b2 in ramp


"Due to dynamic effects in the LHC magnets, hysteresis and saturation effects, the betatron phase advances can drastically changes during the different operation modes of the LHC (injection plateau/snap-back/ramp/flat-top), from a couple of degrees up to 50 degrees depending on the distance between the points where the closed orbit sources occures and the point where the correction is applied. The worst contribution comes from the change of b2 in the main dipoles from injection to flap-top. This effect does not change the overall tune since it has opposite sign in the inner and outer aperture of the ring (see attached document). However, accross a full sector, it translates into a change of phase advance of about 25 degrees in both plane, and twice more between IP1 and IP5.

Other effects, less important but not completely negligible may play some role as the dynamic b2 feed-down effect from the MCS spool-pieces, or the decay of the MQs, each amounting to about 1 degree per sector during Decay and SB. Also PC tracking errors between B2 and B1 may have a much larger impact in terms of changes of phase advances. However all these other effects will also have an effect on the tune such that their impact on local phase advance errors will be minimised if the tune correction is uniformately distributed over the LHC ring. On the other hand, this might not at all be the case, if, as proposed at chamonix, only one single MQT family will be dedicated for this purpose in order to control and minimise the MQT hysteresis effects at injection."