  Following a previous work on the quantization of a massless scalar field in a
spacetime representing the head on collision of two plane waves which fucus
into a Killing-Cauchy horizon, we compute the renormalized expectation value of
the stress-energy tensor of the quantum field near that horizon in the physical
state which corresponds to the Minkowski vacuum before the collision of the
waves. It is found that for minimally coupled and conformally coupled scalar
fields the respective stress-energy tensors are unbounded in the horizon. The
specific form of the divergences suggests that when the semiclassical Einstein
equations describing the backreaction of the quantum fields on the spacetime
geometry are taken into account, the horizon will acquire a curvature
singularity. Thus the Killing-Cauchy horizon which is known to be unstable
under ``generic" classical perturbations is also unstable by vacuum
polarization. The calculation is done following the point splitting
regularization technique. The dynamical colliding wave spacetime has four quite
distinct spacetime regions, namely, one flat region, two single plane wave
regions, and one interaction region. Exact mode solutions of the quantum field
equation cannot be found exactly, but the blueshift suffered by the initial
modes in the plane wave and interaction regions makes the use of the WKB
expansion a suitable method of solution. To ensure the correct regularization
of the stress-energy tensor, the initial flat modes propagated into the
interaction region must be given to a rather high adiabatic order of
approximation.
