A study of a PPM focusing system for a C-band power TWT

In this paper a methodology to determine the suitable magnetic focusing properties that minimizes the electron beam ripple, propagating in a drift region, is presented. The methodology is applied in a typical C-band, power traveling-wave tube (TWT) which needs a 9.0 keV and 0.4 A electron beam with 0.8 mm of beam waist for a right operation. Using numerical simulation tools the beam behavior was investigated in the drift region along of periodic permanent magnet (PPM) without pole pieces and with 10 magnets. Best results were found with magnets with a length of 3.1 mm, an inner radius of 3.4mm, an outer radius of 6.35 mm, and a peak magnetic density flux intensity of 605 mT. In all cases studied the separation between adjacent magnets was set in 1.1 mm. By the using of the present methodology, no hollows in the electron beam were observed and a beam ripple of 1.99% was obtained. Some additional design details are also discussed.