Interim report on the analysis of the microwave power module

PRELIMINARY ANALYSIS OF THE MICROWAVE POWER MODULE PERFORMANCEPeter Ramins, Raymond W. Palmer, Dale A. Force, Ben T. Ebihara,Robert P. Gruber, and James A. Dayton, Jr.National Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135SummaryThe results of a traveling wave tube multistage depressedcollector (TWT-MDC) design study in support of the DARPA/DoDMicrowave Power Module (MPM) Program are described. The studystressed the MDC as a key element in obtaining the required highoverall efficiencies in the MPM application. The results showedthat an efficient MDC, utilizing conventional design andfabrication techniques can be designed for the first generationMPM TWT, which permits a package one wavelength thick (.66" at18 GHz). The overall TWT efficiency goal of 40% for ECMapplications appears to be readily achievable. However, the 50%goal for radar applications presents a considerable challenge.IntroductionThis report summarizes the results of the first stage of theNASA-Lewis Traveling Wave Tube-Multistage Depressed Collector(TWT-MDC) design effort in support of the DARPA/Tri-ServiceMicrowave Power Module (MPM) program. The MPM combines a solidstate driver, vacuum power booster, and power conditioning in asingle module. This first stage of the analysis stressed the MDCas a key element in obtaining the required high overall TWTefficiencies in the MPM application, and addressed the problem ofattaining the required high collector efficiency while meetingthe stringent system size constraints and practical HV, thermaland mechanical design requirements. The MDC was designed to fitinto a first generation MPM (.66" maximum width dimension). TheTWT was modelled only at mid-band and its parameters selected toproduce a representative spent beam (MDC analysis input) in termsof beam size, space charge, and energy spread. No attempt wasmade to design and model a broad-band rf (helical) circuit. Thisapproach was based on the observation that, in general, the MDCefficiency is highly insensitive to the operating frequency assuch, and an MDC designed for the frequency producing maximum rfoutput power works well across octave and wider operating bands.The addition of a spent beam refocuser (SBR) with controlled beamexpansion, as a key element in obtaining adequately high MDCefficiencies, emerged from the particular combination of sizeconstraints, and cooling and HV stand-off requirements.1