Ultrafast melting of Warm Dense Cu studied by x-ray spectroscopy

We present novel experimental results of ultra fast heating of Warm Dense Cu diagnosed by means of x-ray absorption and emission spectroscopy carried out at the Draco laser facility at HZDR in 2021. A thin Cu foil was directly heated to few eV temperature by an ultra short laser pulse (40 fs, 2e15 W/cm2) and probed with variable delay in the range 0.2-20 ps by a laser-driven betatron radiation. This betatron radiation, created by a laser wakefield accelerator, is an unique x-ray source with its ultra short duration and broadband spectrum, therefore ideally suited for studies of non-equilibrium dense plasmas while its high brightness allows for single-shot measurement. The sample is studied via the X-ray absorption spectroscopy in the region above the Cu K-edge. This method provides temporally-resolved information about both the ionic structure of the matter and its temperature during the process of ultrafast heating and melting of the material. The measured spectra are understood and analyzed by using Ab initio simulations and the temporal evoution of heatig and melting is compared to PIC simulations to infer the electron to ion energy transer.