In situ calibration of rotating sensor coils for magnet testing

Abstract

An in situ procedure for calibrating equivalent magnetic area and rotation radius of rotating coils is proposed for testing accelerator magnets shorter than the measuring coil. The procedure exploits measurements of magnetic field and mechanical displacement inside a reference quadrupole magnet. In a quadrupole field, an offset between the magnet and coil rotation axes gives rise to a dipole component in the field series expansion. The measurements of the focusing strength, the displacement, and the resulting dipole term allow the equivalent area and radius of the coil to be determined analytically. The procedure improves the accuracy of coils with large geometrical irregularities in the winding. This is essential for short magnets where the coil dimensions constrain the measurement accuracy.Experimental results on different coils measuring small-aperture permanent magnets are shown. (2012) Geneva, Switzerland Published in Review of Scientific Instruments CERN-ATS-2012-018 February 2012Abstract REVIEW OF SCIENTIFIC INSTRUMENTS 83, 013306 (2012) In situ calibration of rotating sensor coils for magnet testing An in situ procedure for calibrating equivalent magnetic area and rotation radius of rotating coils is proposed for testing accelerator magnets shorter than the measuring coil. The procedure exploits measurements of magnetic field and mechanical displacement inside a reference quadrupole magnet. In a quadrupole field, an offset between the magnet and coil rotation axes gives rise to a dipole component in the field series expansion. The measurements of the focusing strength, the displacement, and the resulting dipole term allow the equivalent area and radius of the coil to be determined analytically. The procedure improves the accuracy of coils with large geometrical irregularities in the winding. This is essential for short magnets where the coil dimensions constrain the measurement accuracy. Experimental results on different coils measuring small-aperture permanent magnets are shown.

Rotating-coil calibration in a reference quadrupole, considering roll-angle misalignment and higher-order harmonics

“…These results show that the calibration error for the classical method [1] depends on the displacements Mx, while the proposed method is more robust. Table 3 shows, the results obtained for a quadrupole magnet with an octupole field harmonic.…”

“…The results obtained by Method 1 (see Ref. [1]) for an ideal quadrupole are shown in Table 1. These results are referred to an reference coil radius of 20 mm, as modeled in ROXIE.…”

“…In general, by considering all the harmonics, the two sets of measured harmonics are linked by the feed-down formula [1]:…”

“…The calibration based on a reference quadrupole magnet [1] is currently used in rotating coil systems. The magnetic-equivalent rotation radius can be obtained in a reference quadrupole field either by measuring the focusing strength, or rotating the coil by a given angle [1] or also before and after a translation in the horizontal (xz) and vertical (yz) planes.…”

“…However, if significant roll and swing misalignment errors are present, this gives rise to significant calibration errors. Moreover, the equations used to calculate the radius is based on the feed-down effect from the quadrupole component [1], by assuming all the contributions of higher-order terms as negligible. This is justified because the dominant field component in the reference magnet is much larger than the field errors of higher-order multipoles [10].…”