Compensation of a magnetic octupole field for the lifetime experiment τSPECT

The neutron lifetime experiment τSPECT at TRIGA Mainz uses full magnetic storage of ultracold neutrons (UCN) to avoid neutron losses on material walls due to upscattering or absorption. Therein the neutrons are radially stored by a Halbach octupole and in longitudinal direction between two high field bumps produced by superconducting coils. The latter also serve to polarise the neutrons as they enter the τSPECT cryostate. Only high field seekers (HFS) are accelerated into the cryostate, the low field seekers (LFS) are being reflected at the magnetic potential wall of the first bump. For the storage of the neutrons in the low field region of the magnetic trap, the HFS need to be transformed into LFS. This is done by an RF coil (birdcage resonator), which irradiates a transverse magnetic field. In their rest-frame, the neutrons experience a rotating field, which adiabatically rotates their spin by 180 ^circle. However this resonator does not work efficiently enough in the high radial gradient field of the magnetic octupole. Therefore a second octupole is built which fits into the first one and ideally compensates the radial magnetic field in the spin-flipping region. In this talk field compensation simulations are shown, the realisation of the inner octupole together with field measurements on the quality of the field compensation.