  A field theory is presented for particles which interact via Coulomb and
hard-core potentials. We apply the method to the one-component plasma (OCP)
with hard cores, consisting of identical particles of fixed charge and diameter
in a neutralizing background, and the symmetric two-component plasma (TCP) with
hard cores, consisting of equal numbers of positively and negatively charged
particles of identical size. We obtain exactly the first few coefficients of a
systematic low-density expansion of the free energy for both models. The OCP
coefficients go over to the classical Abe result in the high-charge (or small
hard-core diameter) limit. The TCP, on the other hand, exhibits diverging
coefficients in the high-charge limit, which is due to the formation of
strongly bound ion-pairs.
