Application of precision magnetic measurements for control of the Duke storage ring
The 1 GeV Duke FEL storage ring is dedicated to drive UV and VUV free electron laser devices. Specifics of this ring include the use of combined function magnets: quadrupole-sextupole and dipole-sextupole. The use of combined function magnets is necessitated by the close spacing of magnetic elements. A discussion on the measurement procedures of these magnets is included in this paper, as well as the data analysis used to create a viable control system for the combined function magnets. The design of the Duke storage ring was driven by the requirement to use existing hardware manufactured for the original ring design at Stanford. The original design called for the use of 'nose' and 'dimple' endpieces attached to the dipoles as the main fixed strength sextupole source. In addition, adjustable sextupole magnets were placed in an 18 cm gap between dipoles and arc quadrupoles. In that design the distance between magnetic poles of dipoles, quadrupoles, and sextupoles was less than 2 cm. It was no surprise that this design uncovered major asymmetric saturation of dipoles and sextupoles. Saturation would cause intolerable orbit distortion and high order nonlinearities in the magnetic field. To solve these problems we decided to: a) replace odd asymmetric endpieces with smooth and symmetric ones; b) remove discrete sextupoles which were saturated anyway; c) create necessary sextupole moments in the quadrupoles using asymmetric excitation; d) place fixed strength sextupole shims in the center of the dipoles.