1. R. W. GOULD and A. W. TRIVELPIECE, Proc. Symposium on Electronic Waveguides, Polytechnic Press, Brooklyn, 1958, p. 215
2. G.D. BOYD, L.M. FIELD and R, W. GOULD, Phys. Rev. 109, 1393 (1958)
3. E.V. BOGDANOV, V. Ia. KISLOV and Z.S. CHERNOV, Radio Eng. Electronics 5, 75 (1960)
4. P. A. STURROCK, Phys. Rev. 112, 1488 (1958)
5. T.L. FERRARI, Proc. 5th Int. Conf. on Ionization Phenomena in Gases, Munich 1961