Currently the premier physics laboratory, of many around the world, is the Large Hadron Collider.  This, the world's largest particle accelerator, is located at the European Organization for Nuclear Research (CERN) straddling the border between France and Switzerland near Geneva.  Bunches of protons (or sometimes heavy atomic nuclei) travel in opposite directions inside a pair of arm sized vacuum tubes bent along an 8+ km diameter circular path, colliding a locations where detectors have been constructed to study what new particles are formed from the high energy.  The actual tubes, with super-conducting bending magnets, and accelerating Radio Frequency klystrons with their Niobium cavities, are located in a tunnel about 100 meters underground below the yellow circle superimposed on the photograph above.  The giant Atlas detector is located at 2 o'clock in this image and the even more massive Compact Muon Solenoid detector is opposite in about the 9 o'clock position.  (click on photo for more)

Wednesday, 10 September 2008 was the special day for particle physics: the startup of the Large Hadron Collider (LHC) at CERN.  For the first time, protons circulated around the entire 27 km LHC ring.

THE incident

During commissioning (without beam) of the final LHC sector (sector 34) at higher current for operation at 5 TeV, a large liquid Helium coolant leak occurred at mid-day Friday 19 September 2008.  The most likely cause of the problem was a deficient electrical connection between two magnets which melted at high current leading to an avalanche of mechanical failures.  There was no risk to people since everyone remains above the underground tunnel when operating.  But that sector of the super cold, super conducting apparatus had to be warmed for repairs.  Followed months to diagnose all shortcomings, re-engineer the components to prevent reoccurrence, rebuild that section of the accelerator, and install upgrades on the rest.

A Winter shutdown is traditionally scheduled from early December until April.  While the LHC does use superconducting magnets and accelerators to maximize operation with a minimum of electrical power, it still uses significant amounts of power.  In Switzerland where many residents and business use electric heat, electricity is priced higher during the Winter.  So accelerators are seldom run in the Winter; experiments are conducted when it is most economical to do so.

A variety of planned upgrades to the LHC and the experiments were planned for the Winter 2008-9 break and additional fail-safe updates were done during the down time.  The large particle detectors continued observing cosmic ray events as they had been doing.  This provided an early way to tune the detectors.  Occasionally cosmic rays collide with the Earth's atmosphere at energies far greater that the LHC will ever provide so this also provides an avenue of ongoing research.

The world's largest manmade particle accelerator was restarted at half power in November 2009  The machine began running at roughly half of its design energy, far less than some physicists would like, and eventually ramp up to higher energies in 2010.

what's going on

We saw something cool today: the LHC is again circulating beam and we saw splash events in CMS pretty much real time.  Splash events are where the beam hits a collimator and splashes particles all over the detector.  The beam is running in one direction only at 450 GeV - the same energy it had on Sept 10 last year. by anonymous on 7 Nov 2009  The first week in December saw the beams declared stable.  The first routine collisions were reported by the end of the second week of December 2009.  The 2009 run ended 16 December.  CMS cooling system leaks were repaired and other improvements made over the Winter.  It was decided to next attempt a long run at 7 TeV (3.5 TeV per beam) lasting perhaps nearly two years with only brief technical shut downs.  The first splash events were recorded late February.  The first time stable squeezed beam at 3.5 TeV including an extra colliding bunch occurred in late April.  This resulted in a factor 10 increase in the instantaneous luminosity.

• See recent information for the LHC beams at CMS e-commentary by Darin Acosta.
  
  
• Study the various LHC design reports which have much detail in pdf format.
  
  
• CERN marked the startup event by inviting press, dignitaries as well as scientists to celebrate around the world.  At Fermilab, there was a Pajama Party in the early morning hours when the first beam circulation took place.  We had four high school teachers and ten students who did live blogging of the event at FermiLab and upload videos: http://cosm.hamptonu.edu/vlhc/doku.php?id=wiki:start:lhco:post:home:postings.
  
  
• You might want to do an experiment using indirect measurement like those found on theVLHC wiki at http://cosm.hamptonu.edu/vlhc/doku.php?id=start:wiki:lhco:im.
  
  
• You might want to calculate the time it takes a proton to go around the 27 km ring at nearly the speed of light.
  
  
• View the LHC Rap at http://www.youtube.com/watch?v=j50ZssEojtM.
  
  
• Watch the documentary The Next Big Bang, about the LHC on the History Channel.    (See http://www.history.com/shows.do?action=detail&episodeId=276858.)
  
  
• Oh, and not unexpectedly, we're still here!  The news media did their usual efforts to raise viewership by publicizing the fear that the LHC might make a mini-black hole which would engulf the entire world, ending our existence.  That had long ago been given due consideration and found highly unlikely.  Equal and higher energy collisions occur naturally when the most energetic cosmic rays crash into the Earth's upper atmosphere.  Those obviously didn't annihilate Earth, so collisions in the LHC are unlike to do so.  And even if such mini-black holes are created, Steven Hawking earlier explained that black holes evaporate by emitting energy, with mini-black holes doing so almost instantaneously.

Position of proton beam after first circulation in LHC.


Profile of first proton bunches (verses time).

additional links

Details of the accident

Investigations at CERN following a large Helium leak into sector 3-4 of the Large Hadron Collider (LHC) tunnel have confirmed that cause of the incident was a faulty electrical connection between two of the accelerator's magnets.  This resulted in mechanical damage and release of Helium from the magnet cold masses into the tunnel.  Sufficient spare components are in hand to restart in 2009.  Measures to prevent a similar incident in the future are being put in place.

The curved sections of the LHC, extending over most of the length of each 3.3 km long sector, are composed of a repeating layout, the elementary cell of which (107 m long) is composed of a horizontally focusing quadrupole magnet, three beam curving dipole magnets (cross section at right→), a vertically focusing quadrupole magnet and another three dipole magnets.  The magnets are electrically powered with the current passing in series throughout the sector.  The magnets, equipped with their Helium vessel and end covers, constitute the cold masses, which, in normal operation, contain superfluid Helium at 1.9 K and 0.13 MPa, and are thermally insulated from the vacuum enclosure.  The neighbouring cold masses are electrically and hydraulically interconnected.  The weight of the cold mass is transmitted to the vacuum enclosure via cold support posts and is further transmitted to the tunnel floor by adjustable support jacks, anchored in the concrete.  The cryogenic system, fed from the cryogenic distribution line through a jumper connection every 107 m at the location of a quadrupole.  Two subsequent sections constitute a vacuum subsector sharing a common insulation vacuum; the insulation vacuum enclosures of neighbouring subsectors are separated by vacuum barriers.  The two beam pipes constitute two other separate vacuum systems.

Summary of the analysis of the 19 September incident at the LHC

1. During the ramping-up of current in the main dipole circuit (strengthening the magnetic fields) at the nominal rate of 10 A/s, a zone of lost superconductivity (with heat generating resistance) developed leading in less than one second to a resistance equivalent to a voltage drop of 1 V at 9 kA.  The power supply, unable to maintain the current ramp, tripped off and the energy discharge switch opened, inserting dump resistors into the circuit to produce a fast shut off of the electric current.  In this sequence of events, the quench detection, power converter and energy discharge systems behaved as expected.  It is certain that this magnet quench was not the cause of the initial event.  During the discharge, many magnet quenches were triggered automatically and the helium from their cold masses was recovered through the self actuated relief valves.
   
   
2. Within one second, an electrical arc developed, puncturing the Helium enclosure and leading to a release of helium into the insulation vacuum of the cryostat.  After 3 and 4 seconds, the beam vacuum also degraded in both beam pipes.  Then the insulation vacuum started to degrade in the two neighbouring subsectors.
   
   
3. The spring-loaded relief discs on the vacuum enclosure opened when the pressure exceeded atmospheric, thus releasing the expensive Helium into the tunnel, but they were unable to contain the pressure rise below the nominal 0.15 MPa in the vacuum enclosure of the central subsector, thus resulting in large pressure forces acting on the vacuum barriers separating the central subsector from the neighbouring subsectors.
   
   

1. After restoring power and services in the tunnel and ensuring mechanical stability of the magnets, the investigation teams proceeded to open up the cryostat sleeves in the interconnections between magnets, starting from the central subsector.  This confirmed the location of the electrical arc, showed absence of electrical and mechanical damage in neighbouring interconnections, but revealed contamination by soot-like dust which propagated over some distance in the beam pipes.  It also showed damage to the multilayer insulation blankets of the cryostats.  The forces on the vacuum barriers attached to the quadrupoles at the subsector ends were such that the cryostats housing these quadrupoles broke their anchors in the concrete floor of the tunnel and were moved away from their original positions, with the electric and fluid connections pulling the dipole cold masses in the subsector from the cold internal supports inside their undisplaced cryostats.  The displacement of the quadrupoles cryostats damaged "jumper" connections to the cryogenic distribution line, but without rupturing its insulation vacuum.
   
   
2. Pending further inspection of the inside of the dipole cryostats, it has been established that the number of magnets to be repaired is at most 5 quadrupoles and 24 dipoles from the three subsectors involved.  But it is possible that more magnets will have to be removed from the tunnel for cleaning and exchange of multilayer insulation.  Spare magnets and spare components appear to be available in adequate types and sufficient quantities to allow replacement of the damaged ones during the forthcoming shutdown.  The extent of contamination to the beam vacuum pipes is not yet fully mapped, but is known to be limited; in situ cleaning is being considered to keep the number of magnets to be removed to a minimum.  The plan for removal/reinstallation, transport and repair of magnets in Sector 3-4 is being established and integrated with the maintenance and consolidation work to be performed during the winter shutdown across the whole CERN facility.  The corresponding manpower resources have been secured.
   
   
3. Once all possible inspections are completed, an analysis of the events will lead to recommendations for future actions to prevent the reoccurrence of this type of initial event, and to mitigate its consequences should it accidentally reoccur.  Although the cause of the initial growth of electrical resistance at the mechanical connection has not yet been established, and knowing that a similar event has not occurred in the test of all other sectors and of their thousands of connections, it has nonetheless been decided that additional measurements to generate early warnings and interlocks, improvements in pressure relief devices and in external anchoring of the quadrupole cryostats with vacuum barrier will be implemented before any further powering of the LHC circuits at high current.

derived from information provided by James.Gillies, 16 October, forwarded by Ken Cecire

page created 18 July 2007
latest revision 10 May 2010
conceived, designed & maintained to support virtual QuarkNet & virtual LHC @ FermiLab

by D Trapp

This project has been supported in part through QuarkNet at FermiLab by the National Science Foundation and the Office of High Energy Physics, Office of Science, U.S.  Department of Energy.  However the choice of content, opinions expressed and any omissions or other errors are those of D Trapp and not the responsibility of QuarkNet, NSF or DoE.