  The design, implementation, and performance of a customized carbon atom beam
source for the purpose of investigating solid-state reaction routes in
interstellar ices in molecular clouds are discussed. The source is integrated
into an existing ultrahigh vacuum setup, SURFace REaction SImulation DEvice
(SURFRESIDE$^{2}$), which extends this double atom (H/D, O, and N) beamline
apparatus with a third atom (C) beamline to a unique system that is fully
suited to explore complex organic molecule solid-state formation under
representative interstellar cloud conditions. The parameter space for this
system is discussed, which includes the flux of the carbon atoms hitting the
ice sample, their temperature, and the potential impact of temperature on ice
reactions. Much effort has been put into constraining the beam size to within
the limits of the sample size with the aim to reduce carbon pollution inside
the setup. How the C-atom beam performs is quantitatively studied through the
example experiment, C + $^{18}$O$_2$, and supported by computationally-derived
activation barriers. The potential for this source to study the solid-state
formation of interstellar complex organic molecules through C-atom reactions is
discussed.
