Bright betatron radiation from direct-laseraccelerated electrons at moderate relativistic laser intensity

Direct laser acceleration(DLA)of electrons in a plasma of near-critical electron density(NCD)and the associated synchrotron-like radiation are discussed for moderate relativistic laser intensity(normalized laser amplitude a0≤4.3)and ps length pulse.This regime is typical of kJ PW-class laser facilities designed for high-energy-density(HED)research.In experiments at the PHELIX facility,it has been demonstrated that interaction of a 1019 W/cm2 sub-ps laser pulse with a sub-mm length NCD plasma results in the generation of high-current well-directed superponderomotive electrons with an effective temperature ten times higher than the ponderomotive potential[Rosmej et al.,Plasma Phys.Controlled Fusion 62,115024(2020)].Three-dimensional particle-in-cell simulations provide good agreement with the measured electron energy distribution and are used in the current work to study synchrotron radiation from the DLA-accelerated electrons.The resulting x-ray spectrum with a critical energy of 5 keV reveals an ultrahigh photon number of 731011 in the 1–30 keV photon energy range at the focused laser energy of 20 J.Numerical simulations of betatron x-ray phase contrast imaging based on the DLA process for the parameters of a PHELIX laser are presented.The results are of interest for applications in HED experiments,which require a ps x-ray pulse and a high photon flux.