Ultrafast Electron Radiography of Magnetic Fields in High-Intensity Laser-Solid Interactions

Using electron bunches generated by laser wakefield acceleration as a probe, the temporal evolution of magnetic fields generated by a $4\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ ultrashort (30 fs) laser pulse focused on solid density targets is studied experimentally. Magnetic field strengths of order $B_0\sim {10}^4\mathrm{T}$ are observed expanding at close to the speed of light from the interaction point of a high-contrast laser pulse with a $10\mathrm{\text{−}}\mu \mathrm{m}$-thick aluminum foil to a maximum diameter of $\sim 1\mathrm{mm}$. The field dynamics are shown to agree with particle-in-cell simulations.

Figure 1

Experimental geometry.

Figure 2

(a) Calibrated electron spectrum and (b) spectrometer image for a typical shot.

Figure 3

(Column a) Measured radiographs taken from laser shots with high contrast and $10\mu \mathrm{m}$ Al at delay timings from 0 to $+1333\mathrm{fs}$. (Columns b–d) Simulated radiographs for each respective delay with defocusing with azimuthal perturbation (b), focusing (c), and defocusing (d). (The color and length scales are the same in each column). Measured radiographs from low contrast, $13\mathrm{\text{−}}\mu \mathrm{m}$ Mylar shots (e), and $10\mathrm{\text{−}}\mu \mathrm{m}$ Al shots (f) at selected delays. Note that the electron beam profile is elliptical before interacting with the target, and the simulated profiles assume radial symmetry for simplicity.

Figure 4

(a) Radiograph from Fig. 3a with location of lineout. (b) Lineout plot with midpoint between peak-to-valley indicated. (c) Radiograph with threshold set to midpoint and a circle fit to curvature to determine feature diameter. (d) Plot of measured feature size versus delay timing taken from analysis of Fig. 3a. The error bars represent the variance in determining the diameter over multiple lineouts as well as the timing uncertainty. The linear fit (solid line) indicates that the velocity of the expanding feature is $(0.98\pm 0.08)c$.

Figure 5

(a–c) Magnetic field component in the $x_3$ direction in tesla (Fourier filtered to remove the laser field) at different times, where $t=0$ is when the peak of the pulse is incident on the target surface, and (d) initial electron density profile with initial pulse envelope superimposed.