The characterization and control of relativistic electron beams emitted from laser-solid interactions are important in the context of laser fusion and time-resolved electron diagnostics. We study the spatial and energy distributions of these electrons in an unusual regime where millijoule pulses, available at 0.5 kHz repetition rate, are focused to relativistic intensities (> 2 x1018W/cm2 for λ = 800 nm). For an Al plasma, the electron temperature is found to scale like Thl[keV] ≈120 ([Iλ2(1018W/cm2*µm2)]) in the 1017 – 2×1018 W/cm2 range (Fig. top right). This is in good agreement with 3-D PIC simulations and with a simple energy flux model that also describe previously obtained scalings in the strongly relativistic regime. In a separate experiment, the energy distribution from SiO2 target is measured for different plasma scale-length. At the optimum scale-length of ~0.5λ, the electrons are emitted in a collimated jet with a non-Maxwellian spectrum peaking around 0.8 MeV (Fig. below). This is the first time monoenergetic beams of relativistic electrons are observed from solid-density interactions at kilohertz repetition rates.
