  In a fusion reactor, a proton and a neutron generated in previous reactions
may again fuse with each other. Or they can in turn fuse with or be captured by
an un-reacted deuteron. The average center-of-mass (COM) energy for such
reaction is around 10 keV in a typical fusion reactor, but could be as low as 1
keV. At this low COM 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 x-ray photons from a nuclear
bremsstrahlung process. The x-ray thus generated 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 semiclassical
approach. The results give a peak near 1.1 keV for the proton-deuteron fusion
and a power-to-the-minus-second law in the spectrum's high-energy limit. An
analysis of some prior tokamak discharge data shows that this phenomenon might
have been observed before and it may lead to new plasma diagnostics which are
more sensitive to the ionic or nuclear degree of freedom. This phenomenon
should also play a role in nuclear astrophysics as one of the sources for
astrophysical x-rays. The process contributes particularly to stellar evolution
in the early stage, where the temperature of proto-stars or the so called
pre-main sequence stars (T Tauri stars, for example), is at a relatively low of
several million degrees Kelvin. An order-of-magnitude calculation was made on
the proton-deuteron fusion rate in young star objects. The estimated x-ray
luminosity from this reaction is found enough in magnitude to account for
experimental ones.
