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| Fig. 1: The indirect drive ignition process within an hohlraum. (Courtesy of the U.S. Department of Energy) |
In the search for a safe and clean energy source to replace coal, oil or nuclear fission, nuclear fusion is an exciting alternative. By combining the nuclei of two isotopes of hydrogen under stellar pressures and temperatures, an enormous amount energy is released, mostly in the form of high energy neutrons. In addition, the supplies of deuterium and tritium necessary to fuel the reaction are vast, given the proper method of breeding tritium within the plant. [1]
There are multiple methods of creating the pressure and temperature criteria necessary for fusion. One method involves magnetic confinement, in which the deuterium and tritium ions are confined by the magnetic field of a toroidal superconducting coil. [1] Another method involves electrostatic confinement of these ions. [2] Perhaps the most recently exciting method is inertial confinement, which applies a force on the deuterium-tritium fuel such that it is uniformly compressed and, in turn, ignited. With experiments well under way, NIF is setting its hopes high with plans to achieve laser inertial confinement fusion-fission energy (LIFE) by 2030, which will combine the benefits of fusion and fission while mitigating the safety and waste concerns normally associated with fission. [3] While LIFE heralds a clean and safe energy alternative, there are many technical barriers to address.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, is one of the few facilities at which the method of inertial confinement is undertaken. NIF uses 192 powerful lasers to compress and heat a deuterium-tritium fuel capsule by indirect drive. By this technique, a laser pulse is shot at the inside walls of the capsule's cylindrical container, known as the hohlraum, thereby causing a burst of x-rays toward the capsule and igniting the fuel. In direct drive, the capsule would instead be compressed and ignited by direct laser light. [4]
In order to increase the gain, or ratio of energy released to energy used, of the indirect drive system, the implosion velocity of the fuel capsule must be high enough (~4×107cm/s), but at lower implosion velocities, another technique may be employed to increase gain even higher, namely fast ignition. In this two-step process, the fuel is first compressed by indirect drive and then ignited by another indirect laser pulse. This second indirect laser pulse drives a surge of relativistic electrons toward the center of the capsule, which in turn ignites the fuel. [5]
This two-step process will, if successfully demonstrated, add more flexibility to NIF's development. First, as stated above, the gain of this reaction will be higher. Second, since fast ignition requires lower energy for compression, the variety of possible lasers that NIF could use will broaden. This might include more efficient lasers and lasers with greater pulsed power. [5]
NIF recently became the first facility to achieve a 500 TW laser shot, a major achievement in the race to achieve a gain of at least 1. [6] Once NIF reaches this goal, it will begin development on a larger project: LIFE. The concept of LIFE is particularly appealing, as LIFE would use neutrons from fusion to fuel fission reactions and potentially dispose of "99.9% of the planet's nuclear waste." [3] Despite these optimistic projections, there remain significant barriers to ignition that NIF or other facilities like it, such as the Laser Mégajoule in France, must address. [7] To put these challenges into perspective, the current system at NIF will, as estimated by simulations, produce a final gain of 17, meaning that of the 1.8 MJ the laser produces before contacting the hohlraum, a maximum of 30.6 MJ can be extracted from the reaction. [4,3] Thus, of the 422 MJ used to run the entire experiment (including energy lost to laser inefficiency), only 30.6 MJ can possibly be extracted by current techniques, giving a gain of less than 1%. [7]
This may be a strikingly low figure, but given the number of technical challenges, it is entirely within reason. Most notable of these challenges is laser inefficiency, which can be mitigated by the development of fast ignition, as mentioned above. A related challenge is shot frequency. In order to meet the demands of a large city such as San Francisco, NIF would need to fire at least 10 shots per second, which is currently impossible, considering the time it takes to cool optical components. [3] And as each capsule of fuel costs around $40,000, this shot frequency is unreasonable on a financial level. [8]
Also a factor in financial viability, the lifetime of the reaction chamber is a major limitation. The first wall of the chamber must sustain a large neutron flux from each reaction. Conventional steel will not survive this radiation for long, but scientists at NIF are well aware of this limitation. Current research suggests the possibility of enhanced steels that could last from 4 to 8 years inside the reaction chamber, but until this technology is within the chamber, the expense of replacing the chamber walls might overshadow the significance of any reaction-related scientific achievements. [3]
Despite any technical or logistical difficulties that NIF and other reactors face, the immense possibilities of fusion will never cease to excite us. Harnessing the power of the sun and its nuclear reactions right here on earth is an incredible feat of engineering, and the suggestion that it might rid the world of all its dependence on fossil fuels and diminish its nuclear waste is a tantalizing prospect. Although this feat of engineering is in many ways in its infancy, the potential benefits of such a technology are so great that the few decades that lie ahead its development are brief moments in comparison to the centuries of clean energy it could provide.
© Rex Garland. 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] J. Kates-Harbeck, "Magnetic Nuclear Fusion," Physics 240, Stanford University, Fall 2010.
[2] B. Klopfer, "The Fusor," Physics 241, Stanford University, Winter 2012.
[3] E. Moses, "Ignition on the National Ignition Facility: a Path Towards Inertial Fusion Energy," Nucl. Fusion 49, 104022 (2009).
[4] R. L. McCrory et al., "Progress in Direct-Drive Inertial Confinement Fusion," Phys. of Plasmas 15, 055503 (2008).
[5] M. Tabak et al., "Review of Progress in Fast Ignition," Phys. Plasmas 12, 057305 (2005).
[6] D. Geere, "Record-Breaking Laser Shot Hits 500 Trillion Watts," Wired Magazine, 17 Jul 12.
[7] R. Hamerly, "Inertial Confinement Fusion," Physics 240, Stanford University, Fall 2010.
[8] I. Sample, "California Fires up Laser Fusion Machine," The Guardian, 28 May 09.