A Trajectory Correction based on Multi-Step Lining-up for the CLIC Main Linac

In the CLIC main linac it is very important to minimise the trajectory excursion and consequently the emittance dilution in order to obtain the required luminosity. Several algorithms have been proposed and lately the ballistic method has proved to be very effective. The trajectory method described in this Note retains the main advantages of the latter while adding some interesting features. It is based on the separation of the unknown variables like the quadrupole misalignments, the offset and slope of the injection straight line and the misalignments of the beam position monitors (BPM). This is achieved by referring the trajectory relatively to the injection line and not to the average pre-alignment line and by using two trajectories each corresponding to slightly different quadrupole strengths. A reference straight line is then derived onto which the beam is bent by a kick obtained by moving the first quadrupole. The other quadrupoles are then aligned on that line. The quality of the correction depends mainly on the BPM's and micro-movers' resolution and on the stability of the quadrupole strengths which should be at least of the order of 0.05 %. Although the beam follows a broken straight line, its offset from the center of the quadrupoles is typically 1.5 µm r.m.s.