<title>Efficiency increase in a traveling wave tube by tapering the phase velocity of the wave</title>

Abstract

The beam-wave interaction in a Cerenkov traveling wave tube is considered theoretically. The system is assumed to operate at 8.75 GHz either as an amplifier or as an accelerator. The injected electrons have a kinetic energy of 0.85 MeV and the beam at the entrance can be prebunched or uniform. To avoid saturation, two kinds of tapering are considered: one which is adaptable, in the sense that the phase velocity follows exactly the particle's velocity; in the other, the dielectric coefficient varies algebraically in space. In the case of an accelerator (Eo equals 5 MV/m, I equals 1 A and d equals 0.8 m) the adaptable tapering causes the system to be almost three times more effective than in a uniform device. An analytical solution corresponding to the case 2(gamma) (r)(2) >> 1, is discussed. The other case considered is when the injected beam has a uniform distribution of phases relative to the wave. The construction of the bunching is illustrated as well as the regime in which the amplitude of the wave increases linearly; our simulation reveals that almost 50% of the interaction region is exploited for the former process. The authors show that by tapering, both the gain and the efficiency increase. When operated as an amplifier (E0 equals 1.0 MV/m, I equals 450 A and d equals 0.3 m) with an optimal linear variation of the dielectric coefficient, the efficiency is improved to about 30% from the 5% in the uniform device.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.