Transverse RF focusing in bunching cells for standing-wave linac

Electron RF linacs for X-ray imaging applications require a small beam radius to achieve fine spatial resolution. By adopting transverse RF focusing in an accelerator, one can achieve a compact system without external magnets. A small beam radius can be obtained by transverse RF focusing with a high accelerating gradient, but the beam energy is usually limited by the available power of RF sources. In contrast, RF phase focusing (transverse RF focusing which depends on the beam phase) is enhanced by modifying the accelerating structure. Since RF phase focusing is effective while the beam velocity is low, the RF phase focusing is enhanced in the built-in bunching section which is used in compact accelerators. This paper reports two approaches to enhance the RF phase focusing. First, the phase velocities of bunching cells are optimized for maximum beam-focusing while preserving beam-bunching. Second, the first bunching cell is formed with a longitudinally asymmetric geometry to suppress the defocusing effect on a portion of the beam slices. In simulations using the PARMELA code, the beam radius was reduced by 50% compared to that produced by on-crest acceleration with longitudinally symmetric cells.