Their observations reveal how liquid hydrogen becomes a plasma and provide information on the material's thermal conductivity and its internal energy exchange, which are important ingredients for planetary models. The scientists present their experiments in the March 12 online edition ofPhysical Review Letters.
The atmosphere of gas giants mainly consists of hydrogen, which is the most abundant chemical element in the universe.
"Yet some of its properties at such extreme conditions remain uncertain despite our very good theoretical models," says Tilo Doeppner, a Lawrence Livermore physicist and co-author on the paper. "It is very challenging to perform quantitative experiments on dense hydrogen at conditions that approach those found in the interior of planets."
The researchers decided to use liquid hydrogen as a sample of the planetary atmosphere since it has a similar mass density as the lower atmosphere of giant gas planets. The scientists used DESY's X-ray laser FLASH to heat the liquid hydrogen almost instantaneously, from minus 253 to around 12,000 degrees Celsius and simultaneously observed the characteristics of the heating process through the increase of the X-ray scattering signal.
Hydrogen is the simplest atom of the periodic table, consisting of a single proton in the atomic nucleus, which is orbited by a single electron. Normally, hydrogen occurs as a molecule consisting of two atoms. The X-ray laser pulse initially heats only the electrons. These slowly transfer their energy to the protons, which are around 2,000 times heavier, until a thermal equilibrium is reached. The molecular hydrogen bonds break during this process, and a plasma of electrons and protons is formed. Although this process takes many thousands of collisions between electrons and protons, the studies showed that the thermal equilibrium is attained in just under a trillionth of a second (a picosecond).
"The equilibration time is directly related to the thermal and electrical conductivities which are crucial parameters to correctly simulate the massive, outward-directed heat flows in giant gas planets and subsequently their radial temperature profiles,"Doeppnerexplains.
An accurate knowledge of the conductivity of hydrogenalsois crucial for correctly modeling the ablator-fuel interface in inertial confinement fusion capsule implosions conducted at LLNL's National Ignition Facility (NIF). Future experiments are currently under development at the Materials at Extreme Conditions (MEC) endstation at SLAC National Accelerator Laboratory, which will address such measurements at NIF-relevant conditions.
"Even though the experiments at FLASH in Hamburg were done at lower hydrogen densities than encountered in NIF implosions, they were an important stepping stone, since they showed us how to advance our understanding of high energy-density plasmas with Free Electron X-ray lasers," Doeppner said.
In addition to current and former Livermore researchers Doeppner, Siegfried Glenzer (now at SLAC), Tammy Ma and Paul Neumayer (now at GSI Darmstadt), this work was carried out in collaboration with scientists from SLAC, UC Berkeley, DESY and European XFEL GmbH, universities of Jena, Rostock and Muenster and Oxford as well as the GSI Darmstadt.