1. 1 Commissioning challenges of the LHC Jörg Wenninger CERN Accelerators and Beams Department Operations group February 2008 LHC overview Magnets Luminosity High ‘energy’ issues Hardware & beam commissioning

2. 2 Rüdiger Schmidt Bullay Oktober 20072 The CERN Beschleuniger Komplex CERN (Meyrin) LHC SPS Control room ATLAS LHCB CMS ALICE 7 years of construction to replace LEP: 1989-2000 in the same 26.7 km tunnel by LHC : 2008-2020+

3. 3 IR6: Beam dumping system IR4: RF + Beam instrumentation IR5:CMS IR1: ATLAS IR8: LHC-B IR2:ALICE Injection ring 2 Injection ring 1 IR3: Momentum collimation (normal conducting magnets) IR7: Betatron collimation (normal conducting magnets) Beam dump blocks LHC Layout  8 arcs.  8 long straight sections (insertions), ~ 700 m long.  beam 1 : clockwise  beam 2 : counter-clockwise  The beams exchange their positions (inside/outside) in 4 points to ensure that both rings have the same circumference ! The main dipole magnets define the geometry of the circle !

4. 4 Top energy/GeV Circumference/m Linac 0.12 30 PSB 1.4 157 CPS 26 628 = 4 PSB SPS 450 6’911 = 11 x PS LHC 7000 26’657 = 27/7 x SPS LEIR CPS SPS Booster LINACS LHC 3 4 5 6 7 8 1 2 Ions protons Beam 1 Beam 2 TI8 TI2 Note the energy gain/machine of 10 to 20 – and not more ! The gain is typical for the useful range of magnets !!! The CERN Accelerator Complex

5. 5 LHC – yet another collider? The LHC surpasses existing accelerators/colliders in 2 aspects :  The energy of the beam of 7 TeV that is achieved within the size constraints of the existing 26.7 km LEP tunnel. LHC dipole field8.3 T HERA/Tevatron ~ 4 T  The luminosity of the collider that will reach unprecedented values for a hadron machine: LHC pp ~ 10 34 cm -2 s -1 Tevatron pp2x10 32 cm -2 s -1 SppbarSpp6x10 30 cm -2 s -1 The combination of very high field magnets and very high beam intensities required to reach the luminosity targets makes operation of the LHC a great challenge ! A factor 2 in field A factor 4 in size A factor 100 in luminosity

6. LHC Magnets & Arc Layout 6

7. The Superconductor 7 Bc Tc  The DIPOLE field of 8.3 Tesla required to achieve 7 TeV/c can only be obtained with superconducting magnets !  The material determines: Tc critical temperature Bc critical field  LHC magnets use NbTi, with a typical usable current density @ 4.2 K 2000 A/mm 2 @ 6T  To reach 8-10 T, the temperature must be lowered to 1.9 K – superfluid Helium: Very low viscosity  coil penetration. Large specific heat (2000 x conductor / unit volume). Large heat conduction (3000 x Cu).

8. The superconducting cable 8  1 mm  6  m Typical value for operation at 8T and 1.9 K: 800 A NbTi cable width 15 mm A.Verweij

9. Dipole magnets 9 Dipole length 15 m The coils must be aligned very precisely to ensure a good field quality (i.e. ‘pure’ dipole) To collide two counter-rotating proton beams, the beams must be in separate vaccum chambers (in the bending sections) with opposite B field direction.  There are actually 2 LHCs and the magnets have a 2-magnets-in-one design!

10. Dipole field map - cross-section 10 Superconducting coil Beam Iron Non-magnetic collars B = 8.33 Tesla I = 11800 A

11. 11 Rüdiger Schmidt11 Beam tube Superconducting coil Non-magnetic collars Ferromagnetic iron Steel cylinder for Helium Insulation vacuum Supports Vacuum tank Weight (magnet + cryostat) ~ 30 tons, Length 15 m

12. LHC arc : not just dipoles… 12  Dipole- und Quadrupol magnets –Provide a stable trajectory for particles with nominal momentum.  Sextupole magnets –Correct the trajectories for off momentum particles (‚chromatic‘ errors).  Multipole-corrector magnets –Sextupole - and decapole corrector magnets at end of dipoles –Used to compensate field imperfections if the dipole magnets. To stabilize trajectories for particles at larger amplitudes – beam lifetime ! ~ 8000 superconducting magnets ae installed in the LHC

13. 13 J. Wenninger - ETHZ - December 2005 13 1232 main dipoles + 3700 multipole corrector magnets (sextupole, octupole, decapole) 392 main quadrupoles + 2500 corrector magnets (dipole, sextupole, octupole) Regular arc: Magnets

14. 14 J. Wenninger - ETHZ - December 2005 14 Regular arc: Cryogenics Supply and recovery of helium with 26 km long cryogenic distribution line Static bath of superfluid helium at 1.9 K in cooling loops of 110 m length Connection via service module and jumper

15. 15 J. Wenninger - ETHZ - December 2005 15 Insulation vacuum for the cryogenic distribution line Regular arc: Vacuum Insulation vacuum for the magnet cryostats Beam vacuum for Beam 1 + Beam 2

16. 16 J. Wenninger - ETHZ - December 2005 16 Regular arc: Electronics Along the arc about several thousand electronic crates (radiation tolerant) for: quench protection, power converters for orbit correctors and instrumentation (beam, vacuum + cryogenics)

17. Tunnel view 17

18. Complex interconnects 18 CERN visit McEwen Many complex connections of super-conducting cable that will be buried in a cryostat once the work is finished. This SC cable carries 12’000 A for the main dipoles

19. Vacuum chamber 19 Beam  envel. ~ 1.8 mm @ 7 TeV 50 mm 36 mm  The beams circulate in two ultra-high vacuum chambers made of Copper that are cooled to T = 4-20 K.  A beam screen protects the bore of the magnet from image currents, synchrotron light etc from the beam. Cooling channel (Helium) Beam screen Magnet bore

20. Luminosity 20

21. Luminosity challenges 21 The event rate N for a physics process with cross-section  is proprotional to the collider Luminosity L: To maximize L: Many bunches ( k ) Many protons per bunch ( N ) A small beam size  u = (    ) 1/2   : characterizes the beam envelope (optics), varies along the ring, mim. at the collision points.  : is the phase space volume occupied by the beam (constant along the ring). k = number of bunches = 2808 N = no. protons per bunch = 1.15×10 11 f = revolution frequency = 11.25 kHz  x,  y = beam sizes at collision point (hor./vert.) = 16  m High beam “brillance” N/  (particles per phase space volume)  Injector chain performance ! Small envelope  Strong focusing !

22. Beam enveloppe 22 Fits through the hole of a needle! ARC ATLAS collision point  The beam size follows the local beam optics : In the arcs the optics follows a regular pattern. In the long straight sections, the optics is matched to the ‘telescope’ that provides very strong focusing at the collision point.  Collision point size (rms, defined by ‘  *’): CMS & ATLAS : 16  mLHCb : 22 – 160  mALICE : 16  m (ions) / >160  m (p)

23. Combining beams for collisions 23  The 2 LHC beams circulate in separate vacuum chambers in most of the ring, but they must be brought together to collide.  Over a distance of about 260 m, the beams circulate in the same vacuum chamber and they are a total of ~ 120 encounters in ATLAS, CMS, ALICE and LHCb.

24. Crossing angle 24 IP  From the point of view of beam dynamics, colliding beams is one of the worst things one can do to a beam due to the very non-linear fields of the opposing beams (diffusion to large amplitudes, lifetime reduction…).  To avoid un-necessary direct beam encounters wherever the beams share a common vacuum: COLLIDE WITH A CROSSING ANGLE IN ONE PLANE !  The price to pay : A reduction of the luminosity due to the finite bunch length of 7.6 cm and the non-head on collisions  L reduction of ~ 17%. Crossing planes & angles ATLAS Vertical 280  rad CMS Horizontal 280  rad LHCb Horizontal 300  rad ALICE Vertical 400  rad 7.5 m

25. Bunch Pattern 25  The nominal LHC pattern consists of 39 groups of 72 bunches (spaced by 25 ns), with variable spacing between the groups to accommodate the rise times of the fast injection and extraction magnets (‘kickers’).  There is a long 3  s hole (  5 )for the LHC dump kicker (see later). 72 bunches

26. Stored Energy & Protection 26

27. The price of high fields & high luminosity… 27 When the LHC is operated at 7 TeV with its design luminosity & intensity,  the LHC magnets store a huge amount of energy in their magnetic fields: per dipole magnet E stored = 7 MJ all magnets E stored = 10.4 GJ  the 2808 LHC bunches store a large amount of kinetic energy: E bunch = N x E = 1.15 x 10 11 x 7 TeV = 129 kJ E beam = k x E bunch = 2808 x E bunch = 362 MJ To ensure safe operation will be a major challenge for the LHC operation crews ! Protection of the machine components from the beam is becoming a major issues for all new high power machines (LHC, SNS, …). > 1000 x above damage limit for accelerator components !

28. Stored Energy 28 Increase with respect to existing accelerators : A factor 2 in magnetic field A factor 7 in beam energy A factor 200 in stored energy

29. Comparison… 29 The energy of an A380 at 700 km/hour corresponds to the energy stored in the LHC magnet system : Sufficient to heat up and melt 12 tons of Copper!! The energy stored in one LHC beam corresponds approximately to… 90 kg of TNT

30. Quench 30  A quench corresponds to the phase transition from the super-conducting to a normal conducting state.  Quenches are initiated by an energy in the order of few mJ –Movement of the superconductor (friction and heat dissipation)  ‚training‘ quenches of the magnets until coils are stable. –Beam losses. –Cooling failures. –...  When part of a magnet quenches, the conductor becomes resistive, which can lead to excessive local energy deposition (temperature rise !!) due to the appearance of Ohmic losses. To protect the magnet: –The quench must be detected. –The energy is the magnet circuit must be extracted. –The magnet current has to be switched off within << 1 second.

31. Quench : discharge of energy 31 Magnet 1Magnet 2 Power Converter Magnet 154 Magnet i Protection of the magnet after a quench: The quench is detected by measuring the voltage increase over coil : R ~ 0 to R > 0 ! The energy is distributed in the magnet by force-quenching using quench heaters. The current in the quenched magnet decays in < 200 ms. The current of all other magnets flows through a ‚bypass‘ diode that can stand the current for 100-200 s. The current of all other magnets is dischared into the dump resistors. Discharge resistor

32. Dump Resistors 32 Those large air-cooled resistors can absorb the 1 GJ stored in a dipole magnet circuit (they heat up to few hundred degrees Celsius).

33. If it does not work… 33 P.Pugnat During magnet testing the 7 MJ stored in one magnet were released into one spot of the coil (inter-turn short)

34. Operational margin of SC magnet 34 Bc Tc 9 K Applied Field [T] Bc critical field 1.9 K quench with fast loss of ~10 10 p ~ 0.01% of total int. Quench with fast loss of ~10 6-7 p ~0.01-0.1 ppm of the total int. 8.3 T / 7 TeV 0.54 T / 450 GeV QUENCH Tc critical temperature Temperature [K] The LHC is ~1000 times more critical than TEVATRON, HERA, RHIC

35. ‘Cohabitation’ 35  Even to reach a luminosity of 10 33 cm -2 s -1, i.e. 10% of the design, requires unprecedented amounts of stored beam energy.  The stored energy will be circulating a few cms from extremely sensitive super-conducting magnets, which can quench following a fast loss of less than 1 part per million of the beam: >>> requires a very large collimation system (> 100 collimators).  LHC commissioning has to be much more rigorous than what was done for previous machines – no shortcuts and dirty ‘tricks’ can be used to go ahead as soon as the intensities exceed a few % of the design. >>> Predictions on the commissioning duration are particularly tricky!

36. Collimation 36 Primary collimator Secondary collimators Absorbers Protection devices Tertiary collimators Triplet magnets Experiment Beam Primary halo particle Secondary halo Tertiary halo + hadronic showers hadronic showers A multi-stage halo cleaning (collimation) system with over 100 collimators has been designed to protect the sensitive LHC magnets from beam induced quenches :  Halo particles are first scattered by the primary collimator (closest to the beam).  The scattered particles (forming the secondary halo) are absorbed by the secondary collimators, or scattered to form the tertiary halo.  Primary and secondary collimators are made of Carbon to survive severe beam impacts !  The collimators must be very precisely aligned (< 0.1 mm) to guarantee efficiency above 99.9% at nominal intensities.  the collimators will have a strong influence on detector backgrounds !!

37. Collimation @ 7 TeV 37 1 mm Opening ~3-5 mm The collimator opening corresponds roughly to the size of Spain !  For colliders like HERA, TEVATRON, RHIC, LEP collimators are/were used to reduce backgrounds in the experiments ! But the machines can/could actually operate without collimators !  At the LHC collimators are essential for machine operation as soon as we have more than a few % of the nominal beam intensity !

38. Beam Dumping System IR6 38 Q5R Q4R Q4L Q5L Beam 2 Beam 1 Beam dump block Kicker magnets to paint (dilute) the beam about 700 m about 500 m 15 fast ‘kicker’ magnets deflect the beam to the outside When it is time to get rid of the beams (also in case of emergency!), the beams are ‘kicked’ out of the ring by a system of kicker magnets and send into a dump block ! Septum magnets deflect the extracted beam vertically quadrupoles

39. Dump Block… 39 Approx. 8 m concrete shielding beam absorber (graphite)  The ONLY element in the LHC that can withstand the impact of the full beam !  The block is made of graphite (low Z material) to spread out the hadronic showers over a large volume.  It is actually necessary to paint the beam over the surface to keep the peak energy densities at a tolerable level !

40. Unscheduled beam loss & protection 40 Beam loss over multiple turns due to many types of failures. Passive protection - Failure prevention (high reliability systems). -Intercept beam with collimators and absorber blocks. Active protection systems have no time to react ! Active Protection - Failure detection (by beam and/or equipment monitoring) with fast reaction time (< 1 ms). - Fire beam dumping system Beam loss over a single turn during injection, beam dump or any other fast ‘kick’. In the event a failure or unacceptable beam lifetime, the beam must be dumped immediately and safely into the beam dump block Two main classes for failures (with more subtle sub-classes): The LHC machine protection (interlock) system is designed for high reliability with a design target of < 1 failure in 1000 years.

41. LHC Commissioning : the road to first beam 41

42. Injector status : beam at the gate to the LHC 42 TV screen at end transfer line Beam image taken less than 50 m away from the LHC tunnel in IR8 (LHCb) ! The LHC injectors are ready after a long battle to achieve nominal beam brightness. Beam was sent down both ~3 km long transfer lines to the LHC.

43. Filling exercise from SPS to LHC 43 Extraction of a high intensity beam from the SPS to the LHC transfer lines was tested towards both rings in the fall of 2007. Ready to go …

44. LHC Machine Commissioning 44 Commissioning of the LHC equipment (‘Hardware commissioning’) has started in 2005 and is now in full progress. This phase includes:  Testing of ~8000 magnets (most of them superconducting).  27 km of cryogenic distribution line (QRL).  4 vacuum systems, each 27 km long.  > 1600 magnet circuits with their power converters (60 A to 13000 kA).  Protection systems for magnets and power converters.  Checkout of beam monitoring devices.  Etc…

45. Hardware commissioning sequence 45 The present LHC commissioning phase is designated as ‘Hardware Commissioning’. The most ‘visible’ job is the commissioning of the magnets and their powering systems. When all magnets of one of the eight LHC sectors installed and interconnected:  Vacuum system is pumped to nominal pressure.  Magnets are cooling down to 1.9 (4.5) Kelvin. The Cool down proceeds in steps, interleaved with electrical tests (HV insulation, ground faults…).  Connection of the power converter to the magnets.  Commissioning of the power converter + interlock system + magnet protection system (low current).  Commissioning of magnet powering + magnet protection system (high current).  Powering of all magnets in a sector to nominal current. This commissioning sequence is run individually on each of the 8 LHC sectors (arcs).

46. Circuit by circuit 46  Each electrical circuit (magnet group + power converter) has to pass a number of tests before it is released as ‘commissioned’.  For the large circuits, there are more than 10 intermediate steps.  Each step is validated by system experts.

47. Commissioning status 47  Installation and interconnections finished for magnets.  Cryogenics : -Sector 78 (IR8  IR7) was cooled down to 1.9 K in June/July 2007. -Sector 45 is presently at 1.9 K. -Sectors 56 and 78 : cool down in progress, expected at 1.9 K mid-March.  Magnet powering : - Commissioning of sector 78 in June/July 2007: >> All circuits with individual magnets commissioned (except triplets). >> The main magnets were commissioned to 1 TeV: limited by electrical non- conformities that were repaired during a warm up in September-October. - Commissioning of sector 45 in progress (until mid-February).  Other systems (RF, beam injection and extraction, beam instrumentation, collimation, interlocks, etc) are essentially on schedule for first beam in 2008.

48. Cool-down 48  From 300K to 80K pre-cooling with 1200 tons of liquid N2 (60 trucks).  LHC He inventory: 100 tons.  The duration of the first cool-down of one arc/sector is ~6 weeks (nominal ~ 2 weeks).  So far this was too optimistic… The cool down of sector 45 took ~8-9 weeks, delayed by heat isolation problems in certain cells etc  The stability of the cryogenics so far proves to be ‘delicate’. Xmas break 01/10/07 1.9 K High field quenches

49. Approaching 7 TeV 49  Main dipole magnet high field ramps for sector 45 started some days ago.  The first ‘re-training’ quenches occurred around 5.6 TeV.  After 3 ramps/training quenches, the magnets have reached 6 TeV.  1 TeV/1500 A to go… 31/01/2008 Current discharge into dump resistors Quench at 10270 A > 6 TeV Injection level

50. Cryo Recovery 50 1.9 K  After a high current/energy quench, the temperature in the affected magnet(s) increases above 20 K.  Recovery is ‘slow’ – few hours. >> Progress is slow !

51. 51 K. Foraz TS- IC-PL 1223344556677881 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 21 22 23 24 25 26 27 28 29 20 30 31 32 33 34 35 Jan. Feb. Mar. Apr. May Jun. Jul. Aug. 2008 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 21 22 23 24 25 26 27 28 29 20 30 31 32 33 34 35 Jan. Feb. Mar. Apr. May Jun. Jul. Aug. 2008 Power limitation It is still possible to have a beam test through 78 around 21st May … Assumes very fast cool-down of each sector (6 weeks) Inner triplet interconnect Flushing Cool-down Warm up Powering Tests CV maintenance LHC Schedule, status Week 03 Focus for 2008: Get beam in at 450 GeV asap !! The HWC will be ‘paused’ to make room for a beam test, presently expected around JUNE! Focus for 2008: Get beam in at 450 GeV asap !! The HWC will be ‘paused’ to make room for a beam test, presently expected around JUNE!

52. Towards beam… 52  “As usual” the latest schedule is VERY aggressive: Assumes parallel commissioning of almost the entire machine, while the initial schedules assumed parallel commissioning of only 1/4 LHC in parallel.  There is a strong push for beam tests @ 450 GeV, even before the magnets are commissioning to nominal field: >>> Get beam in asap at injection.  A beam test of a few weeks at 450 GeV of 1 or 2 sectors or of the entire machine will be performed, possibly as soon as June.  Planning is delicate due to interference with the experiments installation and hardware commissioning.  Various scenarios are been evaluated, including beam tests in certain sectors in parallel with hardware commissioning in others !!

53. LHC machine cycle (commissioning) 53 energy ramp preparation and access beam dump injection phase collisions 450 GeV 7 TeV start of the ramp Squeeze

54. Step by step 54  Beam commissioning will start with single low intensity bunches: Get the beams around and establish an orbit. Capture the beams in the RF systems. Commissioning all beam instrumentation. Understand /check the behaviour of the SC magnets at injection (field drifts due to ‘super-conducting eddy currents’). Etc…  Then there will more or less parallel branches to: Establish operation with both beams together. Increase intensity & number of bunches. Ramp, ‘squeeze’ (focus more at IP). Collide … If all goes very well… 2 months ! Please take this as a LOWER limit…

55. Operation phases 55 ParameterPhase APhase BPhase CNominal k / no. bunches43-1569362808 Bunch spacing (ns)2021-5667525 N (10 11 protons)0.4-0.9 0.51.15 Crossing angle (  rad)0250280  */  * nom )2  11  * (  m, IR1&5)322216 L (cm -2 s -1 )6x10 30 -10 32 10 32 -10 33 (1-2)x10 33 10 34 Beam commissioning will proceed in phases with increased complexity:  Number of bunches and bunch intensity.  Crossing angle (start without crossing angle !).  Less focusing at the collision point. It will most likely take YEARS to reach design luminosity !!!

56. Summary 56 We are getting there at last !!!! After many years of delay the LHC is taking shape in the tunnel. The first sector is approaching 7 TeV. The entire machine could be at 1.9 K around June. Beam is knocking at the door : The injectors are ready. There is a strong push/pressure to inject beam as soon as possible and perform some beam commissioning at 450 GeV – we may have beam in at least one sector in June 2008. Although in the best of all Worlds we could achieve collisions after 2 months, the road to 7 TeV collisions is long.... Be patient !! For more details on the LHC, have a look at the 2 nd Hadron Collider Summer School site : http://indico.cern.ch/conferenceDisplay.py?confId=6238http://indico.cern.ch/conferenceDisplay.py?confId=6238

57. ‘RF fingers’ problems 57  RF bellows are used to maintain electrical contact between adjacent pieces of vacuum chamber (essential for beam stability).  The bellows must cope with the thermal expansion of ~ 4 cm between 1.9 K and room temperature (when the magnets are cooled down/warmed up).  Bellows are installed at every interconnection (1700 in total).

58. At room temperature

59. At operating temperature 59

60. Damaged bellows 60  RF fingers that bend into the beam are a classic problem for accelerators. RHIC suffered a lot from it…  So no excuses for not being careful in design and manufacturing !  And yet it happened ! X-ray imaging of some bellows revealed bend fingers in the sector that was tested in July and warmed up since then for repair.

61. Cause & solution for RF fingers 61  The fingers bend due to a combination of a wrong ‘finger angle’ PLUS a gap between the magnet apertures larger than nominal (still inside specification).  Only few interconnects are affected.  Complete survey of one sector was performed using X-ray techniques.  Repair is not difficult…once the bad fingers are found. >> This problem will stay around until every sector is warmed up again in 200X… ‘Solution’: A small (ping-pong size) ball equipped with a 40MHz transmitter is blown through the beam vacuum pipe with compressed air. Beam position monitors are used to follow the ball as it rolls inside the vacuum chamber until it stops or exits on the other end..