Abstract As a result of the even symmetry of the Sextupole field, it creates the horizontal shift of the averaged position of horizontal and vertical betatron oscillation and the amount of the shift depends on its oscillation amplitude. This shift can be observed with usual slow orbit beam position monitor. At the SPring-8 storage ring, this shift is used to detect the excitation of the betatron oscillation for the interlock system for the protection of the vacuum components from strong radiation of insertion devices. AMPLITUDE DEPENDENT BETATRON OSCILLATION CENTER SHIFT The transverse beam instability drives a horizontal or vertical betatron oscillation, and if this occurs in light sources, the strong synchrotron radiation from insertion devices also oscillates as the beam and cause heat damages on beam pipe components if the radiation continuously hit them. The even symmetry of the sextupole field produces the horizontal shift of the time averaged horizontal position if the horizontal and vertical betatron oscillation exists. Here we call it an amplitude dependent betatron oscillation center shift (ADCS). The ADCS on the sextupole strength can be derived by a canonical perturbation theory as the first order effect by sextupole field [1] as where the symbols with overline are the time averaged values, J z and φ z ( z = x, y ) are the action and the phase, respectively, and related to the position and beta function β z s The sextupole strength is expressed as for the magnetic field
