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In a fine article summarizing the work of Gerald Kulcinski, a University of Wisconsin Nuclear Engineer working on Helium-3 fusion, Eric Hedman got me thinking again about nuclear fusion. Nuclear fusion as a means of generating power for consumers began to get serious attention in the 1970s. Then they said that nuclear fusion was thirty years away, but in the latest timeline from the International Thermonuclear Experimental Reactor (ITER) group, fusion power is still at least another thirty years away. (JET tokamak photograph courtesy of EFDA-JET).

Nuclear fusion is so fascinating to us because it holds the promise of providing clean energy using virtually unlimited fuels. Also, it is big science. Fusion reactors are enormous and enormously expensive. And, as demonstrated by the picture above, they are very photogenic. It also captures the attention of the space enthusiast community because space exploration and nuclear fusion may turn out to mutually provide the means and the justification for their existences.

Also, don’t fail to notice the similarities between nuclear fusion and manned space exploration. The analogy of the two efforts is a good one: both are completely government funded, both are about 30 years from becoming commercially viable, both promise to revolutionize the world, and promotional materials for both tend start with “Imagine…”

Many Paths to Fusion
At its most basic level, nuclear fusion involves pushing two nuclei together with enough force that they fuse. More scientifically, the challenge behind nuclear fusion can be easily understood with the graph on the left (credit: Wikibooks) that plots the potential energy of a particle versus its distance from another nucleus. Initially, the particle is at some distance away, but to reach R=0, a certain amount of energy must be given to the incoming particle to overcome the barrier caused by their electric repulsion (called the Coulomb Barrier). Once the particle overcomes that barrier, it is attracted to the nucleus and fusion occurs. The difference between the potential energy at the top of the barrier and at the bottom of the well is the amount of energy given off during fusion.

So, to fuse particles you must force the nuclei together with enough kinetic energy to overcome the barrier, and there are a number of ways that this has been tried. There are basically three categories of methods: high temperatures and pressures, high velocities, or small separations between the nuclei. Particle accelerators such as the Tevatron at the Fermilab in Chicago (discussed in this research synopsis) use the high velocity approach to initiate fusion. Small nuclear separations can lead to nuclear fusion, but forcing the nuclei together requires tremendous energy except in the muon-catalyzed fusion process (which takes place at room temperature and used to be called Cold Fusion before the Fleischmann and Pons paper, see below). But, as that method is not a viable approach for generating power, I will focus primarily on the high temperature and pressure methods.

Cold Fusion: The Story We’re All Sick Of
Few scientific controversies in recent decades have generated as much hot air as cold fusion. In March 1989, Fleischmann and Pons, two researchers at the University of Utah, reported that they had generated a significant amount of excess heat using some electricity, a palladium catalyst, and heavy water. Their claim caught the attention of both the scientific community and the media, not only because of the import of the announcement but also because of the breaches in scientific best practices involved in publication by press release. Within a few months, the media had largely lost interest and the scientific consensus had fallen overwhelmingly against Fleischmann and Pons. Wikipedia has a summary of the events that is fairly sympathetic to the idea of electrolytic cold fusion. There are results not fully explained from their experiments that have continued to generate a low level of scientific inquiry both in the United States and Japan for the last 15 years. In fact, in 2004 the Department of Energy took up the issue of cold fusion again to see if the field had progressed to the point of having irrefutable results. Its report was inconclusive but not exactly supportive either. Right now, cold fusion is one of pseudoscience’s banner issues, and sites like FreeEnergyNews.com keep that community updated. (image credit: Oak Ridge National Laboratory)

Fusion for Energy: Plasma Confinement, Bubble Collapse, and Laser Beams
At this point there are a number of avenues of fusion energy research, but currently the magnetic plasma confinement (i.e. tokamak), bubble collapse (i.e. sonofusion), and laser ignition (i.e. inertial confinement) methods are receiving the most attention. Laser and sonofusion techniques rely on lasers and acoustic bubble collapse, respectively, to produce the necessary temperature and pressure for nuclear fusion. Laser ignition research is currently being pursued at the Lawrence Livermore National Laboratory’s National Ignition Facility (on wikipedia). Sonofusion, an off-shoot of sonoluminescence research begun in the 1990s hypothesizes that fusion occurs when bubbles generated by acoustic waves in fluid solutions implode violently (image of sonoluminescence credit: Kenneth S. Suslick UIUC). Magnetic plasma confinement fusion research began in the 1970s and has since improved to the point where significant amounts of energy can be produced at almost the same level as energy is input to keep the fusion going. In light of these successes and the preliminary state of research in the other areas, I’ll focus the rest of this entry on magnetic plasma confinement.

Continuing my comparison of manned space exploration and fusion research, the two fusion techniques seemingly most capable of producing excess energy useful for power generation are the laser-ignition and plasma confinement methods . Both methods require massive infrastructure, enormous startup costs, and plenty of opportunity for high-profile failure. The laser-ignition method relies on focusing hundreds of very high power lasers on a tiny pellet of deuterium that then implodes on itself in a fashion not unlike that in a hydrogen bomb. The image on the right is of the 10-meter target chamber at the National Ignition Facility. The combined output of these lasers for the brief pulses they are active is over 1 Petawatt (1×10¹⁵ watts). Plasma confinement methods require enormous magnetic fields on the order of 20 Tesla (thousands of times stronger than Earth’s field) and very large and complicated tritium breeding systems, neutron absorbing blankets, and associated facilities. Government research will need to reduce the costs of these methods by several orders of magnitude before they become commercially viable, and international cooperation is seen as the only means to share the expenses.

Magnetic Plasma Confinement: Tokamaks and ITER
Tokamak fusion reactors have long been considered the most likely means of achieving practical nuclear fusion energy. Their basic design is a torus (or a donut) within which intense magnetic fields confine very hot plasma. There is a long list of tokamak reactor experiments, but the three of note are the biggest and most recent, the TFTR, JET, and ITER tokamaks. The Tokamak Fusion Test Reactor (TFTR) at Princeton University generated the highest temperature and set what was in 1994 a world record for energy generation. The Joint-European Torus (JET), pictured above, currently holds the record for the most energy generated by a controlled fusion reaction. The International Thermonuclear Experimental Reactor (ITER), still in the design phase, holds hope of being the first break-even nuclear fusion reactor in the world. (image credit: ITER)

Like any international scientific and engineering collaboration expected to cost tens of billions of dollars, ITER has been long in planning. First proposed around 1985 as an international diplomatic research effort, final agreement on a construction site was not reached until June of 2005. Construction is expected to begin in 2008 and finish in 2016. ITER is designed to generate 500 MW (about 10 times the record held by JET) and will hopefully produce more energy than is required to keep the plasma heated and confined. The success of the project is by no means guaranteed, however, and many of the criticisms surrounding it have focused on the technical challenges. Other criticisms have noted that the neutrons released in the deuterium-tritium fusion would create secondary radiation within the metallic parts of the reactor chamber. This secondary radiation would create radiological waste disposal problem, and would also shorten the life of the components in the reactor through radiative metal fatigue.

If ITER is largely successful, research done there over the years between 2015 and 2035 will show us what yet needs developing before commercial nuclear fusion is feasible. Following ITER, a hypothetical successor is project DEMO which will produce commercial nuclear fusion energy for the first time. If ITER is unsuccessful for technical reasons, the timeline for nuclear fusion will likely be driven even further back, keeping the “30 years away” projection true no matter when it is said.

Disadvantages of D-T Fusion, or why ³He
The degree of ITER’s success will make or break international acceptance for commercial nuclear fusion, barring any major breakthroughs with other fusion energy efforts. However, there are issues surrounding the enormous release of neutrons inherent in the deuterium-tritium fusion process that will keep the price of fusion energy uncompetitive for a long time to come. This fact is what gives hope to researchers such as Kulcinski. The Moon contains large quantities of ³He deposited by aeons of solar wind bombardment. If we could somehow harvest enough ³He on the Moon and bring it back here to the Earth, we could use the D-³He fusion process to generate energy with absolutely no radioactivity. Theoretical calculations of ³He abundances on the Moon suggest that it may have enough to supply current world energy demand for thousands of years. Even further out, the gas giant planets contain enough ³He to power human civilization for millions of years. ³He would also provide an ideal fuel for fusion drives for spacecraft.

Nevertheless, the means to harvest ³He, as well as the infrastructure to transport it, and the reactors to fuse it with deuterium do not yet exist. Overcoming this set of hurdles would be very symbolic as it would both create economic incentive for manned space exploration, and provide the holy grail for commercial fusion energy. Thus two massively expensive human enterprises could become synergistically profitable.