Most texts about nuclear fusion say that the reason it is so hard to get protons to fuse is the strong Coulombic repulsion between them. However, it seems to me that at close distances, protons should also experience Pauli exchange repulsion, unless their spins happen to be antiparallel. Are there any articles in which the details of this repulsion (and the limitations that it might pose to nuclear fusion) are discussed? Or is there a reason why the exchange interaction is not significant?
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$\begingroup$ this might interest you hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/shellpau.html $\endgroup$– anna vCommented Mar 13, 2023 at 5:26
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The diproton is in fact unstable. The two protons must have anti-aligned spins and the spin-dependence of the strong force leaves it unbound. In order for fusion to occur, the protons must get close together and one of them must undergo a weak interaction and change into a neutron (and form a bound deuteron with aligned proton and neutron spins). It is this combination that is very unlikely.
The Coulomb repulsion alone is not a particularly high barrier to fusion. For example, the fusion of two deuterons will occur at temperatures an order of magnitude lower than the fusion of protons inside a star, despite identical Coulomb repulsion.
