  We study the influence of a background uniform magnetic field and boundary
conditions on the vacuum of a quantized charged spinor matter field confined
between two parallel neutral plates; the magnetic field is directed
orthogonally to the plates. The admissible set of boundary conditions at the
plates is determined by the requirement that the Dirac Hamiltonian operator be
self-adjoint. It is shown that, in the case of a sufficiently strong magnetic
field and a sufficiently large separation of the plates, the generalized
Casimir force is repulsive, being independent of the choice of a boundary
condition, as well as of the distance between the plates. The detection of this
effect seems to be feasible in the foreseeable future.
