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 (E$-o$/ $EQ 5 MV/m, I $EQ 1 A and d $EQ 0.8 m) the adaptable tapering causes the system to be almost three times more effective than in a uniform device. An analytical solution corresponding to the case 2$gamma$-(r)$/$+(2)$/ $VGRT 1, is discussed. The other case considered is when the injected beam has a uniform distribution of phases relative to the wave. The construction of the bunching is illustrated as well as the regime in which the amplitude of the wave increases linearly; our simulation reveals that almost 50% of the interaction region is exploited for the former process. The authors show that by tapering, both the gain and the efficiency increase. When operated as an amplifier (E$-0$/ $EQ 1.0 MV/m, I $EQ 450 A and d $EQ 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.
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