Checking the Polarity of Superconducting Multipole LHC Magnets

This paper describes the design and operation of the "Polarity Checker", a scanning probe designed to check multipole field order, type and polarity of superconducting LHC magnets. First we introduce the measurement method, based on the harmonic analysis of the radial field component picked up by a rotating Hall sensor at different current levels. Then we describe the hardware and the software of the system, which features automatic powering, data acquisition and treatment, discussing the achieved sensitivity and performance. Finally we provide a summary of the test results on the first 505 cryoassemblies, showing how the system was usefully employed to detect some potentially harmful connection errors

I. Introduction

The Large Hadron Collider (LHC) will include about 1750 cryomagnet assemblies, up to almost 16 m long, housing a total of about 10000 superconducting magnets connected in 1612 electrical circuits [1]–[3]. Any construction error leading to an incorrect multipole type or polarity, such as e.g. those due to busbar inversion, connection of a magnet to the wrong circuit or mistakes in the mechanical installation, may seriously compromise LHC operation. To detect this kind of errors all magnets must be checked carefully, which is not easy since the field can be measured only from the outside via cold bore tubes with 50 mm aperture. To correct such faults after the machine has been built could require warming up to room temperature an entire LHC sector.