Study sheds light on elusive warm dense matter

In the last few decades, scientists have been harvesting enormous amounts of data using synchrotron light sources to probe matter at the atomic scale. Now, even brighter light sources have come online — X-ray free-electron lasers. The Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory is one such laser, providing physicists, chemists and biologists with X-ray pulses short enough and bright enough to capture ultrafast snapshots at the atomic scale that can be assembled as "movies" to reveal the time dependence of a variety of phenomena.

In a paper recently published in Nature Photonics, a group of researchers, including scientists from Lawrence Livermore National Laboratory (LLNL), used the LCLS to document the quick transformation of shock-compressed aluminum into warm dense matter.

Warm dense matter is a complex state found at pressures of a few million atmospheres and temperatures of tens of thousands of degrees. Materials exposed to these high pressures play important roles in the physics of planetary formation, material science and inertial confinement fusion research.

"The heating and compression of warm dense matter has never been measured before in a laboratory with such precise timing," said Siegfried Glenzer, distinguished staff scientist at SLAC and former plasma physics group leader at LLNL. "We have shown the detailed steps of how a solid hit by powerful lasers becomes a compressed solid and a dense plasma at the same time. This is a step on the path toward creating fusion in the lab."

The new pioneering experiment used a tabletop nanosecond-pulsed laser to shock-compress a thin aluminum foil, forcing it to a pressure more than 4,500 times higher than the deepest ocean depths and superheating it to nearly four times hotter than the surface of the sun. The team used ultrabright X-ray pulses of LCLS to illuminate the warm dense states in situ and obtained snapshots of the atomic structure with a variety of X-ray scattering diagnostics, yielding a wealth of information.

"The new data provide important benchmarks for future theoretical and numerical descriptions of warm dense matter," said Marius Millot, LLNL physicist and paper co-author. "In addition, these experiments demonstrate that coupling X-ray free-electron lasers with dynamic compression is a new, powerful way to probe matter at extreme conditions, complementing studies with large lasers like Omega or the National Ignition Facility."

In addition to Millot, other LLNL contributors include Tilo Doppner, Sebastien Le Pape, Tammy Ma, Arthur Pak and David Turnbull.