FEATHER: A fast intrapulse feedback system for the JLC

Ground motion at the future JLC detector may affect beam alignment and cause huge luminosity loss. The FEATHER (Feedback AT High Energy Requirements) project addresses this problem by designing a fast intrapulse feedback system that will correct the observed beam offset. Also submitted to the Proceedings of the APP Institute. FEATHER: a fast intra-pulse feedback system for the JLC. Nicolas Delerue JLC Group, KEK Talk given at the 8th Accelerator and Particle Physics Institute APPI (Japan), February 2003 Ground motion at the Future JLC detector may affect beam alignment and cause huge luminosity loss. The FEATHER (FEedback AT High Energy Requirements) project addresses this problem by designing a fast intra-pulse feedback system that will correct the observed beam offset. 1 Need for a fast feedback system Ground motion arises from both natural and human activities. At frequencies higher than a few tens of hertz most of the motion comes from “cultural noise” (ie. human activities). At these frequencies the amplitude of the ground motion, a few nanometers, is comparable to the vertical beam size proposed for the JLC. This means that beams may be misaligned, leading to loss of luminosity (see figure 1) and poorer performances for the collider. The effects of beam misalignment have been studied using CAIN [1]. This problem will partly be addressed during the site selection (see [2]) but it is unavoidable that some noise will remain (at least the noise arising from the accelerator operation). The figure 2 shows the acceptable noise amplitude for various frequency range. Ground motion will also be reduced by various mechanical device, but in the high 0 0.2 0.4 0.6 0.8 1 0 2.5 5 7.5 10 12.5 15 17.5 20 offset (σy) Lu m in os ity lo ss (L /L 0 ) Figure 1: Fraction of the total luminosity lost as a function of the vertical offset of the beams at the interaction point. The horizontal unit, σy, is the vertical size of the beam (a few nanometers). frequency region, an active device is needed to completely correct the beam misalignment. This is the purpose of the fast feedback system proposed by the FEATHER (FEedback AT High Energy Requirement) collaboration [3]. 2 Different models of fast feedback system As the beam travels at the speed of the light, it is not possible to use the position 1 Figure 2: Acceptable noise amplitude at various frequencies and methods that will be used to correct the resulting misalignment. of a bunch to correct the same beam. Furthermore, the beam size is of the order of a few nanometers, and most displacements are well beyond the reach of current Beam Position Monitors (BPM). But after the interaction point (IP) both beams are deflected with a deflection angle defined by their misalignment (see figure 3). Thus it is easier to measure the misalignment on the outgoing beam after the IP to correct the incoming beam. This can be done either on only one arm or on the two arms of the collider. The two arms solution reduces the correction required but is more complicated as good and fast communication between the two systems is required to avoid “opposite” corrections. 2.1 Simple model of feedback A simple system of feedback just needs to read the outgoing beam position from the 0 50 100 150 200 250 0 2.5 5 7.5 10 12.5 15 17.5 20 offset (σy) an gl e (μ -r ad ) Figure 3: Relation between the incoming offset and the deflected angle after the IP. BPM, compute the correction needed and then apply it to the incoming beam with a kicker. The layout of such system can be seen on figure 4.