LHC : construction and operation

1. LHC : construction and operation JörgWenninger CERN Beams Department / Operations group LNF Spring School 'Bruno Touschek' - May 2010 • Part 2: • Machine protection • Incident and energy limits • Commissioning and operation

2. Machine protection

3. 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 (i.e. without damage) we must be able to dispose of all that energy safely ! • This is the role of machine protection !

4. 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

5. 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 15 tons of Copper!! • 90 kg of TNT The energy stored in one LHC beam corresponds approximately to… • 10-12 litres of gasoline • 15 kg of chocolate It’s how easily/quickly the energy is released that matters most !!

6. Machine protection: beam

7. To set the scale.. • Few cm long groove of an SPS vacuum chamber after the impact of ~1% of a nominal LHC beam (2 MJ) during an ‘incident’: • Vacuum chamber ripped open. • 3 day repair. • The same incident at the LHC implies a shutdown of > 3 months. • >> Protection of the LHC must be much stricter and much more reliable !

8. Beam impact in a target Simulation of a 7 TeV LHC beam impact into a 5 m long Copper target The 7 TeV LHC beam can drill a hole through ~35 m of Copper 20 bunches 180 bunches 100 bunches 380 bunches Courtesy N. Tahir / GSI

9. From a real 450 GeV beam… Shoot a 450 GeV beam into a target… 108 plates 30 cm 6 cm 6 cm Cu plate ~20 cm inside the ‘target’. ~0.1% nominal LHC beam A B D C

10. ‘Safe’ beams at the LHC… 2011 ‘Un-safe beam’ ‘Safe beam’ L ~ 21028 cm-2s-1 LHC 2010

11. Schematic layout of beam dump system in 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 ! Ultra-high reliability system !! Septum magnets deflect the extracted beam vertically Beam 1 Kicker magnets to paint (dilute) the beam Q5L Beam dump block Q4L 700 m 15 fast ‘kicker’ magnets deflect the beam to the outside Q4R  500 m Q5R The 3 ms gap in the beam gives the kicker time to reach full field. quadrupoles Beam 2

12. The dump block Simulation beam absorber (graphite) Measurement • 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 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 ! Approx. 8 m concrete shielding

13. Dump line in IR6

14. Dump line

15. Dump installation

16. ‘Unscheduled’ beam loss due to failures 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. Beam loss over a single turn during injection, beam dump. 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 (~millisecond to many seconds) due to many types of failures. • Because of the very high risk, the LHC machine protection system is of unprecedented complexity and size. • A general design philosophy was to ensure that there should always be at least 2 different systems to protect against a given failure type.

17. Failure detection example : beam loss monitors • Ionization chambers to detect beam losses: • N2 gas filling at 100 mbar over-pressure, voltage 1.5 kV • Sensitive volume 1.5 l • Reaction time ~ ½ turn (40 ms) • Very large dynamic range (> 106) • There are ~3600 chambers distributed over the ring to detect abnormal beam losses and if necessary trigger a beam abort !

18. Beam loss monitoring

19. Machine protection and quench prevention: collimation

20. Operational margin of SC magnet Applied Field [T] The LHC is ~1000 times more critical than TEVATRON, HERA, RHIC Bccritical field Bc quench with fast loss of ~106-7 protons 8.3 T / 7 TeV QUENCH Tccritical temperature quench with fast loss of ~few 109 protons Tc 0.54 T / 450 GeV 1.9 K 9 K Temperature [K]

21. Quench levels • The phase transition from the super-conducting to a normal conducting state is called a quench. • Quenches are initiated by an energy in the order of few milliJoules • Movement of the superconductor by several m (friction and heat dissipation). • Beam losses. • Cooling failures. • .. • Example of the energy (mJ/cm3) required to quench an LHC dipole magnet with an instantaneous loss. • Steep energy dependence. • Models are confirmed at 0.45 TeV.

22. Beam lifetime • Consider a beam with a lifetime t : • Number of protons lost per second for different lifetimes (nominal intensity): • t = 100 hours 109 p/s • t = 25 hours 4x109 p/s • t = 1 hour 1011 p/s • While ‘normal’ lifetimes will be in the range of 10-100 hours (in collisions most of the protons are actually lost in the experiments !!), one has to anticipate short periods of low lifetimes, down to a few minutes ! •  To survive periods of low lifetime we must intercept the protons that are lost with very high efficiency before they can quench a magnet : collimation! Quench level ~ 106-7 p

23. Collimation • A 4-stage halo cleaning(collimation) system is installed to protect the LHC magnets from beam induced quenches. • A cascade of more than 100 collimators is required to prevent the protons and their debris to reach the superconducting magnet coils. • The collimators will also play an essential role for protection by intercepting the beams. •  the collimators must reduce the energy load into the magnets due to particle lost from the beam to a level that does not quench the magnets. in front of the exp. Exp. Detectors Courtesy C. Bracco

24. Collimator settings at 7 TeV • At the LHC collimators are essential for machine operation as soon as we have more than a few % of the nominal beam intensity at injection ! The collimator opening corresponds roughly to the size of Spain ! Carbon jaw 1 mm Opening ~3-5 mm RF contact ‘fingers’

25. Collimation performance Measurements of the collimation efficiency from beam loss maps confirms the excellent performance : > 99.9% efficiency ! Cleaning Momentum Cleaning Dump Protection IR2 IR8 IR1 IR5 Collimation team

26. Machine protection: magnets

27. LHC powering in sectors • To limit the stored energy within one electrical circuit, the LHC is powered by sectors. • The main dipole circuits are split into 8 sectors to bring down the stored energy to ~1 GJ/sector. • Each main sector (~2.9 km) includes 154 dipole magnets (powered by a single power converter) and 47 quadrupoles. •  This also facilitates the commissioning that can be done sector by sector ! 5 4 6 DC Power feed LHC 7 3 DC Power 27 km Circumference Powering Sector 8 2 1 Sector

28. Powering from room temperature source… 6 kA power converter Water cooled 13 kA Copper cables ! Not superconducting !

29. …to the cryostat Feedboxes (‘DFB’) : transition from Copper cable to super-conductor Cooled Cu cables

30. Quench detection • When part of a magnet quenches, the conductor becomes resistive, which can lead to excessive local energy deposition (tT rise !!) due to Ohmic losses. • To protect the magnet: • The quench must be detected: this is done by monitoring the voltage that appears over the coil (R > 0). • The energy release is distributed over the entire magnet by force-quenching the coils using quench heaters (such that the entire magnet quenches !). • The magnet current is switched off within << 1 second. 18/04/10 Example of the voltage signals over 5 quenching dipole magnets (beam induced at injection). Threshold of quench protection system (QPS)

31. Quench - discharge of the 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 increaseover coil. • 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 flows through the bypass diode (triggered by the voltage increase over the magnet). • The current of all other magnets is dischared into the dump resistors.

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

33. BLMs Power Permit Beam Permit Cryo RF Stored beam energy Stored magnetic energy access Power permit Experiments Beam permit Software interlocks Power Converters vacuum QPS Collimators Warm Magnets Machine protection philosophy → Authorises beam operation → Requests a beam dump in case of problems → Authorises power on → Cuts power off in case of fault

34. LHC Devices LHC Devices LHC Devices Movable Devices BCM Beam Loss Experimental Magnets Collimator Positions Environmental parameters BTV screens Mirrors Safe Mach. Param. Software Interlocks SEQ CCC Operator Buttons Experiments Transverse Feedback Beam Aperture Kickers Collimation System FBCM Lifetime BTV PIC essential + auxiliary circuits WIC FMCM BLM Access System Vacuum System RF System BPM in IR6 Monitors in arcs (several 1000) Monitors aperture limits (some 100) Magnets Power Converters Doors EIS Vacuum valves Access Safety Blocks RF Stoppers QPS (several 1000) Power Converters ~1500 AUG UPS Cryo OK Beam interlock system Over 20’000 signals enter the interlock system of the LHC that will send the beam into the dump block if any input signals a fault ! Timing Beam Dumping System Beam Interlock System Safe Beam Flag Injection BIS Timing System (Post Mortem) Isn’t it a miracle that it works !

35. Incident of September 19th 2008& Consequences

36. Event sequence on Sept. 19th • Introduction: on September 10th when the first beam made it around the LHC, not all magnets had not been fully commissioned for 5 TeV. • A few magnets were missing their last commissioning steps. • The last steps were finished the week after Sept. 10th. • Last commissioning step of the dipole circuit in sector 34 : ramp to 5.5 TeV. • At ~5.1 TeV an electrical fault developed in the dipole bus bar (the bus bar is the cable carrying the current that connects all magnet of a circuit). • Later traced to an anomalous resistance of 200 nW (should be 0.3 nW). • An electrical arc developed which punctured the helium enclosure. • Secondary arcs developed along the arc. • Around 400 MJ were dissipated in the cold-mass and in electrical arcs. • Large amounts of Helium were released into the insulating vacuum. • In total 6 tons of He were released.

37. Pressure wave • Pressure wave propagates in both directions along the magnets inside the insulating vacuum enclosure. • Rapid pressure rise : • Self actuating relief valves could not handle the pressure. • designed for 2 kg He/s, incident ~ 20 kg/s. • Large forces exerted on the vacuum barriers (every 2 cells). • designed for a pressure of 1.5 bar, incident ~ 10 bar. • Several quadrupoles displaced by up to ~50 cm. • Connections to the cryogenic line damaged in some places. • Beam vacuum to atmospheric pressure.

38. One of ~1700 bus-bar connections Dipole busbar

39. Incident location Dipole bus bar

40. Collateral damage : displacements Quadrupole-dipole interconnection Quadrupole support • Main damage area ~ 700 metres. • 39 out of 154 dipoles, • 14 out of 47 quadrupole short straight sections (SSS) • from the sector had to be moved to the surface for repair (16) or replacement (37).

41. Collateral damage : beam vacuum The beam vacuum was affected over entire 2.7 km length of the arc. Clean Copper surface. Contamination with multi-layer magnet insulation debris. Contamination with sooth.  60% of the chambers  20% of the chambers

42. Quench - discharge of the energy The bus-bar must carry the current for some minutes, through interconnections Power Converter Discharge resistor Magnet 1 Magnet 2 Magnet 154 Magnet i • In case of a quench, the individual magnet is protected (quench protection and diode). • Resistances are switched into the circuit: the energy is dissipated in the resistances (current decay time constant of 100 s). • >> the bus-bar must carry the current until the energy is extracted !

43. Bus-bar joint • 24’000 bus-bar joints in the LHC main circuits. • 10’000 joints are at the interconnection between magnets. • They are welded in the tunnel. • Nominal joint resistance: • 1.9 K 0.3 nΩ • 300K ~10 μΩ For the LHC to operate safely at a certain energy, there is a limit to maximum value of the joint resistance.

44. Joint quality • The copper stabilizes the bus bar in the event of a cable quench (=bypass for the current while the energy is extracted from the circuit). • Protection system in place in 2008 not sufficiently sensitive. • A copper bus bar with reduced continuity coupled to a superconducting cable badly soldered to the stabilizer can lead to a serious incident. Solder No solder X-ray of joint wedge • During repair work in the damaged sector, inspection of the joints revealed systematic voids caused by the welding procedure. bus U-profile bus

45. Normal interconnect, normal operation • Everything is at 1.9 Kelvin. • Current passes through the superconducting cable. • For 7 TeV : I = 11’800 A Magnet Magnet Helium bath copper bus bar 280 mm2 copper bus bar 280 mm2 superconducting cable with about 12 mm2 copper current Interconnection joint (soldered) This illustration does not represent the real geometry

46. Normal interconnect, quench • Quench in adjacent magnet or in the bus-bar. • Temperature increase above ~ 9 K. • The superconductor becomes resistive. • During the energy discharge the current passes for few minutes through the copper bus-bar. Magnet Magnet copper bus bar 280 mm2 copper bus bar 280 mm2 superconducting cable interconnection

47. Non-conform interconnect, normal operation • Interruption of copper stabiliser of the bus-bar. • Superconducting cable at 1.9 K • Current passes through superconductor. Magnet Magnet copper bus bar 280 mm2 copper bus bar 280 mm2 superconducting cable interconnection

48. Non-conform interconnect, quench. • Interruption of copper stabiliser. • Superconducting cable temperature increase to above ~9 K and cable becomes resistive. • Current cannot pass through copper and is forced to pass through superconductor during discharge. Magnet Magnet copper bus bar 280 mm2 copper bus bar 280 mm2 superconducting cable interconnection

49. Non-conform interconnect, quench. • The superconducting cable heats up because of the combination of high current and resistive cable. Magnet Magnet copper bus bar 280 mm2 copper bus bar 280 mm2 superconducting cable interconnection

50. Non-conform interconnect, quench. • Superconducting cable melts and breaks if the length of the superconductor not in contact with the bus bar exceeds a critical value and the current is high. • Circuit is interrupted and an electrical arc is formed. Magnet Magnet copper bus bar 280 mm2 copper bus bar 280 mm2 superconducting cable interconnection • Depending on ‘type’ of non-conformity, problems appear : • at different current levels. • under different conditions (magnet or bus bar quench etc).