Emittance and the design of beam formation, transport, and collection systems in periodically focussed TWT's

This paper describes how emittance can be used to advantage in the design of beam optical systems for TWT's and other devices, and presents progress on the quantitative intercoupling of the beam formation, capture, focussing, transport, and collection problems via a parameter called tunnel emittance (T-emittance). First, a thermal electron beam model for gun-design computer codes is presented. It is necessary to employ a thermal beam model for accurate determination of T-emittance at the beam waist (especially in diode and low-perveance guns). Next, the paper considers the beam capture problem. How the match between the beam and the periodic permanant magnet (PPM) focussing field affects T-emittance growth downstream is described. After this, the effect of tube length on beam transmission is considered. It is shown that T-emittance can grow downstream due to the transfer of nonlinear field energy to transverse beam motion (explaining the observed degradation in transmission in very long tubes). Finally, at the exit of the tube, the grown value of T-emittance can be used as the basis for a spent beam model useful for preliminary single and multistage depressed collector (MDC) simulations. Appendices are presented which relate the quantities used in this paper to those used within the high-energy physics community.