The work continues the studies of dynamic problems in singularly perturbed domains, that were initiated in [1]. The non-stationary Maxwell system is considered, for all times t ϵ R, in a bounded domain with a small cavity. The impedance conditions are given on the boundary. The asymptotics of solutions are derived as the cavity diameter tends to zero. The presented mathematical model can be of use for the diagnostics of plasma filling a cavity resonator and contaminated with metal particles of small size.