Magic finger corrections

2 mm corrector magnets
2 mm corrector magnets
3 mm corrector magnets
3 mm corrector magnets
5 mm corrector magnets
5 mm corrector magnets

Small permanent magnets are arranged at the end of an insertion device such that they reduce the remaining vertical and horizontal integrated magnetic‐field errors as well as multipole errors. They are usually kept in a dedicated container with holes that can house several of these small magnets. Adjustments are made by changing either the size, position, or orientation of each trim magnet. The arrangement of magnets is calculated by an algorithm based on the remaining magnetic-field errors after tuning of the undulator. The field error after installation is then measured by stretched wire measurements and the magnet configuration is adjusted until a sufficiently low error is achieved.

Hoyer, E., S. Marks, P. Pipersky, and Ross Schlueter. Review of scientific instruments 66, no. 2 (1995): 1901-1903.