There are few current research efforts more synonymous with the potential of future technology than fusion power. The coming era of fusion promises potent, clean nuclear energy to meet all of our needs for centuries to come.
Or so proponents say. Researchers have been promising the "fusion breakthrough" for over half a century now. The reality of fusion power may not be as rosy as some would like to paint. While still providing abundant energy on a level current technologies simply cannot match, it is also rife with a number of potential missteps and hazards.
In this article, we'll see exactly where the road to fusion power started, the twists and turns along the way, and where it may eventually lead.
Basics
Fusion is the process by which two atoms combine—"fuse"—to become a heavier element. In the process, some of the mass of the fusing elements is converted into energy. It is the fundamental process that makes the stars shine, so we know enormous amounts of energy can be unlocked with nuclear fusion. In fact, per gram of fuel consumed, fusion can produce ten million times as much energy as burning petrochemicals like oil or gasoline.
One of the greatest stumbling blocks to creating a sustainable fusion reaction is the enormous amounts of pressure and heat needed to make atomic nuclei fuse. The fusion reaction with the lowest temperature needed, deuterium-tritium fusion, requires an environment of over 40 million degrees Kelvin. Such great temperatures are required to overcome the Coulomb barrier, the field of electric repulsion surrounding the protons of the fusing nuclei. The particles must basically be slammed together with enough force to get them close enough for the attractive nuclear strong force to take over. In stars, the immense pressures created at their cores by their own mass helps to overcome the coulomb barrier with lesser temperatures of only a few million degrees Kelvin, but as star-core pressures are a long way from ever being duplicated on Earth, fusion researchers must rely much more on high temperatures to propagate their reactions.
History
Nuclear fusion was first proposed as a theory back in the 1920s. In 1939, the German-American physicist Hans Bethe worked out the mathematics of the energy generation of fusion reactions. Bethe's results closely matched astronomical observations, proving that fusion powered all the stars in the universe. The idea of harnessing fusion energy was bandied about by the scientists of the Manhattan Project during World War Two, and research along those lines led to the creation of the first thermonuclear bombs in the 1950s.
Fusion energy research began in earnest in 1951, when the Atomic Energy Commission established a secret program called Project Sherwood to investigate the feasibility of using a controlled fusion reaction to generate electricity. In 1958, much of that initial research not tied to military applications was declassified in the West at the Atoms for Peace conference in Geneva. Fusion energy research projects sprung up worldwide in the decades that followed.
The first major breakthrough came in the 1960s from the USSR, where researchers created a toroidal magnetic confinement system called a tokamak, based on a design by physicists Andrey D. Sakharov and Igor Y. Tamm, to sustain plasma temperatures in the millions of degrees. In the 1970s, the energy crisis prompted renewed interest in fusion energy in the West, leading among other things to the creation of the US's Tokamak Fusion Test Reactor, which spearheaded US efforts in that direction for years to come. Efforts into Inertial Confinement Fusion were also started in the 1960s and declassified at about the same time as the Tokamak Fusion Test Reactor was being built. In the 1980s the US's Strategic Defense Initiative began another solid push for fusion generators that could function as power sources for space-based missile defenses.
In 1989, a public furor over fusion was sparked with the report of a successful "cold fusion" experiment. Chemists Martin Fleischmann and Stanley Pons at the University of Utah reported that electrolysis experiments with heavy water produced both an excess of heat energy and other byproducts consistent with fusion reactions. However, efforts to reproduce their experiment met with both mixed results and heated controversy. While cold fusion at this time seems to be a dead end, it did help to spark another surge of interest and funding in mainstream fusion research.
Since then, research has continued steadily, whittling away at the barriers of the fusion "break even" point, where a reaction will yield more overall energy than what was used to create and sustain it. Better methods of plasma containment and heating have been developed, reactions have been sustained longer, and newer and better equipment is continually being developed. Today, physicists have a much clearer idea of the plasma dynamics needed to control a reaction, and many are already sketching out a detailed map of the developments needed to move from today's world to the fusion-powered future.
Confinement
Fusion requires conditions that would instantly vaporize any material substance that tried to contain it. Instead, scientists had to develop specialized means of propagating reactions without destroying the machines they used to create them. So far two techniques, magnetic confinement and inertial confinement, have proven the most promising.
Inertial Confinement is simple in concept but very hard to achieve in reality. It quite simply is squeezing the fusion fuel from all sides equally, until the fuel reaches the critical temperature and pressure needed for fusion to occur. However, under these conditions the isotopes of hydrogen and helium used for fusion fuels quickly turn into superheated, very chaotic plasmas, making uniform compression incredibly difficult.
One form of inertial confinement fusion is found in hydrogen bombs, where radiation pressure from a surrounding nuclear fission chain reaction (an A-bomb) is used to compress the deuterium in the bomb's core to fusion conditions. While a proven and very effective technique, it is not a very practical method for creating anything except vast amounts of destruction.
A recently developed, more sophisticated, and far less destructive method of inertial confinement uses an array of many lasers or particle beams, focused on a single small pellet of fuel. The beams are aligned in such a way that the energy from their crossed beams compresses the fuel pellet as well as superheats it, allowing it to achieve fusion conditions. By cycling through fuel pellets rapidly, an inertial confinement fusion reactor might be made into a practical source of electrical power.
The other main means of producing fusion reactions is with magnetic confinement. Magnetic confinement fusion typically uses a tokamak, but there are a small minority of other designs. Similar in configuration to experimental particle accelerators, a tokamak holds a ring of plasma in the doughnut-shaped cradle of powerful, carefully maintained magnetic fields. The constantly looping plasma is superheated to fusion conditions by various techniques, such as high-speed collisions, compressing magnetic fields, and ignition via particle beam.
Deuterium/Tritium Fusion
Deuterium and tritium are both isotopes of hydrogen. Normal hydrogen has a single proton for its nucleus. Deuterium has a proton-neutron pair in its center, and tritium has a proton and two neutrons in its nucleus. Slamming an atom of deuterium and an atom of tritium together in nuclear fusion produces an atom of Helium-4 and a neutron, producing 17.6 million electron volts of energy. This gives us an energy yield of about 338 trillion joules of energy per gram of fuel used, or approximately 80 million times the amount of energy that's released from the detonation of a one-kilogram block of high explosive.
Deuterium-tritium (DT) fusion is the easiest fusion reaction to obtain, requiring the least amount of heat (a "mere" 40 million degrees K) and pressure, but it unfortunately produces a great deal of high-speed neutrons as a byproduct. Neutrons are electrically neutral and therefore are not easily contained in magnetic fields. This presents a serious radiation hazard, requiring very heavy physical shielding. Worse yet, the shielding itself becomes radioactive after extended use and has to be disposed of. The need to control and eventually dispose of hazardous radioactive shielding could well prove to be the greatest stumbling block in selling DT fusion to the public as a safe and viable energy source.
The sources of the fuel for DT fusion may also prove problematic. Deuterium is relatively easy to obtain; about 1 in 5000 water atoms on Earth has a deuterium atom as part of its hydrogen component. Sophisticated sifting of ocean water for deuterium gives modern civilization a potential supply of billions of tons of deuterium. And if ocean water can be considered as a fusion fuel, one gallon of seawater has a potential energy yield equal to 300 gallons of gasoline.
Tritium is another matter. There are no readily available natural sources for tritium on or near Earth, mainly because tritium has a half-life of only 10 years and therefore decays fairly rapidly. However, tritium can be "bred" by bombarding an isotope of lithium, lithium-6, with high-speed neutrons. Lithium-6 makes up about 7.4% of all naturally occurring lithium, giving potential DT fusion reactors an ample supply, but one still very limited and costly to produce compared to deuterium. Because one of the main byproducts of DT fusion is high-speed neutrons, it has been suggested that the inner layer of shielding in a reactor be lined with lithium-6, so that it can in effect help to produce its own fuel. There has also been speculation that in the distant future tritium could be mined from the hydrogen-heavy atmospheres of gas giants. However, along with the problems of the heavy shielding turning radioactive, the relative scarcity of tritium can be the other limiting factor in the commercial viability of DT fusion reactors.
DT fusion would create power mostly by using the high-speed neutrons it generates to produce heat, which in turn would be used to create high-pressure steam to drive turbines.
Bubble Fusion
Bubble fusion is also called sonofusion. Unlike the cold fusion claims of the late '80s and early '90s, bubble fusion actually does hold the promise of creating tabletop fusion generators sometime in the coming century.
In March 2002, in the journal Science, researchers reported that they had created fusion in a canister of deuterated acetone, which is saturated with deuterium. Every five milliseconds, researchers bombarded the canister with neutrons, causing tiny, microscopic cavities to form in the liquid. At the same time, they bombarded the acetone solution with selected frequencies of ultrasound, which causes the cavities to expand to 100,000 times their original size in microseconds, just barely large enough to be spotted with the naked eye. Rusi Taleyarkhan, the principal investigator of this phenomenon and a professor of nuclear engineering at Purdue University, was quoted in an article as comparing this potential energy buildup within the expanding bubbles as the equivalent of stretching a slingshot from Earth to the sun.
When these bubbles spontaneously collapsed a fraction of a second later, they generated heat and pressure within them equivalent to that found in stars. Temperatures as high as 10 million degrees Kelvin and pressures of thousands of atmospheres exist briefly at the heart of the imploding bubbles. This is enough to overcome the coulomb barrier in the deuterium within it, causing the atoms to undergo fusion.
The main advantage of bubble fusion is that while it still generates the extremes needed to create fusion, the bubbles in which they're created are so tiny as to pose no real risk to the outside environment.
At the moment, bubble fusion is seen less as a potential means of energy propagation and more as a means of producing large amounts of localized neutrons. Its first practical applications will be to act as part of portable neutronic sensors, to help synthesize certain substances like tritium, and for some medical radiation therapies. Though it's possible we may see bubble fusion generators someday powering our homes, most agree that decades of research and development has to take place first.
Helium-3 And Fusion
Helium-3 is an isotope of Helium that is deficient one neutron. Fusion reactions using Helium-3 have a number of advantages over DT or Deuterium/Deuterium bubble fusion, the most significant being they produce far less radiation.
Helium-3 is rare on Earth but exists in abundance on the moon, deposited on the surface rocks and soil there over hundreds of millions of years by the solar wind. Some estimates put the total available supply of Helium-3 on the moon at over 1.1 million metric tons, enough to supply the world's current energy needs for thousands of years.
Deuterium-Helium-3 (DH3) fusion and Helium-3/Helium-3 (2H3) fusion will most likely will not become commercially viable source of power until a moonbase is established and wide-scale harvesting operations of the isotope are underway. However, with the recent push by a number of national space agencies to return to the moon and establish a permanent manned presence there, it is also likely that Helium-3 technologies will be fast-tracked in part to help justify the cost of these initiatives. Indeed, it has been speculated that the whole reason the US, China, and other powers are now looking at the moon anew is specifically to acquire its vast stores of Helium-3.
DH3 fusion has the benefit of producing only about one percent of the neutronic radiation of DT fusion, making them safe enough to build right alongside, or even in the midst of, cities. Unfortunately, the reaction produces significantly less energy (about 1/80th) than DT fusion and requires about three to ten times the operational temperature, so there is a trade-off. However, the energy produced is still millions of times that of petrochemical fuels per kilogram of fuel.
Slamming a Deuterium atom and a Helium-3 atom together in a fusion reaction produces one atom of Helium-4, the more common form of Helium, and a proton. As the proton is electrically charged, it is easily manipulated by electromagnetic fields, and a means of electrostatic, as opposed to electromagnetic, containment can be used to propagate the fusion reaction.
In electrostatic confinement, the fusion point is surrounded and contained by powerful positively charged electrical fields. When a high-speed proton is given off by the reaction, it is repulsed by this electrical field. However, this act of repulsion transfers its energy potential from the proton to the surrounding field. Running a voltage through the field converts this potential into electrical current available for use. Unlike other forms of fusion, in which the fusion process is used to create heat, which is then used to generate electricity, DH3 and 2H3 fusion can be used to produce electrical current directly with much less energy loss. In fact, some proponents contend that up to seventy-five percent of the energy released by the fusion process could be harnessed. Thus, while its energy yield is much less than that of DT fusion, DH3 fusion's ability to create current directly with high efficiency helps to greatly offset this.
The main engineering disadvantage of Helium-3 fusion is that it requires an environment upwards of 400 million degrees Kelvin. Because Helium-3 is rare on Earth, its likely that DH3 reactors will first be built in space, especially on future moon bases and settlements, where Helium-3 saturates the surface. In fact, because of its abundance there, Helium-3/Helium-3 reactors become a possibility. On Earth, because Deuterium will likely remain much cheaper and easier to acquire, DH3 reactors will probably always predominate.
Deep Plasma Focus Containment
Deep Plasma Focus (DPF) fusion is also called dense plasma focus, z-pinch, or micropinch fusion, depending on the variation of the idea used. It is designed to create temperatures and pressures for fusion fuel plasmas that cannot be obtained with other confinement techniques.
In simple terms, a DPF reactor uses powerful electrostatic and electromagnetic forces to swirl superheated, super-accelerated plasma into a thin, compressed column—a "pinch"—where the pressure and heat escalate to unheard of levels at the column's thinnest point. The byproduct of a DPF reactor mostly comes out in the form of a beam or jet on the other end of the pinch, one of the reasons DPF is usually discussed more as a form of fusion rocket propulsion than as a source of power. Still, DPF reactors can be made to harness this exhaust to produce electrical power, either through direct current induction or indirectly through heating.
A commercially viable DPF reactor would require enormously powerful electrical and magnetic fields deployed with extreme precision in order to work. With other types of fusion technologies requiring less extreme methods of propagation and containment, DPF fusion will probably not be fully pursued until other types of fusion are already proven viable and already on the market. A more technical and detailed examination of this containment technique is included in the links below.
DPF reactors will most likely be needed to create the more intense types of fusion reactions that rely on temperatures approaching a billion degrees Kelvin or more. These include Hydrogen/Boron fusion, direct Deuterium/Deuterium fusion, and the ultimate energy prize, Proton-Chain fusion.
Hydrogen-Boron (HB) fusion creates three helium atoms and a proton. Like DH3 fusion, HB fusion creates a clean reaction with no radioactive waste and an energy byproduct, a proton, that can be converted directly into electricity via electrostatic confinement, making it a very clean and efficient source of power.
The isotope of Boron used is Boron-11, which has one extra neutron, combined with normal, non-isotopic Hydrogen. Boron is a fairly common element found in both the oceans and in Earth's crust; it is best known as a cleaning agent.
This type of fusion does have some disadvantages, the first being that it needs temperatures of nearly one billion degrees Kelvin to sustain. Also, such energetic reactions produce X-rays and low-energy neutrons as a byproduct, resulting in some radiation hazard in operating an HB reactor, beyond the residual neutrons that are given off by secondary fusion reactions in the plasma.
Deuterium-deuterium (DD) fusion also requires the much higher temperatures that DPF technology can provide. It yields more overall energy than any other fusion reaction yet described, and it has already been established that deuterium is far easier and cheaper to obtain than other fusion fuels. However, like DT fusion, DD fusion produces a good amount of neutronic radiation, requiring heavy shielding that eventually becomes radioactive.
Hydrogen-Hydrogen fusion, or more properly Proton-Chain fusion, uses plain old atomic hydrogen, requiring the insane temperatures and pressures usually only found in the hearts of stars. It releases the most energy of all the fusion reactions discussed here and is the basic fusion process that brings light to the universe.
Proton Chain fusion actually consists of several steps. Two atomic hydrogen nuclei (basically naked protons, as the voracious heat of the fusion environment has long since stripped the atoms of their electrons) collide and fuse, forming a deuterium atom. At this stage, energy is given off as a positron and a neutrino. These deuterium nuclei fuse again with another proton, forming Helium-3 and a burst of gamma rays. Finally, the Helium-3 particles fuse, creating the stable and fusion-resistant Helium-4 nucleus along with two free protons. Like with Hydrogen-Boron fusion, this reaction can produce electrical current directly, but will also need heavy shielding because of the gamma radiation produced. Of course, this excess radiation could be converted to heat, which in turn could help drive electrical turbines.
Muon-Catalyzed Fusion
Highly advanced fusion reactors in the future may take advantage of Muon-Catalyzed Fusion, where the electron of hydrogen fuels is replaced with a muon, which has an identical electrical charge. A muon is 207 times larger than an electron and therefore reduces the classical Bohr radius of an atom by like amount. Thus, atomic nuclei are able to approach each other more closely, and this enhances the likelihood of overlapping wave functions, increasing the probability of fusion.
Muon-Catalyzed Fusion is a well-proven technique in the laboratory and is notable in that it can create fusion at much lower temperatures than other reactions. However, the energy needed to create the muons in the first place offsets any energy gained as an end result. Muon-Catalyzed Fusion, if ever fully developed, will likely be used for specialized applications where quick on-the-spot energy propagation is more important than the cost of the initial energy investment, such as in spaceship propulsion or military applications.
However, if a way can be found to produce muon-enriched hydrogen fuels cheaply, Muon-Catalyzed fusion could lead to a revolution in the way fusion reactions are created. So-called cold fusion would become a reality.
Mr. Fusion And You
"Mr. Fusion" is the device that powered Dr. Brown's time-travelling DeLorean in the Back to the Future movies. In many ways, it represents the ultimate dream of all fusion visionaries: cheap, easy, readily available energy that can easily generate the gigawatts needed for many applications, even breaking the time barrier with a flux capacitor.
So, if fusion power can be made practical, what does that mean for the average person? What will it be like to live in a fusion future?
At first, people would see no dramatic changes in life overall. Fusion power plants would at the beginning require tremendous investments, the usual controversies would surround them (Are they safe? Are they worth it? Not in my backyard!), and their maturation would proceed at a snail's pace, as both investors and governments would do everything they could to ensure public confidence and avoid any potential disasters.
However, once the technology matures and becomes widely available, say twenty-odd years after the initial break-even breakthrough, energy prices would begin to drop dramatically. Because the first fusion plants will likely use DT fusion, disposal of radioactive shielding will remain a problem, especially from a public-relations stand point.
One of the fields that would make immediate use of the fusion technology would be space travel, not only as an energy source but also as a source of propulsion. Far more efficient and powerful than modern chemical rockets, fusion engines would open up space beyond low Earth orbit to human travel in a way earlier eras could never manage. Trips from orbit to the moon could take mere hours, and the solar system would become a true frontier for human exploration and settlement.
The most immediate consequence for the public from fusion space propulsion is that the moon becomes much more readily accessible, and with it, its vast stores of Helium-3. A Helium-3 "pipeline" would be invested in to bring the resources to Earth in large quantities. DH3 fusion powerplants begin to proliferate on Earth, and because the reactions are nearly free of radioactive byproducts, would generally be welcomed with open arms by the public. Within a generation or two, DH3 fusion could become the major provider of energy to the developed world.
After fusion becomes commonplace, more advanced forms of fusion, like those using Deep Plasma Focus containment, will likely be invested in. Deuterium/Deuterium fusion would especially be highly sought after, given its abundant availability from Earth's oceans. A century or so after the initial break-even fusion breakthrough, much of Earth's energy needs could be met using nothing but simple seawater as fuel source.
And what of Mr. Fusion? Will tabletop fusion generators, called "fusors" by speculative technology enthusiasts, ever become a reality?
Though the "cold fusion" technique first announced in 1989 seems currently unworkable, both bubble fusion and muon-catalyzed fusion point the way to how small, portable fusion generators may someday be made to work without vast amounts of shielding or magnetic containment. To power your home for a year or more in the next century, all you may need is a small bottle of deuterium-saturated water or Helium-3 that would snap snuggly onto your fusor unit in your basement.
Welcome to the fusion future.
Further Information
In Print
Return to the Moon: Exploration, Enterprise, and Energy in the Human Settlement of Space, by Harrison H. Schmitt
On the Web
UCB Inertial Fusion Energy Tutorial
Fusion Energy Sciences Program [US Department of Energy]
Nuclear Fusion [Georgia State University HyperPhysics site]
Philo Farnsworth, Fusors, and Intertial Electrostatic Confinement
Bubble Fusion
"Evidence Bubbles Over To Support Tabletop Nuclear Fusion Device" [ScienceDaily]
Helium-3
"There's Helium-3 in them there Moon hills!," by Guy Cramer
"Researchers and space enthusiasts see helium-3 as the perfect fuel source," by Julie Wakefield [Space.com]
Inertial Electrostatic Confinement Overview, by John F. Santarius
Deep Plasma Focus
Dense plasma focus [Wikipedia]
Focus Fusion Society [focusfusion.org]
Hydrogen-Boron vs. Deuterium-Tritium [focusfusion.org]