FUSION


Simplified diagram of a tokamak fusion reactor. Image from fusionscience.org


Deuterium/Tritium Fusion
Tech Level: 12
Deuterium/Helium-3 Fusion
Tech Level: 13
Helium-3/Helium-3 Fusion
Tech Level: 13
Deuterium-Deuterium Fusion
Tech Level: 14
Portable Fusion Generator
Tech Level: 14
Bubble Fusion
Tech Level:15
Deep Plasma Focus Fusion Reactor
Tech Level: 15
Hydrogen/Boron Fusion
Tech Level: 15
Proton Chain Fusion
Tech Level: 16
Muon-Catalyzed Fusion
Tech Level: 16
Tabletop Fusor
Tech Level: 17

Though many of the above reactions have been achieved in laboratory fusion experiments, the Tech Level number indicates when each type of fusion reaction will likely become a commercially-viable source of power.

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

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.

A heavily-filtered image of fusion plasma

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.

The experimental fusion tokamak at the Oakridge National Laboratory

Fusion of elements heavier than isotopes of hydrogen and helium is of course possible, and is going on all the time in the heart of supermassive stars and in stars leaving the main sequence. They are usually not considered practical as a means of power production.


DEUTERIUM/TRITIUM FUSION
Tech Level: 12

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 along with 17.6 million electron volts of energy. This gives us an energy yield of about 3.38 x 10^14 joules per gram of fuel used, compared to the 8.8 x 10 ^13 joules per kilogram of nuclear fission, or nearly four times as much.

Deuterium-tritium (DT) fusion is the easiest fusion reaction to obtain, requiring the least amount of temperature (a "mere" 40 million degrees K) and pressure, but it unfortunately produces a great deal of high-speed neutrons as a byproduct of the reaction. Neutrons are electrically neutral and therefore are not easily contained in magnetic fields. This presents a serious radiation hazard, requiring 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 ocean water 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. 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 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 DT fusion reactor be lined with lithium-6, so that it can in effect help to produce its own fuel. There has also been speculation that tritium can be mined from the hydrogen-heavy atmospheres of gas giants. However, along with the problems of the heavy shielding required, 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 create heat.


DEUTERIUM/HELIUM-3 FUSION
Tech Level: 13

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 (DD) fusion, the most significant being they produce far less radiation. However, Helium-3 fusion processes require up to ten times the temperatures of DT fusion, and produce less energy overall.

Helium-3 is rare on earth, but exists in abundance on the Moon, deposited on the surface rocks and soil 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 harvesting operations 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.

Deuterium-Helium-3 (DH3) fusion has the benefit of producing only about one percent of the neutronic radiation of DT fusion, making DH3 reactors safe enough to build right alongside, or even in the midst of, cities. Unfortunately, the reaction produces significantly less energy (about 1/80th) that of DT fusion and requires about three times the operational temperature, so there is a trade off. However, the energy produced is still millions of times that of petrochemical fuels.

Slamming a Deuterium atom and a Helium-3 atom, or two Helium-3 atoms, together in a fusion reaction produces one atom of Helium-4, the more common form of Helium, and 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.

The fusion point is surrounded and contained by a powerful positively-charged electrical field. When a high-speed proton is given off by the reaction, it is repulsed by the electrical field. However, this act of repulsion transfers its energy potential from the proton to the surrounding field. Running a current through the field converts this potential into electrical energy 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 DH3 fusion produces less overall energy than DT fusion, DH3’s much higher percentage of energy conversion makes up for this.


HELIUM-3/HELIUM-3 FUSION
Tech Level: 13

As fusion is the holy grail of nuclear energy research, Helium-3/Helium-3(2H3) fusion is the holy grail of fusion research. The reaction produces very little harmful radiation or radioactive byproducts, even less than DH3 fusion and it produces currently directly, meaning up to 75% of its total energy potential can be harnessed.

Because Helium-3 is rare on Earth, its likely that 2H3 reactors will first be built in space, especially on future moon bases and settlements, where Helium-3 saturates the dust and surface rocks. On Earth, because Deuterium will likely remain much cheaper and easier to acquire, DH3 reactors will probably always predominate.


DEUTERIUM/DEUTERIUM FUSION
Tech Level: 14

Deuterium-deuterium (DD) fusion, like 2H3 fusion, requires much higher temperatures (400+ million degrees K) than DT fusion. However, it yields more overall energy, and deuterium is far more plentiful, and easier and cheaper to obtain, than Helium-3, making it a very economically desirable form of power, especially on Earth. However, like DT fusion, DD fusion produces neutronic radiation, requiring heavier shielding than the Helium-3 reactions. Though not quite as much as the DT reaction.

A very unusual form of DD fusion currently being researched is using the collapse of bubbles in deuterium-rich water. The process is known as Bubble Fusion or sonofusion, and is discussed below.


PORTABLE FUSION GENERATOR
Tech Level: 14

Gearing up for the higher temperatures and pressures needed for the more advantageous reactions like DH3 and 2H3 fusion will likely teach engineers a great deal about safely generating, containing, and taming fusion plasma, allowing them to scale down a great deal of the technology previously developed for it. Even though some laboratory fusion devices today are fairly small and can be said to be nominally table-top, these are still experimental and a long way from producing the types of energy needed for commercial applications.

Most likely the first portable fusion generators will be the fusion rockets discussed in that section, particularly the gas dynamic mirror fusion rocket. Helium-3 reactions would by far be preferred for interplanetary craft, as they would require far less shielding to protect the crew and there fore could be made much lighter.

Fusion generators will also likely see early deployment on large sea vessels, just as fission generators did in the 20th century. Modular fusion generators will also likely be developed to act as portable power sources for military and disaster relief operations, carried by either truck or cargo plane.


BUBBLE FUSION
Tech Level: 15

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.


DEEP PLASMA FOCUS FUSION REACTOR
Tech Level: 15

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 molded and handled with extreme precision in order to work. With other types of fusion reactions requiring less extreme methods of propagation and containment, DPF fusion will probably not be fully pursued until other types of fusion technology are already proven viable and already on the market. A more technical and detailed examination of this technology is included in the links below.

DPF reactors will most likely be needed to create the more intense types of fusion reactions listed below, both of which rely on temperatures approaching a billion degrees Kelvin or more.


HYDROGEN/BORON FUSION
Tech Level: 15

A Hydrogen-Boron (HB) fusion reactions 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-isotope Hydrogen. Boron is a fairly common element found in both the oceans and in Earth’s crust; it is most commonly 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.


PROTON CHAIN FUSION
Tech Level: 16

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
Tech Level: 16

Fusion reactors may take advantage of Muon Catalyzed Fusion, where the electron of hydrogen fuels is replaced with a muon. 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 reactions at lower temperatures than ordinary fusion reactions. However, the energy needed to create the muons for the reaction offsets any energy gained. If this can be offset, however, by finding a way to produce muons en masse cheaply, Muon-Catalyzed fusion could lead to a revolution in the way fusion reactions are created.


TABLETOP FUSOR
Tech Level: 17

One of the long-held dreams of science fiction is the cheap, light, easy-to-use fusion generator, sometimes called a fusor or fusion battery. Perhaps the best known example was the "Mr. Fusion" device that powered Dr. Brown’s time-travelling DeLorean in the Back to the Future movies.

There are of course a lot of technical obstacles to overcome before fusion reactors become as light, cheap, and easy to use as Mr. Fusion. Rather than downsizing enormous, billion-degree-temperature reactors, 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.

With the bubble fusion generator, the problem would be creating and manipulating enough bubbles to generate the power needed to run usable devices such as a computer, phone, or a car. In large open containers such as those used in the initial experiments, that many bubble forming, expanding, and collapsing in rapid succession my end up interfering with each other.

An alternative is to a create a liquid chamber honeycombed with microtubes, just wide enough to accommodate a single bubble. These microtubes would also contain the vibrational equipment. Neutrons would still bombard the entire chamber from an external source. Each bubble could thus be insulated from the rest, and the power of each bubble could be tapped directly via heat conductors through these tubes. Such micro-engineering capable of handling the continuous expansion and contraction of such high-energy, high-temp microbubbles over long periods of time reliably edges into science fiction territory.

It is also possible to create muon-catalyzed tabletop fusors, but in this case a miniature reactor to create the muons would be needed. This may or may not be separate from main fusor reactor unit itself. An alternative would be that muons could be created in a large centralized facility, and stored and sold in portable "traps". These modular traps would snap onto the fusor unit along with the fusion fuel, and like the fuel would have to be periodically replaced or refilled periodically.


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

http://www.nuc.berkeley.edu/thyd/icf/IFE.html

http://www.ofes.fusion.doe.gov/

http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fusion.html

http://www.direct.ca/trinity/helium3.htm

http://www.space.com/scienceastronomy/helium3_000630.html

http://fti.neep.wisc.edu/iec/inertial_electrostatic_confineme.htm

http://www.rexresearch.com/farnsworth/fusor.htm

http://en.wikipedia.org/wiki/Dense_plasma_focus

http://www.focusfusion.org/

http://www.focusfusion.org/what/deuterium.html


Article added 2006

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