Variable-period permanent-magnet helical undulator

Abstract

We realized a variable-period permanent-magnet helical undulator with high (∼1 T) field amplitude, which is almost constant over undulator periods of 23–26 mm. Each undulator period has four modular sections of iron poles and permanent magnets embedded in nonmagnetic disks with holes along the undulator axis. Modular plates undergo a longitudinal repulsive force from the magnetic field pressure and the spring coils between modular plates. The undulator period can thus be controlled by mechanically changing the end plate longitudinal position. This compact design is suitable for tabletop terahertz free electron lasers.