Nuclear Instruments and Methods in Physics Research A 483 (2002) 383–387 Measurement, sorting and tuning of LCLS undulator magnets Isaac Vasserman*, Shigemi Sasaki, Roger Dejus, Elizabeth Moog, Emil Trakhtenberg, Nikolai Vinokurov 1 Experimental Facilities Division, Argonne National Laboratory, Building 401, 9700 S. Cass Avenue, Argonne, IL 60439, USA Abstract Currently, a Linac Coherent Light Source (LCLS) prototype undulator is under construction. The prototype is a 3.4- m-long hybrid-type undulator with fixed gap of 6 mm. The period length is 30 mm and the number of poles is 226. For this undulator, 450 NdFeB magnet blocks are used. This project does not have demanding requirements for multipole component errors, but the field strength at x ¼ 0 should be as precise as possible to provide proper particle steering and phase errors. The first set of magnetic blocks has been measured. The strength and direction of magnetization of the magnet blocks are measured using a Helmholtz coil system. In addition to this, Hall probe measurements are performed for magnet blocks while they are mounted in a specially designed cassette with vanadium-permendur poles. The magnet blocks will be sorted using these data to minimize errors. Computer simulations show that magnets may be sorted in decreasing strengths with little or no additional tuning of the undulators. r 2002 Elsevier Science B.V. All rights reserved. PACS: 41.60.Cr Keywords: FEL; Udulator 1. Characterization of magnet blocks with Helmholtz coils The requirements for the Linac Coherent Light Source (LCLS) undulator [1] are demanding. To help ensure that these requirements can be met, tight tolerances must be imposed on the magnetic properties of the permanent magnet blocks to be used in the undulators. In addition to requiring that the magnets be strong (a remanence B r of 1.2 T is specified for the NdFeB permanent magnet blocks), they are also required to be uniform. Each individual magnet block is required to have a total magnetic moment that is within 71% of the average total moment. The direction of the total magnetic moment vector is also required to lie within 21 of the mechanical axis of the magnet block. These requirements are checked by measure- ments made using a modernized system of Helmholtz coils [2]. The vendor measures the magnet blocks when they are manufactured, and the blocks are typically measured again by the manufacturer of the undulator magnetic structure. *Corresponding author. Tel.: +1-630-252-9612; fax: +1- 630-252-9303. E-mail address: isaac@aps.anl.gov (I. Vasserman). 1 Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia. 0168-9002/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0168-9002(02)00348-0