Foam mitigates key obstacle in quest for laser fusion

The goal of inertial confinement fusion is to swiftly heat and compress small capsules of hydrogen fuel with a powerful laser. Ideally, the implosion is spherically symmetric: Hydrogen fuses into helium and emits alpha particles, which slam into more hydrogen and trigger a sustained fusion reaction. In practice, however, Rayleigh–Taylor instabilities subvert spherical symmetry. Barely perceptible bumps on the capsules or variations in laser intensity get magnified as the fuel implodes and create fractures through which the hot, dense plasma needed to drive the reaction escapes.

Now an international team of physicists using the OMEGA laser at the University of Rochester in New York has reduced Rayleigh–Taylor instabilities with the help of extremely low-density foam. The researchers spread the foam over a thin sheet of foil (see image below), which served as the target in place of a fuel capsule. Once struck by six 500-joule beams, the foam vaporized instantaneously into a plasma, which smoothed out imperfections in the laser beams before the light reached the foil. The researchers found that a 500 μm layer of 7 mg/cm³ foam reduced the imprint of laser imperfections by a factor of two. The next step is to envelop a round capsule with the foam in an attempt to achieve perfectly spherical compression.

The technique will probably have limited use at the site of the world’s most powerful laser, Lawrence Livermore National Laboratory’s National Ignition Facility. Most NIF energy trials involve illuminating a small cylinder called a hohlraum, which emits x rays that squash the fuel inside. But directly zapping the fuel has become a hot area of research, particularly after NIF failed to reach its goal of achieving ignition—generating more energy than that of the laser—by 2012 (see Physics Today, February 2015, page 24 ). (B. Delorme et al., Phys. Plasmas 23, 042701, 2016, doi:10.1063/1.4945619 .)