Permanent magnet systems for free-electron lasers

We review uses of permanent magnets (PMs) in free-electron lasers (FELs). Recently, PMs have been considered to replace many of the electromagnet (EM) dipoles, quadrupoles, and sextupoles in FEL beamlines and linear collider quadrupoles. PM beamline optics offer several advantages over EMs (Proceedings of the Particle Acceleration Conference, 2001, p. 3218). They are more compact, need no power, and do not require cooling water. In addition, adjustable-strength quadrupoles may have a precisely tuneable magnetic centerline. High pole tip fields (1.5 T in dipoles and 1.2 T in quadrupoles) are easily achieved. PM technology opens up new bend design possibilities. We describe a new, high performance, all-PM bend, the Ballard bend, that is first-order isochronous and doubly achromatic. It is suitable for use in the exhaust beam leg of an energy recovery FEL. We have designed and built a compact, high field sector PM dipole. Measured field profiles agree to 10 ppm of predictions. Compact PM quadrupoles were also designed and built. Measurements of field strength, axial profile, magnetic centerline tuning, and passive temperature compensation of strength and centerline shift agreed very well with predictions.