Evolution of the Machine Impedance Following the ESRF Upgrade to Low Gap NEG Coated Aluminium Chambers

The installation of 5 meter-long, 8 mm vertical aperture insertion device (ID) aluminum chambers coated in house with non evaporable getter material (NEG) is progressing at a rate of one chamber per shutdown. The evolution of impedance with associated consequences on instability thresholds, following the installation of a number of low aperture insertion device chambers, will be reported. In particular, the impedance measurement using beam orbit bump method made it possible to identify and replace the chambers with the highest impedance. Correlation with the evolution of the instability thresholds and the theoretical prediction will be discussed. It was observed that change in vertical aperture has an important effect on the horizontal threshold.