A 'free' neutron is unstable with respect to beta decay, i.e. a neutron by itself will decay into a proton, beta and anti-neutrino, so the reverse action is unlikely. I suspect that electron capture may occur very rarely.
EC does happen in certain proton-rich (neutron deficient) radionuclides, and typically it is a K-electron which is absorbed. In plasmas, the ions and electrons have kinetic energies much, much greater than the binding energy of electrons (in the light elements). At lower energies, electrons recombine with ions to form neutral atoms, which then leak out of plasmas.
Protons (or more generally ions) and electrons interact in plasma, mostly by scattering or producing bremsstrahlung radiation, the latter of which results in energy loss from the plasma.
Fusion plasmas are likely to be D+D or D+T. D+
3He would be preferable since it's an aneutronic reaction, but it requires higher temperatures (with concommitant pressure for a given particle density) and
3He is extremely rare (and thus very expensive). He-3 can be obtained from the beta decay of T (H-3).
Proton-proton fusion has a very low cross-section, and is the basis of one fusion process in stars - PP fusion or PP-chain.
http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html
Otherwise, there are more exotic reaction like p +
11B -> 3
4He, but that requires significantly higher temperature than D+D or D+T.
p + p -> d + e
+ + 1.4 MeV, but
p + d ->
3He + gamma + 5.5 MeV
