ИСТИНА

Our work is devoted to the investigation of hot electrons acceleration in the spatially inhomogeneous plasma with undercritical density under the action of femtosecond laser radiation with peak intensity exceeding 1018 W/cm2. The plasma corona, extended into vacuum up to a few hundreds of microns, was formed onto the surface of solid metal targets under the irradiation of laser pulse with the duration of several nanoseconds, focused onto the surface of matter to the peak intensity around 1012 W/cm2. Varying the time delay between the nanosecond and the femtosecond pulses from ~0 to ~30 ns the latter could interact with plasma corona with varied spatial extent and different density. The experimental diagnostics based on the hard X-ray spectra measurements revealed the two maxima of hard X-ray emission and hot electron temperatures at delays of ~3 and ~15 ns. In these cases the electron temperature raised several times (from hundreds of keV into the MeV range), compared to the interaction with unaffected by ns pulse target surface. The optical shadowgraphy of plasma cloud, formed by the ns pulse showed, that at a delay of few ns the corona is extended up to ~100 microns with clearly observed critical region, whereas at higher delay (>10 ns) only the undercritical long scalelength (>200 microns) plasma is present. The 3D PIC modeling of relativistic laser interaction with long scalelength plasma with varied density profile showed the complex interplay of nonlinear effects of self-focusing, plasma wave formation due to parametric processes in the undercritical (ne~1/4ncr) region and laser pulse collapse. The fast growth of the plasma wave and its subsequent breaking result in formation of high amplitude electric fields, accelerating the particles to the energy of several MeV. For the case, when the plasma density is much lower (corresponding to the delay of ~10 ns and higher) the hot particle generation may be attributed to the wakefield effect.