Results of the LHC Prototype Chromaticity Measurement System Studies in the CERN-SPS

Tune and chromaticity control is an integral part of safe and reliable LHC operation. Tight tolerances on the maximum beam excursions allow excitation amplitudes of less than 30 $\mu$m. This leaves only a small margin for transverse beam and momentum excitations, required for measuring tune and chromaticity. This contribution discusses the baseline LHC continuous chromaticity measurement with results from tests at the CERN-SPS. The system is based on continuous tracking of the tune using a Phase- Locked-Loop (PLL) while modulating the beam momentum. The high PLL tune resolution achieved, made it possible to detect chromaticity changes well below the nominally required 1 unit for relative momentum modulations of only $2·10^{-5}$. The sensitive tune measurement frontend employed allowed the PLL excitation and radial amplitudes to be kept below a few micrometers. These results show that this type of measurement can be considered as practically non-perturbative permitting its use even during nominal LHC operation.