Characterisation, enhancement and impact of temporal intensity contrast in multi-terawatt laser systems

Abstract

Significant features in a laser pulse at focused intensities above 10^12 Wcm−2, such as discrete pulses arriving before the main peak of the laser or unwanted emission from laser amplifiers, can pre-ionise a target before the arrival of the main laser pulse. This produces a ‘preplasma’ which expands from the front of the target and which may then significantly alter the subsequent laser-matter interaction and even supress ion acceleration. In this work, we develop an extended third order autocorrelator diagnostic for ‘artefact-free’ pulse measurement of the sources of the ‘low-contrast’ features in the temporal profile of the Cerberus laser system at Imperial College London, primarily arising from scattered light in chirped pulse amplification subsystems and optical parametric amplifiers. These measurements informed a contrast enhancement scheme using cylindrical dielectric hollow core fibre is shown to be a capable spatial filter at high energies (∼180 µJ, 1.2 ps) which also demonstrates high transmission efficiencies, >75%, approaching that of conventional spatial filters, and demonstrated to be highly effective in removing astigmatism and spatial chirp. We investigated reintroducing a discrete prepulse, measuring the effect this has on the emission of charged particles from thin metallic foils irradiated at relativistic intensities of order 10^19 Wcm−2. In particular, we have observed a factor ∼5 times enhancement in the maximum of proton energies by the addition of a 2.75× 10−4 relative contrast prepulse, 60 ps before the main pulse, with preliminary results providing evidence of a self-focusing enhancement mechanism. Finally, we also describe an off-harmonic optical probe based on a supercontinuum source which can better characterise the dynamics of these measurements close to the target surface at wavelengths not obscured by bright self-emission at the laser-plasma interaction region. A supercontinuum optical probe, with ∼600 nm spectral width, generated from an aqueous zinc chloride salt solution.