ИСТИНА

We report our experimental and numerical results on the generation of the supra-hot electrons with temperature up to a few MeV at the interaction of the high intensity (1018 W/cm2 and above) femtosecond laser pulse with long scale pre-plasma onto the surface of solid targets. In our experiments we used a laser pulse delivered by the Ti:Sa laser system (pulse duration – 40 fs, energy on target – up to 50 mJ, central wavelength – 805 nm, repetition rate – 10 Hz, peak intensity – up to 5x1018 W/cm2). The pre-plasma with variable parameters was formed by introducing an additional pulse from Nd:YAG laser with 6 ns duration and peak intensity around 1012 W/cm2 passing ahead of the main pulse at a delay of 0-20 nanoseconds. Using 3D3V PIC code simulations of laser-plasma interaction we demonstrated the regime of hot particles generation in the undercritical extended pre-plasma, which includes the acceleration of electrons in the quasistatic electric field, formed by the femtosecond pulse, and consequent collapse of the latter, resulting in acceleration of particles to high momentum. Experimentally we achieved the manyfold growth of the hot electron temperature (from 200 to 1600 keV), when the sub-relativistic (several units of 1018 W/cm2) laser pulse is incident onto the extended (up to 100 microns) undercritical pre-plasma, formed by the nanosecond laser pulse onto the surface of solid targets. At the same time the effect is very sensitive to the time delay between the two laser pulses. The maximum hot electron temperature, as well as hard X-ray and gamma emission, is achieved, when the nanosecond pre-pulse is around 0-2 ns ahead of femtosecond pulse. This indicates, that the mechanism of fast particles generation is dependent on the pre-plasma parameters (extent and density).