  A windowless hydrogen gas target of nominal thickness $10^{19}$ cm$^{-2}$ is
an essential component of the DarkLight experiment, which is designed to
utilize the megawatt electron beam at an Energy Recovery Linac (ERL). The
design of such a target is challenging because the pressure drops by many
orders of magnitude between the central, high-density section of the target and
the surrounding beamline, resulting in laminar, transitional, and finally
molecular flow regimes. The target system was assembled and operated at
Jefferson Lab's Low Energy Recirculator Facility (LERF) in 2016, and
subsequently underwent several revisions and calibration tests at MIT Bates in
2017. The system at dynamic equilibrium was simulated in COMSOL to provide a
better understanding of its optimal operation at other working points. We have
determined that a windowless gas target with sufficiently high density for
DarkLight's experimental needs is feasible in an ERL environment.
