CC BY
20LMI-O-3
Effect of the ablation plume pre-formed by ASE on the characteristics of intense femtosecond laser-plasma interactions
V. Tcheremiskine1,2, O. Ranjbar3, A. Volkov3
1Aix-Marseille Univeersity - CNRS, LP Laboratory, Marseille, France
2Lebedev Physics Institute, Photochemical Processes Laboratory, Moscow, Russian Federation 3University of Alabama, Department of Mechanical Engineering, Tuscaloosa, USA
Experimental data collected in recent studies of the Hard X-ray emission produced by intense (1016-1019 W/cm2) femtosecond (30 fs) Ti-Sa laser pulses interacting with a bulk molybdenum target [1,2] are re-examined applying an original method of the multi-pixel treatment of X-ray photon absorption events registered by CCD camera. Absolute energetic spectra of generated Hard X-ray pulses are reconstructed, and dependencies of the energetic conversion efficiency of laser radiation into Mo Ka line emission at 17.4 keV on the laser peak intensity are obtained for various values of the laser pulse temporal contrast (106, 108, and 109) with respect to the intensity of amplified spontaneous emission (ASE) pedestal. The latter exhibits ~2 ns duration and precedes the main femtosecond laser pulse, which is P-polarized and hits the target at 45°. The obtained dependencies allow estimating the fraction of laser energy, which is transferred to hot electrons accelerated in the intense laser field to energies of the order of laser ponderomotive potential. These dependencies are strongly influenced by the target ablation. They provide important information on the character and physical conditions of laser-plasma interaction.
The derived conclusions are supported by results of numerical simulations of the target ablation by the ASE, which are based on a semi-kinetic model of the plasma plume formation, where the absorption of incident radiation occurs due to inverse bremsstrahlung. Our simulations reproduce well the ablation threshold of molybdenum at the ASE fluence reaching ~2 J/cm2, which corresponds to the ASE pedestal intensity of ~1*109 W/cm2. They also show a kind of self-regulated shielding by the ablation plume of the target surface with respect to the incoming ASE due to the collisional absorption of incident radiation in the plume at fluences exceeding the ablation threshold. It results in relatively similar shapes of density profiles for heavy particles (atoms and ions) and for electrons, which are pre-formed near the target surface at fixed laser pulse intensity but at different pulse contrast and correspond to a time instant, when the rising edge (with characteristic duration <10 ps) of the femtosecond laser pulse reaches the target. This is seen in the experiment by close values of the laser peak intensity of ~6*1017 W/cm2 observed at different pulse contrast for the onset of steepening of the electron density profile under influence of the laser radiation pressure. This effect occurs at the surface of critical electron density, where the dominant fraction of laser radiation is absorbed and reflected. Similar shapes of the pre-formed density profiles for electrons and for heavy particles lead to the similar shapes of the electron density profiles resulted due to the ionisation of heavy particles by intense optical field of the femtosecond laser pulse.
References
[1] V. Tcheremiskine and Y. Azamoum, arxiv preprint 1710.07125 (2017). http://arxiv.org/abs/1710.07125
[2] Y. Azamoum, PhD thesis, Aix-Marseille University (2016). http://www.theses.fr/en/s161735
