Magnetic Signatures of Radiation-Driven Double Ablation Fronts

In experiments performed with the OMEGA EP laser system, magnetic field generation in double ablation fronts was observed. Proton radiography measured the strength, spatial profile, and temporal dynamics of self-generated magnetic fields as the target material was varied between plastic, aluminum, copper, and gold. Two distinct regions of magnetic field are generated in mid-$Z$ targets—one produced by gradients from electron thermal transport and the second from radiation-driven gradients. Extended magnetohydrodynamic simulations including radiation transport reproduced key aspects of the experiment, including field generation and double ablation front formation.

Figure 1

Results from a 2D radiation-magnetohydrodynamic simulation of a laser pulse interacting with a copper target show evidence of double ablation front formation. Lineouts of the electron density (black line), electron temperature (red line), and radiation temperature (dashed green line) are taken along the laser axis after 750 ps of laser heating.

Figure 2

Schematic of the OMEGA EP experiment.

Figure 3

Experimental proton images using Cu (a) and $\mathrm{Cu}+\mathrm{Al}$ (b) targets show the formation of two concentric rings of proton accumulation. Magnetic field profiles from corresponding times in the 2D ($r\text{−}z$) cylindrically symmetric simulation (c) reveal the evolution of two, radially distinct magnetic field regions. Similar trends can be observed when comparing lineouts through the experimental proton images (d) and synthetic images generated using the simulation data (e) (lineouts are offset vertically for clarity, with later times in higher positions). Experimental lineouts were taken along the dashed line shown in (a) from the $\mathrm{Cu}+\mathrm{Al}$ results, except at 0.25 ns. Synthetic images were produced with 33.6 MeV protons for 0.25 through 0.75 ns and 32.8 MeV for 1 ns.

Figure 4

2D simulation results for CH (a) and Cu (b) plasmas at a $t_0+0.75\mathrm{ns}$. Magnetic field profiles (blue) are overlaid on the electron density (gray). $T_e$ contours (in units of keV) are shown in red, and select contours of $Z_{\mathrm{eff}}$ are plotted with dashed purple lines. Panels (c) and (d) show radial profiles [along the dashed lines indicated in (a) and (b)] of the magnetic field (blue line), $T_e$ (red line), and Nernst velocity (${\mathbf{v}}_N$, dashed gray line). Magnetic field lineouts are normalized by 2 MG, $T_e$ by 2.25 keV, and ${\mathbf{v}}_N$ by $1.5\times {10}^6\mathrm{m}/\mathrm{s}$.

Figure 5

Proton images (top row) and field reconstruction analysis (bottom row) are compared for the different target materials at $t_0+0.75\mathrm{ns}$. The proton energy is 37.3 MeV for CH, Al, and $\mathrm{Cu}+\mathrm{Al}$, and 30.7 MeV for $\mathrm{Au}+\mathrm{Al}$. Bottom left: the radial lineouts ($J$), normalized by the mean inferred reference profile ($J_0$). Bottom right: the resulting reconstructed field profiles. For Al and $\mathrm{Cu}+\mathrm{Al}$; the results of double-Gaussian fitting are shown with shaded regions.