NIF to shift emphasis after the facility's failure to achieve ignition

By David Kramer

On 30 September the National Ignition Facility's (NIF's) two-year-long National Ignition Campaign (NIC) officially came to an end. Despite multiple attempts on the part of Lawrence Livermore National Laboratory scientists to create miniature laser-driven thermonuclear explosions, they failed to achieve ignition, the point at which the fusion energy yield exceeds the energy required to start the reaction. In addition, Donald Cook, the National Nuclear Security Administration's (NNSA's) deputy administrator for defense programs, admits that NIF also failed to attain an intermediate milestone where alpha particles created in the fusion process start to contribute significant amounts of heat to the plasma.

Neither Cook nor Christopher Deeney, NNSA assistant deputy administrator for stockpile stewardship, would offer a prediction on when those goals might be achieved. Cook notes, however, "we've not identified reasons that say we can't achieve ignition, and we have some intriguing mysteries that are not due to the facility itself but are due to the separation between the modeling and simulations and the experimental data."

The $3.5 billion, two-football-field-sized NIF facility has performed well, Cook says, with the 192-beam laser exceeding its design level of 1.8 megajoules of energy and 500 terawatts of power. The quality of the target capsules and the cryogenic fuel pellets exceeded expectations, and the experiments have produced "an outstanding set of experimental data" thanks to NIF's large suite of diagnostics. But, although the campaign produced a two order of magnitude progress toward ignition, researchers remained one order of magnitude away from their goal at campaign's end.

"Ignition was never the endgame," Cook says. "The endgame is really stockpile stewardship." For NIF's applications to laboratory astrophysics and fundamental science, he adds, "the endgame is just having a very capable facility with diagnostics that can explore things that you can't otherwise go out and measure."

Stockpile stewardship and basic research

With the start of the new fiscal year on 1 October, much more of NIF's time is being turned over to other experiments in support of ensuring the reliability of the nuclear weapons stockpile. The classified work involves equations of state and shock physics of materials, says Cook. "NIF will get us to the [pressure] level of 50 megabars in diamonds, and to the radiative opacity of different materials. With those things, one can do experiments on radiation transport through different materials."

Deeney says the non-ignition experiments, which also include a small but increasing number of unclassified "shots" devoted to basic research, will increase to more than half the NIF total, with ignition accounting for the remainder. Over the last two years, about 80% of the time on NIF had been devoted to ignition-related experiments, he says.

"There's a sizeable backlog of people who want to do experiments for stockpile stewardship that don't fall into the direction that we would take for achieving ignition," Cook says.

In addition to the cost of operating the NIF, the ignition campaign was funded at $109 million a year in fiscal years 2011 and 2012, and NNSA has requested $84 million for ignition in FY 2013. Another $15 million has been requested to carry out the non-ignition experiments at NIF.

The Livermore laser has pursued an indirect drive approach to inertial confinement fusion (ICF), in which laser light is converted to x rays that then drive the implosion. In contrast, other NNSA-funded labs at the University of Rochester and the Naval Research Laboratory have worked at driving implosions directly with a laser. Cook and Deeney say NIF will continue to focus primarily on the indirect drive route.

But one of four working groups established to define a "path forward" to ignition will examine direct drive. "It's always been part of the NIC not to preclude capability of doing direct drive in the future because it's energetically much more appealing if you don't go through conversion of light to x rays into the capsule, just go directly from light to the capsule," Deeney says. The ability to "smooth" laser beams to more symmetrically drive implosions has advanced considerably since NIF construction got underway in the early 1990s, he notes.

A disappointment of the campaign was the discrepancy between ignition experimental results and the computer models of theinteractions between the laser, the target capsules, known as hohlraums, and the pellets inside them containing the fusion fuel. The express purpose of NIF was to validate the extraordinarily detailed nuclear weapons codes that have been developed since underground nuclear tests were halted 20 years ago. "If the [ICF] codes had been accurate, we'd be on this call telling you that ignition has been achieved," admits Deeney. "Clearly the codes are not exactly replicating the reality of the implosion experiments."

Still, he says, there's no cause to worry about the functioning of US nuclear weapons. "It doesn't give us a concern for the stockpile now. We've other mitigating ways to make sure the weapon codes are accurately predicting in those regimes by tying back to the large number of historic nuclear tests," Deeney says. But the ignition campaign "was built on a schedule predicated on the ICF codes being a good guide on how we would get to ignition. We learned that was not the case."

NNSA is expected to deliver a report to Congress by 30 November that details its new path forward to ignition, Deeney says.