“…Re-entrant cavities, particularly of cylindrical symmetry as shown in figure 1, have been extensively investigated at the time of the development of klystrons (Hansen 1939, Hahn 1941, Mayer 1946) and more recently with the advent of Tunnel and Gunn diodes (Bolle 1962, Rivier and Verge-Lapisardi 1970, Wenakada 1970, Williamson 1976.…”