Energy flow relations in multiwave systems with absorption and dispersion

Expressions for the time average energy flow are derived for a linear physical system with an arbitrary number of interacting waves. The system is assumed to be temporally and spatially dispersive and to include losses. Waves constant in time but growing or decaying in space with arbitrarily large factors are considered. The results are used to discuss the energy flow components in a traveling-wave tube and in a piezoelectric semiconductor device.