Phase focusing in linear-beam devices

The phenomenon of phase focusing in linear-beam devices is investigated theoretically by modifying the nonlinear interaction equations for devices such as the traveling-wave amplifier (TWA) to accommodate spatially-varying parameters and then investigating their use under a variety of operating conditions. Phase focusing can be accomplished either by tapering the RF circuit phase velocity or by applying a dc voltage gradient along the beam or structure. Both approximate analytical solutions and general computer solutions are presented for each of the phase-focusing techniques. The analytic solutions yield profiles which are explicit functions of tube length and these are compared with more general computer solutions where space-charge effects are included. An experimental S-band high-power TWA was constructed with a variable-pitch helix designed on the basis of the hard-kernel-bunch theory. Experimental data indicate an efficiency improvement from 18 per cent to 35 per cent and an improvement in the gain-frequency characteristics.