ИСТИНА

The restraint of the level of the contrast below the ionization/ablation threshold is complicated task, especially for a high peak power laser. As a result the action of the pre-pulse onto the target leads to the formation of the plasma corona with varied extent, which changes the interaction physics. In this work we studied the acceleration of hot electrons with four different and typical temporal contrasts: (i) strong, above the ablation threshold, nanosecond ASE accompanied by a few strong femtosecond pulses ahead of the main pulse; (ii) plasma creating femtosecond pre-pulse positioned 12 ps ahead of the main pulse with the ASE below the ablation threshold; (iii) the same as previous with additional strong femtosecond pre-pulse 12 ns ahead of the main pulse, and (iv) artificial 6 ns prepulse with controlled advancing time, intensity, etc. This enabled us to create pre-plasma with different spatial extent and density. We also present data obtained with two laser pulse durations (45 and 350 fs). It was demonstrated, that fast electrons with energies well beyond the ponderomotive potential appeared in the presence of a long, slightly underdense pre-plasma, formed by a pre-pulse. Energy of accelerated electrons undergone further growth, if the laser pulse duration increased from ~45 to ~350 fs at the constant energy fluence. The experimental results are supported by the numerical 2D3V PIC simulations.