Vacuum polarization in a locally static multiply connected spacetime and a time-machine problem

The quantum field theory in a multiply connected locally static spacetime is considered. It is shown that the vacuum-polarization effect in a spacetime with a nonpotential gravitational field results in divergences of the renormalized stress-energy tensor for a quantum field near the Cauchy horizon. Possible applications of this result are discussed in connection with a "time machine" problem.