Energy transport in high temperature, high density plasmas on femtosecond timescales
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In this work we apply femtosecond (fs) microscopy to a solid target (aluminum and copper) irradiated at relativistic intensity (Ipu ≥ 2 × 1018 W/cm2 ) by high-contrast (≥ 1 : 10−9 ), obliquely-incident (θ inc pu = 45◦ ), P- and S-polarized pump pulses (λpu = 0.8 µm, 35 fs) focused to a wavelength-scale spot size (w0 = 0.8 µm). Under these conditions, radiation and hot electrons are the dominant carriers of energy out of the initially photo-excited volume. The mean free paths governing both transport processes exceed the spot size w0, opening the study of ballistic transport of energy into surrounding target material. This femtosecond microscopy experiment, with λ 2 pu pump spot, is well-suited to observe the initial stages, and the radial dimension, of such non-local transport directly on any target material. The physics of this transport is relevant to fast ignition of laser fusion, to generation of ultrashort pulsed x-rays and relativistic proton and ion beams, and to astrophysics. The experimental interaction volume may be small enough that the entire experiment is amenable to large-scale particle-in-cell simulations.