Homogeneous, Micron-Scale High-Energy-Density Matter Generated by Relativistic Laser-Solid Interactions

Short-pulse, laser-solid interactions provide a unique platform for studying complex high-energy-density matter. We present the first demonstration of solid-density, micron-scale keV plasmas uniformly heated by a high-contrast, 400 nm wavelength laser at intensities up to $2\times {10}^{21}\mathrm{W}/{\mathrm{cm}}^2$. High-resolution spectral analysis of x-ray emission reveals uniform heating up to 3.0 keV over $1\mu \mathrm{m}$ depths. Particle-in-cell simulations indicate the production of a uniformly heated keV plasma to depths of $2\mu \mathrm{m}$. The significant bulk heating and presence of highly ionized ions deep within the target are attributed to the few MeV hot electrons that become trapped and undergo refluxing within the target sheath fields. These conditions enabled the differentiation of atomic physics models of ionization potential depression in high-energy-density environments.

Figure 1

(a) Front- and (b) rear-side Cu foil emission irradiated by the 400 nm laser at $2\times {10}^{21}\mathrm{W}/{\mathrm{cm}}^2$. Lines represent three shot averages. Inset of (a): calculated normalized ratio of the $j$ Li-like satellite to $w$ $\mathrm{He}\alpha$ resonance. Gray cross-hatched bars in (b) denote regions outside the dispersion range of the high-resolution spectrometer.

Figure 2

Experimental Cu $K$-shell emission (black with shaded error bars) compared to scram simulations with constant (dashed red) and variable (dash-dotted blue) temperature profiles. (a) Front- and (b) rear-side spectra of $0.5\mu \mathrm{m}$ foil with constant temperature of 3.5 keV. (c) Front- and (d) rear-side spectra of $1\mu \mathrm{m}$ foil with constant temperature of 3 keV. (e) Front- and (f) rear-side (f) spectra of $5\mu \mathrm{m}$ foil with constant and variable profiles (inset).

Figure 3

3D PIC results of 400 nm interaction. (a) Electron temperature and (b) density as a function of depth for 1 (solid), 2 (dashed), and $5\mu \mathrm{m}$ (dash-dotted) targets.

Figure 4

Rear-side spectra of (a) 1, (b) 5, (c) 10, and (d) $15\mu \mathrm{m}$ thick Cu foil targets from 800 (dashed red) and 400 nm (solid blue) interactions.

Figure 5

Front-side emission of $1\mu \mathrm{m}$ Cu foils (black) compared to scram spectra using the SP (dashed red) or EK (dash-dotted blue) models.