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Commissioning challenges of the LHC

Commissioning challenges of the LHC. J örg Wenninger CERN Accelerators and Beams Department Operations group February 2008. LHC overview Magnets Luminosity High ‘energy’ issues Hardware & beam commissioning. 7 years of construction to replace LEP: 1989-2000.

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Commissioning challenges of the LHC

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

  2. 7 years of construction to replace LEP: 1989-2000 in the same 26.7 km tunnel by LHC : 2008-2020+ CMS ATLAS The CERN Beschleuniger Komplex LHC LHCB Control room SPS ALICE CERN (Meyrin) Rüdiger Schmidt Bullay Oktober 2007 2

  3. 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 ! Beam dump blocks IR5:CMS IR6: Beam dumping system IR4: RF + Beam instrumentation IR3: Momentum collimation (normal conductingmagnets) IR7: Betatron collimation (normal conductingmagnets) The main dipole magnets define the geometry of the circle ! IR8: LHC-B IR2:ALICE IR1: ATLAS Injection ring 2 Injection ring 1

  4. The CERN Accelerator Complex Beam 2 5 LHC 6 4 Beam 1 7 3 TI8 SPS 2 8 TI2 Booster 1 protons LINACS Top energy/GeVCircumference/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 CPS Ions LEIR Note the energy gain/machine of 10 to 20 – and not more ! The gain is typical for the useful range of magnets !!!

  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 field 8.3 T • HERA/Tevatron ~ 4 T • The luminosity of the collider that will reach unprecedented values for a hadron machine: • LHC pp ~ 1034 cm-2 s-1 • Tevatron pp 2x1032 cm-2 s-1 • SppbarS pp 6x1030 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

  7. The Superconductor • The DIPOLE field of 8.3 Tesla required to achieve 7 TeV/c can only be obtained with superconducting magnets ! • The material determines: Tccritical temperature Bc critical field • LHC magnets use NbTi, with a typical usable current density @ 4.2 K 2000 A/mm2 @ 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). Bc Tc

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

  9. Dipole magnets • 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! Dipole length 15 m The coils must be aligned very precisely to ensure a good field quality (i.e. ‘pure’ dipole)

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

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

  12. LHC arc : not just dipoles… • 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. Regular arc: Magnets 1232 main dipoles + 3700 multipole corrector magnets (sextupole, octupole, decapole) 392 main quadrupoles + 2500 corrector magnets (dipole, sextupole, octupole) J. Wenninger - ETHZ - December 2005 13

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

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

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

  17. Tunnel view

  18. Complex interconnects 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 CERN visit McEwen

  19. Vacuum chamber • 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. 50 mm 36 mm Beam screen Beamenvel. ~ 1.8 mm @ 7 TeV Cooling channel (Helium) Magnet bore

  20. Luminosity

  21. Luminosity challenges The event rate N for a physics process with cross-section s is proprotional to the collider Luminosity L: 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 • To maximize L: • Many bunches (k) • Many protons per bunch (N) • A small beam size s*u = (b *e)1/2 • b*: characterizes the beam envelope (optics), varies along the ring, mim. at the collision points. • e: is 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 !

  22. Beam enveloppe ATLAS collision point ARC ARC Fits through the hole of a needle! • 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 ‘b*’): • CMS & ATLAS : 16 mm LHCb : 22 – 160 mm ALICE :16 mm (ions) / >160 mm (p) 22

  23. Combining beams for collisions • 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. IP Crossing angle • 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 mrad • CMS Horizontal 280 mrad • LHCb Horizontal 300 mrad • ALICE Vertical 400 mrad 7.5 m 24

  25. Bunch Pattern • 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 ms hole (t5)for the LHC dump kicker (see later). 72 bunches 25

  26. Stored Energy & Protection

  27. The price of high fields & high luminosity… • 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 Estored = 7 MJ • all magnets Estored = 10.4 GJ • the 2808 LHC bunches store a large amount of kinetic energy: • Ebunch = N x E = 1.15 x 1011 x 7 TeV = 129 kJ • Ebeam = k x Ebunch = 2808 x Ebunch= 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 • 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… 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 • 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 Power Converter Discharge resistor Magnet 1 Magnet 2 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. 31

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

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

  34. Operational margin of SC magnet The LHC is ~1000 times more critical than TEVATRON, HERA, RHIC Applied Field [T] Bccritical field Bc Quench with fast loss of ~106-7 p ~0.01-0.1 ppm of the total int. 8.3 T / 7 TeV QUENCH Tccritical temperature quench with fast loss of ~1010 p ~ 0.01% of total int. Tc 0.54 T / 450 GeV 1.9 K 9 K Temperature [K]

  35. ‘Cohabitation’ • Even to reach a luminosity of 1033 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 • 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 !! Experiment Protection devices Primary collimator Secondary collimators Tertiary collimators Triplet magnets Absorbers Tertiary halo hadronic showers Primary halo particle Secondary halo + hadronic showers Beam 36

  37. Collimation @ 7 TeV • 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 ! The collimator opening corresponds roughly to the size of Spain ! 1 mm Opening ~3-5 mm 37

  38. Beam Dumping System IR6 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 Beam 1 Kicker magnets to paint (dilute) the beam Q5L Beam dump block Q4L about 700 m 15 fast ‘kicker’ magnets deflect the beam to the outside Q4R about 500 m Q5R quadrupoles Beam 2 38

  39. Dump Block… • 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 ! beam absorber (graphite) Approx. 8 m concrete shielding 39

  40. Unscheduled beam loss & protection In the event a failureor unacceptable beam lifetime, the beammust bedumpedimmediately and safely into thebeam dump block Two main classes for failures (with more subtle sub-classes): • Passive protection • - Failure prevention (high reliability systems). • Intercept beam with collimators and absorber blocks. • Active protection systems have no time to react ! Beam loss over a single turn during injection, beam dump or any other fast ‘kick’. Active Protection - Failure detection (by beam and/or equipment monitoring) with fast reaction time (< 1 ms). - Fire beam dumping system Beam loss over multiple turns due to many types of failures. 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

  42. Injector status : beam at the gate to the LHC 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. TV screen at end transfer line Beam image taken less than 50 m away from the LHC tunnel in IR8 (LHCb) !

  43. Filling exercise from SPS to LHC 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 • 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 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 • 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 • 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 • 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’. • From 300K to 80K pre-cooling with 1200 tons of liquid N2 (60 trucks). • LHC He inventory: 100 tons. High field quenches Xmas break 1.9 K 01/10/07

  49. Approaching 7 TeV • 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… Quench at 10270 A > 6 TeV 31/01/2008 Current discharge into dump resistors Injection level

  50. Cryo Recovery • 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 ! 1.9 K

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