IR4 Tuning for LHC V6.5

Possible change of the phase advances by means of IR4 under the present new boundary conditions imposed by the beam screens have been examined. If the vacuum chamber aperture corresponds to a primary collimator setting to 7sigma, the maximum change in phase advance is +-0.06 x 2pi in the horizontal plane and +-0.16 x 2pi in the vertical plane simultaneously. For a setting to 6.5sigma, these changes become +-0.12 x 2pi and +-0.20 x 2pi, respectively. The maximum difference of b2s, in any magnet, between the two rings which can be compensated is 1 unit. This assumes that the non-linear chromaticity in physics is compensated well enough with the sextupole families and that the tuning quadrupoles are reserved for operation adjustments. If a larger value of b2s is expected, either special quadrupoles with an inter-axis distance of 420 mm have to be added in IR4 to extend its tunability, or the operational tune-shifts have to be limited because of the use of the tuning quadrupoles to compensate for the difference in b2s.