Aligning the CLIC Beam Delivery System faces two major challenges, the tight tolerances for the emittance preservation and its strong non-linear beam dynamics. For these reasons conventional beam-based alignment techniques, like dispersion free steering, are only partially successful and need to be followed by optimization algorithms based on other observables, like beam sizes

Latina, A

Schulte, D

Tomás, R

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CERN-AB-2008-011ALIGNMENT OF THE CLIC BDS*Aligning the CLIC Beam Delivery System faces two major challenges, the tight tolerances for theemittance preservation and its strong non-linear beam dynamics. For these reasons conventionalbeam-based alignment techniques, like dispersion free steering, are only partially successful and need to befollowed by optimization algorithms based on other observables, like beam sizes.EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN   -   AB DepartmentGeneva, SwitzerlandAugust 2008AbstractCERN-AB-2008-011CLIC-Note -753EUROTeV-Report-2008-040*Work supported by the Commission of the European Communities under the 6th Framework Programme“Structuring the European Research Area”, contract number RIDS-011899.A. Latina,  D. Schulte, R. TomasCERN, Geneva, SwitzerlandPresented atEPAC'08, 11th European Particle Accelerator Conference, Genoa, Italy - June 23-27, 2008Alignment of the CLIC BDS ∗A. Latina, R. Tom·as and D. SchulteCERN, Geneva, Switzerland.AbstractAligning the CLIC Beam Delivery System faces two ma-jor challenges, the tight tolerances for the emittance preser-vation and its strong non-linear beam dynamics. For thesereasons conventional beam-based alignment techniques,like dispersion free steering, are only partially successfuland need to be followed by optimization algorithms basedon other observables, like beam sizes.INTRODUCTIONThe CLIC Final Focus System is based on the local chro-maticity correction scheme presented in [1] which usesstrong sextupoles near the final doublet quadrupoles for thechromatic correction. Extra non-linear elements have beenadded to the CLIC FFS to cancel residual aberrations ofoctupolar and decapolar order [2, 3]. The intrinsic non-linear behavior of this system is illustrated in Fig. 1 wherethree vertical BPM readings are plotted versus energy devi-ation with and without radiation using the simulation codePLACET [5]. Even though linear dispersion is zero theBPMs measure linear and non-linear dispersion due to thenon-linearities and the bunch density distributions. A ran-dom effect of similar order of magnitude is observed whensynchrotron radiation effects are included in the simulation.It has been verified that this effect scales with the squareroot of the number of particles used in the simulation whichtypically is 5 orders of magnitude smaller than in the CLICnominal bunch.TRADITIONAL ALGORITHMSDispersion Free Steering (DFS) [6] is an extended beam-based alignment method that minimizes the dispersion atthe BPMs by using beams with different energies and linearresponse matrices. Applying this method to the full CLICBDS improves the emittance but does not reach a satisfac-tory correction, Fig. 2. However this method proves suc-cessful if applied to the collimation section only, which isthe first part of the BDS, Fig. 3. We conclude that the non-linear behavior of the FFS limits the use of DFS in this finalsection of the BDS. Other methods using the beam size atthe IP, luminosity and a non-linear version of the DFS arestudied below with first applications to the ATF2 for testingand code verification purposes.USING ATF2 AS A TEST BENCH FORCLIC ALIGNMENT ALGORITHMSATF2 is a test facility with the aim of proving the FFSdesign proposed in [1]. Therefore this is an ideal test∗This work is supported by the Commission of the European Commu-nities under the Framework Programme “Structuring the European Re-search Area”, contract number RIDS-011899-0.4-0.3-0.2-0.1 0 0.1 0.2 0.98  0.985  0.99  0.995  1Vertical BPM reading [µm]Relative Energy (E/E0)3 FFS BPM readings (with and without radiation)Figure 1: Simulation of three vertical FFS BPM read-ings versus beam relative energy including radiation effects(points) and without radiation (lines). Apparent linear andnon-linear dispersion is observed plus an important numer-ical effect of the stochastic nature of radiation. 20 30 40 50 60 70 80 90 100 0  20  40  60  80  100ε y [nm]quadrupole indexσbpm=0.1 µm, misalignment 10 µm1-to-1 correction, no multipolesDFS correction, no multipoles1-to-1 correction, multipolesDFS correction, multipolesnominalFigure 2: Vertical emittance versus longitudinal locationafter attempting the alignment of the full CLIC BDS byone-to-one steering and dispersion free steering.bench for the alignment algorithms to be used at CLIC. Afirst simulation campaign of ATF2 alignment and tuninghas been carried out taking into account: H & V Gaus-sian misalignments with σ = 30µm, transverse rolls withσ = 30µrad, relative strength error with σ = 10−4, mea-surement error of σy with σ=2nm and a model of groundmotion based on measurements at the ATF site.All magnet H & V displacements, their roll and theirstrengths are used to minimize the transverse beam sizes at 20 30 40 50 60 70 80 90 100 0  20  40  60  80  100ε y [nm]quadrupole indextest beam 98% nominal energy, ω1/ω0=1e5, σbpm=0.1 µm, misalignment 10 µm1-to-1 correction, no multipolesDFS correction, no multipoles1-to-1 correction, multipolesDFS correction, multipolesnominalFigure 3: Vertical emittance versus longitudinal locationafter successfully aligning only the collimation section ofthe CLIC BDS.the IP using the Simplex-Nelder algorithm [4]. The tun-ing stops when the target beam size is reached or when themaximum number of iterations is exceeded. Fig. 4 showthe initial beam sizes of the uncorrected FFS, the requirednumber of iterations for convergence and the final beamsizes. Initial vertical IP beam sizes up to 4µm are reducedbelow 44nm with less than 8000 iterations (equivalent to≈6 days of ATF2 operation). This performance is simi-lar to other studies using different approaches and differentsimulation codes [7]. Once this simple algorithm has beenverified in ATF2 it is applied to the CLIC FFS.TESTING ALGORITHMS FOR CLICApplying the exact same algorithm to the CLIC FFS thanto the ATF2 machine we obtain the results shown in Fig. 5.This is unrealistic since CLIC will not be equipped witha beam size monitor at the IP. However we observe thatthe convergence of the algorithm is not as good as for theATF2 case, some seeds fail to reach the nominal beam size≈ 2.4nm even though the number of iterations doubled to16000. A possible improvement to this approach consistsin matching the non-linear dispersion to the predicted bythe model, basically a non-linear dispersion free steering.This is achieved by adding the quadratic deviations of themeasured non-linear dispersion at the BPMs to the figure ofmerit used by the Simplex. The results of this approach areshown in Fig. 6 without improving the previous, and sim-pler, approach. Moreover the number of required iterationslargely increases due to the need of specific extra measure-ments with off-energy beams. The complexity of the CLICFFS requires the development of more sophisticated algo-rithms than for the ATF2.Using the luminosity instead of the beam size as the fig-ure of merit is more realistic for the CLIC case since CLICwill not be equipped with a beam size monitor at the IP.The results are shown in Fig. 6 with a performance similar 0 5 10 15 20 25 30 0  0.5  1  1.5  2  2.5  3  3.5  4CountsInitial σy [µm] 0 2 4 6 8 10 12 14 16 0  1000  2000  3000  4000  5000  6000  7000  8000CountsNumber of iterations 0 5 10 15 20 25 30 35 34  35  36  37  38  39  40  41  42  43  44CountsFinal σy [nm]Figure 4: ATF2 simulations: histograms of initial and finalvertical beam sizes (top and bottom) and number of correc-tion iterations (medium) using the Simplex algorithm.or better than when plainly using the beam size. Basically80% of the seeds end with a luminosity above 80% of thedesign luminosity in less than 18000 iterations.CONCLUSIONSIt has been proved via realistic simulations that disper-sion free steering can be used to align the CLIC collima-tion section with an emittance growth below 5%. Thesepure alignment techniques fail in the FFS and more generaltuning algorithms have to be used for this more complexCLIC system. A simple tuning algorithm that works suc-cesfully in the ATF2 is not satisfactory for all the CLICseeds. This and other reasons have triggered a proposal toreduce the ATF2 β∗ by at least a factor 2, increasing thusits chromaticity and also the tuning difficulty [8]. This in-creased tuning difficulty may lay closer to that of the CLICcase, being one step closer to experimentally validate theCLIC FFS in the frame of the ATF2 project. A more refinedversion of the CLIC tuning algorithm including non-lineardispersion free steering does not improve the situation. The 0 1 2 3 4 5 6 7 8 9 0  0.5  1  1.5  2  2.5  3  3.5  4CountsInitial σy [µm] 0 1 2 3 4 5 6 0  4000  8000  12000  16000  20000CountsNumber of iterations 0 5 10 15 20 2  2.2  2.4  2.6  2.8  3  3.2  3.4  3.6  3.8  4CountsFinal σy [nm]Figure 5: Histograms of vertical beam sizes (before and af-ter correction) and iterations taken by the optimization forthe CLIC FFS. Beam size only as figure of merit. Knobs:H&V displacements.current conclussion is that 80% of the seeds converge toa luminosity above 80% of the design case and that algo-rithms need to be improved to reach 100% success.REFERENCES[1] P. Raimondi and A. Seryi, Phys. Rev. Lett. 86, 3779-3782.[2] R. Toma´s, CERN-OPEN-2006-023 and Phys. Rev. ST Accel.Beams 9, 081001 (2006).[3] R. Toma´s, “MAPCLASS”, CERN-AB-Note-017 ABP, 2006.[4] J.A. Nelder and R. Mead, Computer Journal, 1965, vol 7.[5] D. Schulte, et al, CERN/PS 2001 028 (AE), CLIC Note 482.[6] T. Raubenheimer and R. Ruth, Nucl. Instrum. Methods Phys.Res., Sect. A 302, 191 (1991).[7] G. White, “Extraction line + Final Focus System beam-basedalignment and IP spot-size tuning” at the 5rmth ATF2 ProjectMeeting 2007.[8] P. Bambade et al, “Exploring ultra-low β∗ values in ATF2-R&D Programme proposal”, submitted to the ATF2 projectin 2008. 0 1 2 3 4 5 6 7 8 9 0  10000  20000  30000  40000  50000CountsNumber of iterations 0 2 4 6 8 10 12 14 16 2  2.2  2.4  2.6  2.8  3  3.2  3.4  3.6  3.8  4CountsFinal σy [nm]Figure 6: Histograms of vertical beam sizes (before and af-ter correction) and iterations taken by the optimization forthe CLIC FFS. Figure of merit: Beam size plus non-lineardispersion. knobs: H&V displacements plus strengths 0 2 4 6 8 10 12 14 16 14000  16000  18000CountsNumber of iterations 0 2 4 6 8 10 0.4  0.6  0.8  1CountsRelative final luminosity [L0]Figure 7: Histograms of number of iterations (top) and rela-tive final luminosities (bottom) for the tunning of the CLICFFS. knobs: H&V displacements plus strengths

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