LHC performance in 2011

1. LHC performance in 2011 LHC performance in 2011 - LAL/Orsay J. Wenninger CERN Beams Department Operation group / LHC

2. Outline Introduction Proton operation 2011 High intensity issues Outlook 2011 LHC performance in 2011 - LAL/Orsay

3. The Large Hadron Collider LHC Installed in 26.7 km LEP tunnel Depth of 70-140 m Lake of Geneva LHC ring LHC performance in 2011 - LAL/Orsay CMS, Totem LHCb Control Room SPS ring ATLAS, LHCf ALICE

4. - β* = 0.55 m (beam size =17 μm) - Crossing angle = 285 μrad - L = 1034 cm-2 s-1 LHC layout and parameters • 8 arcs (sectors), ~3 km each • 8 long straight sections (700 m each) • beams cross in 4 points • 2-in-1 magnet design with separate vacuum chambers → p-p collisions RF LHC performance in 2011 - LAL/Orsay

5. LHC accelerator complex Beam 2 Beam 1 TI8 TI2 LHC proton path ≥ 7 seconds from source to LHC LHC performance in 2011 - LAL/Orsay The LHC needs most of the CERN accelerators...

6. LHC dipole magnet • 1232 dipolemagnets. • B field 8.3 T (11.8 kA) @ 1.9 K (super-fluidHelium) • 2 magnets-in-one design : two beam tubes with an opening of 56 mm. • Operating challenges: • Dynamicfield changes at injection. • Verylowquenchlevels (~ mJ/cm3) LHC performance in 2011 - LAL/Orsay

7. Vacuum chamber • The beams circulate in two ultra-high vacuum chambers, P ~10-10 mbar. • A Copper beam screen protects the bore of the magnet from heat deposition due to image currents, synchrotron light etc from the beam. • The beam screen is cooled to T = 4-20 K. Beam screen LHC performance in 2011 - LAL/Orsay Magnet bore Cooling channel (Helium)

8. Tunnel View LHC performance in 2011 - LAL/Orsay

9. LHC target energy: the way down When Why • All main magnets commissioned for 7TeV operation before installation • Detraining found when hardware commissioning sectors in 2008 • 5 TeV poses no problem • Difficult to exceed 6 TeV • Machine wide investigations following S34 incident showed problem with joints • Commissioning of new • Quench Protection System • (nQPS) 7 TeV 2002-2007 Design 12 kA 5 TeV Summer 2008 Detraining 9 kA LHC performance in 2011 - LAL/Orsay Late 2008 3.5 TeV Joints Spring 2009 6 kA 1.18 TeV Nov. 2009 nQPS 2 kA 450 GeV

10. LHC target energy: the way up When What • Train magnets • 6.5 TeV is in reach • 7 TeV will take time • Repair joints • Complete pressure relief system • Commission nQPS system 7 TeV 2014/5 Training 6 TeV 2013 Stabilizers LHC performance in 2011 - LAL/Orsay 2011 3.5 TeV nQPS 2010 1.18 TeV 2009 450 GeV

11. Luminosity : collider figure-of-merit The event rate N for a physics process with cross-section s is proprotional to the collider Luminosity L: Design k = number of bunches = 2808 N = no. protons per bunch = 1.15×1011 f = revolution frequency = 11.25 kHz s*x,s*y = beam sizes at collision point (hor./vert.) = 16 mm LHC performance in 2011 - LAL/Orsay • To maximize L: • Many bunches (k) • Many protons per bunch (N) • Small beam sizes s*x,y= (b*e)1/2 • b*: beam envelope (optics) • e: beam emittance, the phase space volume occupied by the beam (constant along the ring) High beam “brillance” N/e (particles per phase space volume)  Injector chain performance ! Small envelope  Strong focusing ! Optics property Beam property

12. Stored energy • Increase with respect to existing accelerators : • A factor 2 in magnetic field • A factor 7 in beam energy • A factor 200 in stored beam energy LHC 2011 LHC performance in 2011 - LAL/Orsay Damage threshold

13. To set the scale… A few cm long groove in a SPS vacuum chamber after the impact of ~1% of a nominal LHC beam (2 MJ) during an ‘incident’ LHC performance in 2011 - LAL/Orsay

14. Collimation 1.2 m beam • To operate at nominal performance the LHC requires a large and complex collimation system • Previous colliders used collimators mostly for experimental background conditions - the LHC can only run with collimators. • Ensure ‘cohabitation’ of: • 100’s of MJ of stored beam energy, • super-conducting magnets with quench limits of few mJ/cm3 • Almost 100 collimators and absorbers. • Alignment tolerances <0.1 mm to ensure that over 99.99% of the protons are intercepted. • Primary and secondary collimators are made of Carbon to survive large beam loss. LHC performance in 2011 - LAL/Orsay

15. Aperture and collimation • During experiments data taking, the aperture limit of the LHC is in the strong focusing quadrupoles (triplets) next to the experiments. • Hierarchy of collimators is essential to avoid quenching super-conducting magnets and for damage protection. • So far we never quenched a magnet with beam ! •  excellent machine and collimation system stability !!! Exp. LHC performance in 2011 - LAL/Orsay Triplet 18 σ Collimation hierarchy Tertiary 15 σ Tertiary 15 σ Dump Protection 10.5σ Secondary 8.8σ Primary 6σ

16. Collimation • Collimator alignment is made with beam and then monitored from the loss distribution around ring. • Beam cleaning efficiencies ≥ 99.98% ~ as designed Beam loss monitor signal LHC performance in 2011 - LAL/Orsay TCT = tertiary coll. at the experiments.

17. Beam dumping system • The dump is the only LHC element capable of absorbing the nominal beam. • Beam swept over dump surface (power load). Dump block • Ultra-high reliability and fail-safe system. Dilution kickers LHC performance in 2011 - LAL/Orsay Extraction septum magnets Extraction kickers 30 cm

18. Outline Introduction Proton operation 2011 High intensity issues Outlook 2011 LHC performance in 2011 - LAL/Orsay

19. The 2010 run • The 2010 run was the ‘learning to handle high intensity’ year. • Progressive intensity ramp up. • Initial operation with isolated bunches, moving to 150 ns spacingin September 2010. • Up to 368 bunches. • Test with 75 and 50 ns beams: •  Electron clouds. • Peak luminosity 2010: LHC performance in 2011 - LAL/Orsay 2×1032 cm-2s-1

20. From 2010 to 2011 • Outcome the Evian (Dec 2010) and Chamonix (Jan 2011) Workshops: • No change of the beam energy: 3.5 TeV. • Reduction of b* better understanding of the machine aperture. • Faster ramp, faster squeeze. • 75 ns or 50 ns bunch spacing. LHC performance in 2011 - LAL/Orsay

21. Bunch filling schemes • The LHC 400 MHz Radio-Frequency system provides 35’640 possible bunch positions every 2.5 ns (0.75 m) along the LHC circumference. • A priori any of those positions could be filled with a bunch… • The smallest bunch-to-bunch distance is fixed to 25 ns: max. number of bunches is 3564 (- some space for the dump kicker beam free region). • Because of the injector flexibility, the LHC can operate with isolated bunches or with trains of closely spaced bunches. • Startup 2010 : up to 50 isolated bunches (separation ≥ 1 ms). • Fall 2010 : 150 ns spacing - up to 368 bunches. • 2011 : start-up with 75 ns, now 50 ns. LHC performance in 2011 - LAL/Orsay = bunch position = filled position 2.5 ns … 25 ns 21

22. 1092 bunches with 50 ns spacing 2 ns Beam 1 Beam abort gap LHC performance in 2011 - LAL/Orsay LHC circumference

23. Ghost bunches • Ghost (parasitic) bunches are usually present between the main bunches (and essentially unavoidable), spaced by: • 2.5 ns : LHC RF system • 5 ns : SPS RF system • 25 ns : PS RF system • Amplitude ~ fraction of %. LHC performance in 2011 - LAL/Orsay ghost bunches 36 bunch train

24. Experimental long straight sections LHC performance in 2011 - LAL/Orsay • The 2 LHC beams are brought together to collide in a ‘common’ region. • Over ~260 m, the beams circulate in the same vacuum chamber where they can potentially be ‘parasitic’ encounters (when the spacing is small enough).

25. b* limits • The focusing at the IP is defined by b* which relates to the beam size s • b* is limited by the aperture of the triplet quadrupoles around the collision point and by the retraction margins between collimators. s2 = b* e • Smaller size s at the IP implies: •  Larger divergence (phase space conservation !) •  Faster beam size growth in the space from IP to first quadrupole ! LHC performance in 2011 - LAL/Orsay 33 mm e = 2.8 mm b* = 11 m 1.5 m Squeeze 90 mm

26. Separation and crossing: example of ATLAS Horizontal plane: the beams are combined and then separated ATLAS IP 194 mm ~ 260 m LHC performance in 2011 - LAL/Orsay Common vacuum chamber Vertical plane: the beams are deflected to produce a crossing angle at the IP to avoid undesired encounters in the region of the common vac. chamber. ~ 7 mm a Not to scale ! 2011 !

27. LHC Progress in 2010-11 at 3.5 TeV Low bunch intensity operation, first operational exp. with MPS Ramping up to 1 MJ, stability run at 1-2 MJ Reach out for the fb-1 ! LHC performance in 2011 - LAL/Orsay 2011 2010 Peak luminosity evolution

28. Stored energy evolution • The stored energy in each beam has been pushed to 74 MJ. • Despite this large stored energy no magnet was ever quenched with circulating beam – well protected by the beam loss monitors ! • Quenches only occurred due to injection failures (with low intensity). 74 MJ LHC performance in 2011 - LAL/Orsay

29. Luminosity 2011 Peak luminosity 1.26×1033 cm-2s-1 - 1092 bunches Integrated proton luminosity 2011 approaching 1 fb-1 LHCb luminosity limited to ~3×1032 cm-2s-1 by leveling (beams collide with transverse offsets) LHC performance in 2011 - LAL/Orsay 50 ns 75 ns

30. Operational cycle • Cycle: • Injection • Ramp • Squeeze • Collide beams • Stable physics beams • Ramp down/cycle • Ramp and squeeze lengths were reduced in 2011 to < ½ hour. • Presently a good turn around is performed in 3 hours – best 2h 15. Beam charge(example for ions) Magnet current [A] LHC performance in 2011 - LAL/Orsay During the ‘squeeze’ phase, the betatron function at the collision points (b*) is reduced to increase the luminosity. Courtesy S. Redaelli

31. Efficiency Intensity 1014 LHC performance in 2011 - LAL/Orsay 20-May Now Luminosity 1.2×1033 cm-2s-1 CMS/ATLAS LHCb

32. Fill length • Luminosity lifetime is typically 25 hours (beam lifetimes > 100 hours). • Optimum fill length ~12-15 hours. • but beams are frequently dumped before due to HW issues. • Ideally we could produce up to ~ 50-60 pb-1/day ~ 350 pb-1/week LHC performance in 2011 - LAL/Orsay

33. Efficiency • On average the LHC is colliding stable beams for physics ~8-10 hours per day (~30-35%). • Aim for improving the efficiency while increasing the luminosity further. • A significant fraction of the inefficiency is related to high intensity ! • Weekly production now around ~200 pb-1. LHC performance in 2011 - LAL/Orsay

34. Outline Introduction Proton operation 2011 High intensity issues Outlook 2011 LHC performance in 2011 - LAL/Orsay

35. High intensity issues • With 50 ns operation and with stored intensity of ~1014 protons (~1000 bunches) a number of issues related to high intensity have started to surface: • Vacuum pressure increases from electron clouds, • Radiation induced failures of critical tunnel electronics, • Heating of the beam screen temperature, • Heating of injection kickers, collimators, • Losses due to (supposed) dust particles, • RF beam loading, • Beam instabilities leading to emittance blow-up. • Those effects have slowed down the pace of the intensity increase. LHC performance in 2011 - LAL/Orsay

36. Vacuum effects • Vacuum pressure increases were first observed around the 4 experiments at the moment the LHC switched to 150 ns train operation in 2010 – issue becomes more critical as the intensity increases and the bunch separation is reduced. • Effects can be suppressed by solenoids (CMS, ALICE stray fields…). • When the first 50 ns beams were injected into the LHC in 2010, the vacuum pressure increases were too large for operation. • Pressures easily exceeded 10-6 mbar (normal is 10-9 or less) leading to closure of the vacuum valves. • Signs of cleaning by beam, with strong dependence on bunch intensity and bunch spacing. • Consistent with the signature of electron clouds. LHC performance in 2011 - LAL/Orsay

37. Electron clouds • … affect high intensity beams with positive charge and closely spaced bunches. • Electrons are generated at the vacuum chamber surface by beam impact, photons… • If the probability to emit secondary e- is high (enough), more e- are produced and accelerated by the field of a following bunch(es). Multiplication starts… • Electron energies are in the 10- few 100 eV range. • The cloud of e- can drive pressure rise, beam instabilities and possibly overload the cryogenic system by the heat deposited on the chamber walls ! •  The cloud can ‘cure itself’: the impact of the electrons cleans the surface (Carbon migration), reduces the electron emission probability and eventually the cloud disappears – ‘beam scrubbing’ LHC performance in 2011 - LAL/Orsay Bunch N+2 accelerates the e-, more multiplication… Bunch N+1 accelerates the e-, multiplication at impact Bunch N liberates an e- e- e- N+2 N+1 N ++++++ ++++++ ++++++ e-

38. 75 and 50 ns Bunch Spacing • There is a strong dependence of electron clouds on bunch spacing. • For this reason operation in 2011 started with 75 ns beams. LHC performance in 2011 - LAL/Orsay Factor 2 between the slope for 50 ns than 75 ns >> importance of bunch spacing Courtesy J.M. Jimenez

39. ‘Scrubbing’ to overcome e-clouds • Store as much beam as you can at injection (run at the limit of the vacuum / beam stability). • Must keep a high intensity and strong cloud activity since more cloud means more cloud cleaning… • Operate for some time, then re-inject fresh beam/higher intensity, always staying at the vacuum / stability limit. • Iterate until conditions are acceptable / good. • This takes some days... • In April 2011 a ~10 days ‘scrubbing’ run at 450 GeV allowed us to prepare the vacuum conditions for operation with 50 ns beams. • Up to 1080 bunches. • Bunch intensities up to 1.5×1011 p. LHC performance in 2011 - LAL/Orsay

40. Vacuum cleaning with beam • Pressure decrease (normalized to intensity) as a function of effective beam time. • Gain one order of magnitude / 15 hours. LHC performance in 2011 - LAL/Orsay Courtesy J.M. Jimenez

41. Blow up from e-clouds Example of bunch by bunch transverse sizes with 804 bunches / beam With strong electron cloud activity… LHC performance in 2011 - LAL/Orsay … and after some time of vac. chamber scrubbing ! LHC 8:30 meeting

42. Radiation induced problems • With the increasing luminosity tunnel electronics starts to suffer from SEE (Single Event Errors). • In particular the quench protection ad cryogenics systems  frequent need for access to reset/repair. Collisions points Collimators LHC performance in 2011 - LAL/Orsay Loss rate S

43. Mitigation of radiation effects • The mitigation of radiation effects is a long term program. • Relocation of equipment away from the tunnel, sometimes to the surface. • Improvement of embedded (FPGA) codes to cope with errors in non-critical parts and avoid reset involving tunnel access (quench protection). • Some ‘light’ work is done in the technical stops. • Radiation effect are expected to affect the LHC performance in the range of 1033 cm-2s-1 – the limit is clearly ‘soft’ : trade-off between peak luminosity and efficiency. LHC performance in 2011 - LAL/Orsay

44. Beam screen temperatures Beam screen as seen by the beam Cold Bore (2K) • The beam screen (BS) shields the magnet cold bore from the beams. • Gases are trapped on the cold bore (colder than BS). • In the presence of beam, the beam screen maybe be heated by • vacuum pressure increase / electron clouds, • RF heating from EM fields. LHC performance in 2011 - LAL/Orsay • In some cases (triplets) out-gassing from the BS has been observed – very careful T control during technical (or cryo) stops. • Part of the effect could be correlated to (too) short bunches at 3.5. • Increased bunch length blow-up.

45. IT beam-screen temperatures • Triplet BS temperature tricky to stabilise at injection and in the ramp. • Fine tuning/manual intervention by cryo operators. • Effect no fully understood. Injection /ramp 3.5 TeV stable beams dump LBOC, BS_Cryo facts BS T (K) 25 K 17 K Courtesy S. Claudet

46. Surprise, surprise ! • Very fast beam loss events (~ millisecond) in super-conducting regions of the LHC have been THE surprise of 2010 – nicknamed UFOs (Unidentified Falling Object). • 18 beams dumps by UFO events in 2010, 10 dumps in 2011. • Beam loss thresholds were increased by factor 5 between 2010-11 (from experience – no quenches). • Most likely small (10’s mm) objects (dust…) ‘entering’ the beam. LHC performance in 2011 - LAL/Orsay • Some events correlated in time and space to roman pot movements. • Possibly re-expelled after charging up by ionization.

47. Example of a UFO (152 bunches/2010) Beam loss monitor post-mortem LHCb IR7 IR1 Arc Arc LHC performance in 2011 - LAL/Orsay s Time evolution of loss 1 bin = 40 ms 0.5 ms Dump trigger (losses exceed threshold)

48. UFO amplitudes • In 2011 ~5000 UFO events have been observed. • 10 events led to a beam dump. • Most of the events are weak and do not pose any problem. • Operation mostly suffers from the tail of events with losses exceeding the loss monitor thresholds (set for quench prevention). • An issue for 7 TeV / beam operation  lower quench thresholds. nominal arc threshold 7 TeV LHC performance in 2011 - LAL/Orsay Events 4905 candidate UFOs at 3.5 TeV. Loss ampl./ loss monitor threshold Courtesy T. Baer

49. UFO rate in 2011 1510 candidate UFOs during stable beams. Signal RS05 > 2∙10-4Gy/s. LHC performance in 2011 - LAL/Orsay 228 b 36 bpi 480 b 72 bpi 768 b 72 bpi 768 b 72 bpi 912 b 108 bpi 1092 b 108 bpi 336 b 72 bpi 480 b 36 bpi 480 b 72 bpi 624 b 72 bpi 336 b 72 bpi 768 b 108 bpi 480 b 36 bpi UFO Rate ~ independent of no. bunches ~ 10 / hour

50. UFO distribution in ring 450 GeV591 candidate UFOs. Signal RS05 > 5∙10-4Gy/s. 3.5 TeV1096 candidate UFOs. Signal RS05 > 5∙10-4Gy/s. LHC performance in 2011 - LAL/Orsay Mainly UFOs around injection kickers (MKI) The UFOs are distributed around the machine. About 7% of all UFOs are around the injection kickers. .