Nuclear fusion

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Nuclear fusion is the process by which lighter elements are fused together to produce heavier ones. It's the primary source of energy for every main sequence star in the galaxy.[1] Artificial fusion is also possible, the most vivid demonstration being the explosion of a thermonuclear weapon.

Contents

[edit] Reactions

An illustration of the proton-proton chain

In many stars, and the Sun in particular, fusion of hydrogen (H) to produce helium (He) is the most common kind of reaction. In the Sun the most common reaction pathway is the proton-proton chain[wp]. First 2 protons collide to produce the deuterium nucleus; in the process one of them turns into a neutron and a positron is released. Then another proton crashes into this nucleus to produce a 3He isotope. Finally 2 3He isotopes collide to produce a helium-4 nucleus.[2] This reaction produces a tremendous amount of energy, but is very slow to occur; on average, it takes a billion years to complete. Only one such fusion reaction occurs in every cubic kilometer of the sun's core per second.

In heavier main sequence stars (e.g. Sirius A), this hydrogen fusion reaction is catalyzed by carbon, in what's called the CNO Cycle. The net result is the same as from the proton-proton chain, but the reaction happens much more quickly (thousands or millions of years average reaction time, instead of billions).

Outside of main sequence stars there are also a number of other possible reactions, including the fusion of helium into carbon. Unfortunately these reactions would not be of significant use to a reactor as they tend to be somewhat less efficient and require even higher temperatures and pressures.

[edit] Potential for energy generation

The deuterium-tritium reaction

As such a convenient source of energy, a fusion reactor that actually produces more energy than it consumes would be of incredible importance. The most studied reaction for this purpose is deuterium-tritium (D-T) fusion, which involves two isotopes of hydrogen, one of which is rare, and the other of which is so rare that it has to be synthesized in a nuclear fission reactor. This reaction has a much higher cross section than the proton-proton reaction found in main sequence stars, and still gives quite a big bang per particle. It has the disadvantage that a neutron is produced, which can cause radioactive contamination. Furthermore, this neutron carries away the lion's share of the energy released by the reaction, and harnessing the energy of a neutron is much trickier than harnessing the energy of a charged particle.

Unsurprisingly, this line of research has attracted many cranks as well as respectable scientists.

Because it releases the most potential energy, and because of it's relatively high cross-section, it's almost certain that any future fusion reactor will rely on the D-T reaction. Unfortunately even such a simple reaction will be enormously expensive, even if it is possible.

[edit] Woo

See the main article on this topic: Fusion woo

The fringe idea most commonly associated with fusion is cold fusion, the claim that self-sustaining nuclear fusion is possible at temperatures far below what should be considered possible. Despite a few hoaxes suggesting the contrary, at present nuclear fusion is understood to require immense pressures and temperatures in order to occur.[3]

[edit] External links

[edit] Footnotes

  1. http://abyss.uoregon.edu/~js/ast122/lectures/lec13.html
  2. http://burro.astr.cwru.edu/Academics/Astr221/StarPhys/ppchain.html
  3. http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/lawson.html#c1
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