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Comparison of Ilc Fast Beam-beam Feedback Performance in the and Modes of Operation
| Content Provider | Semantic Scholar |
|---|---|
| Author | Alabau, Magali |
| Copyright Year | 2007 |
| Abstract | Several feedback loops are required in the Beam Delivery System (BDS) of the International Linear Collider (ILC) to preserve the luminosity in the presence of dynamic imperfections. Realistic simulations have been carried out to study the performance of the beam-beam deflection based fast feedback system, for both and modes of operation. The beam-beam effects in the collisions make both the luminosity and the deflections more sensitive to offsets at the interaction point (IP) than in the case of the collisions. This reduces the performance of the feedback system in comparison to the standard collisions, and may require a different beam parameter optimization. BEAM-BEAM DEFLECTION BASED FEEDBACK SYSTEM Misalignments in the lattice magnets produced by ground motion induce perturbations to the beam trajectory with respect to the ideal trajectory which can increase the transverse beam sizes at the IP and introduce offsets between the beams at the collision point. Several feedback loops are foreseen in the BDS of the ILC to mitigate these effects and to avoid the resulting degradation of the luminosity [1]. To correct the position and the angle of the beams at the IP, fast feedback systems are applied bunch-to-bunch, while slower feedback systems are required to maintain aligned the lattice magnets and to correct the beam trajectories. The main signal used to maintain the beams aligned within half a nanometre at the IP is the transverse kick that the misaligned beams experience during the collision [2]. Beam-beam effects for and collisions In the case of collisions, a bunch passing close to the axis through the electromagnetic field created by the opposite beam is strongly focused, which leads to a enhancement of the luminosity. For collisions, on the other hand, repulsion occurs, which enhances the effective transverse sizes at the IP, reducing the peak luminosity to values only typically about 20% of those for . In addition, in collisions the luminosity is much more sensitive to residual offsets at the IP and the deflection curve as This work is supported by the Commission of the European Communities under the Framework Programme “Structuring the European Research Area”, contract number RIDS-011899. a function of the IP offsets is much steeper than for (see Fig. 1), which can impact the feedback performance. −100 −50 0 50 100 −300 −200 −100 0 100 200 300 y−offset (nm) O ut gi ng y − an gl e (μ ad ) ee collision ee collision Figure 1: Vertical deflection angle versus vertical half beam-beam offset, for and collisions simulated with GUINEA-PIG [3], using ideal Gaussian beam distributions with ILC nominal parameters at 500 GeV in the center-of-mass [4]. SIMPLIFIED IP POSITION FEEDBACK SIMULATION A study of the impact of this steeper deflection curve on the performance of the beam-beam deflection based feedback system compared to collisions was carried out in [5]. In this simplified simulation, offsets of the order of hundreds of nanometres were introduced between the trains (which are delivered with a frequency of 5 Hz), as well as a bunch-to-bunch jitter of the order of a fraction of the beam size. The collision was simulated with GUINEA-PIG [3], and the obtained out-going angle was used to predict the offset of the beam. The correction was carried out bunchto-bunch. The results indicated that the correction of the position is slower for collisions due to the steeper deflection curve, but the correction can be done fast enough to recover the average luminosity over a train. On the other hand, the luminosity loss as function of the bunch-to-bunch jitter for the collisions was a factor 2 greater compared to , due to greater sensitivity to the offsets at the IP. Thus, a different beam parameter optimization, reducing the disruption parameter, was suggested for the case of collisions. In order to verify that the assumptions on the ground motion amplitudes considered in the simplified simulation of the feedback system are acceptable, a more realistic simulation has been carried out, including dynamic imperfections in the BDS magnets, and is presented here. GROUND MOTION EFFECT An important source of magnet displacements is ground motion, which is transmitted to the lattice elements by their support structures. Several ground motion models have been built, based on the results of measurements in different sites, with different levels of noise. These models include ATL diffusive motion, slow systematic motion, natural micro-seismic motion, and fast cultural noise [6]. For the feedback simulation, the elements of both BDS lines have been misaligned applying the intermediate noisy level model B [6]. The time interval used to sample the ground motion was 0.2 s, corresponding to the frequency at which trains are delivered. This simulation has been done with the tracking code PLACET [7]. To check the misalignments produced by this model along the lattice as function of time, the r.m.s. displacements for 50 seeds of the generator were calculated. Fig. 2 shows the difference of the vertical misalignments produced at each element in the electron line with respect to the same element in the positron one, for successive time intervals. 0 0.1 0.2 0.3 0.4 0.5 0.6 0 100 200 300 400 500 600 700 800 r. m .s .[y (e ) -y (e + )] ( μm ) Element (from the BDS entrance to the IP) Ground motion during: 0 s 0.2 s 0.4 s 0.6 s 0.8 s 1 s Figure 2: Difference in misalignment of each BDS element in the line with respect to the same element in the one. Ground motion model B was applied at successive time intervals. The results are the of 50 seeds. Due to the fact that there is a certain spatial coherence in the ground vibration, the sum of the misalignments is bigger than the difference between corresponding elements of the and lines. The simulation of the beam-based IP position feedback system, is only sensitive to the difference between the beams at the IP. Other deviations of the trajectory with respect to the ideal one, should be corrected upstream, with a slower feedback which maintains the magnets correctly positioned along the beam lines, or through appropriately placed magnetic correctors. Such corrections, while not essential to keep the beam in collision at the IP, are needed to maintain the optical quality of the beam spot, and hence the luminosity. BEAM-BASED IP POSITION FEEDBACK SIMULATION For the simulation of the IP position feedback system, after tracking the beams through the BDS lattices misaligned by the ground motion with the code PLACET [7], the beam-beam collision is simulated with the code GUINEAPIG [3] to obtain the outgoing angle that will serve to compute the correction. The beam position of the next bunch is corrected with a kicker located upstream of the IP close to the final doublet (FD). The operation is repeated bunch-tobunch. Fig. 3 illustrates the feedback response for ground motion applied during successive time intervals, for collisions. The average luminosity performance as a function of time is obtained with 50 seeds. The larger a ground motion is applied, the more important are the misalignments in the lattice, and the smaller is the final luminosity which can be recovered with the beam-beam deflection based IP position feedback. Although 70 or 80 of the luminosity can be recovered after 1 s, the deterioration of the beam sizes due to the optical effects caused by upstream misalignments makes it impossible to recover more than 30 or 40% of the luminosity after e.g. 300 s. Other feedback loops are thus required. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 10 20 30 40 50 60 < re la tiv e lu m in os ity > # bunch 1 s 5 s 10 s 30 s 60 s 120 s 240 s 300 s Figure 3: Beam-based IP postition feedback simulation for the collision with ground motion model B applied for successive time intervals along the BDS. The average relative luminosity is calculated over 50 seeds. FEEDBACK SIMULATION INCLUDING IP ANGLE CORRECTION An IP angle correction has been included in the simulation in order to correct the position of the beams along the Final Focus System (FFS), and thus mitigate the beam size increase produced by passing off-axis through the sextupoles, in order to check if the nominal luminosity can be recovered after the correction of the beam offsets at the IP. The angle at the IP is corrected with a kicker located at the entrance of the FFS, at n phase-advance from the IP. The angle is corrected by zeroing the signal in a BPM located at a phase /2 downstream from the kicker [1]. Figs. 4 and 5 illustrate the feedback responses for ground motion applied during successive time intervals, for and collisions respectively, including the IP position and angle correction. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 10 20 30 40 50 60 < re la tiv e lu m in os ity > # bunch 1 s 5 s 10 s 30 s 60 s 120 s 240 s 300 s Figure 4: Beam-based IP position and IP angle feedback simulation for the collision with ground motion model B applied for successive time intervals along the BDS. The average relative luminosity is calculated over 50 seeds. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 50 100 150 200 250 < re la tiv e lu m in os ity > # bunch 1 s 5 s 10 s 30 s 60 s 120 s 240 s 300 s Figure 5: Beam-based IP position and IP angle feedback simulation for the collision with ground motion model B applied for successive time intervals along the BDS. The average relative luminosity is calculated over 50 seeds. The results indicate that about 20 more luminosity can be achieved by correcting the IP angle compared to the case where only the IP position was considered (see Figs. 3 and 4). But, on the other hand, the luminosity cannot be recovered entirely, due to optical effects along the FFS, which increase the beam size at the IP. The correlation between the vertical beam size at the IP and the luminosity is shown in Fig. 6. The luminosity loss is directly related to the increased beam size. No significant residual offset between the beams remains at the |
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| Alternate Webpage(s) | https://www.eurotev.org/reports__presentations/eurotev_reports/2007/e1065/EUROTeV-Report-2007-053.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
