K-shell spectroscopy of nanowire plasmas heated with highly relativistic laser pulses*

The K-shell emission from Ni and Ti near-solid density nanowire array plasmas and solid foils is measured with high spectral resolution at the ALEPH laser facility at Colorado State University using a suite of high-resolution x-ray spectrometers for time-integrated measurements. The addition of a sub-picosecond resolution x-ray streak camera allows for time-resolved measurements of the spectral line emission. The targets are irradiated at highly relativistic intensities, a_o ~ 20, with ultra-high contrast 400 nm laser pulses of ~50 fs duration. The plasma density is varied using arrays of 100 nm diameter nanowire arrays with different wire spacings which allows us to vary the target density from 7% to 30% of solid density. The x-ray yield of the lower density nanowire arrays exceeds that of the solid density foil by a factor of ~5x. 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.