Suppression of amplitude dependent closest tune approach and its behavior under forced oscillations

An amplitude dependent closest tune approach (ADeCTA) was first observed in the LHC in 2012. This potentially harmful effect can be generated by linear coupling in conjunction with octupoles. In this article we demonstrate experimentally and in simulations a configuration of the octupoles to suppress this recently observed effect. Furthermore, ADeCTA is investigated for the first time with driven oscillations. We show analytically that skew quadrupoles and normal octupoles do not generate ADeCTA for forced oscillations in the same way as for free oscillations. This adds an additional constraint to measure it since the forced oscillations, generated by an ac dipole, are the preferred option for optics measurements in the LHC. The analytical result is confirmed by both measurements and simulations.