  We consider dynamical semigroups with unbounded Kossakowski-Lindblad-Davies
generators which are related to evolution of an open system with a tuned
repeated harmonic perturbation. Our main result is the proof of existence of
uniquely determined minimal trace-preserving strongly continuous dynamical
semigroups on the space of density matrices. The corresponding dual W
*-dynamical system is shown to be unital quasi-free and completely positive
automorphisms of the CCR-algebra. We also comment on the action of dynamical
semigroups on quasi-free states.
