Impact of high contrast ratio on 17.4 keV laser-based Kα source for intensities 1016–1019 W/cm2: A foot print of interaction mechanisms

Summary form only given. Laser-driven ultrafast hard x-ray sources based on K
_α
emission from solid targets are nowadays a powerful tool to monitor ultrafast lattice dynamics in condensed matter [1]. Such point-like sources are of great interest in biomedical imaging where spatial resolution and image quality can be highly improved [2]. Therefore, the development of high-flux K
_a
sources with a small size remains important for the ultrafast x-ray community. Especially, with the routinally obtained now femtosecond laser performances (intensities I > 10
¹⁸
W/cm
²
with high contrast pulses >10
⁸
), it is possible to explore interaction regimes where K
_α
emission can be strongly enhanced.In this work, we investigate extensively the influence of two key laser pulse parameters on the Kα emission : the peak intensity and its temporal contrast ratio which is defined as the intensity ratio of the main femtosecond pulse to the ASE pedestal. The experiments are performed in ASUR facility [3], a multi-beam laser platform available at LP3 laboratory. This facility delivers 30 fs, 800 nm laser pulses with a controlled high contrast ratio (CR) from 1.7 × 107 to 3.3 × 109 and peak intensity that ranges from 1016 to 1019 W/cm2 [3]. Our results illustrated in Fig.1 (b) show several original experimentally observed features of the interaction. For instance, in the relativistic regime (I > 2 × 1018 W/cm2), Kα production is almost independent of the contrast ratio. Besides, Kα emission exhibits different scaling laws for each contrast ratio and intensity range. Finally, the total flux of Kα photons achieved in our experiments is as high as 2 × 1010 photon/sr/s (at 10 Hz) at I ~1019 W/cm2 and the Kα conversion efficiency reaches 2 × 10-4 into 2π sr which corresponds to the highest performance reported to date in litterature for a thick molybdenum solid target [4].