Precision Mapping of Laser-Driven Magnetic Fields and Their Evolution in High-Energy-Density Plasmas

The magnetic fields generated at the surface of a laser-irradiated planar solid target are mapped using ultrafast proton radiography. Thick ($50\mu \mathrm{m}$) plastic foils are irradiated with 4-kJ, 2.5-ns laser pulses focused to an intensity of $4\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. The data show magnetic fields concentrated at the edge of the laser-focal region, well within the expanding coronal plasma. The magnetic-field spatial distribution is tracked and shows good agreement with 2D resistive magnetohydrodynamic simulations using the code draco when the Biermann battery source, fluid and Nernst advection, resistive magnetic diffusion, and Righi-Leduc heat flow are included.

Figure 1

Experimental setup.

Figure 2

(a) Proton radiographs of $50\text{−}\mu \mathrm{m}$-thick CH foils taken at $t=t_0+0.40\mathrm{ns}$, $t=t_0+0.70\mathrm{ns}$, and $t=t_0+1.20\mathrm{ns}$. The proton energy $E_p$ used to generate each image is shown. (b) Synthetic proton radiographs calculated from the draco simulations (see text for details).

Figure 3

Simulated density profiles (a) and spontaneous magnetic-field distributions (b) at times $t=t_0+0.40\mathrm{ns}$, $t=t_0+0.70\mathrm{ns}$, and $t=t_0+1.20\mathrm{ns}$. The calculations are axisymmetric about the horizontal axis. The negative sign indicates magnetic fields out of the page. Magnetic fields at the edge of the laser-focal region ($A$) and the edge of the coronal plasma ($B$) are shown (see Sec. 1 of the Supplemental Material for details about regions $A$ and $B$ [46]).

Figure 4

Comparison of the experimentally measured inner and outer ring radii with draco calculations postprocessed with a proton tracking code. Calculated inner (black) and outer (red) ring radii with (solid line) and without (dotted line) Nernst advection and Righi-Leduc heat flow included in the draco model.