  We consider the vacuum-field pressure on boundaries of a metal slab in the
middle of a cavity with perfectly reflecting mirrors adopting the plasma model
for the metal and paying special attention to the surface plasmon polariton
contribution to the pressure. We demonstrate that, with increasing cavity
length, the pressure on a thin (d<<\lambda_P) slab in this system decreases
from the Casimir pressure F_C at zero slab-mirror distances to the non-retarded
force per unit area F_nr=1.19 (d/\lambda_P)F_C in the case of an isolated slab.
In the first case the pressure is entirely due to the photonic modes
propagating through the metal whereas in the second case it is entirely due to
the (nonretarded) surface plasmon modes supported by the free-standing thin
slab. In either case the pressure decreases with the slab thickness. These
considerations indicate that the vacuum-field pressure on a thin metal layer
(and its modal structure) can be in a symmetric cavity significantly influenced
when changing the cavity length.
