Developing EXAFS platform on NIF for measuring bulk temperature of compressed materials*

Large laser facilities have recently enabled compression of materials above TPa pressures. However, measuring the temperature of the compressed materials remains a challenging problem. We are developing a platform on NIF using EXAFS (Extended X-ray Absorption Fine Structure) to probe the bulk temperature of highly compressed materials. EXAFS 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 (Krygier, et al. APL 2020), a high-resolution spectrometer (Stoupin et al. RSI 2021), and a novel shape of crystal to achieve both high throughput and high resolution (Pablant et al. RSI 2021).

Excellent EXAFS data at K-edge have been obtained for Fe compressed up to 8 Mbar and Cu up to 10 Mbar. It is found unexpectedly that the copper temperature is much higher than predicted when adjacent to diamond layer(s), showing the important influence of the sample environment on the thermal state of materials (Sio et al. submitted). 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 at 2-4Mbar. The prospect of such a new capability at NIF for probing thermal states of compressed materials will be discussed.