Cern: Electrical fault caused LHC helium leak

A faulty electrical connection in the world's largest particle accelerator was the cause of a liquid-helium leak that damaged the machine, says Cern

Cern has confirmed that an electrical fault was the cause of a liquid helium leak in its flagship particle accelerator.

The Large Hadron Collider (LHC), built to smash atoms and enable the study of subatomic particles, was shut down on 19 September due to a malfunction, only nine days after it was powered up.

Robert Aymar, director general of the European Organization for Nuclear Research (Cern), told earlier this month that Cern scientists believed an electrical connection had been at fault. A Cern statement on Thursday confirmed that the cause of the incident was a faulty electrical connection between two of the accelerator's magnets. This resulted in mechanical damage, and a release of helium from the magnet cold mass into the LHC tunnel.

"This incident was unforeseen, but I am now confident that we can make the necessary repairs, ensure that a similar incident cannot happen in the future and move forward to achieving our research objectives," said Aymar.

The Large Hadron Collider is housed in a 27km concrete ring which straddles the Swiss and French borders near Geneva. Around a vacuum chamber, dipole magnets are used to keep two opposing proton beams on their paths, while quadrupole magnets focus the beams. The superconductor magnets have an operating temperature of 1.9K, cooled by liquid helium.

The fault occurred while the magnets' electrical field was being increased to guide and focus the proton beams during a collision. A resistive zone developed in an electrical connection, creating an electrical arc which punctured one of the helium enclosures around a magnet. The warming helium expanded in the vacuum enclosure of the central subsector of the pipe, damaging the vacuum barriers separating the central subsector from the neighbouring subsectors.

After letting the magnets return to room temperature, scientists restored power and services in the tunnel, and ensured the mechanical stability of the magnets. Investigation teams then opened up the cryostat sleeves in the interconnections between magnets, starting from the central subsector. This confirmed the location of the electrical arc, and revealed contamination by soot-like dust over some distance in the beam pipes, Cern said in a statement.

There was damage to the multilayer insulation blankets of the cryostats. The forces on the vacuum barriers attached to the quadrupoles at the subsector ends were so great that the cryostats housing these quadrupoles broke their anchors in the concrete floor of the tunnel and were moved out of position.

Cern said that the number of magnets to be repaired is approximately five quadrupoles and 24 dipoles. It is possible that more magnets will have to be removed from the tunnel for cleaning and re-insulation. Cern believes it has adequate numbers of spare magnets and components to replace the damaged ones, and expects the LHC to come back online in April, after the winter shutdown.

Cern experiments shut down from October to April so the Swiss and French electrical grids are not overloaded during the winter months, and in order to carry out maintenance work.

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