Analyzing the Complex Impedances of All LHC Main Dipole Magnets

Abstract

Measuring a superconducting magnet's complex impedance as a function of the frequency is a promising diagnostic tool to investigate its electrical integrity and the potential presence of electrical nonconformities. Such transfer function measurements (TFM) were performed for the first time on a large number of magnets in the Large Hadron Collider (LHC). During this measurement campaign, the impedances of all 1232 superconducting main dipoles installed in the LHC were measured at cold in the frequency range from 1 Hz to 100 kHz. This letter presents the measurement set-up and provides a comparative data analysis of all complex impedance measurements. Distinct groups of magnets showing similar behavior are analyzed, and frequency ranges showing significant impedance variations are identified. Variations in TFM are shown to be correlated to individual features of each magnet, such as manufacturing tolerances in the magnets' beam screens, different materials used in their coil-protection sheets, and variations in the critical current of their superconductors. Moreover, measurements are compared to the simulation results obtained by a recently developed and validated lumped-element network model and show good agreement. Finally, a few magnets are identified as outliers as their measured impedances deviate significantly from the impedances of the other magnets and differ from the simulated values.