Abstract
Optimisation and reproducibility of beams of protons accelerated from laser-solid interactions require accurate control of a wide set of variables, concerning both the laser pulse and the target. Among the former ones, the chirp and temporal shape of the pulse reaching the experimental area may vary because of spectral phase modulations acquired along the laser system and beam transport. Here, we present an experimental study where we investigate the influence of the laser pulse chirp on proton acceleration from ultrathin flat foils (10 and 100 nm thickness), while minimising any asymmetry in the pulse temporal shape. The results show a pm 10% change in the maximum proton energy depending on the sign of the chirp. This effect is most noticeable from 10 nm-thick target foils, suggesting a chirp-dependent influence of relativistic transparency.
Highlights
Optimisation and reproducibility of beams of protons accelerated from laser-solid interactions require accurate control of a wide set of variables, concerning both the laser pulse and the target
To ensure that the intense laser pulse interacts with a steep, overdense plasma, we employed a double plasma mirror (DPM) to reduce the intensity of the Amplified Spontaneous Emission (ASE) pedestal below the ionisation threshold of the target
We focused on the effect of the laser chirp on proton acceleration, by removing higher order terms in the pulse spectral phase originated in the pulse compressor
Summary
Optimisation and reproducibility of beams of protons accelerated from laser-solid interactions require accurate control of a wide set of variables, concerning both the laser pulse and the target. The presence of a controlled amount of preplasma (with density rapidly decreasing below the critical value) at the target surface results in more efficient electron heating, as the laser pulse interacts with a larger volume of plasma electrons and resonant or stochastic absorption processes can take place[19,20]. Such a pre-plasma can be produced by irradiating the target with either a tailored pre-pulse[15,16,17,18] or by the ns-long pedestal produced by Amplified Spontaneous Emission (ASE) in chiped-pulse-amplification laser s ystems[13,14]. A recent experiment in the PW regime has reported a 40% enhancement of the maximum proton energy for an unchirped, slow rising pulse, further increased to a twofold enhancement if combined with a negative chirp[24]
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