  The upscaling of a system of screw dislocations in a material subject to an
external strain is studied. The $\Gamma$-limit of a suitable rescaling of the
renormalized energy is characterized in the space of probability measures. This
corresponds to a discrete-to-continuum limit of the dislocations, which, as a
byproduct, provides information on their distribution when the circulation of
the tangential component of the external strain becomes larger and larger. In
particular, dislocations are shown to concentrate at the boundary of the
material and to distribute as the limiting external strain.
