Improvement of the field quality in the helical wigglers for the OK-5 VUV FEL at Duke

In this paper, we present an attempt to improve the field quality in the pseudo-helical wigglers used in the OK-5 VUV FEL at Duke. One type of “imperfection” of a wiggler intrinsic for any magnetic system and affecting the dynamic aperture is non-linear focusing. We tried to find an approach to reduce affect of the wiggler on dynamic aperture. Another kind of imperfection of the field in the pseudo-helical wigglers are caused by asymmetry of design. In our case the asymmetry of the coils allows the presence of small average quadrupole and octupole fields. We used small asymmetric cuts in the poles to eliminate these terms. The magnetic field in the regular part of the wiggler was simulated with the use of 3D code “MERMAID”. All four wigglers have been fabricated and measured. Magnetic measurements show good agreement with the simulations.