Twin rotating coils for cold magnetic measurements of 15 m long LHC dipoles

We describe here a new harmonic coil system for the field measurement of th e superconducting, twin aperture LHC dipoles and the associated corrector m agnets. Besides field measurements the system can be used as an antenna to localize the quench origin. The main component is a 16 m long rotating shaf t, made up of 13 ceramic segments, each carrying two tangential coils plus a central radial coil, all working in parallel. The segments are connected with flexible Ti-alloy bellows, allowing the piecewise straight shaft to fo llow the curvature of the dipole while maintaining high torsional rigidity. At each interconnection the structure is supported by rollers and ball bea rings, necessary for the axial movement for installation and for the rotati on of the coil during measurement. Two such shafts are simultaneously drive n by a twin-rotating unit, thus measuring both apertures of a dipole at the same time. This arrangement allows very short measurement times (typically 10 s) and is essential to perform cold magnetic measurements of all dipole s. The coil surface and direction are calibrated using a reference dipole. In this paper we describe the twin rotating coil system and its calibration facility, and we give the typical resolution and accuracy achieved with th e first commissioned unit.