Beam-based alignment of sextupoles with the modulation method

Alignment of sextupoles is vitally important for rings such as the B-factory and damping rings. A precision of 100 μm or less is required to produce low emittance and large dynamic aperture. Beam-based alignment is crucial for such precision. We present a new technique to measure the magnetic center of sextupole magnet using beam. It is based on the so-called “K-modulation” which has been applied to quadrupoles. We use auxiliary coils on sextupoles which are connected to produce a quadrupole field. By modulating the quadrupole field with some frequency, we observe a sizable modulation of closed orbit whose amplitude is proportional to the offset of beams from magnetic center, thus giving information on the center of the sextupole. In the experiment we could measure the center with a precision of less than 50 μm. The present method is applicable to sextupoles connected in series to a common power supply because it induces no voltage to main coils. We report on the experimental results and discuss on the effects of eddy current, asymmetry of chamber and eventual precision