Composite period undulator to improve the wavelength tunability of free electron lasers

Abstract

A novel magnet configuration for permanent magnet undulators is proposed together with the operational procedures to improve the wavelength tunability of free electron lasers (FELs). The magnetization axes of the magnet blocks in the new configuration are inclined at a certain angle to those of the conventional Halbach configuration according to a particular rule. As a result, a composite-period magnetic field is generated, which contains two field components having two different periods, i.e., the fundamental and double periods. Moving the top and bottom magnet arrays toward the opposite directions along the undulator axis, it is possible to eliminate either of the two field components and thus the period is selectable. Simple analysis shows that the operation with the composite field is also useful for FELs despite that the gain length grows slightly due to the effects of the nonsinusoidal field profile. An example of the new undulator scheme applied to expanding the tunability of the SPring-8 x-ray FEL is presented together with discussions on technical issues.