EXAFS for probing thermal states of compressed materials at NIF

EXAFS (Extended X-ray Absorption Fine Structure) refers to the oscillatory modulations in x-ray absorption spectra above an absorption edge, generated by interference between photoelectron waves and scattering by neighbor atoms. EXAFS is sensitive to temperature of materials because ionic thermal motion reduces the coherence of the interference, leading to decay of the modulations. This talk will give a brief overview of the EXAFS platform at NIF, including development of a bright continuum backlighter [1], a high-resolution spectrometer [2], and a novel shape of crystal to achieve both high throughput and high resolution [3]. Excellent EXAFS data at K-edge have been obtained for Fe compressed up to 8 Mbar and Cu up to 10 Mbar. The EXAFS measurements at L-edge are more challenging due to smaller EXAFS amplitude. The high x-ray flux at NIF has enabled L-edge measurements for higher-Z materials such as Ta and Pb. The prospect of such a new capability at NIF for probing thermal states of compressed materials will be discussed.