Spatial Uniformity of Laser-Accelerated Ultrahigh-Current MeV Electron Propagation in Metals and Insulators

The evolution of laser-generated MeV, MA electron beams propagating through conductors and insulators has been studied by comparing measurement and modeling of the distribution of MeV protons that are sheath accelerated by the propagated electrons. We find that electron flow through metals is uniform and can be laser imprinted, whereas propagation through insulators induces spatial disruption of the fast electrons. Agreement is found with material dependent modeling.

Figure 1

(a) Schematic of experiment, with laser focus on target at full power. The target is a smooth $48\mathrm{\mu }\mathrm{m}$ thick Au foil. (b),(c) Distribution of protons in RCF layers (energy as marked). Typical beam half-angle is $10°$; i.e., $p_{\perp }/p_{\parallel }\sim 0.17$. (d) Gaussian electron sheath used in the 3D model, and (e) simulated RCF output.

Figure 2

(a) Astigmatic laser focus on a smooth $48\mu \mathrm{m}$ thick Au target at $\sim 4\times {10}^{18}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. (b) Distribution of protons in RCF. (c) Sheath used in the 3D model, obtained from (a) convolved with a $60\mu \mathrm{m}$ (FWHM) Gaussian PSF. (d) Simulated RCF output. (e) Ellipticity of simulated protons [$A/B$ in (d)] versus FWHM of PSF. Horizontal dashed lines denote measured ellipticity.

Figure 3

3D PIC simulation of proton acceleration from elongated laser focal spot. (a) Laser focal spot [FWHM of major or minor axes are, respectively, $13.3\mu \mathrm{m}$ and $3.3\mathrm{\mu }\mathrm{m}$], incident at ${10}^{19}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ on a $1\mu \mathrm{m}$ thick target at $50n_{\mathrm{c}\mathrm{r}}$, with a rear side proton layer of $0.05\mu \mathrm{m}$. (b) Angular distribution of the protons (2.3–3 MeV) at 250 fs after the beginning of the interaction ($p_{\perp }/p_z=0.17$, corresponds to $10°$).

Figure 4

(a) Aberrated laser focal; (b) RCF distribution of protons from a smooth $48\mu \mathrm{m}$ thick Au target. (c) Simulated RCF [same energy range as (b)] assuming a sheath obtained from (a) convolved with a $60\mu \mathrm{m}$ FWHM Gaussian PSF.

Figure 5

RCF images of protons (8 MeV layer) from $\sim 10\mu \mathrm{m}$ Gaussian laser spot incident on (a) $100\mu \mathrm{m}$ thick smooth glass target, (b) $10\mu \mathrm{m}\mathrm{A}\mathrm{u}+50\mu \mathrm{m}\mathrm{C}\mathrm{H}$ target, (c) $50\mu \mathrm{m}\mathrm{C}\mathrm{H}+10\mu \mathrm{m}\mathrm{A}\mathrm{u}$ target, and (d) $2\mu \mathrm{m}\mathrm{A}\mathrm{u}+80\mu \mathrm{m}\mathrm{C}\mathrm{H}+2\mu \mathrm{m}\mathrm{A}\mathrm{u}$ sandwich target.

Figure 6

3D PÂRIS simulation of 500 keV electron beam (1 J, 300 fs, $5\mu \mathrm{m}$ radius, $20°$ divergence) propagating in (a)–(c) a $10\mu \mathrm{m}\mathrm{A}\mathrm{u}/2.5\mu \mathrm{m}\mathrm{A}\mathrm{l}$ (with a resistivity artificially increased tenfold) $/12.5\mu \mathrm{m}\mathrm{A}\mathrm{l}$ sandwich target and (d)–(f) a solid ($25\mu \mathrm{m}$) gold target. The simulation box is $50\times 50\times 25\mu {\mathrm{m}}^3$; initial temperature is 0.03 eV. (a),(d) Beam current distribution at the end of the simulation box (normalized to $e{n}_c$) and (b),(e) azimuthal magnetic field (side view) in tesla, shown at 500 fs. (c)–(f) Simulated proton RCF distribution with sheath given by (a),(d).