Time resolved high resolution K-shell spectroscopy of Ti nanowire plasmas heated with highly relativistic laser pulses

High resolution (E/DE > 5000) spectra were taken from Ti nanostructured arrays irradiated at highly relativistic intensities (>10²¹Wcm^-2) at the ALEPH laser facility at Colorado State University using a suite of crystal x-ray spectrometers. The x-ray emission was simultaneously observed with time integrated and time resolved crystal spectrometers, the latter with sub picosecond temporal resolution. The laser targets consisted of TiO₂ nanotubes, ~100nm in diameter with ~20nm wall thickness, arranged with different spacings corresponding to 7%, 15% and 27% of solid density. The time integrated spectrometer, comprised of three individual spherically bent crystals, observed emission from the He-a, He-b and Ly-a line series while the time resolved spectrometer observed emission from the He-a and He-like intercombination line. Doppler and Stark broadened profiles were studied systematically versus plasma density along with density sensitive features such as the He-b. Time-resolved x-ray emission reveals that the lower density nanowire arrays reach higher temperatures and radiate for longer durations of time (~25 ps) whereas the higher density nanowire arrays radiate for shorter times, converging with the x-ray emission of solid density foils. The experimental results will be compared to three-dimensional particle-in-cell simulations and detailed atomic physics code computations.