  Light dark matter in the context of dark sector theories is an attractive
candidate for the dark matter thought to make up the bulk of the mass of our
universe. We explore here the possibility of using a low-pressure,
negative-ion, time projection chamber detector to search for light dark matter
behind the beam dump of an electron accelerator. The sensitivity of a 10 m long
detector is several orders of magnitude better than existing limits. This
sensitivity includes regions of parameter space where light dark matter is
predicted to have a required relic density consistent with measured dark matter
density. Backgrounds at shallow depth will need to be considered carefully.
However, several signatures exist, including a powerful directional signature,
which will allow a detection even in the presence of backgrounds.
