A contribution to the development and characterization of rotating-coil magnetometers

  • Ein Beitrag zur Entwicklung und Charakterisierung von Drehspulen-Magnetometern

Rogacki, Piotr; Hameyer, Kay (Thesis advisor); Russenschuck, Stephan (Thesis advisor)

Düren : Shaker Verlag (2022)
Book, Dissertation / PhD Thesis

In: Aachener Schriftenreihe zur elektromagnetischen Energiewandlung 45
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2021


Electromagnets are essential components of particle accelerators and are required to generate a very homogeneous magnetic field to limit orbit distortions and resonances in the particle beam. Magnetic measurements are necessary to determine the field parameters of the accelerator magnets with sufficient accuracy. The increase in the requirements for the magnets for new particle accelerators (for example, the High Luminosity Large Hadron Collider (HL-LHC) upgrade project with respect to the baseline Large Hadron Collider (LHC)) necessitates development and upgrades of magnetic measurement systems. Moreover, a growing number of smaller particle accelerators are being used for medical, industrial, and scientific purposes. Even though magnets for these accelerators may have lower accuracy requirements, they still need to me measured to intercept tolerances and errors in the production process. This calls for measurement systems with reduced complexity that can be designed and constructed without expert knowledge. The main goal of the thesis is to study the possibility of increasing the accuracy and functionality of the induction-coil systems, with particular focus on rotating-coil magnetometers. By investigating the sources of uncertainty in the magnetic measurement systems, as well as their design principles, the conflicting objectives (accuracy and reduced complexity) can be addressed simultaneously. A novel approach to a rotating-coil scanner is proposed and validated, based on the requirements of the HL-LHC magnets. The rotating-coil scanner developed in this thesis is based on a multi-layer Printed Circuit Board (PCB) with an array of printed coils, instead of conventional, wound coils. A high degree of control over the track positioning on the PCB allows improving the calibration accuracy, necessary to meet the requirements for local measurements of the magnetic flux density. To obtain the required accuracy in the measurement of the longitudinal field center, the scanner is equipped with retroreflectors, mounted directly on the PCB and surveyed by a laser tracker. The retroreflectors enable a precise measurement of the field axis position. High-resolution encoders are employed to significantly shorten the measurement times and to extend the scanner’s capability to measurements of magnets in AC excitation. The developed system is thus suited for measurements of the axis in higher order magnets, such as sextupoles and octupoles. A detailed microscopic and geometrical study of the PCB proves that, with a geometric scan of several reference points on a board, it is possible to calculate the induction-coil surface relying on the design values, with an accuracy of 300 ppm, comparable with the standard calibration in a reference magnet. With the proposed approach, the coil rotation radius can be calibrated with at least one order of magnitude higher accuracy, compared to the calibration in a reference quadrupole magnet. Apart from the increase in accuracy, this significantly reduces the need for calibration targets. By modeling the effect of mechanical errors on the final measurement accuracy, it is demonstrated that the use of the PCB coils allows a significant relaxation of mechanical constraints and tolerances without the loss of measurement accuracy, thanks to the improved calibration and stability, as well as efficient compensation schemes. Experimental validation and metrological characterization confirm that the combination of the proposed solutions improves the accuracy and extends the functionality of the rotating-coil magnetometers, and in particular allows fulfilling all accuracy requirements for HL-LHC magnets. The new system is matching or exceeding the capabilities of the state-of-the-art systems, while combining the functionalities of three different measurement approaches, i.e., rotating-coil magnetometers, stretched wire systems, and the legacy ‘AC-mole’ system developed at Conseil Européen pour la Recherche Nucléaire (CERN). This is achieved without sacrificing the flexibility and robustness of the system necessary to operate in an industrial environment. A quantification of the electronic noise in the system reveals that the impact of acquisition electronics and signal transmission on the overall system uncertainty is no longer negligible compared to the other error sources (capture uncertainty due to mechanical vibrations, and uncertainties due to calibration errors). Therefore, a further increase of precision in measurements of superconducting magnets at ambient temperature may require reduction of the signal transmission noise or an upgrade of the electronics. Modeling and verifying the uncertainty due to an acquisition without integration shows that the bandwidth requirements on the acquisition electronics can be significantly reduced without diminishing the systems accuracy below the requirements of the HL-LHC magnets. A further approach to the design of rotating-coil systems thus becomes feasible, where the simplified acquisition electronics would be installed directly in the coil shaft or on the PCB. Moreover, it is shown that with an appropriate signal treatment it is possible to omit the angular encoder in the system design without a significant loss in accuracy and only partially limiting the functionality. This finding is important for measurements in high magnetic fields, for example in superconducting magnets at cryogenic conditions, where to date the encoders must be placed outside the field domain, often far from the induction coils.