the interaction structure with integral coupling and bunching section and method of use of the present invention produces backward wave oscillations in an rf structure which combines ballistic bunching and extended beam - wave interaction in a complex resonator assembly . the complex extended interaction structure includes a five - gap electromagnetically - coupled cavity structure with a coaxial section inserted between the first and second cavities . the first cavity serves as a buncher cavity while the four subsequent cavities serve as energy cavities . in the coaxial section , beam and wave propagate in separate channels . the field in the buncher cavity is coupled to the four subsequent energy extraction cavities through the wave channel between the inner and outer conductors of the coaxial section , while the electron beam drifts along a cylindrical channel cut through the inner conductor of the coaxial section . the complex extended interaction structure of the present invention provides a drift channel to allow ballistic bunching of electrons . since the buncher cavity is strongly coupled to the energy extraction cavities , the first gap exerts a large modulating voltage on the electrons , sufficient to produce a tightly bunched electron beam upon entrance into the energy extraction cavities .