Time Resolved Ni K Shell Spectroscopy of Nanowire Arrays Irradiated at Highly Relativistic Intensities

Ni K shell emission from near-solid density nanowire arrays and solid density foils was spectrally and temporally resolved using an x-ray streak camera with sub-picosecond temporal resolution coupled to a Von Hamos crystal spectrometer. The targets were 100nm diameter Ni arrays with various fractions of solid density (7{\%}, 15{\%} and 24{\%}) and were irradiated with high contrast (\textgreater 10$^{\mathrm{12}})$,~ \begin{figure}[htbp] \centerline{\includegraphics[width=0.08in,height=0.17in]{290620201.eps}} \label{fig1} \end{figure} $=400$nm, 45fs laser pulses focused to intensities \textgreater 10$^{\mathrm{21}}$Wcm$^{\mathrm{-2}}$. These results were compared to the emission from solid density Ni foils. The duration of the Ni He- \begin{figure}[htbp] \centerline{\includegraphics[width=0.09in,height=0.17in]{290620202.eps}} \label{fig2} \end{figure} line was measured to decrease from 21 ps for an initial electron density of 1.6x10$^{\mathrm{23}}$cm$^{\mathrm{-3}}$ to 5 ps for a solid foil with an initial electron density of 2.4x10$^{\mathrm{24}}$cm$^{\mathrm{-3}}$. The increased time duration of the x-ray emission from the lower density nanowire arrays is shown to be a consequence of the increased volumetric heating of the plasma which delays the hydrodynamic expansion. These results are in good agreement with hybrid three-dimensional particle-in-cell/radiative hydrodynamic simulations.~