Multiple trim magnets, or "magic fingers" for insertion device field integral correction

Abstract

Multiple trim magnets (MTMs), also known as ''magic fingers,'' are an arrangement of magnets for reducing integrated magnetic-field errors in insertion devices. The idea is to use transverse arrays of permanent magnets, hence the name ''multiple trim magnets,'' above and below the midplane, to correct both normal and skew longitudinal magnetic-field integral errors in a device. MTMs are typically installed at the ends of an ID. Adjustments are made by changing either the size, position, or orientation of each trim magnet. Application of the MTMs to the ALS undulators reduced both the normal and skew longitudinal field integral errors, over the entire 20 mm×60 mm ''good field region,'' of the beam aperture by as much as an order of magnitude. The requirements included corrections of field and gradients outside the multipole convergence radius. Additionally, these trim magnet arrays provided correction of the linear component of the integrated field gradients for particles with trajectories not parallel to the nominal beam axis. The MTM concept, design, construction, tests that demonstrated feasibility, and magnetic-field integral reduction of ALS undulators are presented. © 1995 American Institute of Physics.