1. Volkov, A.A., Kozlov, G.V., and Prokhorov, A.M., Infr. Phys., 1989, vol. 29, no. 24, p. 747.
2. Golant, M.B., Alekseenko, Z.T., Korotkova, Z.S., et al., Prib. Tekh. Eksp., 1969, no. 3, p. 231.
3. Gershenzon, E.M., Golant, M.B., Negirev, A.A., and Savel’ev, K.S., Lampy obratnoi volny millimetrovogo i submillimetrovogo diapazonov voln (Backward Wave Tubes of Millimeter and Submillimeter Wavelength Ranges), Devyatkov, N.D., Ed., Moscow: Radio i Svyaz’, 1985.
4. Foner, S., Phys. Rev., 1957, vol. 107, p. 683.
5. Rudashevskii, E.G., Prokhorov, A.S., and Velikov, L.B., IEEE Trans. on Microwave Theory and Techniques, 1974, vol. MTT-22, p. 1064.