19 Dec 2012
Three-year plan shifts emphasis towards nuclear stockpile stewardship and fundamental science, though ignition remains a key goal.
A new report for US Congress and the Department of Energy (DOE) on the future priorities for California’s National Ignition Facility (NIF) has outlined a three-year plan aimed primarily at developing a better understanding of the physics underlying the fusion ignition process.
For the past two years, the team at the giant laser facility near San Francisco has been pursuing an intensive campaign to demonstrate fusion with ignition. And although senior representatives from the laboratory have repeatedly expressed their confidence of eventually meeting that goal, ignition has not yet been achieved.
The report, written by the National Nuclear Security Administration (NNSA) and entitled “NNSA’s Path Forward to Achieving Ignition in the Inertial Confinement Fusion Program”, concludes that it is still too early to say whether or not the ultimate goal can be achieved at NIF.
As reported previously by optics.org, that is no fault of the high-energy laser or optical system built to do the job, but because experimental data simply does not match up with the behavior that had been predicted by the complex codes developed to model the physics of fusion with ignition.
“A key goal of NIF to discover discrepancies between codes and experiments has been demonstrated clearly,” summed up the NNSA’s Thomas D’Agostino in the report. “The disagreement between NIF experimental data and codes and models reflects an inadequate understanding of key physics issues.”
Stockpile stewardship priority
As a result, the emphasis at the facility between now and 2015 will shift towards improving those models until they match up with what has been measured experimentally. While that does not mean the end of ignition experiments, it does mean that more of the beam-time at NIF will be used to simulate nuclear weapons (so-called “stockpile stewardship”) and for fundamental science, including some non-ignition work.
After three years of work to better understand the physics of ignition, the NIF program will then be assessed again at the end of fiscal year 2015.
Since construction was completed in March 2009 and the first work geared directly towards ignition took place in September 2010, the 192-laser system at NIF has performed some 37 cryogenic implosion experiments and has set numerous world records for high-energy laser pulses.
“Power and energy have exceeded initial design specifications,” states the NNSA report, adding that the laser has performed reliably and with great precision. “Target quality is superb, and diagnostics are returning experimental data of unprecedented quality.”
However, the target “hohlraum” is not quite behaving in the manner expected under the colossal pressures generated by the near-2 megajoule pulses designed to compress it with perfect symmetry and generate fusion with energy gain.
Neutron yields remain a factor of three to ten times below what is required to initiate the alpha heating process and a subsequent propagating “burn”, or ignition. “The fuel is compressing to one-half the pressure that predictions would require for ignition,” says the report.
Ignition “critical” for stewardship
While the NNSA report emphasizes that achieving ignition remains one of NIF’s key goals – it is regarded as critical for accurate simulation of the behaviour of nuclear weapons – the lab’s priorities will shift noticeably.
As a result, only 40% of shot allocations in 2013 are expected to be for ignition, while 50% will be dedicated to high-energy density/stockpile stewardship work. The remaining 10% of shots will be used for “non-ignition” fundamental science, and the new report makes no mention of potential future applications in energy generation.
“Developing and utilizing NIF’s unique capabilities for weapons physics applications remain important objectives for the weapons programs, regardless of whether ignition is achieved in the near future,” it states.
While the mismatch between experimental evidence and model calculations is not seen as a problem for current weapons understanding, if ignition is to remain elusive, it would have a serious impact on the weapons science envisaged for the site:
“Weapons scientists will be unable to explore experimentally the potential impacts of ageing on thermonuclear ignition and burn for some warhead life extension design or component options,” NNSA adds. “Therefore, the resulting weapon analyses may have associated uncertainties [that are] larger than they would be otherwise.”