Ultra-High Energy Density Relativistic Plasmas by Ultrafast Laser Irradiation of Aligned Nanowire Arrays

Long-lived plasmas that are simultaneously dense and hot (multi-keV) have been created by spherical compression with the world's largest lasers, and by supersonic heating of volumes with densities on the order of N$_{ec}$ using multi-kJ lasers pulses. We demonstrate volumetric heating of near-solid density plasmas to keV temperatures using ultra-high contrast $\lambda =$ 400 nm femtosecond laser pulses of only 0.5 J energy to irradiate arrays of vertically aligned nanowires with 12{\%} average solid density. X-ray spectra show that irradiation of Ni and Au nanowires arrays with relativistic intensities ionizes plasma volumes several micrometers in depth to the He-like and Co-like (Au 52$+)$ stages respectively. He-$\alpha $ line emission greatly exceeds that of the Ni K$\alpha $ line. This volumetric plasma heating approach creates a new laboratory plasma regime in which extreme plasma parameters can be accessed with table-top lasers. The increased hydrodynamic-to-radiative lifetime ratio is responsible for a great increase in the x-ray emission.