Effect of temporally modified ultra-short laser pulses on ion acceleration from thin foil targets

Abstract

We demonstrate the significant enhancement of ion energy from the interaction of intense, chirped pulse amplification based ultrashort laser pulse with thin foil targets by adjusting the laser temporal characteristics with a simple adjustment of grating pair separation inside the pulse compressor. The laser pulse was focused to a maximum intensity of 3.2 × 1019 W/cm2 on to thin metallic foil targets of submicron thickness. The pulse duration was varied from 25 to 500 fs by adjusting the laser pulse compressor grating separation for fixed laser pulse energy (2.1 J). Contrary to the conventional belief, we found that the ion acceleration is not optimum at the shortest laser pulse duration and, thus, at highest intensity as predicted by the previous intensity scaling data. Instead, stretched pulses are better suited for efficient ion acceleration. In addition to this, we found a contrasting effect with respect to the sign of the laser chirp for almost the same laser pulse duration. In particular, we show 70% enhancement in maximum ion energy (from 6 MeV to 10 MeV) with positively chirped 250–350 fs stretched pulses. On the contrary, negatively chirped pulses show gradual monotonic reduction in ion energy and flux. Independent electron energy spectra measurement along the laser forward direction exhibits a strong correlation with proton data. The temporal pulse skewness arising due to displaced grating separation is believed to be responsible for this asymmetric proton acceleration behaviour. Efficient absorption of positively modified skewed pulses (shallow rise time and sharp fall) along with the characteristic ion acceleration time can qualitatively explain the experimental result. The present study can be very much important for efficient ion acceleration based on modern day compact, ultra-short pulse 100 TW to PW class lasers.