Multipole-field measurements by sampling oblong apertures of accelerator magnets

The rotating search coil is a commonly used tool to measure magnetic fields of accelerator magnets. The coil intercepts the magnetic flux at a radius given by the dimensions of the measurement shaft that comprises a set of search coils for the analog bucking of the main signal from the dipole field component. For magnets of a rectangular aperture with large aspect ratio (>3:1) the cylindrical domain covers only a portion of the magnet bore. As the field representation is dominated by measurement errors outside that cylindrical domain, a sampling technique is required. The method presented in this paper trades the precision in the measurements against the precision in the shaft positioning and arrives at a multipole representation that covers the entire bore of the magnet.