  We study the ground-state structures of identical classical point charges
with Coulomb interactions, confined between two symmetric parallel charged
walls. For the well understood homogeneous dielectric case with no
electrostatic images, the charges evenly condense on the opposite walls,
thereby forming a bilayer Wigner crystal; five structures compete upon changing
the inter-wall separation. Here, we consider a dielectric jump between the
walls and a solvent in which charges are immersed, implying repulsive images.
Using recently developed series representations of lattice sums for Coulomb
law, we derive the complete phase diagram. In contrast to the homogeneous
dielectric case, the particles remain in a hexagonal Wigner monolayer up to a
certain distance between the walls. Beyond this distance, a bifurcation occurs
to a sequence of Wigner bilayers, each layer having a nonzero spacing from the
nearest wall. Another new phenomenon is that the ground-state energy as a
function of the wall separation exhibits a global minimum.
