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

Abstract

High resolution K-shell emission from Ni solid and near solid density nanowire arrays was measured at the ALEPH laser facility using a newly commissioned high resolution x-ray spectrometer. The spectrometer consists of a spherically bent Ge crystal coupled to a CCD. The targets were irradiated with high contrast laser pulses using the 400nm second harmonic beamline which operates at highly relativistic intensities, ao ~ 20. The ion and electron density of the plasmas were varied using arrays of 100nm diameter nanowire arrays with different wire spacings which varied from 7% of solid density to 24%. This parametrizes the collisional and radiative properties of the plasma as well as the temperature. The time integrated, high resolution K-shell spectra, coupled with moderate resolution, time resolved x ray emission reveal 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 less than 10 ps, converging with the x-ray emission of the solid density Ni foil. Overall, the x-ray yield of the lower density nanowire arrays exceeds that of the solid density foil by a factor of ~5x. In contrast to this, the average charge state of the solid density foils is greater than that of the nanowire arrays due to the larger collisional ionization rates which occur at solid density. The experimental results will be compared to detailed transient atomic physics computations coupled with radiative hydrodynamic simulations with initial conditions provided by three-dimensional particle in cell simulations.