Beam-based alignment of the SuperKEKB linac quadrupoles

The positron linac of SuperKEKB contains accelerating structures surrounded by quadrupoles without nearby beam-position-monitors (BPMs) or steering magnets. In this situation, standard techniques for beam steering or beam-based-alignment are not efficient. We here present a method to determine the magnetic centers of individual SuperKEKB linac quadrupoles. This method is independent of any BPM offsets. The beam position at the quadrupole under study is scanned, and for each position the quadrupole strength is varied, so as to find the location of zero deflection. Unlike other method used in the past, this technique does not rely on any transfer matrices. Interpolation is possible even to quadrupoles without BPM by assigning ‘virtual BPMs’. We show that the misalignment of the quadrupoles with BPM in the bore can be determined with an rms error of ≤37μm; for quadrupoles without proper BPMs the error is σ≤112μm.