Correcting the LHC β* at collision

To correct the β* at the main collision points (IP1 and IP5) simultaneously for the two counterrotating proton beams in the Large Hadron Collider (LHC), a set of specific quadrupoles in the noncommon part of the machine is used. Due to the antisymmetric optics, several quadrupoles on each side of the insertion have to be employed. The change of β* is accomplished by incrementing the quadrupole gradients. This set of increments is referred to as β* tuning knob. The increments were calculated by rematching β* in a range of ±20 % about the nominal value. Linear curves were fitted to the variation of increments to construct a linear tuning knob. This was done for each plane using MAD 8 and repeated with MAD X. The linear behaviour and the orthogonality of the knobs were investigated for the LHC lattices V6.2 and V6.4. Different field errors were introduced in the lattice and the correction efficiency of the knobs was studied.