1. M. S. Rabinovich and A. A. Rukhadze, Fiz. Plazmy,2, 715 (1976).
2. V. N. Shevchik, G. N. Shvedov, and A. V. Soboleva, Wave and Oscillatory Phenomena in Electron Fluxes at Superhigh Frequencies [in Russian], Saratovsk Univ. Press (1962).
3. A. N. Didenko, Ya. E. Krasik, S. F. Perelygin, and G. P. Fomenko, Pis'ma Zh. Tekh. Fiz.,5, No. 6, 321 (1979).
4. R. A. Mahaffey, P. Sprangle, J. Golden, and C. A. Kapetankos, Phys. Rev. Lett., 39, 843 (1977).
5. C. K. Birdsall and W. B. Bridges, Electron Dynamics of Diode Regions, Academic Press, New York (1966).