NIF Experiment Challenges Standard Ionization Models

When hydrogen fuel is compressed to extreme densities and temperatures in NIF inertial confinement fusion (ICF) implosions, a very dense plasma—a chaotic mixture of freely moving ions and unbound, or free, electrons—is created. Researchers need to understand the properties of this material in order to design successful ignition experiments, as well as to model the state and evolution of planets and dwarf stars.

A recent Discovery Science experiment on NIF, however, indicates that widely used models describing the ionization balance in a hot dense plasma—the number of bound compared to unbound electrons—underestimate the amount of ionization that occurs when material is subjected to the kinds of pressures and temperatures created by converging shocks driven by NIF’s laser beams.

The experiment, reported in a Physical Review E paper published online on July 21, used 176 laser beams to create pressures exceeding 100 megabars (100 million atmospheres) in a solid polystyrene (plastic, or CH) sphere. The sample ionization state was measured by NIF’s new x-ray Thomson scattering (XRTS) platform (see “Measuring Ionization at Extreme Densities”) using ~9-keV (about 9,000-electron-volt) x rays produced by a zinc backlighter foil located 7.5 millimeters from the center of the CH sphere and irradiated by an additional 16 laser beams. The experiment approached the conditions needed to dislodge the electrons closest to the nucleus, known as K-shell electrons, from their orbits.

“We can see from these results that the current ionization model is off at high pressures,” said LLNL physicist Tilo Döppner, a member of the experimental team. “The experiment shows higher ionization in CH than the common model would predict. This is important, because many thermodynamic quantities depend on the ionization state of the material.”

The findings are of special significance for ICF experiments, Döppner said, because knocking electrons from the innermost atomic shells reduces the absorption of the plastic ablator, and that can cause errors in radiographic measurements of the remaining mass of imploding ICF capsules.

The experiment was performed as part of the GigaBar Equation of State (EOS) campaign of the NIF Discovery Science program. Döppner said the development of the XRTS platform, including the construction of a new high-efficiency gated spectrometer, required “a lot of hard and dedicated work by a large number of people. X-ray Thomson scattering is a challenging diagnostic technique,” he said, “but this new NIF data point (for ionization balance measurements) has already triggered new theoretical work in dense plasma physics.”

Future experiments will drive hollow CH capsules to even higher compression, pressures approaching one gigabar (one billion atmospheres), and electron densities of 10²⁵ per cubic centimeter. “This is a very interesting plasma state because of its high degree of degeneracy and correlations,” Döppner said, “potentially with sufficient energy density to fully ionize carbon—and it’s only possible at NIF.”

Former UC Berkeley postdoc Dominik Kraus, who worked on the experiment at NIF, was lead author of the paper. He and Döppner were joined by LLNL researchers Annie Kritcher, Benjamin Bachmann, Rip Collins, Dan Kalantar, Nino Landen, Tammy Ma, Sebastien Le Pape, Joe Nilsen, and Damien Swift and by collaborators from the University of Warwick and the Atomic Weapons Establishment in the UK, SLAC National Accelerator Laboratory, Washington State University, and the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany.

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