Normal conducting linac designs for low beam loading

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 (2.856 GHz, 5.712 GHz and 11.424 GHz) 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.