Arc cells to minimise the effects of the systematic multipoles in LHC

In LHC the main contribution to the dynamic aperture is presently (September 1998) that of the ``systematic multipole component per octant''. These components originate from the existence of several fabrication lines which can have different systematic manufacturing errors. It was decided at the Montreux workshop (list of participants in reference [1] that the dipoles belonging to a given octant come from a single given fabrication line. It is shown in this note that, for this kind of perturbation, a certain choice of the phase advances per cell allows a cancellation of most of the non-linear resonances to first order in multipole strengths, whatever the multipole index. This is the meaning of the word ``minimise'' in the title of the note. The efficiency of the method is established by means of a simple tracking test.