Combined function magnets using double-helix coils

We describe a technology for creating easy-to-manufacture combined function magnets. The field is produced by double-helix coils in which the axial path of the windings is defined by a sinusoidal function containing the superposition of the desired multipoles. The result is a magnet that can contain, for example, a pure dipole field with superimposed multipole fields whose magnitude relative to the dipole field can be easily controlled to any level. We show how low level (i.e. 0.1% -1%) modulation amplitudes of the superimposed multipoles can be used as built-in or "free" correction coils to compensate for iron saturation effects or geometrically-induced multipoles. The combined function winding can also be used to superimpose a dipole and quadrupole winding where the quadrupole integral of Gdl can be adjusted to any level desired over the length of the main dipole magnet. In this way a "free" quadrupole can be obtained within a dipole. The characteristics of this type of combined function magnet are also discussed.