We present recent experiments on the development and use of betatron x-ray radiation for high energy density science experiments at the Linac Coherent Light Source (LCLS). Betatron x-rays were used to probe the relaxation of warm dense matter (created with intense optical or x-ray radiation) back to equilibrium. In these conditions, we performed x-ray absorption near edge structure measurements of iron and silica.
High Energy Density (HED) science laser and free electron laser facilities such as the Linac Coherent Light Source (LCLS) are now uniquely able to recreate in the laboratory conditions of temperature and pressure that were thought to be only attainable in the interiors of stars and planets. To diagnose such transient and extreme states of matter, the development of efficient, versatile and fast (sub-picosecond scale) x-ray probes has become essential for HED science experiments. Betatron x-ray radiation, a source driven by laser-wakefield accelerated electrons, holds great promise in this field of research. We present recent pump-probe experiments performed at the LCLS. At LCLS, we have recently commissioned the betatron x-ray source driven by the MEC short pulse laser (1 J, 40 fs). The source is used as a probe by investigating the X-ray absorption near edge structure (XANES) spectrum at the K- or L-edge of several materials (iron and silicon oxide) driven to a warm dense matter state (temperature of a few eV, solid densities). The driver is either LCLS itself or optical lasers. With these experiments we are able to study, with subpicosecond resolution, the electron-ion equilibration mechanisms in warm dense matter.
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