  We report on a windowless, high-density, gas flow target at Jefferson Lab
that was used to measure $r_p$, the root-mean-square charge radius of the
proton. To our knowledge, this is the first such system used in a fixed-target
experiment at a (non-storage ring) electron accelerator. The target achieved
its design goal of an areal density of 2$\times$10$^{18}$ atoms/cm$^2$, with
the gas uniformly distributed over the 4 cm length of the cell and less than 1%
residual gas outside the cell. This design eliminated scattering from the end
caps of the target cell, a problem endemic to previous measurements of the
proton charge radius in electron scattering experiments, and permitted a
precise, model-independent extraction of $r_p$ by reaching unprecedentedly low
values of $Q^2$, the square of the electron's transfer of four-momentum to the
proton.
