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The LHC Machine

The LHC Machine. J örg Wenninger CERN Beams Department Operations group Hadron Collider Summer School - June 2009. Part 2: Machine Protection Incident 19 th September 2008 Commissioning and operation. Machine Protection. The price of high fields & high luminosity….

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The LHC Machine

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  1. The LHC Machine JörgWenninger CERN Beams Department Operations group Hadron Collider Summer School - June 2009 • Part 2: • Machine Protection • Incident 19th September 2008 • 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 the energy is released that matters most !!

  6. Machine protection: the 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 during an ‘incident’: • vacuum chamber ripped open. • 3 day repair. • The same incident at the LHC implies a shutdown of > 3 months. • >> The protection of the LHC must be much stricter and much more reliable !

  8. LHC beam impact : energy deposition 7 TeV LHC beam into a 5 m long Copper target 20 bunches 180 bunches 380 bunches 100 bunches N. Tahir / GSI

  9. Beam impact in target : density The 7 TeV LHC beam is able to drill a hole through ~35 m of Copper 20 bunches 180 bunches 100 bunches 380 bunches N. Tahir / GSI

  10. From a real 450 GeV beam Cu plate ~20 cm inside a ‘target’. ~0.1% nominal LHC beam A B D C

  11. Machine protection: how do we get rid of the beam?

  12. 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 about 700 m 15 fast ‘kicker’ magnets deflect the beam to the outside Q4R about 500 m Q5R The 3 ms gap in the beam gives the kicker time to reach full field. quadrupoles Beam 2

  13. The dump block • This is 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 ! beam absorber (graphite) Approx. 8 m Beam dump at 450 GeV concrete shielding Sept. 2008

  14. Dump line in IR6

  15. Dump line

  16. Dump installation CERN visit McEwen 16

  17. ‘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 against beam damage is of unprecedented complexity and size. • A general design philosophy was to ensure that there should always be 2 (or more than 2) different systems to protect against a given failure type.

  18. 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 !

  19. 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 10’000 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)

  20. Machine protection and quench prevention: collimation

  21. 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 protons 8.3 T / 7 TeV QUENCH Tccritical temperature quench with fast loss of ~4x109 protons Tc 0.54 T / 450 GeV 1.9 K 9 K Temperature [K]

  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 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 background reduction. •  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

  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. Machine protection: magnets

  26. 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

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

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

  29. Quench • A quench is the phase transition from the super-conducting to a normal conducting state. • 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. • ... • 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: a voltage 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 has to be switched off within << 1 second.

  30. 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.

  31. 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).

  32. Incident of September 19th 2008

  33. Event sequence on Sept. 19th • Introduction: on September 10th the LHC 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 main dipole circuit in sector 34 : ramp to 9.3kA (5.5 TeV). • At 8.7kA 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). • 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.

  34. Inter-connection Note the bellows that are needed to compensate for a 4.5 cm contraction per magnet between room temp. and 1.9 K. Insulation blankets Vac. chamber Dipole busbar LHC Status - Planck09 - Padova,IT

  35. 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.

  36. Incident location Dipole bus bar LHC Status - Planck09 - Padova,IT

  37. Collateral damage : displacements Quadrupole-dipole interconnection Quadrupole support • Main damage area ~ 700 metres. • 39out 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).

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

  39. What triggered the incident?

  40. Dipole magnet protection - again • 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 ! Bus-bar must carry the current for some minutes, through interconnections DFB DFB Magnet 2 Magnet 4 Magnet 152 Magnet 154 Magnet 1 Magnet 3 Magnet 5 Magnet 153 Energy Extraction: switch open Energy Extraction: switch open Power Converter

  41. One of ~1700 bus-bar connections

  42. Bus-bar joint (1) • Superconducting cable embedded in Copper stabilizer. • Bus bar joint is soldered (not clamped). • Joint resistance ~0.35 nW(@ 1.9 K). • Protection of the joint during quench relies on good joint quality. ~2 cm

  43. Bus-bar joint (2) • A post-mortem analysis of the data from the sector with the incident revealed the presence of a 200 nW anomalous resistance in the cell of the primary electrical arc. This acted as a heat source that quenched the superconducting cable. • >> Unfortunately the evidence is destroyed… • An inspection of accessible joints revealed non-conformities like poor soldering and/or reduced electrical contact as in the example to the right.

  44. Bad soldering surprise • Few cm long voids are created during soldering process – discovered during the repair of sector 34. • SnAg solder can flow out during soldering – present in MOST joints. X-ray of joints in sector34

  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).

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