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| Fig. 1: Diagram of a Tokamak. (Source: Wikimedia Commons) |
We need to change our energy system. High levels of carbon dioxide emissions resulting from the burning of fossil fuels are altering the Earth's climate in ways we may not be able to predict or adapt to. Many of the brightest minds on Earth are looking for solutions to this energy conundrum, and many of these thinkers place their faith in harnessing the power of the sun through solar photovoltaic cells. While photovoltaic power is an impressive technology, it's really a secondary method of capturing energy generated by nuclear fusion in the Sun's core. If we could recreate fusion in a controlled environment here on Earth, we would have a plentiful, carbon-free source of power for the foreseeable future. Unfortunately, there are a plethora of challenges facing fusion researchers, many resulting from the apparent necessity for extremely high temperatures in order to ignite fusion reactions. In this report, we'll explore some of the paths that researchers are pursuing in an effort to master controlled nuclear fusion.
We will first cover Magnetic Confinement Fusion, one of two major branches of research. Magnetic confinement fusion, like most strategies for nuclear fusion, involves heating hydrogen until it becomes plasma in order to overcome the enormous repelling electrostatic forces. [1] The plasma is contained by a strong magnetic field, usually created by a structure called a tokamak (Fig. 1). [1] While magnetic confinement fusion holds promise, it's very challenging to keep the plasma confined. Confinement times are on the order of a few seconds, so the plasma is not stable enough to begin a fusion reaction.
Inertial Confinement Fusion is the other major branch of research being pursued. While magnetic confinement fusion uses magnets to confine the hydrogen plasma, ICF uses an array of lasers to hold the hydrogen in place. [2] As shown in Fig. 2, the lasers blast away the outer layer of heavier isotopes of hydrogen, which cause inward-traveling shockwaves that compress and implode the center of the fuel pellet. [2] There are competing theories about the best way to apply these lasers to the pellet, but so far no research team has been able to apply the lasers uniformly to the pellet in an energy-positive way. [2] The key to ICF is applying the lasers uniformly across the surface of the pellet, because if the application is not uniform, the plasma will escape confinement and any chance of fusion will be lost. [2]
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| Fig. 2: Simplified diagram of inertial confinement fusion. (Source: Wikimedia Commons) |
One of the more interesting alternative methods being explored is fusion through sonoluminescence, or sonofusion. Sonoluminescence refers to the phenomenon of converting sound into heat and light inside a bubble. [3] Some scientists believe that nuclear fusion occurs during sonoluminescent events, but the evidence is unclear. [3] Our current understanding of plasma physics would indicate that fusion is not occurring, but alternative explanations for the energy released during sonoluminescence are also wanting. [3] For now, sonoluminescence is not much more than a curiosity.
The final technology we will explore is the fusion-fission hybrid. During a fusion reaction, a excess neutrons are released. The fusion-fission hybrid aims to use those neutrons to begin a fission chain reaction that will continue to produce energy. [4] While this technology sounds promising, in it's current form the vast majority of the energy produced comes from the fission reaction, so it's not necessarily a superior technology to our current best fast breeder designs. [4] Until we can tackle fusion on its own, the fusion-fission hybrid will not be a viable option.
Nuclear fusion represents the promise of carbon-free, abundant electricity without the production of hazardous nuclear waste. Because of this incredible upside, research will and should continue on many possible methods for controlling the reaction. However, all of the technologies that we currently are exploring have serious deficiencies, and a breakthrough in fusion does not seem to be on the near horizon.
© Dominick Francks. 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] H. P. Furth, "Magnetic Confinement Fusion," Science 249, 1522 (1990).
[2] R. Islam, "Inertial Confinement Fusion: A Promising Alternative? Physics 241, Stanford University, Winter 2015.
[3] R. T. Lahey, Jr., R. P. Taleyarkhan, and R. I. Nigmatulin, "Sonofusion - Fact or Fiction?" Rensselaer Polytechnic Institute, 2 Oct 05.
[4] W. Manheimer, "The Fusion Hybrid as a Key to Sustainable Development," J. Fusion Energy 23, 223 (2004).
