In early May, scientists at Lawrence Livermore National Laboratory will resume their quest to attain a major milestone on the path to controlled nuclear fusion. Using the 192-beam laser at the National Ignition Facility, the researchers aim to implode a bb-sized sphere of deuterium and tritium. If the powerful illumination is sufficiently uniform, the nuclei will fuse into helium and release many times more energy than it took to initiate the reaction.
The National Ignition Campaign, as the experimental program is formally known, got under way last fall, 18 months after construction of the $3.5-billion NIF was declared finished in March 2009. In the NIC experiments, pulses from the lasers are directed through ports into a 10-meter-diameter target chamber onto a dime-sized cylindrical target. The beams enter the cylinder, known as a hohlraum, through windows located at each end. For ignition to occur, the scientists must tune to within nanoseconds the timing of multiple shocks that are delivered within a single pulse of the laser. If the timing or the power applied by the shocks is slightly off, the sphere will implode unevenly into a pancake or sausage shape and fizzle.
The complexity of the NIC challenge became evident over the winter when scientists found that surrogate fuel pellets weren't behaving the way they had predicted when they hit them with laser pulses. They were flattening instead, says Edward Moses, LLNL's principal associate director for NIF and photon sciences. The scientists eventually traced the problem to an unanticipated formation of frost on the hohlraum windows. The targets must be cooled to 20 K to keep the fusion fuel frozen, Moses explains. But because of its large size, the degree of vacuum that the target chamber can maintain is limited. Some moisture was condensing on the glass and distorting the angles of the laser beams as they entered the hohlraum. The solution turned out to be installing a second layer of glass over the hohlraum windows with an air gap between the two—essentially creating a storm window.
The frost problem was officially declared fixed on 23 January, the day before Steven Koonin, DOE undersecretary for science, visited the lab for a scheduled review of the ignition campaign. Koonin has taken an intense interest in the ignition campaign; he convened a panel of experts, chaired by himself, to review its progress. Critics, including the Natural Resources Defense Council, have complained that the Koonin committee does not hold its meetings in public. Last fall, Koonin commissioned a separate study from the National Research Council to recommend whether DOE should initiate a research program on inertial fusion as a potential energy source. The NRC committee is scheduled to deliver its interim recommendations this summer.
In a 10 March memorandum summarizing his January visit to NIF, Koonin called ignition 'a goal of overriding importance for the DOE.' He expressed some disappointment that progress since his previous visit in mid-October 'was not as rapid as I had hoped.' In addition to the frost, other operational problems encountered during the period were dust in the targets and trouble with the layering of the deuterium, tritium, and hydrogen cryogenic fuel mixtures used for the targets. Koonin said that discrepancies had arisen between measurements obtained from experimental shots and predicted values. On the other hand, he lauded the 'control and consistency' achieved by the laser and said the target diagnostics were 'working extremely well.' In sum, he said 'The degree of experiment control that has been achieved is an outstanding accomplishment and is fundamental to ignition success.'
Solving the frost problem
John Lindl, NIF's chief scientist, says the results that Koonin saw during his review were from experiments performed before the storm-windows were installed. Lindl welcomed the DOE official's comments on reproducibility. 'One of the huge challenges for NIF was to achieve the level of shot-to-shot reproducibility that is required for ignition,' he says. No other high-power laser facility has been able to reproduce results with such precision, he says, with shot-to-shot results varying only a few percent.
Repeatable results are crucial because producing meaningful amounts of energy will require firing targets every few seconds. 'Eventually they will have to be able to do [ignition] at will,' notes Stephen Dean, president of Fusion Power Associates, an industry group that promotes fusion energy.
Ignition experiments were halted temporarily in mid-February to allow other scheduled weapons physics and science experiments to proceed at NIF. The NIC is scheduled to resume around 1 May, and run through July 1.
Lindl is confident that with the frost problem solved, 'we will be able to go in and get a precision optimization of the early part of the pulse, which is necessary to get high density. Then we will apply that to the cryo-layer implosion to see whether we can get the improvement that we were anticipating.'
Moses says the frost had also caused the fuel-layering problems Koonin mentioned. Before they came up with the storm window idea, experimenters took measures to limit the frost that inadvertently prevented them from producing the precisely layered fusion fuel crystals they needed, says Moses. With the storm windows installed, 'the layering is looking much better,' he says, but he cautions that further results are needed before declaring the issue resolved.
Although Koonin wrote in the March memo that 'having targets with the desired attributes available on schedule has been limiting,' Moses and Lindl praise General Atomics, which manufactures them. Moses says the targets are designed and delivered to tolerances within microns. Adds Lindl, 'We have targets that will allow us to precisely adjust each piece of the [laser] pulse, from early time all the way up to peak power.'
Although the laser is designed to produce 1.8 megajoules, the maximum achieved to date has been 1.5 MJ. 'We've been running the machine where the users want it,' Moses says, adding that he hopes to fire the laser at 1.8 MJ this fall for experiments not related to NIC. He points out that NIF is on or slightly ahead of a long-established schedule to reach 1.8-MJ operation during fiscal year 2012.
It isn't clear yet when ignition will be achieved, and some skeptics remained convinced that NIF won't meet the goal. One persistent critic of NIF, Stephen Bodner, who formerly directed a 'direct-drive' laser-fusion research program at the Naval Research Laboratory, maintains that LLNL's failure to take into account unwanted physics phenomena such as dielectronic recombination and fast-electron generation will prevent NIC from achieving the required implosion symmetry. 'Fundamentally, the indirect-drive target is a clumsy and inelegant solution to laser fusion,' Bodner says. Whatever the case, it's clear that the project will be taking longer than the early success optimists were predicting when the campaign began.
David Kramer
