Field Quality Analysis as a Tool to Monitor Magnet Production

Field harmonics offer a powerful tool to examine the mechanical structure of accelerator magnets. A large devia- tion from the nominal values suggests a mechanical defect. Mag- nets with such defects are likely to have a poor quench perform- ance. Similarly, a trend suggests a wear in tooling or a gradual change in the magnet assembly or in the size of a component. This paper presents the use of the field quality as a tool to moni- tor the magnet production of the Relativistic Heavy Ion Collider (RHIC). Several examples are briefly described. Field quality analysis can also rule out a suspected geometric error if it can not be supported by the symmetry and the magnitude of the measured harmonics. I. INTRODUCTION Field harmonics are measured warm (at room temperature) in all superconducting accelerator magnets to determine if these are acceptable for RHIC (1). These warm harmonics are also a reflection of the magnet geometry and can be used to monitor the mechanical structure of the magnet. The current passing through the cable gives the magnetic signal which reflects the coil geometry. The image current of the coil due to iron reflects the geometry of the iron yoke. The field har- monics are the Fourier analysis of this signal. A large depar- ture from the nominal value may indicate an unacceptable part or assembly of the magnet. This paper presents the use of field harmonics as a tool to determine possible defects or ir- regularities in the magnets. The presence and absence of sev- eral harmonics is used to determine the nature, size and loca- tion of the defects. In most cases, the warm (room tempera- ture) measurements were sufficient to pinpoint the defect, but in some cases cold measurements were also used. The field harmonics in RHIC magnets are expressed in terms of the normal and skew harmonic coefficients, bn and an in "units", defined by the following expansion In this section, a few selected examples (cases) during the RHIC magnet production (2) are given, where the field qual- ity analysis was used to determine a defect in the magnet. A systematic analysis of the harmonics, when they deviated sig- nificantly from their typical values, determined the defect. In some cases, further measurements were required to find the exact nature and location of the defect. The necessary steps were taken either to remove the defect or to prevent it from occurring in subsequent magnets.