Session BO4: HEDP Laboratory Astrophysics and Shocks

Charged particle transport in laser-driven magnetized turbulence: understanding the intergalactic voyage of ultra-high energy cosmic rays

How charged particles and turbulent magnetic fields interact is key to understanding the journey of cosmic rays through space. In this talk we report on numerical simulations and laser-driven experiments at the OMEGA Laser Facility that measure the propagation of energetic particles through random magnetic fields in a turbulent plasma. We characterize their angular diffusion and recover their mean free path and associated diffusion coefficients. The OMEGA experiments we present constitute the first laboratory probe of particle diffusion through magnetized turbulence in the absence of a mean background field and complement laboratory studies of energetic particle propagation in diffuse plasmas, where there is a strong guide field. The experiments emulate the propagation of ultra-high energy cosmic rays through the intergalactic medium, critical for interpreting anisotropies in their arrival direction and for constraining the distance of their sources   

Modeling of Laser-Generated Plasmas in a MG Magnetic Field

Experiments coupling a plasma generated by a laser pulse with fields generated by pulsed power have shown that localized plasma structures (disk or ring) are present long after the pulse has ended.[1] We model the effects of coupling a pulse of intensity ( ~10¹⁵ W/cm²) with a field in the azimuthal direction of  ~4 MG generated by axial current in a rod in 2 spatial dimensions. We are able to see localized structures that exist in the presence of the azimuthal field that are not seen when no field (current) is present. These localized plasma structures resembling a disk continue to expand in both the axial and radial directions. The pulse initially expels the field from the plasma; however, later in time the field generated in the localized structure reaches the same order of magnitude as the initial field. We also investigate how the parameters β (beta) and χ (Hall) vary as the localized structure evolves in time.

Proton radiography of a highly asymmetric laser-driven magnetic reconnection geometry

In experiments performed at the OMEGA EP laser facility at the Laboratory for Laser Energetics, the typical laser-driven magnetic reconnection geometry was adapted such that one high intensity pulse (I ≈ 10¹⁹ Wcm^-2) was focused alongside a moderate intensity UV long pulse (I ≈ 10¹⁴ Wcm^-2) on foil targets. First, the long pulse ablates a region of the target and misaligned temperature and density gradients in the plasma plume generate an azimuthal magnetic field via the Biermann battery mechanism. After the Biermann field develops, the high intensity pulse arrives on target and produces a relativistic, highly magnetized plasma (σ = B²/μ₀n_em_ec² ≥ 1) which sweeps across the target surface with velocities near the speed of light. Proton radiography captures the evolution of the strong, impulsive magnetic field generated by the high intensity pulse and its interaction with the relatively slowly evolving Biermann battery fields. Quantitative measurements of the magnetic field dynamics will be presented, as well as 2D and 3D particle-in-cell simulation results.   

Progress on astrophysical collisionless shock experiments on Omega and NIF

We report the latest experimental progress on our laboratory experiments using the Omega and NIF lasers to investigate the dynamics of high Mach number collisionless shock formation in two interpenetrating plasma streams. It is believed that in astrophysical environments such shocks generate and amplified magnetic fields and accelerate cosmic rays. Counterstreaming plasma flows generated by laser ablation have been probed on the Omega laser using proton radiography, showing the formation, growth, and merger of filamentary magnetic field structures associated with the Weibel instability in a collisionless regime [1]. Recently, higher-energy experiments using the NIF laser showed enhanced heating when the flow interaction transited from collisional to collisionless flows [2]. The latest experiments on NIF considerably reduced collisionality and increased interaction volume. Significantly enhanced neutron production and high energy electrons were observed. We will present these recent experimental results. [1] C. M. Huntington, et al., Nature Physics, 11, 173 (2015); [2] J. S. Ross et al., Phys. Rev. Lett., 118, 185003 (2017).