Generation of energetic protons from thin foil targets irradiated by a high-intensity ultrashort laser pulse

The results of investigations of fast proton generation from thin foil targets irradiated by 1-ps laser pulses of intensities up to 1.5×1017 W/cm2 are reported. The characteristics of forward-emitted proton beams produced from both single- and double-layer targets have been determined by the time-of-flight method for various thicknesses, atomic numbers (Z) and structure of the target. It is found that using a double-layer target, containing high-Z front layer and low-Z hydrogen-rich back layer, allows to obtain significantly higher energies and a current of protons as well as shorter proton pulse duration than in the case of a commonly used single-layer target. Both the maximum and the mean proton energies as well as the proton current are correlated with the yield of hard X-rays emitted from the target and they increase with the increase in the Z number of the front layer. For maximizing the energies and/or the current of protons and for minimizing the proton pulse duration both total target thickness and high-Z layer thickness must be carefully selected regarding particularly the hot electron range in the target and a possible overheating of the back target surface by the electron heat wave generated by the prepulse and the leading edge of a laser pulse. If the target thickness is smaller than the characteristic path length of the heat wave, the proton energies can be a decreasing function of laser energy. The angular divergence of a proton beam emitted from a properly prepared double-layer target are rather low (⩽30°) which results in the high proton current density in a far expansion zone (∼0.4 mA/cm2 at 1 m from Au/polystyrene target) in spite of relatively low energy (<0.5 J) and intensity of a laser pulse.