Increased x-ray conversion efficiency from ultra high contrast, relativistic laser pulse irradiation of large aspect ratio, vertically aligned nanowires*

Recent experiments at Colorado State University have shown that the effective trapping of clean, Joule-level fs laser pulses of relativistic intensity in arrays of high aspect ratio aligned nanowire creates multi-kev, near solid density, large scale (\textgreater 4um deep) plasmas. The drastically decreased radiative life time and increased hydrodynamic cooling time from these plasmas increases the x-ray conversion efficiency. We measured a record conversion efficiency of \textasciitilde 10{\%} into hv\textgreater 1KeV photons (2pi steradians), and of 0.3{\%} for hv\textgreater 6KeV. The experiments used Au and Ni nanowires of 55nm, 80nm and 100nm in diameter with 12{\%} of solid density irradiated by high contrast (\textgreater 10$^{\mathrm{12}})$ pulses of 60fs FWHM duration from a frequency doubled Ti:Sa laser at intensities of I$=$5x10$^{\mathrm{19}}$Wcm$^{\mathrm{-2.\thinspace }}$We also present preliminary results on x-ray emission from Rhodium nanowires in the 19-22KeV range and demonstrate the potential of this picosecond X-ray source in flash radiography.