Adiabatic invariants for electromagnetic waves guided by time-varying plasma structures

Propagation of an electromagnetic wave along a planar waveguiding structure containing gaseous/solid-state plasma slabs with time-varying free carrier density is considered in the case when the time-scale of density variations is much greater than the wave period. Both ionization and deionization (recombination, attachment) processes are included. General relations describing frequency shifting and energy losses of a guided wave are derived for arbitrary open/closed waveguiding structure. Adiabatic invariants, i.e., combinations of wave energy and frequency conserving in the course of temporal variations of the waveguiding medium, are found. Detailed analyses are given for surface waves guided by the boundary of a time-varying plasma half-space and a plasma slab.