Trim Magnet Polarities, Dispersion, and Response Data in Sector 23

During the first LHC injection test, on 10 August 2008, the horizontal dispersion measured with beam 1 at the end of Sector 23 differed from the model prediction close to point 3. Beam-based polarity checks performed during the same period indicated an inversion of the trim quadrupole QTL11.R2. Combined with other evidence, like electrical drawings and earlier Hall-probe measurements of warm magnets, this gave rise to the hypothesis of a systematic error. Indeed a model inversion of all trim quadrupoles (QT or QTL) attached to a defocusing main quadrupole (actually the odd-numbered trim quadrupoles in Sector 23) reproduced the dispersion measurement. An analysis of kick-response measurements independently revealed an optics error left of point 3, and confirmed the inversion of the odd-numbered trim quadrupoles in this sector. After changing the polarity of the suspected set of quadrupoles prior to the second injection test on August 24, the measured dispersion nicely traced the model prediction.