  A system of $n$ screw dislocations in an isotropic crystal undergoing
antiplane shear is studied in the framework of linear elasticity. Imposing a
suitable boundary condition for the strain, namely requesting the non-vanishing
of its boundary integral, results in a confinement effect. More precisely, in
the presence of an external strain with circulation equal to n times the
lattice spacing, it is energetically convenient to have n distinct dislocations
lying inside the crystal. The result is obtained by formulating the problem via
the core radius approach and by studying the asymptotics as the core size
vanishes. An iterative scheme is devised to prove the main result. This work
sets the basis for studying the upscaling problem, i.e., the limit as
$n\to\infty$, which is treated in [17].
