OPERATION OF THE LARGE HADRON COLLIDER WITH HEAVY IONS

Transcription

1 OPERATION OF THE LARGE HADRON COLLIDER WITH HEAVY IONS R. Bruce CERN - AB/ABP, Geneva, Switzerland also at MAX-lab, Lund University, Sweden

2 Outline Introduction: CERN and the LHC, physics motivation, injector chain Pb 82+ operation in the LHC Luminosity Key parameters Optics Expected time evolution of luminosity, intensity, emittance Performance limits with ions Collimation Nuclear electromagnetic interactions and their effects (BFPP and EMD) Present status Summary R. Bruce 1

3 CERN CERN is the world s largest particle physics centre, founded in 1954 Located on the border between France and Switzerland Switzerland France LHC 2600 employees, 7900 visiting scientists, 500 universities, 80 countries R. Bruce 2

4 LHC (Large Hadron Collider) Largest accelerator/collider (together with LEP) and highest energy ever Design started in the early 1980 s, approved in 1995 Built in old LEP tunnel Will collide 7 TeV protons 0.57 PeV Pb 82+ nuclei (fully stripped ions) 4 interaction points (IPs) with experiments ATLAS, CMS, LHCb, ALICE 8 straight sect., 8 arcs R. Bruce 3

5 Some LHC figures 27 km circumference 100 m underground 362 MJ stored energy per proton beam 1 TB of data/second (ions) 9300 magnets 1.9 K working temperature of superconductors atm vacuum pressure Total cost: 6 billion CHF, out of which 4.6 B CHF for the machine R. Bruce 4

6 Physics motivation PROTONS Find Higgs particle (ATLAS, CMS) Explain origin of mass Search for supersymmetric particles (ATLAS, CMS) Unification of forces Dark matter Balance between matter and antimatter in the universe Symmetry breaking in b-meson decay (LHCb) HEAVY IONS Find and study quark-gluon plasma (ALICE) State of matter believed to have existed in the early universe. No colour confinement R. Bruce 5

7 Ion injector chain Ion source, 2.5 A KeV, 27+ Provide highest possible intensity of Pb 29+ RFQ + Linac 3, 4.2 A MeV, 54+ Adapt to LEIR injection energy strip to Pb 54+ LEIR, 72.2 A MeV, 54+ Accumulate and cool Linac3 beam, Electron cooler Prepare bunch structure for PS Necessary since PSB falls short by a factor 30 in intensity/emittance ratio PS, 5.9 A GeV, 82+ Strip to Pb 82+ Oldest machine at CERN SPS, 177 A GeV, 82+ Define filling scheme of LHC T12 n-tof * TT10 North Area * AD pbar p TT2 LINAC2 COMPASS SPS LHC ISOLDE PSB LINAC3 Pb ions E0 E1 E2 LINAC 3 PS T18 LEIR * CNGS neutrino East Area R. Bruce 6

8 Outline Introduction: CERN and the LHC, physics motivation, injector chain Pb 82+ operation in the LHC Luminosity Key parameters Optics Expected time evolution of luminosity, intensity, emittance Performance limits with ions Collimation Nuclear electromagnetic interactions and their effects (BFPP and EMD) Present status Summary R. Bruce 7

9 Luminosity Luminosity is defined as the number of reactions per cross section between colliding beams Can be calculated from accelerator parameters, general case: Parameters in luminosity Number of particles per bunch Ν b Number of bunches per beam k b Relativistic factor γ Normalised emittance ε n Beta function at the IP β * Hour glass factor R Case with equal beams and β functions: R. Bruce 8

10 LHC nominal key parameters The LHC will run ~1 month/year with heavy ions, starting with Pb 82+ After 2-3 years proton-pb collisions, after 5 years lighter ions 362 MJ Energy 28 times beyond what is presently accessible new regime, not only in the experimental study of nuclear matter, but also in the beam physics of hadron colliders R. Bruce 9

11 LHC Early vs Nominal scheme Early beam to provide safety margins against dangerous beam losses (see later) and to facilitate comissioning Parameter Units Nominal Early Beam Energy per nucleon Initial Luminosity L 0 TeV/n cm - 2 s No. bunches Bunch spacing ns β * m 0.5 (same as p) 1.0 Number of Pb ions/bunch Transv.. norm. RMS emittance μm Longitudinal emittance ev s/charge Luminosity half-life life (1,2,3 expts.) H 8, 4.5, 3 14, 7.5, R. Bruce 10

12 LHC Optics - arcs Modular approach with arcs and insertions Same arc optics for ions and protons FODO lattice in the arcs phase advance approximately π/2 in both planes R. Bruce 11

13 Main magnets 1.9 Κ 8.33Τ 11850Α 7MJ R. Bruce 12

14 LHC Optics - experiments Final focussing system around collision points to minimize the beam size Crossing angle to avoid parasitic encounters (serious problem for protons) Separation bump to turn on/off central collision Ion optics similar to proton optics in IP1 and IP5 Different ion optics in IP2 with smaller β* Optical functions around the ALICE experiment R. Bruce 13

15 Collider performance Time evolution of emittances and bunch population described by coupled non-linear diff. equations Radiation damping Intra-beam scattering multiple scattering Inelastic scattering Burnoff from luminosity Can be solved numerically Courtesy of J.M. Jowett R. Bruce 14

16 Synchrotron Radiation LHC first heavy ion storage ring where synchrotron radiation has a significant impact on beam dynamics Radiation damping with respect to protons in same ring, same magnetic field Radiation damping for Pb is twice as fast as for protons Many very soft photons Critical energy in visible spectrum This is fast enough to overcome IBS at full intensity t p ÅÅÅÅÅÅÅÅÅÅÅ t ion 2 1 Radiation damping enhancement for all stable isotopes Courtesy of J.M. Jowett Lead is (almost) best, deuteron is worst. Z R. Bruce 15

17 RB2 Luminosity evolution: Nominal scheme 5 μ μ 10 7 exêm 4 μ μ μ μ Transverse emittance An ideal fill, starting from design parameters giving nominal luminosity têh Lê c m -2 s -1 1 μ μ μ μ μ Burnoff dominated by unwanted processes (BFPP and EMD, see later) Nb 6μ μ μ μ μ μ 10 7 Particles per bunch No. of experiments: n = 0, 1, 2, 3 Luminosity têh exp têh BPM visibility threshold Increasing number of experiments reduces beam and luminosity lifetime. Courtesy of J.M. Jowett R. Bruce 16

18 RB3 Average luminosity with with 3h 3h turn-around time, in in ideal fills fills starting from nominal initial luminosity. Maximum of of curve gives optimum fill fill length. Example: average luminosity Average luminosity depends strongly on time taken to dump, recycle, refill, ramp and re-tune machine for collisions. Beams will probably be dumped to maximise average L before BPM visibility threshold is reached. H c m -1 s -1 L 8 μ μ μ μ Average Luminosity t run HhL No. of experiments: n = 0, 1, 2, 3 Courtesy of J.M. Jowett exp R. Bruce 17

19 Outline Introduction: CERN and the LHC, physics motivation, injector chain Pb 82+ operation in the LHC Luminosity Key parameters Optics Expected time evolution of luminosity, intensity, emittance Performance limits with ions Collimation Nuclear electromagnetic interactions and their effects (BFPP and EMD) Present status Summary R. Bruce 18

20 Performance limits Ultraperipheral and hadronic interactions of highly-charged beam nuclei will cause beam losses Collimation inefficiency, direct limit on beam current Bound-free pair production (BFPP) at the IP, direct limit on luminosity Direct luminosity burn-off of beam intensity by BFPP and electromagnetic dissociation (EMD) processes dominates luminosity decay LHC ion operation will start with early beam with 10 times fewer bunches to obtain safety margins but still useful luminosity R. Bruce 19

21 Collimation in the LHC Collimation system essential to protect machine from particles that would be lost causing magnet quenches or damage Stored energy in an LHC proton beam: 362 MJ This corresponds to: 77.4 kg TNT 400 ton TGV train cruising at 150 km/h R. Bruce 20

22 Collimation of ions N 2 βε x δx Necessary condition to hit secondary collimator: Secondary collimator (shower) Primary collimator (scatterer) x δx' > ( 2 2 N ) 2 N1 ε N, γ REL β TWISS (J.B. Jeanneret PRSTAB , 1998) N 1 βε Ions in the LHC: δx'> 7μrad RMS MCS angle of 2.76 A TeV Pb 82+ ions on graphite: ~4.7 μ rad/m 1/2 ~2 m of collimator needed to give necessary kick Nuclear interaction length of 2.76 A TeV Pb 82+ ions on graphite: ~2.5 cm (compare protons: 38 cm) Electromagnetic dissociation length: ~19 cm Ions are likely to undergo nuclear fragmentation before the necessary angle is obtained! R. Bruce 21

23 Collimation of ions (2) Production of isotopes (Pb 207, Pb 206, Tl 203 etc) with different Z/A ratio (different rigidity), not intercepted by secondary collimator, assuming the same collimation optics as for protons σ (barn) A Fragmentation cross sections for 2.76 A TeV Pb 82+ on a C target (simulated with FLUKA) Fragments follow the locally generated dispersion. May be lost downstream, causing heat deposition in superconducting magnets. 75 Z R. Bruce 22

24 LHC Collimation Example Loss map after IR7 (betatron cleaning section). Collision optics, standard collimator settings. Courtesy G. Bellodi Special simulation combining optical tracking and particle-matter interaction in collimators Used to locate additional beam loss monitors for ion runs R. Bruce 23

25 Remarks on Ion Collimation Probably the major limit for LHC ion luminosity Nevertheless: Conventional (1996) quench limit (tolerable heat deposition in superconducting magnet coils) now appears pessimistic This is a soft limit: losses only get to this level if, for some reason, the single-beam (not including collisional) losses reach a level corresponding to a lifetime of 12 min. Simulations benchmarked with real beams LHC collimator in SPS (2007) - good agreement Earlier data from RHIC - consistent Phase II Collimation upgrade needed for p-p Looking at what can be included for ions New ideas: cold collimators, crystals, magnetic collimation, optics changes, high-z primary collimators, R. Bruce 24

26 Pair Production in Heavy Ion Collisions Racah formula (1937) for free pair production in heavy-ion collisions Z + Z Z + e + e + Z σ = γ + π b for Pb-Pb LHC Z1 Z2 α r b for Au-Au RHIC e 224 PP log ( 2 CM ) L 4 Cross section for Bound-Free Pair Production ( BFPP) (several authors) Z + Z + + e + Z ( Z e ) s, K 2 1/2 has very different dependence on ion charges (and energy) 5 2 σpp Z1 Z2 [ Alog γ CM + B] 7 Z Alog γ CM + B for Z1 = Z2 We We use use BFPP BFPP values values from from Meier Meier et et al, al, Phys. Phys b for Cu-Cu RHIC Rev. Rev. A, A, 63, 63, (2001), (2001), includes includes detailed detailed 114 b for Au-Au RHIC calculations calculations for for Pb-Pb Pb-Pbat at LHC LHC energy energy 281 b for Pb-Pb LHC Large contribution to luminosity decay! BFPP can limit luminosity in heavy-ion colliders, S. Klein, NIM A 459 (2001) R. Bruce 25

27 Luminosity Limit from BFPP Pb + Pb Pb + Pb + e γ One-electron ions follow dispersive orbits out from IP Lost in localized spot Induced heating might cause magnet quenches Longitudinal Pb Longitudinal Pb ion ion distribution on screen distribution on screen IP2 IP2 Secondary Pb Secondary Pb beam beam emerging from IP and emerging from IP and impinging on beam impinging on beam screen screen Beam Beam screen screen Main Pb Main Pb 82+ beam beam R. Bruce 26

28 Consequences for the LHC 281 khz loss rate at nominal L 25 W heating power in dispersion suppressor dipole magnet Monte-Carlo of shower in FLUKA Revised estimates of quench limit (thermodynamics of liquid He and heat transfer) suggest magnets are close to quench limit (ongoing work!) Quench possible within estimated uncertainties Quench limit, Monte Carlo, BFPP cross section, Additional BLMs installed around IPs to monitor these losses in LHC operation Alleviation possible through redistribution Unwrapped inner coil Unwrapped outer coil R. Bruce 27

29 Alleviation of BFPP losses Alleviation possible through redistribution of losses Off-momentum orbit oscillates with dispersion function Existing orbit correctors used to introduce bump(s), so that a fraction of the losses escapes further downstream Disadvantage: sensitive to orbit distortions, difficult to tune at first impact location Larger off-momentum β at second impact We could lose everything there since the spot size is larger R. Bruce 28

30 Test of LHC methodology at RHIC Parasitic measurement during RHIC Cu-Cu run Loss monitors setup as for LHC Just visible signal! Compared predictions and shower calculations as for LHC Reasonable agreement R. Bruce et al, Phys. Rev. Letters 99:144801, 2007 We still need to benchmark quench limit (in LHC!) View towards PHENIX R. Bruce 29

31 Installed BLMs in the LHC R. Bruce 30

32 Electromagnetic dissociation Similar process: Electromagnetic dissociation Major contribution to luminosity decay Loss of one or several neutrons Collimators The change in magnetic rigidity is smaller than for BFPP 1-neut. EMD particles intercepted by collimation system 2-neut. process has smaller cross section (factor 5), so no danger of quench IP2 BFPP Nominal beam 1-n EMD 2-n EMD R. Bruce 31

33 Other limits on performance Total bunch charge is near lower limits of visibility on beam instrumentation, particularly the beam position monitors Must always(!) inject close to nominal bunch current Intra-beam scattering (IBS, multiple Coulomb scattering within bunches) is significant but less so than at RHIC where it dominates luminosity decay Emittance blowup due to IBS is compensated by synchrotron radiation damping! Vacuum effects (losses, emittance growth, electron cloud ) should not be significant R. Bruce 32

34 Outline Introduction: CERN and the LHC, physics motivation, injector chain Pb 82+ operation in the LHC Luminosity Key parameters Optics Expected time evolution of luminosity, intensity, emittance Performance limits with ions Collimation Nuclear electromagnetic interactions and their effects (BFPP and EMD) Present status Summary R. Bruce 33

35 2007: Injector chain commissioned for protons and early ion beam Status of the LHC 2008: Experimental detectors finalized June 2008: beam pipe closed in ATLAS (photo) July 2008: vacuum August 2008: cooldown to 1.9 K R. Bruce 34

36 First beam September : First proton beam in the LHC. Without RF, debunching in ~ 25*10 turns, i.e. roughly 25 ms Successful RF capture on injection plateau within 3 days With RF capture Without RF capture R. Bruce 35

37 LHC incident September : Electrical connection fault followed by helium leak. Technical stop For more information about the incident, see the press release: Or see the more detailed report here: 2_.pdf Startup after repairs scheduled to spring 2009 Earliest possibility for ion runs in end of R. Bruce 36

38 Outline Introduction: CERN and the LHC, physics motivation, injector chain Pb 82+ operation in the LHC Luminosity Key parameters Optics Expected time evolution of luminosity, intensity, emittance Performance limits with ions Collimation Nuclear electromagnetic interactions and their effects (BFPP and EMD) Present status Summary R. Bruce 37

39 Summary With 0.57 PeV Pb 82+ ions, the LHC will open up a new regime, not only in the experimental study of nuclear matter, but also in the beam physics of hadron colliders. Main physics goal in ion programme is to find and study quarkgluon plasma Ion beam accelerated by a chain of injectors before injection into the LHC Modular construction with 8 straight insertions and 8 arcs Beam and luminosity lifetimes of a few hours expected The most serious performance limit for ions believed to be collimation inefficiency BFPP might also cause problems nominal heat load predicted close to quench level Proton beam successfully injected in the LHC on September , RF capture working on injection plateau Incident with faulty electrical connection followed by a helium leak on September 19. Technical stop until spring R. Bruce 38

40 Acknowledgements This talk sketched some aspects of the work of many people, over many years, in Ions for LHC and LHC Projects, in CERN and many collaborating institutes around the world. Particular thanks for slide material to: J.M. Jowett, S. Gilardoni, G. Bellodi, H. Braun, C. Carli, L. Evans, W. Fischer, D. Kuchler, D. Manglunki, S. Maury Thank you for your attention R. Bruce 39