BEAM BASED MEASUREMENTS OF FIELD MULTIPOLES IN THE RHIC LOW BETA INSERTIONS AND EXTRAPOLATION OF THE METHOD TO THE LHC

The multipolar content of the dipoles and quadrupoles is known to limit the stability of the beam dynamics in super-conducting machines like RHIC and even more in LHC. The low-beta quadrupoles are thus equipped with correct-ing coils up to the dodecapole order. The correction is planned to rely on magnetic measurements. We show that a relatively simple method allows an accurate measurement of the multipolar field aberrations using the beam. The principle is to displace the beam in the non-linear fields by local closed orbit bumps and to measure the variation of sensitive beam observables. The resolution and robustness of the method are found appropriate. Experimentation at RHIC showed clearly the presence of normal and skew sex-tupolar field components in addition to a skew quadrupolar component in the interaction regions. Higher-order com-ponents up to decapole order appear as well. 1 PRINCIPLE OF THE MEASUREMENT To avoid a strong perturbation of the beam dynamics, the beam is moved locally to large amplitudes using a closed orbit bump. The non-linearity inside the orbit bump acts by feed-down to all lower orders. Its order and magnitude are deduced from the variation of suitable observables with the bump amplitude. This method is extrapolated from [1] where it was used to measure the azimuthal dependence of a parasitic skew gradient in LEP. In this paper, we consider the feed-downs to dipole and quadrupole orders. Measurement of the Orbit Perturbation We select as an observable the rms orbit perturbation measured outside the bump. The sign of the field perturbation is recovered by fitting an oscillation. The rms orbit z ̃ is given by: z ̃ = fz(cn, zco) = β̄zar