  Using semiclassical WKB-methods, we calculate the rate of electron-positron
pair-production from the vacuum in the presence of two external fields, a
strong (space- or time-dependent) classical field and a monochromatic
electromagnetic wave. We discuss the possible medium effects on the rate in the
presence of thermal electrons, bosons, and neutral plasma of electrons and
protons at a given temperature and chemical potential. Using our rate formula,
we calculate the rate enhancement due to a laser beam, and discuss the
possibility that a significant enhancement may appear in a plasma of electrons
and protons with self-focusing properties.
