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Description
A tunable permanent-magnet quadrupole based on two independently rotatable Halbach rings has been developed for compact, low-energy beam transport systems. Previously characterized with Hall-probe measurements, the same prototype is here studied using a high-precision rotating-coil bench at CERN, in the frame of a long-standing collaboration with the Wigner Institute. The integrated gradient, field orientation, and higher-order multipole components were measured over the full tuning range by scanning the relative rotation angle of the two rings. The quadrupole field is well described by a simple two-parameter model, allowing extraction of the individual ring strengths and azimuthal offsets. After correction for global alignment errors, residual multipole components remain within ±40 units over the operating range. Measurements under synchronous ring rotation show that higher-order multipoles rigidly follow the quadrupole field, indicating negligible hysteresis and minor mechanical coupling effects. These results confirm the excellent field quality and robustness of the concept and provide quantitative input for optimization and use in ion-source front-end beamlines.
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