The managers of the National Ignition Facility (NIF), a giant laser fusion lab in California, have admitted to Congress that they don't understand why the $3.5 billion machine is not working. And they cannot guarantee that it will ever work.
"At present, it is too early to assess whether or not ignition can be achieved at the National Ignition Facility," wrote Thomas P. D'Agostino, administrator of the National Nuclear Security Administration (NNSA) in a report requested by Congress that was submitted last week.
NIF, which started operation in 2009 at the Department of Energy's (DOE's) Lawrence Livermore National Laboratory (LLNL), attempts to create a burning fusion plasma—the same process that powers the stars—by blasting a small capsule of hydrogen fuel with a powerful laser. At the time, managers confidently predicted that they would be able to achieve ignition—a self-sustaining fusion burn that produces more energy than the laser pulse that sparked it—by 30 September 2012.
That deadline passed with NIF researchers still some way from their goal. Congress had mandated NNSA to submit a report within 2 months if that deadline was not met, explaining the barriers to success, how they could be overcome, and what impact the failure would have on the stockpile stewardship program. NIF's principle function is to help the agency maintain readiness of the U.S. nuclear arsenal by recreating miniature nuclear blasts so that researchers can verify their computer simulations and test components for blast-hardness.
The lab's 2-year program to achieve ignition, known as the National Ignition Campaign, relied heavily on work with earlier lasers and computer modeling to develop a design for the target and the shape of the laser pulse. Those models predicted that NIF should have achieved ignition by now. Although the laser itself, the diagnostic instruments, and the target fabrication have all met or exceeded specifications, the physics of the implosions was unexpected. "Experimental data demonstrate that the physics underlying ignition implosions are not predicted accurately by the simulation codes that were used to design ignition targets and to predict their performance," the report says.
"Mother nature kind of won on this one," says Mary Hockaday, deputy associate director for weapons physics at DOE's Los Alamos National Laboratory in New Mexico and one of the lead authors of the report. "We've been humbled, but we haven't given up."
The NNSA report now proposes a 3-year program to better understand why the actual implosion does not agree with the models. And 3 years may not be enough. "The three-year plan culminates in a comprehensive review at the end of FY 2015. At that time, NNSA will have an assessment of the likelihood and schedule for achieving ignition," the report says.
Significantly, the report also says that the new program should explore alternatives to the indirect drive approach used at LLNL, in which the lasers are shone into a gold cylinder the size of a pencil eraser that surrounds the peppercorn-sized target. This "hohlraum" heats up, emits x-rays, and these compress the target. "There is already a qualitative understanding of the reasons for the NIF failure. Obtaining quantitative information will be difficult. There are severe limitations on what can be diagnosed inside the gold hohlraum," says a laser fusion expert who doesn't want to be identified.
Researchers at the University of Rochester's Laboratory for Laser Energetics have championed an alternative approach in which the laser beams shine directly onto the target capsule. The new program says more work should be done on this technique in case indirect drive proves impractical. Another backup could be a technique devised with the Z machine at DOE's third weapons lab, Sandia National Laboratory in New Mexico, which uses huge electrical pulses to crush fusion fuel magnetically.
The report says that the failure, thus far, to achieve ignition will not undermine the safety of current weapons. Results from the underground test program (which ended in 1992) combined with monitoring the condition of the weapons themselves assure their continued viability, the report states. What it does mean, however, is that NNSA will not be able to rely on computer models and simulation to predict the behavior of weapons it wishes to modify.
The next steps are up to Congress. Many legislators are concerned about the cost of operating NIF and have questioned its value for stockpile stewardship.