Acceleration of multiply charged ions by a high-contrast femtosecond laser pulse of relativistic intensity with the front surface of a solid target

It is shown that the acceleration efficiency of protons and multiply charged ions (and also the charge composition of the latter) accelerated backwards under irradiation of the front surface of thick solid targets by high-power femtosecond laser radiation with an intensity of 2 × 10¹⁸ W cm^-2 is determined by the contrast of this radiation. Thus, highly ionised ions up to C⁶⁺, Si¹²⁺ and Mo¹⁴⁺ are recorded on polyethylene, silicon and molybdenum targets at a contrast of 10^-8, the ions with charges up to C⁵⁺, Si¹⁰⁺ and Mo¹⁰⁺ possessing an energy of more than 100 keV per unit charge. In the case of a metal target, the acceleration efficiency of protons is significantly reduced, which indicates cleaning of the target surface by a pre-pulse. The measurements performed at a contrast increased by two-to-three orders of magnitude show the presence of fast protons (up to 300–700 keV) on all targets, and also a decrease in the energy and maximum charge of multiply charged ions.