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Abstract: To drive future X-ray FELs, normal-conducting linacs with a wide range of rf are being considered. In this paper, we present accelerator structure and high power rf system designs at three rf (2.856GHz, 5.712GHz and 11.424GHz) that are optimized for single bunch and multi-bunch linac operation with very low beam loading. General scaling laws for the main linac parameters are derived to serve as guidelines to optimize such linacs at other frequencies. The linac costs at the three rf are also discussed. High rf linacs are shown to provide compact, low cost and reasonably efficient drivers for X-ray FELs. [Copyright &y& Elsevier]

Abstract: Electron multipacting in a coaxial waveguide of the cold part of TTF-III power coupler has been observed at SLAC to be correlated with vacuum pressure growth and delayed with respect to the start of the rf power pulse. Analysis of the electron signal, gas pressure and rf power has been performed. It is found that at a given rf power level, the multipacting electron saturation current is proportional to the K th power of gas pressure, where K is positive and only determined by the applied rf power and gas species. The K value is found much larger at the resonant mulipacting power than that at the non-resonant power. A model of the physics explains the delay time of the electron signal. All of the model predictions have been experimentally demonstrated. [Copyright &y& Elsevier]