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Linear collider test facility: ATF2 final focus active stabilisation pertinence

Abstract

International audienceBeam motion at the Interaction Point (IP) of ATF2 has to be less than 10nm relative to the instrumentation used for measurements. Due to ground motion (GM), the beam can pass off-axis through the quadrupoles of the beam line and hence be deflected. It was shown in previous studies that good spatial coherence of the GM over a few meters makes the relative motion of the Final Doublets (FD) small enough for the tolerance not to be exceeded. However, since the coherence drops rapidly with distance, other quadrupoles further upstream can be expected to induce significant effects. In this paper, an evaluation taking into account all ATF2 quadrupoles is presented, using a GM generator with parameters tuned to dedicated measurements done recently along the ATF2 beam line and propagating to the IP with the optical transfer matrices. It was shown that although large IP beam motion can indeed be induced by some specific upstream quadrupoles, the combined effect of all is small because of compensations. The tolerance can thus be achieved without specially stabilising these quadrupoles

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