An induction-coil magnetometer for mid-plane measurements in spectrometer magnets

Induction-coil magnetometers are among the most common devices for measuring both static and transient magnetic fields in accelerator and spectrometer magnets. Recent developments have included an induction-coil array mounted on a sledge, which is translated on the mid-plane of normal-conducting spectrometer magnets. This device, subsequently denoted as the moving fluxmeter, is used to derive the field homogeneity in the magnet center, as well as the gradients in the fringe-field areas. Induction coils capture the magnetic flux through the surface that is traced out by the coil windings. A deconvolution is necessary to recover the flux density from the measured voltage signal. The key idea of this article is to combine the advantages of small and large induction coils to optimize the sensitivity function in the frequency domain. In this way, an information loss due to ”blindeye” frequencies can be avoided and signals can be deconvoluted with ease. While the coil design and metrological characterization is inspired by the theory of the rotating-coil magnetometer, the sensitivity function needs to be expressed in terms of longitudinal spatial instead of angular frequencies. Consequentially we are working with Fourier transforms, instead of Fourier series of periodic signals. The coil sensitivity function, i.e., the convolution kernel, is optimized in the relevant frequency range by the precise layout of the coil-turns on a printed circuit board (PCB). Measurement results are presented that validate the concept and prove its advantages with respect to the classical coil design.