  This paper describes the operation of the Coherent CAPTAIN-Mills (CCM)
detector located at the Lujan Neutron Science Center (LANSCE) at Los Alamos
National Laboratory (LANL). CCM is a 10-ton liquid argon (LAr) detector located
20 meters from a high flux neutron/neutrino source and is designed to search
for sterile neutrinos ($\nu_s$) and light dark matter (LDM). An engineering run
was performed in Fall 2019 to study the characteristics of the CCM120 detector
by searching for coherent scattering signals consistent with $\nu_s$'s and LDM
resulting from $\pi^+$ and $\pi^0$ decays in the tungsten target. New parameter
space in a leptophobic dark matter model was excluded for DM masses between
$\sim2.0$ and 30 MeV. The lessons learned from this run have guided the
development and construction of the new CCM200 detector that will begin
operations in 2021 and significantly improve on these searches.
