Evaluation of the Multi-Bunch Kink Instability in ILC Head-on Collisions

An interaction region with head-on collisions is considered an alternative to the baseline configuration of the International Linear Collider which includes two interaction regions with finite crossing-angles (2 and 20 mrad). Although more challenging from the point of view of the beam extraction, the head-on scheme is favoured by the experiments because it allows an easier optimisation of the detector configuration, particularly in the forward region. The optics of the head-on extraction is revisited, as compared to the TESLA TDR, by separating the e+ and e- beams horizontally, first by electrostatic separators operated at their LEP nominal field and then using a defocusing quadrupole of the final focus beam line. In this way the septum magnet is protected from the beamstrahlung power. The influence of parasitic collisions is shown to lead to a region of stable collision parameters.