Oct 30, 2020

Astrophysics and Cosmic PhenomenaPhysics and Astronomy96 references33 citations

TL;DR

This study presents the first laboratory evidence of proton energization via shock surfing (SSA) in a quasi-perpendicular, super-critical, magnetized collisionless shock. Using laser-driven plasma experiments and kinetic simulations, the authors demonstrate that protons from the ambient plasma are accelerated to hundreds of keV within the first 2–3 ns of shock formation, confirming SSA as a dominant low-energy ion acceleration mechanism in astrophysical environments such as supernova remnants and the solar wind termination shock.

ABSTRACT

Charged particles can be accelerated to high energies by collisionless shock waves in astrophysical environments, such as supernova remnants. By interacting with the magnetized ambient medium, these shocks can transfer energy to particles. Despite increasing efforts in the characterization of these shocks from satellite measurements at the Earth's bow shock and powerful numerical simulations, the underlying acceleration mechanism or a combination thereof is still widely debated. Here, we show that astrophysically relevant super-critical quasi-perpendicular magnetized collisionless shocks can be produced and characterized in the laboratory. We observe characteristics of super-criticality in the shock profile as well as the energization of protons picked up from the ambient gas to hundreds of keV. Kinetic simulations modelling the laboratory experiment identified shock surfing as the proton acceleration mechanism. Our observations not only provide the direct evidence of early stage ion energization by collisionless shocks, but they also highlight the role this particular mechanism plays in energizing ambient ions to feed further stages of acceleration. Furthermore, our results open the door to future laboratory experiments investigating the possible transition to other mechanisms, when increasing the magnetic field strength, or the effect induced shock front ripples could have on acceleration processes.

Motivation & Objective

• To experimentally demonstrate proton energization by collisionless shocks under astrophysically relevant conditions.
• To validate shock surfing (SSA) as a dominant low-energy ion acceleration mechanism in quasi-perpendicular, super-critical shocks.
• To bridge the gap between astrophysical observations and laboratory-scale experiments by replicating key shock parameters (Mach number, magnetic field, ion Larmor radius).
• To investigate the transition from SSA to other acceleration mechanisms under varying magnetic field strengths or shock front structures.
• To provide direct experimental validation of kinetic simulations predicting early-stage ion acceleration via SSA.

Proposed method

• Laser-driven plasma expansion into a preformed hydrogen gas target to generate a collisionless shock in a controlled, externally applied magnetic field.
• Optical probing via interferometry to measure plasma density and shock front evolution in real time (spatial and temporal resolution ~100 µm and ~1 ns).
• Magnetic field strength (~1.5 T) applied to ensure collisionless conditions by making the ion Larmor radius larger than the shock width.
• Proton energy spectra measured using time-resolved ion spectrometry to infer acceleration efficiency and spectral index.
• Kinetic simulations using SMILEI and FLASH codes to model plasma dynamics, shock structure, and particle trajectories, with varying resolution and ion-to-electron mass ratios.
• Comparison of experimental data with simulations to isolate the role of shock surfing versus other mechanisms (e.g., DSA, SDA).

Experimental results

Research questions

• RQ1 Can shock surfing (SSA) be experimentally observed as a dominant proton acceleration mechanism in a laboratory-generated, super-critical, quasi-perpendicular collisionless shock?
• RQ2 What is the timescale and energy gain of proton energization during the early evolution of a laser-driven collisionless shock?
• RQ3 How do shock parameters (Mach number, magnetic field strength, shock velocity) influence the efficiency of SSA?
• RQ4 To what extent does the shock structure (e.g., foot, ramp, pre-shock density profile) correlate with kinetic simulation predictions?
• RQ5 Can the observed proton energy spectrum be explained by SSA, and how does it compare to theoretical expectations for DSA or SDA?

Key findings

• Protons were accelerated to energies exceeding 300 keV within the first 2–3 ns of shock formation, confirming early-stage ion energization via shock surfing.
• The experimental proton energy spectrum followed a power-law with index p = −4.28, consistent with a short-lived shock; simulations with longer duration (t = 5.1 ns) yielded p = −2.27, approaching values seen in astrophysical DSA.
• Kinetic simulations confirmed that shock surfing dominates over shock drift acceleration (SDA), as the ion Larmor radius (~0.8 mm) was larger than the shock width (~0.2 mm), a key condition for SSA.
• The shock was super-critical (Mms ≳ 2.7) with a magnetosonic Mach number of ~3.0, and the ion collision mean free path (λmfp ≈ 10 mm) exceeded the interaction scale, confirming collisionless conditions.
• The presence of a shock foot structure (~0.5–1 mm upstream) was experimentally observed and matched simulations, indicating ion reflection and cyclic shock front dynamics.
• Simulations showed that higher magnetic field strength increased acceleration efficiency, supporting the theoretical expectation that SSA is more effective in stronger B-fields.