A technician at the National Ignition Facility inspects the laser's target {credit}LLNL/NIF{/credit}

Last autumn, the world’s most powerful laser missed a major milestone in its drive to produce thermonuclear fusion. Now, the findings of an independent peer-review panel lay out in detail why achieving that goal is turning out to be so difficult.

The US$3.5-billion National Ignition Facility (NIF), at the Lawrence Livermore National Laboratory in Livermore, California, is designed to crush tiny pellets of hydrogen isotopes until they fuse into helium. The goal is to release more energy than goes into the pellet and, in doing so, to roughly mimic conditions inside a modern nuclear warhead.

That was the goal, but a six-year “ignition campaign” came up short in September, sparking introspection among scientists, federal officials and congressional funders. Introspection in Washington inevitably leads to reports, and in November and December, a series of reviews of the project were released — including plans to shift the giant laser facility away from ignition work and towards weapons.

Now, a peer review of the project has been made public by the US National Nuclear Security Administration (NNSA), the government body that oversees the NIF. That review, by independent scientists, is the last in a series convened by Steven Koonin, former undersecretary of science at the US Department of Energy.

The new review doesn’t differ too much from previous ones, but it does provide a pithy summary of some of the problems. In particular, it notes that scientists at the NIF have had trouble controlling the symmetry of their laser-driven implosion, and the ways in which hot and cold fuels mix together. The committee also noted that computer codes just aren’t good enough.

Perhaps more interestingly, the committee seemed to be split over whether ignition would actually ever be achievable. “Some reviewers were optimistic while others remain highly skeptical as regards for the prospects of future ignition,” the report says.