# Fusion

The term fusion is often used as shorthand for nuclear fusion, but it is also a general term describing the merging of two or more entities into a single entity. The term "heat of fusion" describes the additional heat energy that is required to convert a solid that is already at its melting point into a liquid at the same temperature.

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 levelsthe 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).

Solar Cycle

Fusion powers the Sun and other stars. The process is called the proton-proton cycle, [often shortened to p-p cycle] The first step involves the fusion of two hydrogen into Deuterium
1H + 1H --> 2H + e+ + ν + 0.42MeV
The e+ immediately annihilates with one of the hydrogen's electrons and their mass energy is carried off by two gamma ray photons.
e+ +e- --> 2γ + 1.02 MeV
After this the deuterium produced in the first stage can fuse with another hydrogen to produce 3He
2H +1H --> 3He +γ + 5.49 MeV
Finally the 3He produced can fuse together to make 4He 3He +3He -->4He +1H +1H + 12.86 MeV
The whole cycle releases a net energy of 26.7 MeV per cycle, and the sun processes { someone put the right number in here please} cycles every second.

Another way this happens is the CNO cycle in heavier stars.

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)

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 hte matter to be in in a plasma state. There are three principle mechanisms for confining these hot plasmas - magnetic, inertial and gravitational.

Fusion is the best option for a truly sustainable or long term energy source, the fuel is virtually inexhaustible and readily available throughout the world (deuterium can be taken from ocean water and a thimble full of deuterium is equivalent to 20 tons of coal in energy production). 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 millions or billions 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.)

The abundant, relatively cheap, and extremely energy-dense fuel supply, lack of grerenhouse emissions, and much lower radioactive waste levels suggest that if we were were to succeed in creating a controlled fusion reactor our energy problems would be solved. Unfortunately, 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.

See fusion power

Historical development of Fusion Knowledge

• 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 succesively 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.
• 1958 - American, English and Soviet scientists began to share previously classified fusion research, as their countries declassified controlled fusion work (an amazing development considering the Cold War political climate of the time)
• 1968 - Results from the T-3 Soviet magnetic confinment device, called a tokamak, which Igor 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.
• 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 England produces 16 MW of fusion power, which is roughly their break even point where they were producing as much fusion power as they were putting in to heat the plasma and sustain the reaction.