Possible Scenarios for an LHC Upgrade F. Ruggiero, F. Zimmermann, CERN Rationale of LHC Upgrade LHC Performance Limitations Upgrade Scenarios & Options

time scale of an LHC upgrade courtesy J. Strait radiation damage limit ~700 fb-1 time to halve error integrated L L at end of year ultimate luminosity design luminosity (1) life expectancy of LHC IR quadrupole magnets is estimated to be <10 years due to high radiation doses (2) the statistical error halving time will exceed 5 years by 2011-2012 (3) therefore, it is reasonable to plan a machine luminosity upgrade based on new low-b IR magnets before ~2014

Chronology of LHC Upgrade Studies Summer 2001: two CERN task forces investigate physics potential (CERN-TH-2002-078) and accelerator aspects (LHC Project Report 626) of an LHC upgrade March 2002: LHC IR Upgrade collaboration meeting http://cern.ch/lhc-proj-IR-upgrade October 2002: ICFA Seminar at CERN on “Future Perspectives in High Energy Physics” March 2003: LHC Performance Workshop, Chamonix http://ab-div.web.cern.ch/ab-div/Conferences/Chamonix/2003/ 2004: CARE-HHH European Network on High Energy High Intensity Hadron Beams http://care-hhh.web.cern.ch/care-hhh/

LHC performance limitations beam dumping system compatible with ultimate intensity of 1.7x1011 /bunch, increases to 2.0x1011/bunch could be tolerated with reduced safety margin or after moderate upgrade detector architecture in their present configurations, the CMS and ATLAS detectors can accept a maximum luminosity of 3-5x1034 cm-2s-1 collimation & machine protection machine protection is challenging: beam transverse energy density is 1000 times that of the Tevatron; simple graphite collimators may limit maximum transverse energy density to ½ the nominal value in order to prevent collimator damage; closing collimators to 6s yields an impedance at the edge of instability; a local fast loss of 2.2x10-6 of the beam intensity quenches nearby arc magnets electron cloud additional heat load on beam screen; its value depends on beam & surface parameters; at 75-ns spacing no problem anticipated; initial bunch populations at 25-ns spacing will be limited to ½ nominal value beam-beam limits total current; upgrade may be necessary limits luminosity; detector upgrade in parallel with accelerator upgrade, which could allow moving low-b quads closer to the IP limits total current & b* may constrain minimum bunch spacing limits Nb/e, & crossing angle; compensation schemes may help

electron cloud Photo-electrons created at the vacuum pipe are accelerated by proton bunches up to 200 eV and cross the pipe in about 5ns; slow or reflected electrons survive until the next bunch; depending on vacuum-pipe surface conditions (SEY) and bunch spacing, this may lead to an electron cloud build up with implications for LHC beam stability, emittance growth and heat load on the cold LHC beam screen.

Simulated average arc heat load due to electron cloud and LHC cooling capacity as a function of bunch population for different values of the maximum secondary emission yield. Nominal or ultimate LHC intensity and 25 ns spacing are probably ok for well conditioned surfaces.

electron cloud blue: e-cloud effect observed red: planned accelerators experience at several storage rings suggests that the e-cloud threshold scales as Nb~Lsep; possible LHC upgrades consider either smaller Lsep with constant Nb, or they increase Lsep in proportion to Nb longer fewer more intense bunches more ‘ultimate’ bunches

predicted e-cloud heat load vs. bunch spacing on a vertical log scale change in dmax appears as ~constant vertical shift nominal LHC Simulated average arc heat load due to electron cloud for nominal LHC bunch intensity as a function of the bunch spacing, for two values of the maximum secondary emission yield dmax. Elastically reflected electrons are included.

saturation of e- build up for high bunch intensities ~average energy of secondary electrons e- line density 109 m-1 the electron cloud density saturates and stays almost constant when the bunch intensity is doubled from the beam-beam limit value for two IPs of 2.3x1011 to 4.6x1011 Nb= 4.6x1011 2.3x1011 time 10 ms

schematic of reduced electron cloud build up for a super- bunch; most e- do not gain any energy when traversing the chamber in the quasi-static beam potential negligible heat load [after V. Danilov]

beam-beam: long-range collisions LHC: 4 primary interaction points … … and npar~32 long-range collision points around each primary IP long-range collisions: perturb motion at large betatron amplitudes, where particles come close to opposing beam cause ‘diffusive’ (or dynamic) aperture (Irwin), high background, poor beam lifetime increasing problem for SPS, Tevatron, LHC, i.e., for operation with larger # of bunches dynamic aperture due to long-range collisions minimum crossing angle higher bunch charge, more bunches or smaller b* all require larger crossing angle to maintain the same dynamic aperture

beam-beam: tune shift tune shift from head-on collision (primary IPs) tune shift from long-range collisions increases with reduced bunch spacing or crossing angle limit on xHO limits Nb/(ge) d: normalized separation, xHO / IP no. of IPs DQbb total SPS 0.005 3 0.015 Tevatron (pbar) 0.01-0.02 2 0.02-0.04 RHIC 0.002 4 ~0.008 LHC (nominal) 0.0034 2 (4) ~0.01 conservative value for total tune spread based on SPS collider experience

Schematic of a super-bunch collision, consisting of ‘head-on’ and ‘long-range’ components. The luminosity for super-bunches having flat longitudinal distribution is ~1.4 times higher than for conventional Gaussian bunches with the same beam-beam tune shift and identical bunch population (see LHC Project Report 627)

fundamental luminosity equations below beam-beam limit, luminosity is reduced for long bunches and large qc (1) HV crossing in 2 IPs no linear tune shift due to long-range collisions, total linear tune shift also reduced by a factor Fbb~F: (2) 1/F combine (1) + (2): at the beam-beam limit, luminosity can be increased by increasing bunch length or qc a) higher injection energy would allow larger (ge) and hence more intensity & luminosity b) another possibility to achieve higher luminosity is to operate with large crossing angle (either ‘Piwinski regime’ or ‘superbunches’) K. Takayama et al., PRL88, 2002 F. Ruggiero, F. Zimmermann, PRST-AB 5, 2002

ultimate Relative increase in LHC luminosity versus bunch length (or crossing angle) for Gaussian and flat (super-)bunches at constant beam-beam tune shift with alternating crossings in IP1 and IP5

luminosity upgrade: baseline scheme 1.0 reduce sz by factor ~2 using higher frf & lower e|| (larger qc ?) 0.58 A qc>qmindue to LR-bb increase Nb increase F BBLR compen-sation yes bb limit? crab cavities reduce qc (squeeze b*) 2.3 no 0.86 A reduce b* by factor ~2 new IR magnets use large qc & pass each beam through separate magnetic channel 4.6 0.86 A if e-cloud, dump & impedance ok increase nb by factor ~2 simplified IR design with large qc luminosity gain 9.2 beam current 1.72 A

luminosity upgrade: Piwinski scheme decrease F reduce b* by factor ~2 new IR magnets 1.0 increase szqc 0.58 A superbunches? flatten profile? increase Nb reduce #bunches to limit total current? yes no 7.7 15.5 luminosity gain ? 0.86 A 1.72 A beam current

additional considerations total current limited? (e.g. by e-cloud, machine protection, dump) fewer bunches with more charge give higher luminosity, but also increase the event pile up minimum b*: depends on IR magnets, Q’ correction (more critical for larger Dp/prms) & collimator settings integrated luminosity ~Tbb/(Tbb+Tturnaround): reduce Tturnaround by increasing Einj (SuperSPS), which reduces injection time and snapback BBLR compensation + SuperSPS larger intensity at larger en: L L*2 more luminosity with flat (long) bunches capability of experiments, e.g., bunch structure

upgrades to LHC injector complex possibility being considered also for CNGS beams is to upgrade the proton linac from 50 to 120-160 MeV, to overcome space charge limitations at injection into the PS booster; then ultimate LHC intensity would be easy to achieve and a further 30% increase would be possible with same emittance & filling time SPS equipped with s.c. magnets (‘SuperSPS’) & upgraded transfer lines allow LHC injection at 1 TeV instead 0.45 TeV; this option can increase peak LHC luminosity by nearly a factor of 2 at constant beam-beam parameter Nb/e, in conjunction with LR beam-beam compensation schemes; reduces turnaround time & increases integrated luminosity; first step in view of LHC energy upgrade (energy swing reduced by factor of 2) s.c.linac could replace booster, or FFAG based injector?

new IRs goal: reduce b* by factor 2-5 T. Sen et al., PAC2001 T. Taylor, EPAC02 J. Strait et al., PAC2003 F. Ruggiero et al., EPAC04 goal: reduce b* by factor 2-5 options: NbTi ‘cheap’ upgrade, NbTi(Ta), Nb3Sn new quadrupoles new separation dipoles maximize magnet aperture, minimize distance to IR factors driving IR design: minimize b* minimize effect of LR collisions large radiation power directed towards the IRs accommodate crab cavities or beam-beam compensators compatibility with upgrade path

IR ‘baseline’ schemes short bunches & minimum crossing angle & BBLR crab cavity triplet magnets triplet magnets BBLR short bunches & minimum crossing angle & BBLR crab cavities & large crossing angle

alternative IR schemes dipole magnets dipole triplet magnets triplet magnets dipole first & small crossing angle dipole first & large crossing angle & long bunches or crab cavities reduced # LR collisions collision debris hits D1 N. Mokhov et al., PAC2003

‘cheap’ IR upgrade short bunches & minimum crossing angle & BBLR in case we need to double LHC luminosity earlier than foreseen triplet magnets BBLR short bunches & minimum crossing angle & BBLR each quadrupole individually optimized (length & aperture) IP-quad distance reduced from 23 to 22 m NbTi, b*=0.25 m possible

bunch structure more (&shorter) bunches nominal & ultimate LHC plus: can use crab cavities event pile up tolerable more (&shorter) bunches upgrade path 1 concerns: e-cloud LRBB impedance nominal & ultimate LHC ~12.5 ns 25 ns upgrade path 2 longer (&fewer) bunches ? 75 ns plus: no e-cloud? less current concerns: event pile up impedance super-bunch concerns: huge event pile up plus: no e-cloud less current transitions by bunch merging or splitting; new rf systems required in all cases

example parameter sets baseline ‘Piwinski’ super-bunch

LHC upgrade scenarios LHC phase 0: maximum performance w/o hardware changes LHC phase 1: maximum performance with arcs unchanged LHC phase 2: maximum performance with ‘major’ changes Nominal LHC performance at 7 TeV corresponds to DQbb=0.01 with L=1034 cm-2s-1 in IP1 and IP5 (ATLAS and CMS), halo collisions in IP2 (ALICE) and low-luminosity in IP8 (LHC-b) phase 0 baseline collide beams only in IP1&5 with alternating H-V crossing increase Nb up to beam-beam limit L=2.3x1034 cm-2s-1 increase dipole field to 9T (ultimate field) Emax=7.54 TeV phase 0 Piwinski 4. increase longit. emittance & bunch length, e.g., sz=15.2 cm 5. increase crossing angle by ~10% 6. increase Nb up to beam-beam limit L=3.6x1034 cm-2s-1

Comparison of tune footprints, corresponding to betatron amplitudes extending from 0 to 6 s, for nominal LHC (red-dotted), ultimate (green-dashed), and large Piwinski parameter configuration (blue-solid) with alternating H-V crossing only in IP1&5. (Courtesy H. Grote)

modify insertion quadrupoles and/or layout b*=0.25 m Possible steps to increase luminosity with hardware changes only in the LHC insertions and/or injector complex include: phase 1 baseline modify insertion quadrupoles and/or layout b*=0.25 m increase crossing angle by ~1.4 increase Nb up to ultimate intensity L=3.3x1034 cm-2s-1 halve sz with high harmonic system L=4.6x1034 cm-2s-1 double number of bunches (and increase qc!) L=9.2x1034 cm-2s-1 (excluded by e-cloud?) phase 1 Piwinski modify insertion quadrupoles and/or layout b*=0.25 m increase crossing angle by ~60% optionally merge every 3 bunches 75-ns spacing 3. increase Nb up to beam-beam limit L=7.2x1034 cm-2s-1 phase 1 superbunch 2. collide 1-A superbunches with large qc L=9x1034 cm-2s-1

phase 2: luminosity & energy upgrade modify injectors to significantly increase beam intensity and brilliance beyond ultimate value (possibly together with beam-beam compensation schemes) equip SPS with s.c. magnets, upgrade transfer lines, and inject at 1 TeV into LHC install new dipoles with 15-T field and a safety margin of 2 T, which are considered a reasonable target for 2015 and could be operated by 2020 beam energy around 12.5 TeV

Sketch of the common coil design for a double aperture dipole magnet; the coils couple the two apertures and can be flat (no difficult ends). One of the most difficult challenges will be to make the magnets at a reasonable cost, less than 5kEuro/(double)T.m say, including cryogenics, to be compared with 4.5 kEuro/(double)T.m for the present LHC.

Nb3Sn block-coil dipole reached 16 T field

summary & recommendations for future studies and R&D (1) nominal LHC performance is challenging: learn how to overcome e-cloud effects, inject, ramp, and collide 3000 high-intensity bunches, protect s.c. magnets, safely dump the beams, etc. Upgrades in beam intensity are a viable option, require R&D for cryogenics, vacuum, RF, beam dump, and injectors, and operation with large crossing angles (2) radiation limit for IR quads (~700 fb-1) reached by 2013? new triplet quadrupoles with high gradient and larger aperture (or alternative IR layouts) are needed for luminosity upgrade; opening the quads has advantage of letting radiation through (3) further studies needed to specify field quality of IR magnets, required upgrades of instrumentation, collimation, and machine protection; to reduce collimator impedance during b-squeeze and physics conditions, triplet aperture should be large (4) experimental studies on e-cloud, long-range, and strong-strong beam-beam effects are important, as well as MDs in existing hadron colliders with large Piwinski parameter and many (flat) bunches; international collaboration (US-LARP/CARE) is welcome/needed for LHC machine studies/commissioning (5) beam-beam compensation schemes with pulsed wires would reduce tune footprints and loss of dynamic aperture due to long-range collisions; experimental validation is underway

(6) interesting possibilities currently under study to pass each beam through separate final quadrupoles include: alternative separation schemes with separation dipoles in front of the triplet quadrupoles, collision of long bunches with large crossing angle, normal bunches at large crossing angle with crab cavities; luminosities ~1035 cm-2s-1 (7) super-bunch and ‘large Piwinski angle’ options are interesting for large crossing angles, can potentially avoid electron-cloud effects, and minimize the cryogenic heat load; one could inject a bunched beam, accelerate it to 7 TeV, and then use a multiple harmonic rf system to form 30-900 longer bunches; the larger the number of bunches, the smaller is the event pile up in the experiment (8) crab cavities are attractive, likely raise beam-beam limit, and allow for separate magnet channels; first experience will be gained at KEKB from 2005; viability for hadron beams (emittance growth due to rf phase noise) should be explored (9) major & sustained R&D effort on new s.c. materials and magnet design needed for any LHC performance upgrade; foster & extend collaboration with other labs: new low-b quadrupoles with high gradient and larger aperture based on Nb3Sn superc- conductor require 9-10 years for short-model R&D and component development, proto- typing and final production (10) increased 1-TeV injection energy into the LHC in conjunction with beam-beam compensation schemes would yield an integrated luminosity gain >2; a pulsed SuperSPS (and new s.c. transfer lines) or cheap low-field booster rings in the LHC tunnel could be the first step for an LHC energy upgrade

thank you for your attention!