Characterization of axially-symmetric magnetic elds

In solenoids for particle accelerators, the magnetic field is usually mapped by means of 3D Hall-sensing systems through a burdensome and costly procedure. A further problem arises from a coherent treatment between the beam physics requirements, the qualification of numerical models, the design and manufacturing of the magnet, and the magnetic measurements. For example, when the magnet is misaligned with respect to the longitudinal direction of the mapper, the fringe field shows spurious components. A method was therefore developed for measuring the magnetic field of axisymmetric magnets by exploiting their inherent symmetry. The method yields a measurement of the magnetic flux linked with a pair of sensing coils as a function of their longitudinal position. An induction transducer, sensitive to the longitudinal and radial components of the solenoid under test, has been designed and constructed. A transport system moves the transducer along the magnet axis, covering the full length of the magnet and including the fringe field. The induced voltage is acquired and integrated digitally in order to yield the flux linkage as a function of the linear position, measured by a laser interferometer. The re-parametrization related to intrinsic relationship between the motion time and the linear translation position along the magnet axis relaxes the requirements on the homogeneous motion of the transport system. After a proof-of-principle demonstration, the uncertainty sources affecting the measurement system were evaluated and their influence on the result assessed. In particular, the measurement model for the translating-coils method is formulated and a Sequential Monte Carlo algorithm, based on Bayesian inference, is then used to dynamically estimate the distribution of the output quantity, starting from the knowledge of the distributions of all the uncertainty sources considered in the model. According to the indications of the uncertainty analysis results, a translating-coils system was prototyped and integrated in the existing CERN platform, which includes Fast Digital Integrators (FDIs), an encoder board, a motor controller, and the Flexible Framework for Magnetic Measurement (FFMM). The system was validated in a reference solenoid by pointing out more than satisfying agreement of the results. Finally, the system has been employed in challenging measurements on the field, resulting in a reliable and accurate method for the characterization of axially-symmetric magnet systems.