Pulse-shortening in Electron Linear Accelerators and E11 Type Modes

VARIOUS authors have discussed pulse-shortening in high-current electron linear accelerators, suggesting that the phenomenon is due to the excitation of an E 11 mode in the iris-loaded wave-guide by the electron beam. The E 11 mode, having deflecting properties, is believed to impart transverse momentum to the electrons, causing them to strike the wave-guide and be lost. In particular, Crowley-Milling et al. 1 attribute the interaction of the E 11 wave with the beam to the fact that the wave has a negative group velocity, on the grounds that only in such a backward wave do the magnetic and electric forces not cancel each other at the velocity of light.