Bright tunable femtosecond x-ray emission from laser irradiated micro-droplets

It is demonstrated that bright femtosecond X-rays can be obtained by irradiating a moderate laser onto a helium micro-droplet. The laser ponderomotive force continuously sweeps electrons from the droplets and accelerates them forward. The electrons exposed in the outrunning laser field oscillate transversely and emit photons in the forward direction. The total flux of photons with energies above 1 keV is as high as 109/shot which is about 10-fold enhancement compared with betatron oscillation under similar laser conditions. The maximum achieved peak brightness is up to 1021 photons/s/mm2/mrad2/0.1%BW. By adjusting laser and droplet parameters, we can get tunable X-rays with required brightness and energy.