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Impact of the pulse contrast ratio on molybdenum Kα generation by ultrahigh intensity femtosecond laser solid interaction

Abstract : We present an extended experimental study of the absolute yield of K α x-ray source (17.48 keV) produced by interaction of an ultrahigh intensity femtosecond laser with solid Mo target for temporal contrast ratios in the range of 1.7 × 10 7-3.3 × 10 9 and on three decades of intensity 10 16-10 19 W/cm². We demonstrate that for intensity I ≥ 2 × 10 18 W/cm² K α x-ray emission is independent of the value of contrast ratio. In addition, no saturation of the K α photon number is measured and a value of ~2 × 10 10 photons/sr/s is obtained at 10 Hz and I ~10 19 W/cm². Furthermore, K α energy conversion efficiency reaches the same high plateau equal to ~2 × 10 −4 at I = 10 19 W/cm² for all the studied contrast ratios. This original result suggests that relativistic J × B heating becomes dominant in these operating conditions which is supposed to be insensitive to the electron density gradient scale length L/λ. Finally, an additional experimental study performed by changing the angle of incidence of the laser beam onto the solid target highlights a clear signature of the interplay between collisionless absorption mechanisms depending on the contrast ratio and intensity. Optimization of ultrafast x-ray emission from plasmas produced by femtosecond high intensity laser-solid interaction is today under strong investigation by many groups around the world 1,2. The interest towards developing intense and compact ultrashort x-ray sources is motivated by important scientific applications like time-resolved x-ray diffraction 3,4 or x-ray absorption spectroscopy for advanced high-resolution diagnostics of materials driven to extreme thermodynamic conditions 5. Furthermore, these sources provide powerful tools in societal applications such as phase contrast imaging for developing highly resolved diagnostics for biology 6 and medicine 7. Laser-driven ultrashort hard x-ray pulses based on K α radiation from solid targets are studied since decades 8,9. Nevertheless, the constant improvement of femtosecond laser performances 10 enables to explore a wide range of regimes of laser interaction where K α emission can be strongly enhanced. In fact, laser pulses of intensities >10 18 W/cm² and pulse duration <30 fs with controlled high contrast ratios (>10 8) are today routinely obtained in laboratory using a single laser source with moderate peak power (~10 TW). K α x-ray emission is induced by the produced hot electrons due to the laser energy absorption by the plasma formed during the interaction. Therefore, K α spectral lines radiation is governed by the energy distribution of the hot electron population accelerated by the laser electric field and/or the laser-induced plasma field. Key parameters to generate efficient hot electrons for K α line production are mainly the laser electric field (i. e. intensity) and the fraction of the pulse energy converted into hot electrons. This fraction depends on the laser energy coupling to the plasma. The latter is driven by several collisionless absorption mechanisms which relative importance is controlled by the laser pulse parameters such as intensity, duration, temporal contrast ratio, angle of incidence, polarization, etc. 9,11-15. For an obliquely incident p-polarized pulse, the laser energy absorption also relies on the electron density gradient scale length L of the plasma formed on the target before the main pulse. L is defined as =       − () L 1 n dn dx 1 e e , n e being the electron density and x the axis in the direction normal to the target surface. The parameter L is strongly dependent on the temporal contrast ratio CR, defined by = CR I /I peak background. CR is the ratio between the peak pulse intensity I peak and the intensity of the background I background present before the main pulse. The background consists of nanosecond amplified spontaneous emission (ASE) pedestal or any pre-pulses.
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Y. Azamoum, V. Tcheremiskine, R. Clady, A. Ferré, L. Charmasson, et al.. Impact of the pulse contrast ratio on molybdenum Kα generation by ultrahigh intensity femtosecond laser solid interaction. Scientific Reports, Nature Publishing Group, 2018, 8 (1), ⟨10.1038/s41598-018-22487-3⟩. ⟨hal-02137909⟩

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