After an unsuccessful campaign to demonstrate the principles of a futuristic fusion power plant, the world’s most powerful laser facility is set to change course and emphasize its nuclear weapons research.
For the past six years, scientists and engineers at the US National Ignition Facility (NIF) have been working flat out to focus 192 laser beams on a gold-lined ‘hohlraum’ capsule, just a few millimeters long, containing a pellet of hydrogen isotopes. As 500 terawatts of laser power hits the capsule, it generates X-rays that blast into the pellet, causing the atoms of deuterium and tritium inside to fuse. The fusion converts a tiny amount of their mass into a burst of energy (see ‘The NIF’s fusion strategy’).
The goal of the National Ignition Campaign (NIC) is reflected in its name: ‘ignition’, in which the fusion reaction generates as much energy as the lasers supply. Success, NIF officials say, could pave the way to developing a power plant that would implode nearly 1,000 pellets a minute (see Nature 483, 133–134; 2012). But unexpected technical problems left the NIF well short of its goal when the campaign finally ended in September.
Now federal officials and the US Congress are preparing to set a new direction for the US$3.5-billion facility at the Lawrence Livermore National Laboratory in California. A series of reports commissioned by the government, Congress and the University of California, which administers the lab, are all due later this month. They are expected to outline plans to cut its time for ignition research from 80% to 50% and to give the National Nuclear Security Administration (NNSA), which is responsible for maintaining the US nuclear arsenal, a more central role in determining the NIF’s priorities. The NNSA is planning to emphasize experiments that mimic conditions inside nuclear weapons, generating data to validate the computer codes used to check that the nation’s warheads remain viable — essential work, given the voluntary moratorium on underground testing that began in 1992.
Nobody has given up on ignition, declares Donald Cook, deputy administrator for defense programs at the NNSA. But a new program for generating net energy will take a slower, more methodical approach. “We’re now going to get right into the science of what issues are preventing ignition and work through them,” he says. “But we believe that’s going to take a fair amount of work.”
Significant progress has already been made towards ignition, according to physicist Robert Byer at Stanford University in California, who is leading the University of California’s review of the NIF. “The laser itself has been quite remarkable,” he says. One shot can deliver 1.85 megajoules of energy, roughly what the lab originally promised. The instruments used to study the pellet are also performing well, he says.
Yet on the basis of data obtained from the imploding pellets, researchers think that they are still far from reaching the conditions necessary for ignition. One problem seems to be that too much of the laser light is scattering back out of the capsule. Another is that the pellet is being squeezed asymmetrically, which lowers the pressure at its center. The asymmetry also causes the isotopes to mix unevenly, lowering the temperature in the pellet. “Nature pushes back: that’s my shorthand version of what’s going on,” Byer says.
Nature isn’t the only one pushing back — the NIF’s funders in Congress also want answers. “We’re disappointed,” says one congressional staff member, who spoke to Nature only on condition on anonymity. Critics say that the lab’s enthusiastic promotion of the idea that laser fusion could generate electrical power led many in Congress to believe that they were funding an energy project, when in fact laser fusion is decades from producing electricity. “The lab overemphasized and oversold the energy aspect of the NIF, at the expense of the very important and successful work it was doing in stockpile stewardship and basic science,” says a senior scientist familiar with the NIF program.