Experimental study of fast electron generation in intense short duration laser solid interaction at grazing incidence

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Abstract
We report a study on the generation of relativistic fast electrons during the interaction of an ultrashort (45 fs) laser pulse with a variety of targets (Al, Cu, Mo, Si, polyvinyl chloride, and Perspex), at a nearly grazing angle of incidence (∼70°) at an intensity of ∼1.2 × 1018 W/cm2. The fast electron emission direction was found to be dependent on the laser pulse amplified spontaneous emission (ASE) contrast, its extent, and laser pulse duration. For a 45 fs laser pulse and an ASE extent of 1 ns, the electron emission direction changes from a nearly target surface (∼7° ± 2°, divergence: ∼30°) to a specular direction (∼20° ± 2°, divergence: ∼20°) when the contrast was decreased from ∼10−7 to ∼10−6. It further shifts toward target normal with an increase in the ASE duration from 1 ns to 5 ns. For an ASE contrast of ∼10−7 and ∼10−6 with a 1 ns duration, preplasma density scale lengths of ∼30 μm and ∼45 μm were estimated, respectively, from the HELIOS hydrodynamic code. For a longer ASE duration in the range of 1–5 ns, the density scale length was in the range of ∼45–140 μm. Further, for a 45 fs pulse and a 1 ns ASE level, the electron beam charge was found to be higher in the case of Mo (∼52 pC) and Si (∼122 pC) targets compared to the other targets (∼23–34 pC). A further increase in the charge of the electron beam of ∼1.45× (∼75 pC) and ∼2.7× (∼141 pC) was also observed with Mo for a longer pulse duration of ∼170 fs (ASE 1 ns) and a larger ASE duration of 5 ns at (45 fs pulse), respectively. The electron beam properties were found to be independent of laser polarization (p, s, and circular) except a slightly higher charge (∼1.5×) was observed in the case of p-polarized laser light. The fast electron spectrum showed a maximum energy of 1 MeV with electron temperatures of ∼390 keV and ∼320 keV for the target surface and specular direction, respectively. Electron acceleration and their emission are explained in terms of applicable mechanisms in preformed plasma on the target surface considering the role of surface fields which is also established by the 2D particle in cell simulation.

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