Ballistic radial energy transport in micron-scale plasmas under high energy density conditions.

In this work we apply fs microscopy to solid targets (Al, Cu) irradiated at relativistic intensity ($I_{pu} \ge 2 \times 10^{18}$ W/cm$^{2}$) by high-contrast (better than $1:10^{-10}$), obliquely-incident ($\theta^{inc}_{pu} = 45^{\circ}$ and $70^{\circ}$), \textit{P}- and \textit{S}-polarized pump pulses ($\lambda_{pu} = 0.8$ $\mu$m, 34 fs) focused to a {\it wavelength-scale} spot size ($w_0 = 0.8$ $\mu$m). 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 $w_0$, opening the study of ballistic transport of energy into surrounding target material. Our fs microscopy experiment, with $\lambda_{pu}^2$ 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. Our interaction volume may be small enough that the entire experiment is amenable to large-scale particle-in-cell simulations.