The Sun and other stars rely on gravitational compression to overcome the coulombic repulsion between atoms and power their fusion. To mimic that compression, scientists use the world’s most powerful laser at Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) to squeeze isotopes of hydrogen—deuterium and tritium—in a 2-mm-wide capsule. Conceptually, the experiment is simple: NIF trains 192 laser beams (blue in the figure) into a 1-cm-tall cylinder, known as a hohlraum, in which the capsule is suspended. In response, the hohlraum’s walls produce a flux of soft x rays. Within 8 nanoseconds, that flux compresses the capsule into a “hot spot” half the width of a human hair at a temperature of 100 million kelvin and a pressure of 350 billion atmospheres.
The NIF experiments strive to convert the kinetic energy of that implosion into internal energy faster than any other process can quench the fusion. To date, that’s been an elusive goal. In two papers, one in Nature and another in Nature Physics, the NIF collaboration reports a more modest achievement: creating a burning plasma in four experiments conducted between late 2020 and early 2021. A burning plasma is the regime in which the fusion reactions themselves—not the compression—are the primary source of heat for the plasma. And it’s an essential precursor to ignition, the state in which the heat added by the alpha particles outstrips all the losses. The resulting thermal instability triggers a nonlinear rise in temperature, which sustains and propagates the burn.
The NIF collaboration stopped short of claiming ignition from the four shots reported in their two papers—or from a fifth, record-energy-releasing event on 8 August 2021. But all the experiments were deeply in the burning-plasma regime. To reach it, the NIF scientists increased the size of the imploding capsule by 10–15%. To combat the loss of symmetry during compression, the scientists swapped out capsule materials—replacing the plastic shell with diamond. And with diamond three times as dense as plastic, the outer shell became more absorptive to x rays and more efficient at coupling compressive energy to the fuel. It was also thinner, which allowed the researchers to use shorter laser pulse lengths, down to 8 ns from 20 ns in previous experiments. That too improved the efficiency at which x rays compressed the capsule. To further optimize the experiment’s design, the scientists explored modifications to the hohlraum.
As chief scientist for the laboratory’s Inertial Confinement Fusion Program, Omar Hurricane says, “Having reached a burning plasma, we are now on the verge of ignition. Turning the experiment into a useful energy source, though, is a very long way off.” (A. B. Zylstra et al., Nature 601, 542, 2022; A. L. Kritcher et al., Nat. Phys., 2022, doi:10.1038/s41567-021-01485-9.)
