Flux shunts for undulators

Undulators for high-performance applications in synchrotron radiation sources and periodic magnetic structures for free-electron lasers have stringent requirements on the curvature of the electron´s average trajectory. Undulators using the permanent magnet hybrid configuration often have fields in their central region that produce a curved trajectory caused by local, ambient magnetic fields such as those of the Earth. The 4.6 m long Advanced Light Source (ALS) undulators use flux shunts to reduce this effect. These flux shunts are magnetic linkages of very high permeability material connecting the two steel beams that support the magnetic structures. The shunts reduce the scalar potential difference between the supporting beams and carry substantial flux that would normally appear in the undulator gap. Magnetic design, mechanical configuration of the flux shunts and magnetic measurements of their effect on the ALS undulators are described