Structure of self-generated magnetic fields in laser-solid interaction from proton tomography

Abstract

Strong magnetic fields are naturally self-generated in high-power, laser-solid interactions through the Biermann-battery mechanism. This work experimentally characterizes the 3D location and strength of these fields, rather than path-integrated quantities, through multi-view proton radiography and tomographic inversion on the OMEGA laser. We infer magnetic fields that extend several millimeters off the target surface into the hot, rarefied corona and are sufficient to strongly magnetize the plasma ($Ω_{e}τ_e \gg 1$). The data is used to validate MHD simulations incorporating recent improvements in magnetic transport modeling; we achieve reasonable agreement only with models with re-localization of transport by magnetic fields. This work provides a key demonstration of tomographic inversion in proton radiography, offering a valuable tool for investigating magnetic fields in laser-produced plasmas.

Figures (9)

• Figure 1: Experimental setup showing the different proton backlighter source positions (colored circles) in successive shots. The backlighter angle $\theta$ is defined relative to the target normal.
• Figure 2: Experimental proton radiographs at $t=1.4~$ns from (a-d) 15 MeV protons and (e) 3 MeV protons, as the target is tilted about the $y$-axis in successive shots. Darker regions received higher proton fluence. The red boxes in (b,c) show where lineouts were taken in Figs. \ref{['fig:deflection_comp']}(d,e).
• Figure 3: (a) Sample straight-line proton trajectories from the different backlighter sources, projected onto cylindrical coordinates. The target is positioned on the $z=0$ plane and irradiated from the positive $z$ direction. The black box shows the inversion domain. (b) Zoomed-in view of a single proton trajectory from the $\theta=45\degree$ backlighter through the inversion domain. The trajectory is discretized on a 2D grid with 150$~\mu$m resolution and the pathlength in each cell is shown by the blue color. The red line shows the proton trajectory.
• Figure 4: (a) Toroidal magnetic field from extended MHD simulation at full field strength and (b) extracted from the tomographic inversion. All fields have been smoothed over 100 $\mu$m and the color limits have been adjusted to compare the coronal fields. (c) Radial lineout of $|B_\phi|$ at $z=1.5~$mm for experiment, relocalized simulation, and maximally suppressed simulation using a recent model of Biermann suppression by nonlocal effects davies_nonlocal_2023. (d) Electron hall parameter estimated from experimental magnetic fields and simulated density and temperature. The solid, dashed, and dotted lines are isocontours of $\Omega_e \tau_e=$ (1, 100, 2000), respectively.
• Figure 5: Lineouts of the proton deflection on the detector in the (a-c) radial direction for front and back views and in the (d,e) horizontal direction along the x-axis for $\theta=45\degree$ and $67\degree$ views from the regions outlined in Fig. \ref{['fig:data']}. The experimental data (black circles) are compared to synthetic deflection from inversions that include coronal fields (red lines) and omit coronal fields (blue lines).