On 17 June Physics Today’s David Kramer wrote a revealing story about the latest setback for the National Ignition Facility (NIF). A new report from the National Nuclear Security Administration (NNSA) confirms that nearly four years after the nuclear fusion lab’s self-imposed deadline, NIF is still nowhere close to achieving ignition—a reaction that generates more energy than that of the laser used to initiate the reaction.
When David first told me the news, I immediately thought about my visit to the Lawrence Livermore National Laboratory facility in February 2013, four months after the missed deadline. I came away from the tour impressed by the laser technology but more baffled than ever about the NIF leadership’s 2009 ignition guarantee.
It doesn’t take a historian to understand that efforts for controlled fusion don’t tend to go as smoothly as planned. Lo and behold, once NIF was turned on in 2010, scientists quickly realized that their simulations of how hydrogen capsules would implode when struck by a laser-driven burst of x rays were alarmingly off. Among other problems, the capsules weren’t imploding evenly, which short-circuited the fusion reaction. NIF researchers responded by doing what scientists are supposed to do—take measurements to explore what’s going on and adjust predictions accordingly—but meeting the deadline became a pipe dream. I got the sense during my visit that these scientists felt that no matter how much insight they gained into the factors preventing ignition, they would be labeled as failures for missing a deadline they didn’t set.
Credit: Jason Laurea, NIF
The facility’s scientists have made laudable achievements since the September 2012 deadline came and went. The laser pulse was adjusted to deliver an ultraquick burst, leading to a swifter and more compact implosion. A late 2013 trial produced about 14 kJ of energy, 3 kJ more than the laser energy absorbed by the capsule. (Of course, the 14 kJ didn’t nearly make up for the roughly 1800 kJ of laser energy that never got absorbed.) About half the yield resulted from alpha heating, the crucial mechanism for achieving a sustained, self-perpetuating reaction.
Despite the incremental progress, the chances of achieving ignition at NIF or any other laser facility are dwindling. “Barring an unforeseen technical breakthrough and given today’s configuration of the NIF laser, achieving ignition on the NIF in the near term (one to two years) is unlikely and is uncertain over the next five years,” the new report says. “Currently there is no known configuration, specific target design, or approach that will guarantee ignition on the NIF.” The language is more pessimistic than that of NNSA’s 2012 report, which stated that it was “too early to assess whether or not ignition can be achieved at the NIF.”
The facility’s importance for nuclear weapons research will at least allow for limited opportunities to continue inching toward ignition. NIF engineers are tinkering with the fuel capsules and the cylindrical hohlraums that house them. Custom diagnostic instruments now extract as much information as possible from each laser shot; that data, in turn, help improve the simulations.
There’s plenty of blame to go around for NIF coming up short, but it’s a shame that the facility’s scientists were pretty much doomed to fail from the first shot.
