Extreme heating with an x-ray free-electron laser

The border between condensed-matter physics and plasma physics, illustrated by the phase diagram below, is home to a little-understood state called warm dense matter (WDM), in which thermal and Coulomb energies become comparable. (See the article by Paul Drake, Physics Today, June 2010, page 28 .) Such matter is thought to inhabit the inner cores of giant planets and is formed during the compression stage of inertial confinement fusion experiments. To study WDM, researchers want to controllably create the stuff with uniform temperature and density in the lab. But of the usual heating techniques, optical lasers lack sufficient penetration depth, ion beam pulses last too long, and laser-induced shocks are limited to a narrow slice through temperature-density space. Now, Anna Lévy of Marie and Pierre Curie University, Patrick Audebert of École Polytechnique, and their collaborators have shown that x-ray free-electron lasers (XFELs) are just the tool for the job. At SLAC’s Linac Coherent Light Source, the group used a focused XFEL pulse to heat a 0.5-μm-thick silver foil to temperatures greater than 100 000 K. Then they simultaneously probed the foil’s front and back surfaces with IR pulses. The results showed that temperature and density profiles were nearly uniform throughout the foil. Next on the agenda is to investigate other WDM properties such as ionic structure and electrical and thermal conductivities. (A. Lévy et al., Phys. Plasmas 22, 030703, 2015, doi:10.1063/1.4916103 ; figure adapted from Basic Research Needs for High Energy Density Laboratory Physics, US Department of Energy, Office of Science and National Nuclear Security Administration, 2010.)