Non-Maxwellian electron distributions resulting from direct laser acceleration in near-critical plasmas

The irradiation of few-nm-thick targets by a finite-contrast high-intensity short-pulse laser results in a strong pre-expansion of these targets at the arrival time of the main pulse.The targets decompress to near and lower than critical densities with plasmas extending over few micrometers,i.e.multiple wavelengths.The interaction of the main pulse with such a highly localized but inhomogeneous target leads to the generation of a short channel and further self-focusing of the laser beam.Experiments at the Glass Hybrid OPCPA Scaled Test-bed(GHOST)laser system at University of Texas,Austin using such targets measured non-Maxwellian,peaked electron distribution with large bunch charge and high electron density in the laser propagation direction.These results are reproduced in 2D PIC simulations using the EPOCH code,identifying direct laser acceleration(DLA)[1]as the responsible mechanism.This is the first time that DLA has been observed to produce peaked spectra as opposed to broad,Maxwellian spectra observed in earlier experiments[2].This high-density electrons have potential applications as injector beams for a further wakefield acceleration stage as well as for pump-probe applications.

Extreme brightness laser-based neutron pulses as a pathway for investigating nucleosynthesis in the laboratory

With the much-anticipated multi-petawatt(PW)laser facilities that are coming online,neutron sources with extreme fluxes could soon be in reach.Such sources would rely on spallation by protons accelerated by the high-intensity lasers.These high neutron fluxes would make possible not only direct measurements of neutron capture andβ-decay rates related to the r-process of nucleosynthesis of heavy elements,but also such nuclear measurements in a hot plasma environment,which would be beneficial for s-process investigations in astrophysically relevant conditions.This could,in turn,finally allow possible reconciliation of the observed element abundances in stars and those derived from simulations,which at present show large discrepancies.Here,we review a possible pathway to reach unprecedented neutron fluxes using multi-PW lasers,as well as strategies to perform measurements to investigate the r-and s-processes of nucleosynthesis of heavy elements in cold matter,as well as in a hot plasma environment.

A tabletop, ultrashort pulse photoneutron source driven by electrons from laser wakefield acceleration

Relativistic electron beams driven by laser wakefield acceleration were utilized to produce ultrashort neutron sources.The experiment was carried out on the 38 fs,~0.5 J,800 nm Ti:Sapphire laser in the 10 TW UT 3 laser lab at University of Texas at Austin.The target gas was a high density pulsed gas jet composed of 90%He and 10%N 2.The laser pulse with a peak intensity of 1.5×10^(18) W/cm^(2) interacted with the target to create a cylindrical plasma channel of 60 mm radius(FWHM)and 1.5 mm length(FWHM).Electron beams of~80 pC with the Gaussian energy distribution centered at 37 MeV and a width of 30 MeV(FWHM)were produced via laser wakefield acceleration.Neutron fluences of~2.4×10^(6) per shot with hundreds of ps temporal length were generated through bremsstrahlung and subsequent photoneutron reactions in a 26.6 mm thick tungsten converter.Results were compared with those of simulations using EPOCH and GEANT4,showing agreement in electron spectrum,neutron fluence,neutron angular distribution and conversion rate.