Recent Advances in the Measurement of Chromaticity via Head-Tail Phase Shift Analysis

A so-called "Head-Tail" monitor has been operational in the CERN-SPS for a few years. The measurement of chromaticity using such a monitor relies on the periodic dephasing and rephasing that occurs between the head and tail of a single bunch for non-zero chromaticity. By measuring the turn-by-turn position data from two longitudinal positions in a bunch it is possible to extract the relative dephasing of the head and the tail, and so to determine the chromaticity. Until recently this technique had suffered from an unexplained "missing factor" when compared to conventional chromaticity measurements. This paper explains the source of this factor and also reports on the considerable experimental, simulation and analysis effort that has qualified the technique for use in the LHC.