Observation of anomalous heating of compressed materials by adjacent diamond layers using EXAFS measurements

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