Proton bremsstrahlung and its radiation effects in fusion reactors

Protons and neutrons are emitted in many fusion processes of light nuclei. In a fusion reactor, a proton and a neutron thus generated may again fuse with each other. Or they can in turn fuse with or be captured by an un-reacted nuclear fuel, for example deuterium. The average center-of-mass energy for such reaction is around 10 keV in a typical fusion reactor. At this low energy, the reacting nucleons are in an s-wave state in terms of their relative angular momentum. The single-gamma radiation process is thus strongly suppressed due to conservation laws. Instead the gamma ray released is likely to be accompanied by soft X-ray photons from a nuclear bremsstrahlung process. The generated soft X-ray has a continuous spectrum and peaks around a few hundred eV to a few keV. The average photon energy and spectrum properties of such a process are calculated with a semi-classical approach, with the explicit example of proton–neutron capture. This phenomenon may have been observed in some prior tokamak discharge experiments, and its interpretation is complicated by the presence of electron bremsstrahlung. However, it also opens up the possibility of new plasma diagnostics which are more sensitive to the ionic or nuclear degree of freedom.