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LHC Incident On 19 September: A report on the chronology of events
:: 25 October, 2008
On 19 September 2008,It is the failure of an electrical connection between two superconducting magnets which is the cause of the incident on September 19 in a section of the Large Hadron Collider. The incident occurred nine days after the first movement of the particle beam in the ring, forced CERN to put the LHC ''stand by'' at least until spring 2009. Here are the technical report published by CERN.
Investigations conducted at CERN in the wake of a major leak in the helium 3-4 tunnel of Large Hadron Collider (LHC) confirmed that the incident was caused by a faulty electrical connection between two magnets in the accelerator. This failure led to mechanical damage in cold mass helium tunnel.
The proper security procedures were in force, safety systems worked as intended and no one was endangered. CERN has enough spare parts for the LHC could restart in 2009 and arrangements are taken to prevent a similar incident from reoccurring in the future.
"This incident is an unforeseen event, said Robert Aymar, director general of CERN, but I am now convinced that we can make the necessary repairs, ensure that a similar incident can not recur and continue to pursue our research objectives. "
During feeding trials circuit of the main dipoles 3-4 in the LHC, the failure of an electrical connection occurred in an area between a dipole and quadrupole (LHC is equipped with 1234 dipoles and 392 quadrupole magnets that the magnetic field guides the beams of particles within the ring of 27 km in circumference), resulting in mechanical damage and a leak of helium cold mass of magnets to the tunnel. The proper security procedures were in force, safety systems worked as intended and no one was endangered.
After the time needed to heat the magnets at a value close to room temperature, inspections have begun and a number of clear findings are now established. The investigation continues and more complete findings will be presented later.
The following summary provides a brief description of the chain of events that occurred on September 19 at around noon. A more detailed technical report can be downloaded (PDF).
During the growing intensity of the current in the circuit of the main dipoles at 10 A / s (nominal), a resistive zone was formed, resulting in the emergence of a resistive voltage of 1 V to 9 kA less than a second . Electrical power, unable to maintain current growth, and a switch discharge energy, began introducing in the circuit resistance energy absorption to cause a rapid drop in magnets current. During this sequence of events, detection systems quench the magnets in power supply and discharge of energy have worked as planned. We can say that the first event did not quench the origin of a magnet, prior to discharge quickly. During discharge, many resistive transitions were triggered automatically in the magnets of the arc and helium to cool their bodies had been recovered via the automatic drain valves.
In less than a second, an electrical arc was formed, piercing the walls of helium, which spilled into the vacuum insulation cryostat. After 3 and 4 seconds, the vacuum is also altered in the beam tubes 2 and 1 respectively. The vacuum insulation began to be altered in two adjacent sub-sectors.
The discs against excess of the vacuum chamber were open when the pressure exceeded the atmospheric pressure, releasing the helium in the tunnel. However, they could not maintain the pressure below the nominal value of 0.15 MPa in the vacuum of sub-central sector. Large forces are exerted on the vacuum barriers separating the sub-central sector.
Having restored the power supply and services in the tunnel, and have provided the mechanical stability of magnets, investigation teams have opened the sleeves protecting the interconnections between magnets, starting with the sub-central sector. This operation has confirmed the location of the arc and showed the lack of electrical and mechanical damage in the neighboring interconnections, but showed contamination by a deposit similar to the soot that spread over a distance in the beam tubes. She also revealed damage to the thermal insulation multilayer cryostats.
The forces exerted on the barriers to the empty set quadrupole end of the sub-sector were such that cryostats containing these quadrupoles were ripped from their roots in the soil of the tunnel and moved from their original position, while the electrical connections and the fluid connections exerted on the masses of cold dipoles of this sub-sector tensile strength which displaced from their internal supports cold inside the cryostats remained in place. Moving cryostats quadrupoles damaged fittings "jumpers" linking the cryogenic distribution line, without breaking the vacuum insulation.
Pending the inspection of internal cryostats dipole, it was determined that repairs are at the 5 quadrupole and 24 dipoles assigned three sub-sectors. However, it may be necessary to extract other magnets in the tunnel to clean and change the multilayer insulation. Magnets and spare components of all types required are available in sufficient numbers to allow during the winter shutdown of the facilities of CERN, the replacement of damaged parts. The extent of the contamination suffered by the vacuum beam tubes is not yet fully assessed, but we know that this contamination is limited, there are plans to conduct an in situ cleaning to reduce to a minimum the number of magnets to move. A plan for re-extraction, transport and repair of magnets in the sector 3-4 is being developed and these operations will be integrated into the work of maintenance and consolidation perform during the winter stop on the all facilities of the CERN. The resources of labor have been provided.
Once all inspections have been made possible, a detailed analysis of events lead to the development of arrangements in the future to prevent this type of initial event recurrence and to mitigate its consequences if it accidentally happen again. Although the cause of the initial increase of the strength of the connection concerned has not yet been established, and knowing that a similar event did not occur in the tests performed on all other sectors and their thousands connections, it was decided that, before feeding again high intensity circuits of the LHC, an additional tool will be used to trigger alarms and necessary enslavement, the number and size of discs will be reviewed and anchorages on the ground of improved quadrupole fitted cryostats of barriers to empty.
Tags: LHC , superconducting magnets , Large Hadron Collider , particle beam , ,
