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The interaction of ultraintense laser pulses with solids is largely affected by the plasma gradient at the vacuum–solid interface, which modifies the absorption and ultimately, controls the energy distribution function of heated electrons. A micrometer scale-length plasma has been predicted to yield a significant enhancement of the energy and weight of the fast electron population and to play a major role in laser-driven proton acceleration with thin foils. We report on recent experimental results on proton acceleration from laser interaction with foil targets at ultra-relativistic intensities. We show a threefold increase of the proton cut-off energy when a micrometer scale-length pre-plasma is introduced by irradiation with a low energy femtosecond pre-pulse. Our realistic numerical simulations agree with the observed gain of the proton cut-off energy and confirm the role of stochastic heating of fast electrons in the enhancement of the accelerating sheath field.

We report measurements of parametric instabilities and hot electron generation in a laser intensity regime up to 6 x 10(15) W/cm(2), typical of the shock ignition approach to inertial fusion. Experiments performed at the PALS laboratory in Prague show that the incident laser energy losses are dominated by Stimulated Brillouin Scattering (SBS) rather than by Stimulated Raman Scattering (SRS) or Two-Plasmon Decay (TPD). Results are compared to hydrodynamics simulations using a code that includes self-consistent calculations of non-linear laser plasma interactions and accounts for the laser intensity statistics contained in the beam speckles. Good agreement is found for the backscattered SRS light, and for temperature and flux of hot electrons. The effect of high-intensity speckles on backscattered SRS is also underlined numerically and experimentally. Copyright (C) EPLA, 2017