340. Thermonuclear fusion: some basic facts about thermonuclear reactions

When I wrote Ten open problems in physics, the ultimate plan behind the post was that I first list those problems and then discuss every single one of them to some details – just to learn something new and relevant about each of the problems would already be enough fun for me to consider this idea seriously. As I said before, discussing important open problems is a) fun, b) it makes physics interesting and c) it makes it also relevant.

Somehow, the plan got crippled (I am lazy), and so far I was only able to discuss the only problem in the list to some extent – the problem of turbulence. Although I did not finish with turbulence yet, let me switch to something else: problem N 7 in the list – thermonuclear fusion and recall…

Some basic facts about thermonuclear reactions

Thermonuclear reactions are reactions between light atomic nuclei that proceed at very high temperatures, K. Wikipedia has a great article about nuclear fusion, and it would be stupid to reproduce it here, so I’ll try to be as original as possible in the given context

Thermonuclear reactions belong to the class of processes (actually, relatively rare) where nuclei overcome Coulomb repulsion and get close enough in order for weak interactions to cease to be negligible.

In practice, the latter means that once the Coulomb interaction gets overcomed, the system basically falls into the deep potential well on the picture above and gets restructured with subsequent (kinetic) energy release.

Weakly bound nuclei get transformed into strongly bound ones. Since nuclei with the largest binding energy per nucleon are located in the middle part of the Mendeleev table, the most typical thermonuclear reactions are fusions of lighter nuclei with production of heavier ones. Such reactions as   (decay of light nuclei with production of heavier ones) are also possible in Nature, though.

In Nature, the Coulomb barrier gets usually overcomed in two ways (correspondingly, we will classify thermonuclear reactions as A-reactions and B-reactions).

In the first case, the basic idea is to lower the Coulomb barrier in order to overcome it. The potential can be deformed for example due to high pressure (this naturally happens in really dense stars with ), screening of the Coulomb field of the proton by captured negatively charged muon living on the Bohr orbit (so called muon catalisis) etc. etc. This is what is called cold fusion – since you don’t need to keep kintic energy of the nuclei too high in order to start the fusion.

In the second case, the (undeformed) Coulomb barrier can be overcomed by very high kinetic energy of nuclei (for example, you collide these nuclei on LHC or a much less powerful device) or high temperature (essentially, the same story since temperature is a characteristic kinetic energy of particle in plasma) etc. etc. This is a  non-elegant, unsportsman-like way to start thermonuclear fusion but that’s how it typically happens in stars like our Sun.

Sometimes, it seems that we find reactions which belong to neither type A nor B, like in the effect observed by Fleischmann and Pons in 1989 The work of those two guys is considered pseudoscience nowadays but – who knows… according to Wikipedia

“Triple tracks” in a CR-39 plastic radiation detector claimed as evidence for neutron emission from palladium deuteride, suggestive of a deuterium-tritium reaction. On 22-25 March 2009, the American Chemical Society held a four-day symposium on “New Energy Technology”, in conjunction with the 20th anniversary of the announcement of cold fusion. At the conference, researchers with the U.S. Navy’s Space and Naval Warfare Systems Center (SPAWAR) reported detection of energetic neutrons in a palladium-deuterium co-deposition cell using CR-39, a result previously published in Die Naturwissenschaften.

By the way, if we have an expert here on NEQNET reading my post, can please comment on those experiments?

So, why one should be interested to learn about thermonuclear fusion? There are several reasons: a) as I said above, thermonuclear fusion is the main mechanism of the energy release in stars and Sun in particular, b) it does seem that thermonuclear synthesis is the way of the future for the global economics – it is cheap once we learn how to control it and it should make Greenpeace happy.

Next time I am going to talk about rates of thermonuclear reactions and will try to keep the level of the discussion basic.