  It has recently been demonstrated that a program of parasitic electron-beam
fixed-target experiments would have powerful discovery potential for dark
matter and other new weakly-coupled particles in the MeV-GeV mass range. The
first stage of this program can be realized at Jefferson Laboratory using an
existing plastic-scintillator detector downstream of the Hall D electron beam
dump. This paper studies the physics potential of such an experiment and
highlights its unique sensitivity to inelastic "exciting" dark matter and
leptophilic dark matter scenarios. The first of these is kinematically
inaccessible at traditional direct detection experiments and features potential
"smoking gun" low-background signatures.
