Inertial fusion energy could have a bright future, despite the failure of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL; Livermore, CA) to ignite a fusion target by the target date of Sept. 30, 2012, says a National Research Council (NRC) report issued Feb. 20, 2013. The panel writes that the potential benefits of inertial fusion—including a carbon-free energy generation without the fuel limitations or large volume of high-level radioactive waste from fission—“justify it as part of the long-term U.S. energy R&D portfolio.” But that’s going to take time, and commercial inertial-fusion reactors are decades away.
The immediate priority is resolving why the giant laser at LLNL failed to achieve ignition with indirect-drive targets despite delivering pulses that theoretical models of the process predicted would be sufficient (see figure). Doing that and modifying NIF and targets to optimize performance “will likely take significantly more than a year,” the panel wrote. They also want to test direct-drive fusion at full NIF power. And they say that now is too early to make a final decision among technologies competing for use in an inertial-fusion energy demonstration.
Scaling model down to inertial fusion
The discrepancies stem from the nature of the models, which were developed to describe the physics of thermonuclear weapons. “The codes may not be wrong, but we’re using a driver different than what the codes are based on,” says panel co-chair Gerald Kulcinski, director of the fusion technology institute at the University of Wisconsin (Madison, WI). Problems could arise from scaling the weapons codes down to the much smaller dimensions of the laser-target experiments, and from performing the laser experiments in a vacuum, unlike weapon tests.
The most advanced drive system, and the only one yet tested on a megajoule scale, is the frequency-tripled solid-state laser with indirect drive used in NIF. The National Nuclear Security Administration built NIF for stockpile stewardship tests of the US nuclear arsenal, which simulate weapon tests by directing laser beams through the top and bottom of a centimeter-scale metal cylinder to produce a blast of x-rays that implodes the fusion target. That is less energy-efficient than directly illuminating the fusion target with the laser light, but when NIF was designed in the early 1990s indirect drive offered more uniform target illumination—crucial for symmetrical implosions.
The panel recommended direct-drive experiments whether or not NIF succeeds in igniting indirect-drive targets. Direct-drive experiments with the Omega laser at the University of Rochester (Rochester, NY) have demonstrated the uniform illumination needed for good target compression. Models predict that this can be scaled to higher laser energy, but “experiments must be done at full NIF scale” to validate the models, says Robert McCrory, director of Rochester’s Laboratory for Laser Energetics. The first tests will use polar indirect drive, which aims the beams at two poles of the target, because that can be done by modifying NIF’s beam delivery system.
The next big step for inertial fusion would be to move to a driver with a sustained repetition rate of 5 to 15 shots per second, a vast increase over the few single shots per day possible with NIF. Kulcinski says the most advanced driver technology is solid-state frequency-tripled lasers. Livermore proposed building a tripled solid-state laser testbed called Laser Inertial Fusion Energy (LIFE) that would replace the powerful flashlamp pumps used in NIF with high-power pump diodes for a high repetition rate. The panel says that demonstrating a full-scale diode-pumped beamline would be “a critical step toward laser-driver development” for such a testbed.
Alternate technologies
The leading backup laser technology is krypton fluoride, exemplified by the 5 kJ NIKE laser at the Naval Research Laboratory (Washington, DC). The 249 nm krypton fluoride (KrF) output wavelength is shorter than the 351 nm third harmonic of NIF, which may have some advantages for direct-drive fusion. The broader KrF bandwidth is attractive for smoothing out laser-plasma instabilities, and the laser can generate sub-nanosecond pulses for shock ignition of fusion plasmas.
The panel also recommended serious consideration of two nonlaser technologies—heavy ion beams and magnetic pinches driven by high-current pulses—largely as alternatives if laser problems prove intractable. The Department of Energy has studied heavy-ion fusion in the past, but abandoned its program a decade ago; although heavy-ion accelerators are efficient, they also are very expensive. Magnetized-liner inertial fusion, in which pulsed power produces a magnetic field that heats and compresses fusion fuel, is attractive because it offers a simple target geometry and high efficiency, but the technology is immature.
“The report does a good job at looking at many approaches to fusion,” says McCrory. It lays out a technology roadmap, with ignition followed by reproducible modest gain, then reactor-scale gain, cost-effective targets, and then combining those with the high repetition rate needed for energy generation. The time scale is uncertain, but includes driver evaluation and selection, and development of a fusion test facility with generation capacity of a few hundred megawatts. If all goes well, a gigawatt-scale demonstration plant would follow in 10 to 15 years.
Meeting those goals will require overcoming big technological challenges besides understanding inertial-fusion physics and developing drivers. Target fabrication must become a mass-production industry. Radiation-resistant materials must be developed for target chambers.
But the biggest challenge in the current economic climate will be finding the money to support a major new energy program. So far, the quest for inertial-fusion energy has largely piggybacked on the nuclear-weapons program, but the weapons program does not need a fusion test facility. Building that will require a major new energy research program, and that will not be easy to get started.
