The beam delivery system (BDS) for the 3 TeV version of the Compact Linear Collider (CLIC) has two main design types. One type is referred to as the local scheme, as it is approximately one kilometer shorter and corrects the chromaticity in both planes. The other type is referred to as the traditional scheme, and separates the chromaticity correction of each plane into different areas. The expectation early in the studies is that the traditional scheme would be easier to tune. However, it appears that this is not the case. Previous proceedings have shown the complications in tuning the traditional scheme. This work will address the problems experienced in tuning simulations for the traditional BDS and describe the current state of these simulation

Bodenstein, RM

Burrows, PN

Marin, E

Bodenstein, Ryan M

Burrows, Philip N

Oxford University Research Archive

CHALLENGES AND STATUS OF TUNING SIMULATIONS FOR CLICTRADITIONAL BEAM DELIVERY SYSTEMR.M. Bodenstein∗, P.N. Burrows, John Adams Institute, Oxford, UKE. Marin, CERN, Geneva, SwitzerlandAbstractThe beam delivery system (BDS) for the 3 TeV version ofthe Compact Linear Collider (CLIC) has two main designtypes. One type is referred to as the local scheme, as it isapproximately one kilometer shorter and corrects the chro-maticity in both planes. The other type is referred to as thetraditional scheme, and separates the chromaticity correc-tion of each plane into different areas. The expectation earlyin the studies is that the traditional scheme would be easierto tune. However, it appears that this is not the case. Pre-vious proceedings have shown the complications in tuningthe traditional scheme. This work will address the problemsexperienced in tuning simulations for the traditional BDSand describe the current state of these simulation.INTRODUCTIONThe design of the Compact Linear Collider (CLIC) [1]has evolved to reflect progress in accelerator research and de-velopment as well as recent developments in particle physics.The final focus section (FFS) of CLIC’s beam delivery sys-tem (BDS) has gone through several design iterations. Allof these iterations fall under two basic layouts: the localchromaticity correction scheme [2], which simultaneouslycorrects the chromaticity of both planes, and the traditionalchromaticity correction scheme, which separates the cor-rection of each plane into two different regions of the FFS[3]. The basic layouts of the two designs can be seen inFig. 1. One of the most important differences between thesedesigns is that the local scheme is over 1 km shorter than thetraditional scheme for the 3 TeV machine, measuring 450 mand 1460 m, respectively. This influences the constructioncosts, as a longer machine would cost more to build. Despitethe large difference in overall length, both FFSs are capableof achieving approximately the same luminosity in idealconditions.The focus of this work is a continuation of the work pre-sented in [4], which involved the single-beam tuning of theCLIC 3 TeV traditional FFS. There will be a very brief re-view of the tuning process, as well as previous tuning effortsfor this lattice. The discussion will then focus on the most re-cent efforts, and will conclude with what the current resultsmean, especially when compared to other FFSs.TUNING SIMULATION RECIPEDue to the very small beam size at the interaction point(IP) and the one-shot nature of the collisions in linear col-liders, the requirements placed on controlling the beam are∗ ryan.bodenstein@physics.ox.ac.ukFigure 1: General layouts of two FFS systems [3].very stringent. The BDS, and specifically the FFS, providesthe last opportunity to control the beam prior to collision.Therefore, the BDS must be tuned carefully to maximizethe luminosity achieved by the machine. For these tuningsimulations, the programs PLACET [5] and GUINEA-PIG[6] are used [7].The first step in the tuning simulation process [8] is tomisalign the components of the beamline. For the studiespresented in this work, only transverse static offsets of upto 10 μm were applied randomly to each component. One-to-one (121) tuning is then performed, which uses dipolesto steer the beam back toward its nominal trajectory. Uponcompletion of 121 tuning, a first stage of dispersion freesteering (DFS) is performed to bring the dispersion backtoward the design values. The next step is to adjust linearknobs which help to minimize first-order aberrations andmaximize the luminosity. Then, after applying the linearknobs, some of the random seeds will need to go through asecond stage of dispersion-free steering, while others willhave to go through another set of hybrid DFS knobs whichminimize dispersion growth while maximizing luminosity[9]. Then, the machines will go through a series of iterationswhich apply linear knobs, non-linear (2nd and higher-order)knobs, and dispersion free steering. For single beam tuningwith only static transverse offsets, the goal of the tuning isto have 90% of random seeds reach 110% of the nominalluminosity.In general, this approach works for the early stages oftuning. However, after some time this procedure no longershows improvement. At this point, custom tuning knobsare required to make further improvements. The processof designing and applying these custom tuning knobs isdiscussed in [4] for the traditional FFS. To summarize briefly,higher-order aberrations in each plane are identified, andknobs are designed to address these aberrations. Once theseknobs are applied, and several iterations of various knobs areperformed, new aberrations are identified and more customknobs are created.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-MOPML010MOPML010412ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.01 Circular and Linear CollidersA03 Linear CollidersFigure 2: β vs. ω1 vs. horizontal beam size averaged over40 seeds. Minimum indicated by blue cross.This method was shown to be successful for the localFFS [10, 11], but only achieved minor improvements in thetraditional FFS [4].REVISITING THE FIRST STAGESLast year [4], we started an investigation into why thetraditional FFS did not appear to improve after several iter-ations, especially when compared to the local FFS. It wasdecided to start at the beginning and re-examine basic func-tions and formulas used during the early stages of the tuningprocess, which are described in detail in [8].Two weighting parameters used for the 121 and first stagedispersion free steering steps were suspected to be less thanoptimal. To check them, parameter scans were performed.These weighting functions (β for 121 tuning andω1 for DFS)were scanned to see which would minimize the overall beamsize (thus maximizing luminosity) after the first stage DFSbut before any tuning knobs were applied. The beam sizescans included scanning the values of β = 0 and ω1 = 0. Ifβ = 0, this means that 121 tuning and the first stage DFSwould not occur. If ω1 = 0, 121 tuning can still take place,but the first stage DFS tuning will not occur. During theanalysis, the smallest beam sizes were recorded with thecorresponding weighting parameters. When including all ofthe data acquired, the smallest beam size occurred when ω1= 0 and β = 1 for both planes. If the data where the weightingparameters equalled 0 is omitted from the analysis, then thesmallest beam sizes in each plane occurred for differentvalues; β = 2 and ω1 = 5 for the horizontal plane and β = 8and ω1 = 1 for the vertical plane. However, the beam sizesfor both planes were far smaller when ω1 = 0 and β = 1 forboth planes. the results of these scans can be found in [4].Before restarting the tuning procedure with the new pa-rameters, a series of new scans were performed where thetuning process was allowed to continue past the stage withlinear knobs. These results are shown in Figs. 2 and 3 forthe x and y planes, respectively. Similarly to the previousscans, the minimum beam size occurred when ω1 = 0. Thisindicates that the first stage of DFS may be detrimental tothe overall tuning procedure for the traditional FFS.Figure 3: β vs. ω1 vs. vertical beam size averaged over 40seeds. Minimum indicated by blue cross.Figure 4: Comparing the results for the first tuning iterationusing the old weighting parameters and the new.RESTARTING THE TUNINGUsing the results from the parameter scans, the tuningprocedure was restarted in the same manner which gave thebest results previously. The only change from the previoustuning efforts was the use of the new weighting parametersfound from the scans, which meant that the first stage DFStuning was effectively turned off.Figure 4 shows an apples-to-apples comparison of thefirst iteration of tuning using the old and new parameters.Despite the parameter scans finding the smallest beam sizeat the new parameters, the tuning performed far worse.To be sure that the new parameters were detrimental tothe tuning, a second iteration was performed in the samemanner as previous studies. Using the saved machine statusfrom the first iteration, the second iteration cycles throughlinear knobs, second-stage DFS (or knobs) and non-linearknobs. No custom knobs are used at this point. As can beseen in Fig. 5, this actually results in decreasing the overallluminosity for most machines. This behavior is similar tothat described previously at the Linear Collider Workshop inMorioka [12]. Prior to the development of the second stage,hybrid DFS tuning described in [9], each tuning iteration forthe traditional lattice unexpectedly decreased the luminosityachieved. This was previously remedied with the use of thehybrid DFS knobs, but it appears that this is no longer thecase.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-MOPML01001 Circular and Linear CollidersA03 Linear CollidersMOPML010413ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Figure 5: Comparison of iterations 1 and 2 using the newweighting parameters.DISCUSSION AND COMPARISONThe procedures tested in this study have failed to improvethe luminosity to an adequate level. Even in the simplestcase of single-beam tuning with static transverse offsets, thetraditional FFS has failed to reach the goal of 90% of ma-chines reaching 110% of the nominal luminosity. The bestresults have accomplished approximately 45% of machinesreaching 110% of the nominal luminosity and 100% of ma-chines reaching ≥ 75% [4, 12]. Attempts to address thishave thus far been detrimental to the progress made previ-ously. Furthermore, attempts to understand why this latticefails to respond to the methods that have been so success-ful elsewhere [10, 11] have been unfruitful. One possibleexplanation is the existence of Brinkmann sextupoles [13]in the traditional FFS which do not exist in the local FFS.Creating new tuning knobs which use these sextupoles maybe necessary, but this has yet to be studied.Meanwhile, the local FFS has had much more success[11]. For single-beam tuning with only static misalignments,90% of machines achieved 102% of the nominal luminosityfor the local FFS, which is reasonably close to the goal.Furthermore, [11] also shows that it is capable of meetingthe more difficult goals of two-beam tuning under morerealistic conditions. For two-beam tuning with only staticimperfections, the local FFS reaches 97% of the nominalluminosity with 90% of machines. When including dynamicimperfections in addition to the static, two-beam tuning forthe local FFS achieves 89% of the nominal luminosity with90% of machines.CONCLUSIONThese studies have been performed in an attempt to findthe most feasible and cost-friendly design for the CLIC beamdelivery system. Initially, the longer, traditional FFS for theCLIC BDS was thought to have been more feasible (albeitmore expensive to construct). However, these studies, alongwith others for the local FFS, have shown this to not be thecase.The local chromaticity correction FFS is not only morefeasible to tune using the standard methods [11], but it isalso less expensive to construct due to its shorter length.The only way in which the traditional chromaticity correc-tion scheme would be favorable to the local scheme is if it iscapable of reaching a significantly higher overall luminosityon average or could tune significantly more quickly. How-ever, this work shows that, under the same conditions andgeneral methods of tuning, the traditional FFS is incapableof reaching the same luminosity as the local FFS. It is possi-ble that new and different tuning procedures could improvethe traditional FFS’s luminosity, but the current methodsand procedures have failed in this. More study would berequired for tuning the traditional FFS to form any definiteconclusions.ACKNOWLEDGEMENTSThe authors would like to thank the FONT group at theUniversity of Oxford for providing meaningful feedback andsupport during these studies. Additionally, we would liketo thank Fabien Plassard for his time and assistance in thiswork.REFERENCES[1] P. Burrows et al., “Updated baseline for a staged compactlinear collider,” CERN, Geneva, Tech. Rep. CERN-2016-004, 2016.[2] P. Raimondi and A. Seryi, “A Novel final focus design forfuture linear colliders,” Phys. Rev. Lett., vol. 86, pp. 3779–3782, 2001. doi: 10.1103/PhysRevLett.86.3779.[3] H. G. Morales and R. T. García, “Final-focus systems formulti-tev linear colliders,” Phys. Rev. ST Accel. Beams,vol. 17, no. 101001, 2014.[4] R. Bodenstein, P. Burrows, E. Marin, F. Plassard, and R.Tomás, “Tuning simulations for the clic traditional beamdelivery system,” Proceedings of IPAC17, Copenhagen, Den-mark, MOPIK108, 2017.[5] A. Latina, Y. Levinsen, D. Schulte, and J. Snuverink, “Evo-lution of the tracking code placet,” Proceedings of IPAC13,Shanghai, China, MOPWO053, 2013.[6] D. Schulte, “Beam-beam simulations with guinea-pig,” Pro-ceedings of ICAP98, Monterey, California, USA, 1998.[7] “The placet and guinea-pig++ projects,” http://clicsw.web.cern.ch/clicsw/[8] A. Latina and P. Raimondi, “A novel alignment procedurefor the final focus of future linear colliders,” Proceedings ofLINAC2010, Tsukuba, Japan, MOP026, 2010.[9] J. Snuverink, R. Bodenstein, A. Latina, and R. Tomás, “Ad-vanced bba techniques for the final focuses of future linearcolliders,” Proceedings of IPAC16, Busan, South Korea, TH-PMR046, 2016.[10] E. Marin, A. Latina, R. Tomás, and D. Schulte, “Final focussystem tuning studies towards Compact Linear Collider fea-sibility,” Phys. Rev. Accel. Beams, vol. 21, no. 1, p. 011 003,2018. doi: 10.1103/PhysRevAccelBeams.21.011003.[11] E. Marin, A. Latina, D. Schulte, R. Tomás, and J. Pfingstner,“Tuning of clic-final focus system 3 tev baseline design un-der static and dynamic imperfections,” presented at IPAC18,Vancouver, Canada, MOPMF043, 2018.[12] R. Bodenstein, “Single-beam tuning of the clic 3 tev tradi-tional lattice,” Presented at LCWS 2016, Morioka, Japan,2016.[13] R. Brinkmann, “Optimization of a final focus system for largemomentum bandwidth,” DESY-M-90-14, 1990.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-MOPML010MOPML010414ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.01 Circular and Linear CollidersA03 Linear Colliders

Challenges and status of tuning simulations for CLIC traditional beam delivery system

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