Physics Today David Kramer | 17 June 2016
More than three years after the deadline passed for obtaining a sustained, high-energy-yield nuclear fusion reaction at the National Ignition Facility (NIF), the US Department of Energy is still unsure whether the $3.5 billion laser can ever attain that milestone. Much as it did in 2012, the agency has established a new, less ambitious goal for NIF several years hence: to determine whether the machine can ever achieve its eponymous goal, and if not, why not.
“The question is if the NIF will be able to reach ignition in its current configuration and not when it will occur,” states a May report prepared by DOE’s National Nuclear Security Administration (NNSA). The reassessment of progress toward ignition at the Lawrence Livermore National Laboratory facility was conducted three years after the NNSA suspended its formal two-year-long ignition campaign in September 2012. Ignition, the threshold at which more energy results from a fusion reaction than is required to spark it, is an essential determinant in whether inertial confinement fusion (ICF) could ever become a source of fusion power.
Despite the report’s assurances that much progress has been made toward ignition since 2012, the NNSA appears no closer to committing to ignition on NIF than it was then. In a December 2012 report to Congress, the agency found “no compelling information suggesting that the [NIF’s] indirect-drive approach cannot achieve ignition.” Still, then-NNSA administrator Tom D’Agostino said it was “too early to say whether or not ignition can be achieved at the NIF.”
In a new plan for the ICF program, the NNSA establishes a goal, with a deadline of 2020, to “determine the efficacy of reaching ignition on NIF.” That contrasts sharply with the virtual assurances of ignition that were made by proponents in 2009, when NIF began operating. Although ignition experiments continue at NIF, they have been interspersed with experiments designed to deepen understanding of other nuclear weapons–related phenomena, such as the behavior of materials under extreme pressures and densities.
Since 2012 NIF’s 1.8 MJ laser has nearly doubled the frequency of shots, the machine’s diagnostics have been improved, and progress has been made on identifying key impediments to ignition, the new report states. NIF’s indirect-drive approach focuses 192 beams on a cylinder, or hohlraum, containing a tiny capsule of fusion fuel. The hohlraum converts the light to x rays, which implode the capsule.
In the meantime, the University of Rochester’s Omega laser and Sandia National Laboratories’ Z machine—both also supported by the NNSA’s inertial confinement fusion program—continue research on alternative approaches to ignition. Omega, a glass laser like NIF, uses direct drive, which brings beams to impinge directly on targets; Z uses electromagnetic fields to produce implosions.
The NNSA review says computer models and codes predicting that NIF would attain ignition conditions “are not capturing the necessary physics to make such predictions with confidence. A lack of appreciation for this, combined with a failed approach to scientific program management, led to the failures” in the ignition campaign.
Although the performance of NIF’s targets containing fusion fuel continues to improve, “currently, there is no known configuration, specific target design, or approach that will guarantee ignition on the NIF,” says the review.
Stephen Bodner, a former director of the laser fusion program at the US Naval Research Laboratory (NRL), has been a vocal critic of NIF since before its construction began. In a 1995 paper published in Plasma Physics and Controlled Fusion, Bodner predicted that the highly intense NIF laser would create instabilities in the plasma. That, plus the formation of unpredictable magnetic fields, would prevent the symmetrical implosions required for ignition.
“Basically [the report] is confirmation of what I predicted in 1995,” Bodner says. “It took the community 21 years, and many billions of dollars, to vindicate my predictions. So sad.”
Regardless of whether ignition is achieved, there are other compelling nuclear weapons stewardship questions concerning the properties of thermonuclear plasmas with multi-megajoule yields, the NNSA report says. Planned Russian and Chinese laser facilities may surpass NIF’s capabilities, it warns, and in an era without nuclear testing, a source capable of producing 500 MJ of fusion energy “will be essential for the health of the [weapons] program.” Such energy yields are unlikely to be achieved within the next decade but should be considered an ultimate goal, the report says.
Bodner argues, however, that NIF’s regimes of temperature, ionization, pressure, density, and radiation spectrum are fundamentally different from those that occur in a nuclear weapon. “To extrapolate the regime in the laboratory that they’re using to anything in nuclear weapons would be outrageously irresponsible,” he says. “They should not be using any of that science in the nuclear weapons program.”
David Crandall, a former NNSA scientist who helped oversee NIF, disagrees. He says the realization that the codes predicting ignition were wrong has instilled a new level of caution among weapons scientists about extrapolating from data sets of nuclear tests. “That piece of reality was extremely important to the weapons program,” he says. Further, Crandall says, new methods have been developed for using NIF-generated fast neutrons to test weapons codes. For those techniques, neutron yield is more important than ignition. Also, he explains, experiments at NIF have already provided important new information about the behavior of plutonium at high pressures.
John Edwards, associate director for the NIF’s ICF program, declines to say whether he’s optimistic or pessimistic about ignition at NIF. Progress since 2012 includes the first ever laboratory demonstration of the alpha heating process, in which thermal energy is supplied by the helium nuclei that result from fusion. “But there are obstacles which we are quite open about,” Edwards acknowledges. Researchers think they can overcome the instabilities inside the hohlraums by making the cylinders larger; the question is whether NIF’s energy is sufficient to drive the larger targets, he says.
Reconfiguring NIF to perform direct-drive experiments is being evaluated by a University of Rochester–led team. But that will require a major revamp costing several hundred million dollars.
Bodner argues that solid-state lasers like NIF and Omega won’t work; he says the krypton fluoride gas laser—two of which he helped build while at the NRL—is the best option for an ignition driver. The KrF laser in a direct-drive mode produces a broader bandwidth beam that can be “smoothed” to eliminate asymmetric hot spots, he says. But the NNSA’s plan doesn’t include KrF lasers among its driver candidates.
Stephen Dean, president of the nonprofit Fusion Power Associates, says DOE’s justification for NIF has shifted from the energy-relevant milestone of achieving ignition to a focus mainly on weapons research. “They don’t want to be held to ignition,” he says. Dean sees a parallel with DOE’s magnetic fusion program. In 1980 the project was sold as an energy program with a 2000 deadline for construction of a working fusion power plant; today it’s classified as a science program.
“You have people working who were goal-oriented,” Dean says. “And when the program doesn’t accomplish those goals, there’s a scramble to do something to save it.”
Correction (20 June 2016): Stephen Bodner was incorrectly identified as the head of the plasma physics program at NRL. He was, instead, the head of the laser fusion program at NRL.