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We report on the characterization of plasmas produced by ultrashort laser irradiation of 500 A thick layers of NaF buried at varying depths in plastic to reduce plasma gradients. The targets are irradiated with 130 fs , ≈170 mJ laser pulses (λ=400 nm ) at an intensity of ≈2×10 18 W cm −2 . The data are spectrally and temporally resolved, with resolutions of λ/Δλ≈1000 and ≳500 fs , respectively. We use X-ray emission spectroscopy to assess the electron density, Ne, and temperature, Te, in the plasma. The density is measured from Stark broadened line profiles of the He-like 1 1S– 3 1 P , He β , and H-like Lyβ lines of Na, while the temperature is determined from the intensity ratio of Na Heβ to Lyβ, and also from the dielectronic satellites to these lines. We find peak densities and temperatures of ≳10 23 cm −3 and ≈400 eV , respectively, at 2– 4 ps after the laser pulse. The plasma conditions plateau near these values for 5 ps after that. Atomic data for the kinetics simulations are generated with the HULLAC suite of codes. The Stark broadened line profiles, with full accounting of satellite transitions, are computed with the TOTAL code. Self-absorption effects are included along the observation line of sight using the radiation transport code CRETIN. Steady-state, non-LTE equilibrium plasma conditions are demonstrated suggesting that buried-layer experiments can be used as a test bed to study equation of state and opacity properties in hot, near-solid density matter. |
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