Hiroyuki Hama
Laboratory of Nuclear Science, Tohoku University
Optical klystrons employing helical undulators are favorable device to enhance the FEL gain without considerable increase of cavity mirror degradation [1]. There are, however, additional focusing effects for the beam due to multipole magnetic fields, which lie on the curvilinear coordinate system. It is well known that the multipole effect coming from the insertion devices modifies the betatron oscillation and then breaks the lattice symmetry down. This is finally harmful for the beam stability because of resonance excitations. Particularly for relatively low energy storages ring such as FEL drivers, the effect of the undulators is crucial. Because of geometrical restriction, inhomogeneity of transvers magnetic fields is normally strong in the helical optical klystrons, which means the multipole fields are not able to be disregarded. The study evaluates the multipole fields in a helical optical klystron for the UV-FEL on the 600 MeV storage ring at Okazaki, and discusses characteristics using experimental results of the betatron tune shifts.
[1] H. Hama, Nucl. Instr. and Meth. A 375 (1996) 57.
