Nuclear fusion

In physics, nuclear fusion is a process where two small

nuclei join together to form a larger nucleus. This process occurs because the binding energy per nucleon of small nuclei such as ¹H, ²H and ³He is small compared to the binding energy per nucleon of the nuclei that they make when they fuse.

Because nuclear energy levels are many orders of magnitude larger than atomic energy levels the energy released is much larger than that for chemical reactions. For example, the ionization energy of hydrogen is 13.6 eV, compare that number with the figures given later for hydrogen fusion (which are up to 1000 times greater).

Nuclear fusion is the energy source that causes stars to shine, and is one of the processes in the hydrogen bomb. For many years, considerable theoretical and experimental effort has gone into tapping fusion power, initially for electricity generation and possibly as an extremely efficient spacecraft propulsion system.

Some argue that fusion is the best option for a truly sustainable or long term energy source because the fuel is virtually inexhaustible and readily available throughout the world (deuterium can be taken from water and a thimble full of deuterium is equivalent to 20 tons of coal in energy production, so that a medium size lake contains enough deuterium to supply a nation with energy for centuries using fusion). It is argued that power plant operation will be inherently safe without the risk of long-lived radioactive waste (Much less radioactive waste results from fusion than from fission or coal plants. During the D-T reaction, neutrons are released which cause the reactor vessel to become radioactive. This radioactivity can be greatly reduced by using "low activation" materials. Such materials would have half-lives of tens of years, rather than the tens of thousands of years for radioactive waste produced from fission.). Fusion will be environmentally sound without atmospheric pollutants or contribution to global warming (compared to fossil fuels where 64 lbs of CO₂ is produced per American per day from fossil fuel usage.)

However, critics point out that it is far from clear that nuclear fusion will indeed be economically competitive with other forms of power. It is possible that fusion advocates are making some of the same mistakes in creating unrealistic economic projections that fission advocates have made in the past. When the cost of the plant is factored in, it is not clear that fusion will be cheaper than traditional forms of power, and although there are many economic estimates of the cost of fusion power, these estimates can give wildly different answers as to the economic viability of fusion power, depending on what the input assumptions of the models are. Fusion advocates would counter that it is very dificult to predict these future costs, especially as they depend upon political climates which would set regulatory standards, and was a large source of the rising price of fission power, for instance. It has also been argued, although most economists would disagree, that it is dificult to weigh an increased economic cost with the environmental advantages of fusion.

Fusion does also have potential safety concerns. Although there intrinsically would be no danger of a runaway fusion reaction (a meltdown) and any malfunction would result in a rapid shutdown of the plant, there are possible scenarios which are safety concerns. In 1973 the American Association for the Advancement of Science (AAAS) pointed out several concerns for a fusion power plant, including the possibility of a tritium leak, lithium fire or the accidental release of magnetic energy. These concerns would need to be addressed as part of any reactor design.

Unfortunately, there are still significant barriers standing between current scientific understanding and technological capabilities and the practical realization of fusion as an energy source, and it is far from clear that an economically viable fusion plant is even possible. It is an extremely difficult task to harness a 100 million degree plasma in an economically efficient way, so a working reactor is still many years down the road and is an active part of plasma physics research.

A substantial energy barrier opposes the fusion reaction. The long range Coulomb repulsion between the nuclei is offset by the attractive strong nuclear force. The problem becomes one of bringing the nuclei sufficiently close for the strong nuclear force to overcome the Coulomb barrier. The cross section combines the effects of the potential barrier and thermal velocity distribution of the nuclei into an effective area for fusion collisions. The cross section forms an equation

f=nσν

where n is the density of nuclei, σ is the cross section, ν is the thermal velocity, and f is the frequency of fusion producing collisions. Increasing any of these three quantities will increase the collision frequency. The cross section is also itself a function of thermal energy in the nuclei. Cross section increases from virtually zero at room temperatures up to feasible magnitudes at temperatures of 10 - 100 keV. At these temperatures, the fusion reactants exist in a plasma state.

In order for fusion reactions to occur, the particles must be hot enough (temperature), in sufficient number (density) and well contained (confinement time). This can be quantified by what is commonly called the fusion triple product nTτ or pτ where p=nT. For reasonable fusion reaction rates the temperature and density require the matter to be in in a plasma state. There are several mechanisms for confining these hot plasmas - magnetic, electrostatic, inertial and gravitational.

Gravitational confinement is how stars work. However, there are engineering difficulties in building reactors. No-one knows how to create the necessary confinement field without using impractically large masses, or unavailably dense materials.

Inertial confinement is basically squeezing the plasma with external photons (huge lasers or in the case of a thermonuclear weapon the intense x-rays emitted from a nuclear fission explosion) or particles (usually heavy ions). Inertial confinement schemes are inherently not steady state and a reactor design would require repetitions of inertial reactions (small explosions)

Electrostatic confinement uses grids, usually spherical in a sort of large vacuum tube, to concentrically accelerate ions to a focus. Since one electron volt equals 11,604 degrees, this approach can achieve fusion with practical voltages. This is the approach taken by the Farnsworth-Hirsch Fusor. Fusors are accelerators, not heated bottles. They therefore achieve consistent high-energy collisions, which makes them far more efficient. Also, unlike linear accelerators, they recycle ions which do not fuse, because the ions scatter elastically and are refocused by the field.

Fusors are attractive for many reasons. They theoretically should be able to use fusion reactions that are impractically energetic for a thermal reactor, such as the proton - Boron 11 reaction, which generates only high energy alpha particles with no gamma or neutrons. Some designs could electrostatically decelerate ionized reaction products, achieving direct conversion of nuclear power to electricity, at efficiencies above 90%. Fusors are mostly vacuum, so they should have a mass low enough to be a practical power source for vehicles, including aircraft and spacecraft. Fusors might even be portable: operating fusors have been built with vacuum chambers as small as 30cm. Finally, fusor research uses relatively small amounts of equipment. It should therefore be inexpensive compared to the other forms of fusion.

However, reseearchers believe that fusors do not achieve break-even because the ions collide with the grids, which wastes energy. Some people, attracted by the advantages and the low research costs, believe that this problem might be both more valuable and more easily solved than the problems that have surfaced in inertial and magnetic confinement thermal fusion rectors.

Magnetic confinement of a thermally accelerated plasma is the most highly-developed form of fusion reactor. It uses the electromagnetic properties of a plasma to confine the particles with a magnetic field. A magnetic field confines a particle in two dimensions, rather than three, so magnetic confinement devices (for example tokamaks and stellarators) are toroidal, so that the particles can be confined for long periods of time.

Magnetically-confined plasmas have to be heated. Only a small fraction of the most energetic ions actually achieve the temperatures necessary to fuse.

Plasma Heating

In an operating fusion reactor, part of the energy generated will serve to maintain the plasma temperature as fresh deuterium and tritium are introduced. However, in the startup of a reactor, either initially or after a temporary shutdown, the plasma will have to be heated to 100 million degrees Celsius. In current tokamak (and other) magnetic fusion experiments, insufficient fusion energy is produced to maintain the plasma temperature. Consequently, the devices operate in short pulses and the plasma must be heated afresh in every pulse.

Ohmic Heating: Since the plasma is an electrical conductor, it is possible to heat the plasma by passing a current through it; in fact, the current that generates the poloidal field also heats the plasma. This is called ohmic (or resistive) heating; it is the same kind of heating that occurs in an electric light bulb or in an electric heater. The heat generated depends on the resistance of the plasma and the current. But as the temperature of heated plasma rises, the resistance decreases and the ohmic heating becomes less effective. It appears that the maximum plasma temperature attainable by ohmic heating in a tokamak is 20-30 million degrees Celsius. To obtain still higher temperatures, additional heating methods must be used.

Neutral-Beam Injection involves the introduction of high-energy (neutral) atoms into the ohmically -- heated, magnetically -- confined plasma. The atoms are immediately ionized and are trapped by the magnetic field. The high-energy ions then transfer part of their energy to the plasma particles in repeated collisions, thus increasing the plasma temperature.

Magnetic Compression: A gas can be heated by sudden compression. In the same way, the temperature of a plasma is increased if it is compressed rapidly by increasing the confining magnetic field. In a tokamak system this compression is achieved simply by moving the plasma into a region of higher magnetic field (i.e., radially inward). Since plasma compression brings the ions closer together, the process has an additional benefit of facilitating attainment of the required density for a fusion reactor.

Radio-frequency Heating: High-frequency waves are generated by oscillators outside the torus. If the waves have a particular frequency (or wavelength), their energy can be transferred to the charged particles in the plasma, which in turn collide with other plasma particles, thus increasing the temperature of the bulk plasma.

See fusion power

Solar Cycle

Fusion powers the Sun and other stars. In stars the size of the sun or smaller, the proton-proton chain predominates; in larger stars, the CNO cycle is the dominant reaction.

Some other fusion reactions which are interesting for building a terrestrial reactor are:

(D is a shorthand notation for deuterium (H²), and T is short for tritium (H³))

D-D reaction (both reactions are equally likely to occur)

D + D -_50%-> T (1.01 MeV) + p (3.02 MeV)

D + D --> He³ (0.82 MeV) + n (2.45 MeV)

D-T reaction (good for reactors because cross section peaks at lower temperature ~50 keV)

D + T --> He⁴ (3.5 MeV) + n (14.1 MeV)

D-He³ reaction

D + He³ -_51%-> He⁴ + n + p + 12.1 MeV

D + He³ -_43%-> He⁴ (4.8 MeV) + D (9.5 MeV)

D + He³ -_6%-> He⁵ (2.4 MeV) + p (11.9 MeV)

Historical Development of Fusion

1929 - Atkinson and Huetermans used the measured masses of light elements and applied Einstein's discovery that E=mc² to predict that large amounts of energy could be released by fusing small nuclei together.
1939 - Hans Bethe won the Nobel Prize in physics (awarded 1968) for quantitative theory explaining fusion
shortly after World War II and the success of the Manhattan Project the hydrogen bomb was built, which released large amounts of fusion energy from a reaction ignited by a fission trigger
1951 - Argentina publicly claimed that they had harnessed controlled nuclear fusion (these claims were false), sparking a responsive research effort in the U.S.
- Lyman Spitzer started the Princeton Plasma Physics Laboratory (or PPPL) which was originally codenamed Project Matterhorn - most early work was done on a type of magnetic confinement device called a stellarator.
- James Tuck, an English physicist, began research at Los Alamos National Laboratory (LANL) under the codename of project Sherwood, working on pinch magnetic confinement devices. (Some people claimed that the project was named Sherwood based on Friar Tuck)
- 1952 Edward Teller expanded hydrogen bomb research at Lawrence Livermore National Laboratory (LLNL) and began studying inertial confinement using high powered lasers.
1958 - American, English and Soviet scientists began to share previously classified fusion research, as their countries declassified controlled fusion work as part of the Atoms for Peace conference in Geneva (an amazing development considering the Cold War political climate of the time)
1967 - Demonstration of Farnsworth-Hirsch Fusor appears to generate neutrons in a nuclear reaction.
1968 - Results from the T-3 Soviet magnetic confinment device, called a tokamak, which Igor Yevgenyevich Tamm and Andrei Sakharov had been working on - showed the temperatures in their machine to be over an order of magnitude higher than what was expected by the rest of the community. The western scientists visited the experiment and varified the high temperatures and confinement, sparking a wave of optimism for the prospects of the tokamak as well as construction of new experiments. which is still the dominant magnetic confinement device today.
1978 - The European Community (with Sweden and Switzerland) launched the JET (tokamak) project in the UK
1988 - The Japanese tokamak, JT-60 came online
March 1989 - some scientists announced that they achieved cold fusion - causing fusion to occur at room temperatures. However, they made their announcements before any peer review of their work was performed, and no subsequent experiments by other researchers revealed any evidence of fusion.
1993 - The TFTR tokamak at Princeton (PPPL) does experiments with 50% deuterium, 50% tritium, which eventually produces as much as 10 MegaWatts of power from a controlled fusion reaction.
1997 - The JET tokamak in the UK produces 16 MW of fusion power. This is roughly their break even point. They were produced as much fusion power as they were using to heat the plasma and sustain the reaction.
2002 - Claims and counter-claims are published regarding bubble fusion, in which a table-top apparatus is reported as producing small-scale fusion in a liquid undergoing acoustic cavitation.