Generation of bright γ-ray/hard x-ray flash with intense femtosecond pulses and double-layer targets

We study a method to generate collimated gamma-ray/hard X-ray flash with high energy conversion efficiency and high peak brilliance by shooting a double-layer target with a readily available ultra-intense laser pulse. The target is composed of a homogeneous tens-of-micrometer-thick slightly underdense plasma (SUP) slab in front of an ultrathin foil. For optimal targets, particle-in-cell numerical simulations reveal that non-linear 'Thomson scattering from directly accelerated superponderomotive electrons leads to the generation of a gamma-ray flash with a peak brilliance of 1.6 x 10(22) pholons/s/mm(2)/0.1%BW at the MeV energy level at a laser intensity of 8.5 x 10(2) W/cm(2). The dependence of the emission on the target parameters is systematically studied. Compared with using slightly overdense plasma, the use of SUP results in the enhancement of the photon number and brightness of the gamma-ray flash by a factor of 10 and 40, respectively. The underlying physics of the enhancement is discussed.