Helium fusion

Helium fusion is a kind of nuclear fusion, with the nuclei involved being helium.

The fusion of helium-4 nuclei (alpha particles) is known as the triple-alpha process, because fusion of just two helium nuclei only produces beryllium-8, which is unstable and breaks back down to two helium nuclei with a half life of 1×10⁻¹⁶ to 2.6×10⁻¹⁶ seconds. If the core temperature of a star exceeds 100 million kelvins (100 megakelvins), as may happen in the later phase of red giants and red supergiants, then a third helium nucleus has a significant chance of fusing with the beryllium-8 nucleus before it breaks down, thus forming carbon-12. Depending upon the temperature and density, an additional helium nucleus may fuse with carbon-12 to form oxygen-16, and at very high temperatures, additional fusions of helium to oxygen and heavier nuclei may occur (see alpha process).

The fusion of helium-3 with itself or with helium-4 occurs during the fusion of hydrogen in main sequence stars (see proton-proton chain), and is not ordinarily referred to as helium fusion.

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*Fusion powerThe actual ratios of fusion to Bremsstrahlung power will likely be significantly lower for several reasons. For one, the calculation assumes that the energy of the fusion products is transmitted completely to the fuel ions, which then lose energy to the electrons by collisions, which in turn lose energy by Bremsstrahlung. However because the fusion products move much faster than the fuel ions, they will give up a significant fraction of their energy directly to the electrons. Secondly, the plasma is assumed to be composed purely of fuel ions. In practice, there will be a significant proportion of impurity ions, which will lower the ratio. In particular, the fusion products themselves must remain in the plasma until they have given up their energy, and will remain some time after that in any proposed confinement scheme. Finally, all channels of energy loss other than Bremsstrahlung have been neglected. The last two factors are related. On theoretical and experimental grounds, particle and energy confinement seem to be closely related. In a confinement scheme that does a good job of retaining energy, fusion products will build up. If the fusion products are efficiently ejected, then energy confinement will be poor, too.

The temperatures maximizing the fusion power compared to the Bremsstrahlung are in every case higher than the temperature that maximizes the power density and minimizes the required value of the fusion triple product. This will not change the optimum operating point for 21D-31T very much because the Bremsstrahlung fraction is low, but it will push the other fuels into regimes where the power density relative to 21D-31T is even lower and the required confinement even more difficult to achieve. For 21D-21D and 21D-32He, Bremsstrahlung losses will be a serious, possibly prohibitive problem. For 32He-32He, p+-63Li and p+-115B the Bremsstrahlung losses appear to make a fusion reactor using these fuels with a quasineutral, anisotropic plasma impossible. Some ways out of this dilemma are considered—and rejected—in Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium by Todd Rider. [13] This limitation does not apply to non-neutral and anisotropic plasmas; however, these have their own challenges to contend with.

References

*cite journal |last=Ray |first=Alak K. |authorlink= |coauthors= |year=2004 |month= |title=Stars as thermonuclear reactors: their fuels and ashes |journal=arΧiv e-print |volume= |issue= |pages= |id=arXiv|astro-ph|0405568 |url= |accessdate= |quote=