Beam-based Alignment of the LSCL Final Focus Sextupoles

The strong demagnification inherent in final focus systems requires local cancellation of the resulting chromaticity.Strong sextupole pairs separated by a -I transform are positioned x12 in betatron phase away from the Interaction Point (IP) in order to cancel chromatic aberrations primarily due to the final quadrupoles.Sextupole alignment is critical in order to provide orthogonal tuning of the chromaticity and, in the case of the SLC, to limit the third and higher order optical aberrations generated from misaligned and 'nested' horizontal and vertical sextupole pairs.Reported here is a novel technique for aligning the beam centroid to the sextupole centers, which uses measurements of the critically dependent parameter -the beam size at the IP.Results for the SLC final focus sextupoles are presented, where a resolution of ~50 pm is achieved.