Data on Laser-Plasma Instabilities Could Improve Fusion Experiments

New research from the University of Rochester could lead to more accurate computer models for simulations of laser-driven implosions. Researchers at the the university’s Laboratory for Laser Energetics (LLE), along with their colleagues at Lawrence Livermore National Laboratory and the Centre National de la Recherche Scientifique, have demonstrated how laser beams modify the conditions of the underlying plasma as they move through it, affecting the transfer of energy in fusion experiments.

In laser-driven inertial confinement fusion (ICF) experiments, short beams consisting of intense light pulses are used to heat and compress hydrogen fuel cells. Ideally, this process results in a greater release of energy than the amount of energy used to heat the cells.

Laser-driven ICF experiments require that many laser beams propagate through a plasma of free-moving electrons and ions to deposit their radiation energy precisely at their intended target. As the beams move through the plasma, they interact with it in ways that can complicate the intended result.

To accurately model the laser-plasma interaction, scientists need to know exactly how the energy from the laser beam interacts with the plasma. While researchers have offered theories about the ways in which laser beams alter a plasma, no theory has been demonstrated experimentally.

The Rochester team conducted experiments to make highly detailed measurements of the laser-heated plasmas. The results showed that the distribution of electron energies in a plasma was affected by the interaction of the energies with the laser radiation and could no longer be accurately described by prevailing models.

Researchers used the Omega Laser Facility at the University of Rochester’s Laboratory for Laser Energetics to make highly detailed measurements of laser-heated plasmas. Courtesy of J. Adam Fenster/University of Rochester.

The new research shows that laser-plasma interaction strongly modifies the transfer of energy. “New inline models that better account for the underlying plasma conditions are currently under development, which should improve the predictive capability of integrated implosion simulations,” LLE scientist David Turnbull said.

For the past decade, researchers have used computer models that have generally assumed that the energy from the laser beams interacts in a type of equilibrium known as Maxwellian distribution — an equilibrium one would expect in the exchange when no lasers are present.

Scientists predicted almost 40 years ago that lasers alter the underlying plasma conditions. In 1980, a theory was presented that predicted these non-Maxwellian distribution functions in laser plasmas due to the preferential heating of slow electrons by the laser beams. In subsequent years, scientists predicted that the effect of these non-Maxwellian electron distribution functions would change how laser energy is transferred between beams. But lacking experimental evidence to verify that prediction, researchers did not account for it in their simulations.

A model that accounts for the non-Maxwellian electron distribution function could improve the predictive capability of crossed-beam energy transfer modeling and spur inquiry in other areas affected by non-Maxwellian electron distribution functions, such as laser absorption, heat transport, and x-ray spectroscopy.

The research was published in Nature Physics (www.doi.org/10.1038/s41567-019-0725-z).