The collimation efficiency for Pb ion beams in the LHC is predicted to be
lower than requirements. Nuclear fragmentation and electromagnetic dissociation
in the primary collimators create fragments with a wide range of Z/A ratios,
which are not intercepted by the secondary collimators but lost where the
dispersion has grown sufficiently large. In this article we present
measurements and simulations of loss patterns generated by a prototype LHC
collimator in the CERN SPS. Measurements were performed at two different
energies and angles of the collimator. We also compare with proton loss maps
and find a qualitative difference between Pb ions and protons, with the maximum
loss rate observed at different places in the ring. This behavior was predicted
by simulations and provides a valuable benchmark of our understanding of ion
beam losses caused by collimation.Comment: 12 pages, 20 figure

A. Bertarelli

C. Bracco

C. A. Bertulani

E. B. Holzer

E. B. Holzer

G. Bellodi

H. H. Braun

J. M. Jowett

R. Bruce

R. W. Assmann

R. W. Assmann

S. Gilardoni

S. Redaelli

T. Weiler

Physical Review Special Topics. Accelerators and Beams

English

arXiv

The collimation efficiency for Pb ion beams in the LHC is predicted to be lower than requirements. Nuclear fragmentation and electromagnetic dissociation in the primary collimators create fragments with a wide range of Z/A ratios, which are not intercepted by the secondary collimators but lost where the dispersion has grown sufficiently large. In this article we present measurements and simulations of loss patterns generated by a prototype LHC collimator in the CERN SPS. Measurements were performed at two different energies and angles of the collimator. We also compare with proton loss maps and find a qualitative difference between Pb ions and protons, with the maximum loss rate observed at different places in the ring. This behavior was predicted by simulations and provides a valuable benchmark of our understanding of ion beam losses caused by collimation

Bruce, R

Assmann, Ralph Wolfgang

Bellodi, G

Bracco, C

Braun, H H

Gilardoni, S

Holzer, E B

Jowett, John M

Redaelli, S

Weiler, T

Zamantzas, C

CERN Document Server

Measurements of heavy ion beam losses from collimation

Assmann, R W

LHC-PROJECT-REPORT-110928Aug2008R. Bruce*, R. Assmann, G. Bellodi, C. Bracco,  H. H. Braun, S. Gilardoni, E.B. Holzer,J.M. Jowett, S. Redaelli, T. Weiler, C. ZamantzasEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsMEASUREMENTS OF HEAVY ION BEAM LOSSES FROMCOLLIMATIONThe collimation efficiency for 208Pb82+ ion beams in the LHC is predicted to be lower than requirements.Nuclear fragmentation and electromagnetic dissociation in the primary collimators create fragments with awide range of Z/A ratios, which are not intercepted by the secondary collimators but lost where thedispersion has grown sufficiently large. In this article we present measurements of loss patterns caused by aprototype LHC collimator in the CERN SPS. The loss maps show a qualitative difference between 208Pb82+ions and protons, with the maximum loss rate observed at different places in the ring. This behaviour waspredicted by simulations and provides a valuable benchmark of our understanding of ion beam losses causedby collimation.Large Hadron Collider ProjectAbstractLHC Project Report 1109CERN,CH-1211 Geneva 23Switzerland* also at MAX-lab, Lund University, SwedenCERN, Geneva, SwitzerlandPresented atEPAC'08, 11th European Particle Accelerator Conference, Genoa, Italy - June 23-27, 2008MEASUREMENTS OF HEAVY ION BEAM LOSSESFROM COLLIMATIONR. Bruce∗ , R. Assmann, G. Bellodi, C. Bracco, H.H. Braun, S. Gilardoni, E.B. Holzer,J.M. Jowett, S. Redaelli, T. Weiler, C. Zamantzas, CERN, Geneva, SwitzerlandAbstractThe collimation efficiency for 208Pb82+ ion beams inthe LHC is predicted to be lower than requirements. Nu-clear fragmentation and electromagnetic dissociation in theprimary collimators create fragments with a wide range ofZ/A ratios, which are not intercepted by the secondarycollimators but lost where the dispersion has grown suf-ficiently large. In this article we present measurements ofloss patterns caused by a prototype LHC collimator in theCERN SPS. The loss maps show a qualitative differencebetween 208Pb82+ ions and protons, with the maximumloss rate observed at different places in the ring. This be-haviour was predicted by simulations and provides a valu-able benchmark of our understanding of ion beam lossescaused by collimation.INTRODUCTIONThe collimation in the LHC [1] of 208Pb82+ ions is ex-pected to be less efficient than for protons [2], becauseions have a high probability of fragmenting in the pri-mary collimators. This produces isotopes (e.g. 207Pb,203Tl and others) with a different charge to mass ratio fromthe main beam, and therefore a different magnetic rigidity(Bρ)(1+ δ), if (Bρ) is the rigidity of the original beam andδ is given byδ =Z0A0AZ(1 + δkin)− 1 (1)where (Z0, A0) are the charge and mass number of the orig-inal ion, (Z,A) those of the fragment and δkin the fractionalmomentum deviation per nucleon. These ions follow thedispersion function generated downstream from the colli-mator and may be lost later in the machine, outside thecollimation insertion, possibly quenching superconductingmagnets.To study the LHC ion collimation inefficiency, a seriesof simulation studies have been done [2, 3]. Since a largefraction of the systematic error in those simulations comesfrom the generation and tracking of the fragmented ions,an experiment on ion collimation in the SPS has been per-formed. The results, presented in the following, have beencompared to simulations, not only in terms of loss locationsbut with the goal of reproducing the absolute value of thelosses measured by the SPS beam loss monitors (BLMs).We also make a brief comparison with protons.∗ also at MAX-lab, Lund University, Sweden. roderik.bruce@cern.ch5200 5250 5300 5350 5400s m20406080100COLL. BL520 BL521 BL522 BL523dx dmΒy mΒx mFigure 1: The β-functions of the SPS just downstream ofthe collimator together with the locally generated disper-sion dx from the collimator. The locations of the BLMsand the collimator are indicated by vertical lines.EXPERIMENTAL SETUPA prototype of a secondary LHC collimator has been in-stalled in the SPS [4]. It consists of a pair of 1 m long CFCgraphite jaws, which can be moved independently to inter-cept the beam in the horizontal plane. The optical functionsin the vicinity of the installation are shown in Fig. 1 and thehorizontal aperture in Fig. 2.Moving the collimator into the beam creates losses,which are recorded by 216 BLMs placed around the ring.The BLMs are ionization chambers mounted close to lat-tice quadrupoles. Losses are read out and integrated overevery machine cycle (18 s in the case of ions). Figs. 1 and 2show the s-values of the four BLMs (called BL520, BL521,BL522 and BL523) immediately downstream of the col-limator. Beam loss data were collected during 208Pb82+dedicated ion runs in late 2007 with a 106.4 GeV/nucleoncoasting beam. The transverse normalized RMS emittancewas approximately 1 μm and the injected intensity around6×107 ions. The collimator, kept parallel to the beam, wasmoved in steps, generally 0.1−1 mm, but sometimes up to10 mm. Data were also taken with tilted jaws and at lowerenergy, which we intend to present elsewhere.SIMULATION SETUPIn order to simulate the particle propagation through theLHC lattice together with the particle-matter interactionsin the collimators, a specialized program, ICOSIM [2], hasbeen developed. ICOSIM tracks particles through a lat-tice, defined in external files, using a linear matrix formal-ism but chromatic effects at leading order and sextupolesin thin kick approximation are included. The collimatorinteractions are, for simulations in this article, treated byexternal calls to the Monte Carlo program FLUKA [5, 6].5250 5275 5300 5325 5350 5375 5400s m86422468x cmBL520 BL521 BL522 BL523Δ0.50Δ0.40Δ0.30Δ0.20Δ0.15Δ0.10Δ0.05Δ0.04Δ0.03Δ0.02Δ0.01Figure 2: Dispersive orbits of fragmented ions producedin of one of the collimator jaws, shown together with theaperture. The vertical dashed lines indicate the location ofthe four BLMs closest downstream.Figure 3: The geometry as implemented in FLUKA aroundthe monitor BL520, which is located around 30 m down-stream of the collimator in the SPS.The simulations done for the LHC were performed using aless detailed method based on tabulated cross sections. Weintend to publish comparisons with this method elsewhere.The BLM signals depend not only on the number of ionslost nearby, but also on the mass of the ions, the distributionof impact parameters and the amount and type of materialthey have to traverse before reaching the monitor. In orderto accurately simulate this for a quantitative comparisonwith data, the particle-matter interaction of the lost ionsneeds to be taken into account.As discussed later, the main loss location is right down-stream of the collimator. Thus the 3D geometry of the mag-netic elements around BL520, BL521 and BL523 was im-plemented in FLUKA, as illustrated for BL520 in Fig. 3.BL522 was not included, since the losses predicted and ob-served there were negligible. The momenta and impact co-ordinates of all particles lost within a 15 m interval of eachBLM were recorded in ICOSIM and fed as starting condi-tions into FLUKA and the resulting energy deposition inthe N2 gas inside the BLMs was converted to dose in Gy.RESULTSA typical example of a loss map in the SPS ring, mea-sured with coasting 208Pb82+ beam, is shown in Fig. 4,together with the corresponding simulated loss map. Thedetector background, consisting of noise and other beamlosses that are not caused by the collimator movement, hadto be subtracted. As background data we used the loss mapfrom the machine cycle before the collimator movement. Asimilar approach was already used in Ref. [4]. The only sta-1000 2000 3000 4000 5000 6000 7000s m020 00040 00060 00080 000no. lost part.SIMULATIONS1000 2000 3000 4000 5000 6000 7000sm1.02.03.0losses mGyMEASUREMENTS5150 5200 5250 5300 5350 5400 5450sm1.02.03.0losses mGyBL521BL520 BL523Figure 4: Example of simulated ICOSIM (top) and mea-sured (middle) 208Pb82+ ion loss map for the whole SPSring. The bottom part shows a closeup of the loss peakin the measurements, with the names of the BLMs withthe highest signals indicated. The simulated losses werebinned in 5 m intervals. The collimator is located ats = 5222 m, just upstream of the large loss peak.ble loss locations, clearly separable from the background,are located right downstream of the collimator, both in sim-ulation and measurement.Fig. 5 shows the simulated and measured BLM signalsaround the maximum, normalized to 1010 lost particles andaveraged over several machine cycles where the collimatorjaws were kept parallel when moved into the beam. Thepattern from the measurements is very similar to the sim-ulation and is due to the dispersive orbits of ion fragmentswith different values of δ starting at the collimator jaws, asshown in Fig. 2. Fragments satisfying −0.2 < δ < −0.08are lost near the aperture limitation at s = 5277 m, closeto BL521. Fig. 6, showing the spectrum of δ of all ionsexiting the collimator, demonstrates that this correspondsto a large fraction of the fragments, so that this monitor isexpected to show a high signal.At BL523, only fragments with a magnetic rigidity muchcloser to the original 208Pb82+ ion are lost (δ ≈ −0.02).This is close to what can be expected in the cold regionsof the LHC. In the vicinity of BL520, however, the lossesare not only dispersive: light fragments with large verticalbetatron angles are also lost there. Fig. 7 shows the massspectrum of the particles lost close to each BLM.The magnitudes of the simulated signals agree well withmeasurements, although they are lower. The discrepanciesare however well within estimated error margins associ-520 521 523BLM0.00.20.40.60.8signal Gy1010 particlessimulatedmeasured520 521 523BLM0.000000.000050.000100.000150.000200.000250.00030signal Gy1010 particlessimulatedmeasuredFigure 5: Average measured ion (top) and proton (bottom)loss map from the BLMs downstream of the collimatorcompared with simulations normalized to 1010 lost parti-cles. The standard deviation between different measure-ments is indicated.ated with the uncertainty in the shower simulation (approx-imately a factor 2) and the uncertainties in the tracking, theimpact distribution of halo particles on the collimator andthe fragmentation cross sections in the collimator, whichare hard to quantify.The simulated ratio between the losses at BL521 andBL523 shows an excellent agreement with measurements,while the expected relative loss at BL520 is lower. Thiscould be due to the fact that BL520 is located only 30 mdownstream of the collimator, meaning that it might seetraces of shower particles from the collimator that are notproperly taken into account in the simulation.COMPARISON WITH PROTONSFor the sake of comparison, proton runs in the SPSwere also simulated with ICOSIM and FLUKA and com-pared with measurements from 2007 with coasting beamat 270 GeV (emittances x ≈ 2.6 μm, y ≈ 4 μm, inten-sity ≈ 1012, typical jaw steps ≈ 0.2 mm). Measured andsimulated loss maps are shown in Fig. 5. The ratio be-tween the higher peaks agree well with measurements andit is clear that there is a significant qualitative differencebetween proton and ion loss patterns: the maximum signalfor protons was found at BL520, while in the ion runs itwas found on BL521. This can be understood from the factthat the δ of the protons is much lower, which means thatlarge betatron angles caused by multiple scattering are themain loss mechanism instead of dispersion. This differenceis a striking parallel to the expected behaviour in the LHC,and the ability of the simulations to quantitatively predict itin the SPS provides an extremely valuable benchmark.0.6 0.4 0.2 0.0 0.2 0.4Δ100 000200 000no. nucleonsFigure 6: The distribution of δ of all ion fragments comingout of the collimator. The heights of the bars show thenumber of nucleons belonging to ions having δ within acertain interval.0 50 100 150 200A103105no. nucleonsBL5200 50 100 150 200A103105no. nucleonsBL5210 50 100 150 200A102104no. nucleonsBL523Figure 7: The A distribution of ion fragments lost within15 m upstream of each BLM. The heights of the bars showthe number of nucleons from ions having a certain A.CONCLUSIONSMeasurements and simulations with ICOSIM andFLUKA show that beam losses induced by the collima-tor in the SPS are qualitatively different for 208Pb82+ ionsand protons. Ion losses are mainly due to large values of δand the protons’ due to large angles, resulting in differentloss patterns. Quantitatively, the simulated loss maps cor-respond well to the measured ones, both in the magnitudeof the signal and the ratio of losses between different loca-tions. This confirms and strengthens our knowledge of ionbeam losses related to collimation, which is vital for ourunderstanding of what to expect in the LHC.Acknowledgements: The authors would like to ex-press their gratitude to: G. Arduini, B. Dehning, A. Fer-rari, M. Magistris, D. Manglunki, E. Metral, G.I. Smirnov,M. Stockner, V. Vlachoudis and the SPS operators.REFERENCES[1] R. Assmann et al., MOPLT005, Proc. of EPAC04 (2004)[2] H. Braun et al., MOPLT010, Proc. of EPAC04 (2004)[3] R. Bruce et al., MOPLS007, Proc. of EPAC06, (2006)[4] S. Redaelli et al, TUPLS130, Proc. of EPAC06 (2006)[5] A. Fasso et al., CERN-2005-10, 2005[6] A. Fasso et al, arXiv:hep-ph/0306267 (2003)

Crossref

The collimation efficiency for Pb-208(82+) ion beams in the LHC is predicted to be lower than requirements Nuclear fragmentation and electromagnetic dissociation in the primary collimators create fragments with a wide range of Z/A ratios, which are not intercepted by the secondary collimators but lost where the dispersion has grown sufficiently large. In this article we present measurements and simulations of loss patterns generated by a prototype LHC collimator in the CERN SPS. Measurements were performed at two different energies and angles of the collimator. We also compare with proton loss maps and find a qualitative difference between Pb-208(82+) ions and protons, with the maximum loss rate observed at different places in the ring. This behavior was predicted by simulations and provides a valuable benchmark of our understanding of ion beam losses caused by collimation

Bruce, Roderik

Assmann, R. W.

Bellodi, G.

Bracco, C.

Braun, H. H.

Gilardoni, S.

Holzer, E. B.

Jowett, J. M.

Redaelli, S.

Weiler, T.

Lund University Publications

The collimation efficiency for ^{208}Pb^{82+} ion beams in the LHC is predicted to be lower than requirements. Nuclear fragmentation and electromagnetic dissociation in the primary collimators create fragments with a wide range of Z/A ratios, which are not intercepted by the secondary collimators but lost where the dispersion has grown sufficiently large. In this article we present measurements and simulations of loss patterns generated by a prototype LHC collimator in the CERN SPS. Measurements were performed at two different energies and angles of the collimator. We also compare with proton loss maps and find a qualitative difference between ^{208}Pb^{82+} ions and protons, with the maximum loss rate observed at different places in the ring. This behavior was predicted by simulations and provides a valuable benchmark of our understanding of ion beam losses caused by collimation

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Measurements of heavy ion beam losses from collimation

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