Superconducting double-helix accelerator magnets

We describe an important contribution to accelerator magnet technology based on the concept of modulating the helical turns of solenoid coils to produce pure multipole fields of any order. Calculations show that these configurations inherently produce virtually error free fields of the desired multipole order in a large fraction of the aperture in the two dimensional cross section without the presence of iron. The characteristics of one such configuration, the double-helix dipole (DHD), are described. It is also explained how the novel geometry of the double-helix coils simplifies the manufacturing, eliminates complex coil parts, and thus significantly reduces the cost of the magnets in comparison to the conventional cosine theta (racetrack design) coils. This has been demonstrated by the design and construction of a prototype dipole that produces a 4T field in an 80 mm aperture (without iron).