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| Fig. 1: Inertial confinement fusion process. The fast fusion technique hits the compressed core with a burst of laser energy, heating the core to the point of igniting a fusion reaction. (Source: Wikimedia Commons) |
Fast ignition is a technique applied to experiments using Inertial Confinement Fusion (ICF) to ignite fusion reactions, commonly for power generation applications. Modern ICF research facilities, such as the National Ignition Facility, direct arrays of lasers onto a single, millimeter-sized pellet of fuel, commonly composed of deuterium and tritium. Given the sudden extreme transfer of energy, the outer layer of the fuel pellet explodes outward. If the ignition lasers have impacted the pellet symmetrically, as shown in Fig. 1, the resulting shock waves compress the pellet's core to conditions similar to those at the center of a star, and the resulting heat initiates a fusion reaction throughout the remaining fuel, [1,2] ICF techniques involve two main approaches:
Direct energy transfer, in which each laser beam directly heats the surface of the fuel pellet. [1] However, the approach can suffer from minute inconsistencies in laser positioning or beam properties which affects the maximum temperature and pressure at the pellet's core. [3]
Indirect energy transfer, in which each beam heats a small cylinder, or hohlraum, in which the fuel pellet is encased. The heated hohlraum then releases X-rays which in turn heat the fuel pellet evenly, in exchange for a loss in energy efficiency. [1,2]
In both cases, fast ignition is being explored as a viable addition that could increase the performance of inertial confinement fusion reactions.
Achieving the necessary pressure and temperature to initiate a fusion reaction through the precise application of symmetric shock waves remains technically challenging. Fast ignition relaxes these constraints by providing an extremely short (on the order to picoseconds) burst of energy directly to pellet core at the moment of maximum density, starting the fusion reaction. [3] In doing so, less energy is expended in heating the pellet outer shell and even, synchronized heating is less of an issue.
However, its addition does not make ICF a commercially viable energy solution, and fast ignition carries its own set of challenges.
ICF facilities continue to be large, extremely costly, and often fail to meet desired operating criteria. Foremost among them is the National Ignition Facility (NIF) at Lawrence Livermore National Labs, which was completed in 2009 after almost three decades of development. [4,5] After two years of attempting to initiate a fusion reaction, the DOE concluded that the theoretical models on which the entire design had been based were not accurate enough to leave much hope of succeeding in the future. [6] In any case, the 1.1 MJ laser used by the NIF would consume too much energy to make the process cost-effective for power generation. [5]
Central to the effectiveness of fast ignition itself is how its high-energy pulse is delivered to the core of the deuterium-tritium pellet. One method concentrates the heating pulse on one point of the pellet's circumference. However, this incurs a loss when the pulse is spent heating the fuel material between the application point and the core and energy is diverted away from the desired path. The University of Osaka's GEKKO XIII laser, among other installations, instead uses a gold cone inserted into the fuel pellet, providing direct access to the dense core for the heating beam. Although the "cone-in-shell" procedure does allow for the pellet core to become dense enough to spark a fusion reaction, and despite simplifying the application of the fast ignition beam, the cone alters the symmetry of the spherical pellet and affects the compression behavior of the outer shell-induced shock waves. [7]
Surmounting such technical challenges is appealing to ICF research scientists because reliable fast ignition could make the process suitable for commercial applications. In 2016, a group using the OMEGA laser setup at the University of Rochester observed significant efficiency improvements when using the cone-in-shell fast ignition technique with kJ-scale lasers, increasing measured neutron yield by a factor of approximately 2.4 at full power. [8] Given that any useful future fusion ignition device would have to be as efficient as possible, there is substantial research into the best delivery mechanism for the fast ignition pulse. The fast ignition realization experiment (FIREX) at the University of Osaka developed a kilotesla magnetic field to guide the heating pulse in the place of a physical gold cone embedded in the particle, although a full-scale power generation test could take twenty years or more to achieve. [9] Fast ignition does not yet place us on the cusp of an energy renaissance, but as the technique matures, efficient ICF may be possible.
© Jean-Luc Watson. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. Heller, "On the Path to Ignition," Lawrence Livermore National Laboratory, Science and Technology Review, March 2013, p. 11.
[2] R. Islam, "Inertial Confinement Fusion: A Promising Alternative?," Physics 241, Stanford University, Winter 2015.
[3] M. Roth et al., "Fast Ignition by Intense Laser-Accelerated Proton Beams," Phys. Rev. Lett. 86, 436 (2001).
[4] E. I. Moses, "The National Ignition Facility and the National Ignition Campaign," IEE Trans. Plasma Sci. 38, 684 (2010).
[5] M. Baumer, "Why Has the National Ignition Facility Failed to Live Up to Its Name?," Physics 241, Stanford University, Winter 2015.
[6] "2015 Review of Inertial Confinement Fusion and High Energy Density Science Portfolio," US National Nuclear Security Administration, DOE/NA-0040, May 2016.
[7] R. Kodama et al., "Fast Heating of Ultrahigh-Density Plasma as a Step Towards Laser Fusion Ignition," Nature 412, 798 (2001).
[8] L.C. Jarrot et al., "Visualizing Fast Electron Energy Transport into Laser-Compressed High-Density Fast-Ignition Targets," Nature Phys. 12, 499 (2016).
[9] H. Azechi et al., "Present Status of Fast Ignition Realization Experiment and Inertial Fusion Energy Development," Nucl. Fusion 52, 104021 (2013).
