Improving Magnet Aperture by Estimation of Errors due to the Influence of Temperature Gradients during Magnet Axis Measurements

The LHC magnet axes are measured from both ends of the magnet. These two redundant measurements are combined to get a reliable measurement result and to compensate calibration errors. When the two measurements are put together, we observe a "saw tooth" effect due to the fact that the two measurements are, in general, not identical. It can be seen that this effect is larger than expected from the accuracy of the measurements which motivated the study. Effects of temperature gradients in the cold bore tube of the magnet during measurements have been observed [6] and we show that this effect is the most probable explanation for our observations of the geometry measurements in the vertical plane. The aim of this work is to present an algorithmic approach to filter this effect and improve the measurement results. The effect is relatively small but for some magnets the displacement is up to -0.3 mm. The magnet positioning is controlled to 0.1 mm. Our analysis shows that by applying this correction we can insure the best positioning of the magnets (including the spool pieces) in the tunnel in the vertical plane.