Development of a High Efficiency Output Circuit for a Spatial Modulation High Power Microwave Amplifier

75053-High power, pulsed relativistic klystrons, presently being developed for linear collider applications, operate with electron beam energy densities of 300-500 joules/cm2 and with RF bunch charge densities of approximately 40 nC/cm3. The space charge gradients generated during the bunching process can exceed 15 MV/m, thereby limiting the maximum efficiency that can be demonstrated as the wavelength is reduced. A spatio-temporal modulated high power electron beam, arranged to induce and continuously interact with an energy extracting RF field, could generate microwave power at high efficiency without bunching the beam. Unfortunately, achieving high conversion efficiency with a spatio-temporal modulated beam requires a beam diameter that is a small fraction of the operating wavelength. Therefore, at short wavelengths, the demonstration of both high power and high efficiency is typically contraindicated. This project will develop a high-efficiency means of extracting radio frequency power from a spatially modulated beam without requiring a very small beam-diameter-to-wavelength ratio. Phase I will design a new output structure that can (1) provide an energy extracting field with a uniform intensity distribution over an interaction region that is large compared to the beam cross section and (2) maintain synchronism with the spatially modulated beam. In Phase II, the prototype microwave amplifier will be engineered, fabricated, and high power tested. Commercial Applications and Other Benefits as described by the awardee: A new high power amplifier tube, capable of operating at high frequency, should be suitable for linear collider applications as well as any commercial applications that would benefit from a high efficiency radio frequency source.