Inertial fusion energy gets a nod

Despite scientists’ failure to achieve ignition at the massive laser fusion facility at Lawrence Livermore National Laboratory, inertial fusion energy (IFE) warrants continued research, concluded a committee of experts convened by the National Research Council. A report released on 20 February said that ignition of a fuel pellet at LLNL’s National Ignition Facility (NIF) is ‘the minimum technical accomplishment that would give confidence that commercial IFE may be feasible.’ The panel predicted that it will likely be ‘significantly more than a year before the discrepancies between theory and experiment are fully understood and needed adjustments are made to optimize the performance of the target fuel capsules containing deuterium and tritium.’

Although ignition wasn’t achieved at NIF during the two-year-long experimental campaign that ended 30 September 2012, the committee said that doesn’t lessen the long-term technical prospects for IFE. It noted that none of the expert reviews of the ignition campaign had ruled out ignition at NIF, and that it would be a modest step from ignition at NIF to ignition on the scale required for IFE.

‘The realization of inertial fusion energy would be a tremendous achievement capable of satisfying the world’s ever-growing need for power without major environmental consequences,’ said Ronald Davidson, professor of astrophysical sciences at Princeton Plasma Physics Laboratory and the committee cochair. ‘These possibilities form an extremely compelling rationale to continue R&D efforts toward this goal.’

It is premature to choose from among the numerous candidate drivers the one that is best suited for imploding fuel capsules in an IFE demonstration plant, the report said. Among those candidate drivers are diode-pumped lasers such as those at NIF, heavy ions, krypton fluoride lasers, and pulsed power.

Because of the relevance to nuclear weapons of inertial fusion, it is currently funded by the Department of Energy’s National Nuclear Security Administration. DOE has no coordinated IFE program, however. If ignition is attained, an independent IFE program should be established, the report recommended, since the technological needs for energy, such as high-repetition-rate drivers, mass-producible targets, and target chamber materials, will continue to diverge from weapons program requirements.

Steven Koonin, the former DOE undersecretary for science who requested the NRC study in late 2010, says the agency ought to consolidate the energy-specific R&D elements of inertial fusion into a discrete program now. ‘Where it sits in the department is open for discussion,’ he adds. Koonin, now director of New York University’s Center for Urban Science and Progress, says he would hold off on increased funding for IFE. ‘If you don’t have ignition, you have no program for energy production.’ With the report in hand, and if that milestone is reached, the DOE now has a plan to proceed to IFE, he notes.

In addition to developing drivers, the fusion fuel capsules, or targets, are a major engineering challenge that will need to be addressed in moving from NIF to a working IFE power plant. The tiny cylindrical chambers that hold fusion fuel are currently manufactured from gold using a technique that does not extrapolate to mass production, although concepts for producing targets at 100 000 times today’s rate have been developed, the report said. While NIF is capable of imploding only one or two targets per day, an IFE plant would consume as many as one million targets per day. Even so, the committee concluded that target fabrication ‘does not represent an obvious insurmountable obstacle for IFE.’

Another challenge will be making reliable, long-lived target chamber walls that can cope with the surface erosion and other problems caused by charged particles, target debris, x rays, and neutron bombardment.

The committee found that the nuclear proliferation risks associated with IFE ‘are real, but are likely to be controllable.’ Greater risk is associated with the indirect drive approach to inertial fusion employed at NIF than with the direct drive process, which is used at experimental facilities such as the OMEGA laser at the University of Rochester. That is due to the former’s similarity to the secondary stage of thermonuclear weapons. But that concern may soon become moot: The panel noted that information concerning data, simulations, and knowledge about indirect drive, much of which is currently classified by the US government, is likely to become widely available before long.