Protons that collide at the interaction points of the FCC-hh may contribute to the background in the subsequent detector. Due to the high luminosity of the proton beams this may be of concern. Using DPMJET-III to model 50 TeV protonproton collisions, tracking studies have been performed with PTC and MERLIN in order to gauge the elastic and inelastic proton cross-talk. High arc losses, particularly in the dispersion suppressor regions, have been revealed. These losses originate mainly from particles with a momentum deviation, either from interaction with a primary collimator in the betatron cleaning insertion, or from the proton-proton collisions. This issue can be mitigated by introducing additional collimators in the dispersion suppressor region. The specific design, lattice integration, and the effect of these collimators on cross-talk is assessed

Abelleira Fernandez, Jose L.

Appleby, RB

Krainer, AM

Langner, AS

Rafique, H

English

Oxford University Research Archive

PROTON CROSS-TALK AND LOSSES IN THE DISPERSIONSUPPRESSOR REGIONS AT THE FCC-hh∗H. Rafique†, R. B. Appleby, Cockcroft Institute and The University of Manchester, Manchester, UKA. M. Krainer, A. S. Langner, CERN, Geneva, SwitzerlandJ. L. Abelleira, John Adams Institute, University of Oxford, Oxford, UKAbstractProtons that collide at the interaction points of the FCC-hhmay contribute to the background in the subsequent detector.Due to the high luminosity of the proton beams this maybe of concern. Using DPMJET-III to model 50 TeV proton-proton collisions, tracking studies have been performed withPTC and MERLIN in order to gauge the elastic and inelas-tic proton cross-talk. High arc losses, particularly in thedispersion suppressor regions, have been revealed. Theselosses originate mainly from particles with a momentumdeviation, either from interaction with a primary collimatorin the betatron cleaning insertion, or from the proton-protoncollisions. This issue can be mitigated by introducing addi-tional collimators in the dispersion suppressor region. Thespecific design, lattice integration, and the effect of thesecollimators on cross-talk is assessed.INTRODUCTIONThe Future Circular Collider (FCC-hh) aims to accelerateand collide two counter rotating 50 TeV proton beams attwo major and two minor experiments. The FCC-hh layoutis shown in Fig. 1. The nominal and ultimate luminositiesof the FCC-hh are 5 · 1034cm−2s−1 and 30 · 1034cm−2s−1respectvely. Given this high proton energy and luminosity,collision debris must be properly handled. Here we assessthe cross-talk, i.e. the effect on the downstream detectorfrom the collision debris. The debris comprises mostly ofphotons, pions, protons, other charged hadrons, and muons,in order of abundance. Photons are ignored for cross-talkstudies as they will not reach the next detector. The numberof pions produced is significant, however together with othercharged hadrons these do not have the required rigidity to betransported in the accelerator, and are therefore lost almostimmediately in aperture restrictions. Muons are of concernas they have a large mean free path and can travel kilometresin dense materials - this is discussed in [1]. Here we focus onprotons, of particular interest in cross-talk studies becausethey may be transported by the accelerator to the proceedingdetector.PROTON CROSS-TALKUsing the upgraded version of the DPMJET-III [2] eventgenerator within FLUKA [3], we simulate the proton col-lisions at interaction point A (IPA), with vertical crossing.These protons have a large energy range, as shown in Fig. 2.∗ Work supported by the HORIZON 2020 project EuroCirCol, grant agree-ment 654305.† haroon.rafique@manchester.ac.ukFigure 1: FCC layout. The main experimental interactionregions are located at A and G, and the low luminosity onesat L and B.We separate these protons into two regimes, elastic protonswhich have an energy greater than 49.95 TeV, and inelasticwhich have an energy less than this.Figure 2: Proton energy distribution 3 m downstream of IPAfor 106 pp collision events.Using PTC [4] and MERLIN [5] we perform tracking ofboth elastic and inelastic protons to assess the cross-talk.The elastic protons nearly all reach IPB, as their energy andposition spread is similar to that of the machine proton bunch.This will lead to some emittance growth. The inelastic pro-tons are more interesting. At nominal luminosity under 2inelastic protons will reach IPB per IPA bunch crossing, atultimate settings this rises to ≈ 9. The mean energy of theseProceedings of IPAC2017, Copenhagen, Denmark TUPIK03701 Circular and Linear CollidersA01 Hadron CollidersISBN 978-3-95450-182-31763 Copyright©2017CC-BY-3.0andbytherespectiveauthorsprotons is 49.89 TeV. It is unlikely that these few protonswill be of concern in terms of cross-talk.LOSSESInelastic protons are mostly lost in the short straight sec-tion, and dispersion suppressor (DS) regions post IPA. Theenergy correlation loss map for inelastic protons is shownin Fig. 3. These simulations treat all apertures as black ab-sorbers. The power loss map is shown in Fig. 4. We seethat the majority of low energy protons are lost in aperturerestrictions close to the IP as expected, only higher energyprotons travel as far as the dispersion suppressor, where theyare nearly all lost. Power loss on the D1 and D2 separationand recombination dipoles is large, FLUKA studies of en-ergy deposition and shielding is foreseen in the near future.As the losses in the DS (after s ≈ 800 m) region comprisesof higher energy protons, these are of concern.Figure 3: Inelastic proton energy correlation loss map perelement. The TAS is located at 40 m, D1 at 230 m, D2 at425 m, the dispersion suppressor region begins around 800m, and s = 0 is IPA.Figure 4: Inelastic proton power loss map per element.DISPERSION SUPPRESSORPROTECTIONThe cold dispersion suppressor regions in the FCC-hh area critical point in the optics since they are a bottleneck for off-momentum particles, as the dispersion is rapidly rising [6].To reduce the losses in the cold regions to a cleaning in-efficiency of ≈ 3 · 10−7 [7] a new dispersion suppressor col-limation system has been integrated into the current lattice.This collimation system consists of two ‘TCLD’ collimators,one in cell 8 and one in cell 10 of the dispersion suppressorat the point where the dispersion is rapidly rising. With theadditional collimators in place it can be shown that the coldlosses from off-momentum beam particles can be managed.Particle showers induced by these protons impacting uponthe collimator jaws are therefore the main concern [8].The DS regions in the current FCC-hh lattice version 7have been modified to include drift space for the placementof the TCLD collimators in front of the first quadrupolein cell 8 and cell 10. Important for the positioning is arelatively small β function and a rising dispersion. Thisgives the possibility of a bigger jaw gap so not to violate thecollimation hierarchy, while still being able to intercept theoff-momentum particles.Simulation of the dispersion suppressor region losses withthe additional collimators has been done in two steps. First,using the MERLIN code [5] to generate and track the pri-mary protons, in order to ensure that all relevant particles getcaught by the collimation system, and to create a distributionof particles that hit the collimator for shower simulations.Second, using the FLUKA simulation code [3] with a sim-plified geometry of the DS regions that includes a modelof the first downstream quadrupole, to simulate the showerdevelopment and energy deposition in the cold magnet coils.The main focus of these studies has been on the DS re-gion after the betatron cleaning insertion (IPJ) since this hasbeen identified as the region with the biggest cold lossesduring short beam lifetime scenarios. This is not the casefor nominal physics operation; here the largest losses comefrom collision debris around the experiments (IPA, IPG). Toensure that a DS collimation design similar to the one underinvestigation for IPJ can also be used around the experiments,simulations with collision debris particles have been carriedout.ENERGY DEPOSITION SIMLUATIONSMERLIN was used to track the collision debris inelasticprotons, including the TCLD collimators in the DS regionpost-IPA. This provides an input distribution for FLUKAshower and energy deposition studies. Treating aperture re-strictions as black absorbers MERLIN simulations confirmthat the TCLDs post-IPA intercept all remaining inelasticprotons. For the shower simulations only cell 8 has beenconsidered, as the load on the collimator in cell 10 is smallerand therefore also the energy deposition in the downstreammagnets should be less. A 1 m long collimator has beenplaced in the drift before the quadrupole as close to the begin-TUPIK037 Proceedings of IPAC2017, Copenhagen, DenmarkISBN 978-3-95450-182-31764Copyright©2017CC-BY-3.0andbytherespectiveauthors01 Circular and Linear CollidersA01 Hadron Collidersning of the drift as possible, considering space for cryostatconnections. This gives the shower particles space to moveaway from the beam axis and potentially hit the magnet yokeinstead of the coils. A 3D model of the geometry is shownin Fig 5.Figure 5: 3D FLUKA model used for shower simulationsin the dispersion suppressor. The green objects are first theTCLD collimator, followed by a 50 cm long mask, both madeof Inermet180. Following this is the first quadrupole in theDS. The particles are loaded 63 cm before the collimator.Since the collimator has to act as an absorber, Inermet180has been chosen as the jaw material, this is also used inthe absorbers currently installed in the LHC. The half-gapsfor the collimators were set to 35 σ. This has proven tobe tight enough to intercept the particles, but wide enoughnot to violate the collimation hierarchy. As the momentumcollimation system in the FCC-hh is a work in progress, thereis a possibility that the momentum cleaning hierarchy mightbe violated. This has to be investigated when the momentumcleaning insertion is finalized. For the quadrupole a simplemodel with coils consisting of a mixture of 50% niobium-3-tin and 50% copper is used. Additionally simulationsincluding a mask of 50 cm Inermet180 directly in front of themagnet have been performed. Figure 6 shows the maximumenergy deposition along the length of the quadrupole.Figure 6: Maximum energy deposition per cm3 in 5 cm binsalong the first quadrupole in cell 8 after IPA for baseline andultimate configuration.Without a mask the energy deposition rises rapidly inthe first few centimeters of the magnet. The mask is ableto stop almost all particles that hit the face of the magnetcoils, and the main losses come from shower particles thattravel through the beampipe and hit the magnet on the inside.Considering magnet limits of ≈ 5 - 10 mW cm−3 [9] protec-tion without the mask should be sufficient for the baselineconfiguration of the FCC-hh. For ultimate parameters how-ever the mask is required to provide sufficient protection,this includes consideration of errors of up to a factor of 2.The mask could also provide flexibility for future luminosityupgrades.CONCLUSIONProton cross-talk has been assessed in the FCC-hh usingDPMJET-III in FLUKA as an event generator, as well asPTC and MERLIN for tracking and loss location simulations.It has been found that elastic protons will all reach the pro-ceeding IP, causing some emittance growth. The inelasticprotons reach the next IP at a rate of 2 - 8 protons (at IPB)per IPA bunch crossing. This is deemed negligible. What isof concern is the loss of inelastic protons in the short straightsection and dispersion suppressor post-IP. The inner tripletquadrupoles, recombination, and separation dipoles are thesubject of detailed energy deposition and shielding studies toensure adequate performance. Losses in the dispersion sup-pressor thus become the main concern. For the dispersionsuppressors around the experiments a simple collimation de-sign consisting of two 1 m Inermet180 collimators is enoughto stop the cold magnets from quenching. When including asimulation error of a factor of 2, a 50 cm mask is enough tocompensate. Further studies must be performed to ensurethat the momentum collimation hierarchy is not violated.With the dispersion suppressor collimators in place post-IP,the inelastic proton cross-talk is negated.REFERENCES[1] J. L. Abelleira et. al., “Cross-Talk Studies Between FCC-hhExperimental Interaction Regions”, presented at IPAC’ 17,Copenhagen, Denmark, May 2017, paper TUPVA036, thisconference.[2] A. Fedynitch, “Cascade equations and hadronic interactionsat very high energies”, Ph.D thesis, CERN-THESIS-2015-371, KIT, Dept. Phys., 2015.[3] A. Ferrari, P.R. Sala, A. Fasso and J. Ranft, “FLUKA: amulti-particle transport code”, CERN-2005-10, 2005.[4] P. K. Skowronski et. al., “Advances in MAD-X using PTC”,LHC-PROJECT-Report-1016, 2007.[5] MERLIN on github: https://github.com/MERLIN-Collaboration/MERLIN[6] James Molson et. al., “Simulation of the FCC-hh collimationsystem”, in Proc. IPAC’16, Busan, Korea, May 2016, paperTUPMR054, pp.1381-1383.[7] Daniel Schulte, “FCC-hh machine layout and optics”, FCCWeek 2016, Rome, 2016.[8] M. Fiascaris, “Betatron collimation efficiency”, FCC Week2016, Rome, 2016.[9] Ezio Todesco, Private Communication.Proceedings of IPAC2017, Copenhagen, Denmark TUPIK03701 Circular and Linear CollidersA01 Hadron CollidersISBN 978-3-95450-182-31765 Copyright©2017CC-BY-3.0andbytherespectiveauthors

Proton cross-talk and losses in the dispersion suppressor regions at the FCC-hh

Protons that collide at the interaction points of the FCC-hh may contribute to the background in the subsequent detector. Due to the high luminosity of the proton beams this may be of concern. Using DPMJET-III to model 50 TeV proton-proton collisions, tracking studies have been performed with PTC and MERLIN in order to gauge the elastic and inelastic proton cross-talk. High arc losses, particularly in the dispersion suppressor regions, have been revealed. These losses originate mainly from particles with a momentum deviation, either from interaction with a primary collimator in the betatron cleaning insertion, or from the proton-proton collisions. This issue can be mitigated by introducing additional collimators in the dispersion suppressor region. The specific design, lattice integration, and the effect of these collimators on cross-talk is assessed

Rafique, Haroon

Appleby, Robert Barrie

Krainer, Alexander

Langner, Andy Sven

Abelleira, Jose

CERN Document Server

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Proton cross-talk and losses in the dispersion suppressor regions at the FCC-hh

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