Magnetic Measurements of the Higher-Order Corrector Magnets for the High-Luminosity Upgrade of the Large Hadron Collider at CERN

This report describes the development of two measurement systems for the room temperature magnetic measurements of the corrector package for the High-Luminosity upgrade of the Large Hadron Collider at the European Laboratory for Particle Physics, CERN. Both the physical setup and the post-processing procedures of the systems are described, together with measurements used for determining the optimal set of measurement parameters. The rotating single-stretched wire measurement system was developed for the measurements of the magnets’ magnetic axis and azimuthal orientation. It comprises a taut conductive wire that is moved on a circular trajectory, measuring the magnetic flux density of the AC-powered magnet. The harmonic content of the flux density is used to find the axis and orientation of the magnet. A translating coil fluxmeter was developed for the measurements of the longitudinal center and magnetic length of the corrector magnets. It comprises a set of induction coils, tangentially positioned onto a measurement head. It is manually moved through the magnetic field, giving the longitudinal field profiles of the magnets, which is used to locate longitudinal centers and magnetic lengths. The developed systems were tested to characterize their measurement uncertainty, which is requested to be within pre-defined limits for the measurement campaign. To this end, a single, ten-pole corrector magnet was measured with both systems. To fully characterize the rotating single-stretched wire method, a more powerful magnet was also tested, and the results compared to measurements with a well-known measurement system. Lastly, the systems were used to measure the present assembly of the corrector package, to provide feedback to the assembly and fabrication processes, and for further validation of the measurement systems. The validation measurements showed the ability of the systems to determine the measurands, and gave their spread from several measurements. The rotating single-stretched wire proved to yield the magnetic axis with an uncertainty of ±20 μm, which when referred to the physical magnet is increased to ±170 μm. The system was able to measure the roll angle with an uncertainty of ±0.06 mrad. This is well beneath the measurement requirements. The measurements with the translating coil fluxmeter proved the system to have a spread of less than ±1 mm for the measurements of both the longitudinal center and the magnetic length. This is ten times lower than the required uncertainty. As such, both systems were deemed suitable for further measurements. The present assembly of the corrector package was measured with both systems. The measurements with the rotating single-stretched wire method showed offsets of the magnetic axis within the expected deviations for all the magnets, even though they were rather large. The measured roll angles were found to exceed the expectations by a factor eight. The latter points are most likely the results of misalignment of the magnets during the assembly phase, as this is not yet complete. For the measurements using the translating coil fluxmeter, it was proven to have an uncertainty of less than the requirements with the entire corrector package powered in series, even at lower field values. The deviations were within the expectations for both the longitudinal center and the magnetic length for nearly all the magnets. However, the magnetic length of the quadrupole corrector was found to be higher than expected. This could be explained by low precision in the design phase, and the uncertainty introduced by the mechanical movement of the measurement head. The measurement systems both showed great promise for future measurement campaigns. The rotating single-stretched wire performed better than what could be expected, and will be used in the future measurements of the corrector packages. The translating coil fluxmeter was deemed sufficiently precise and accurate for these measurements, and as such could be used in its current state for further measurements. However, reducing the mechanical noise from the movement of the measurement head is advised for the future development.