LHC restart back on track

On March 30, teams working on the Large Hadron Collider resolved a problem that had been delaying the restart of the accelerator.

A short circuit to ground occurred on March 24 in one of the connections with an LHC magnet. LHC magnets are superconducting, which means that they can maintain a high electrical current with zero electrical resistance. To be superconducting, the LHC magnets must be chilled to almost minus 460 degrees Fahrenheit.

The short circuit ocurred between a superconducting magnet and its diode. Diodes help protect the LHC’s magnets by diverting electrical current into a parallel circuit in the event of a quench, which is when the magnets lose their superconductivity and change from a superconducting to a conducting state.

When teams discovered the problem, all eight sections of the LHC were already cooled to operating temperature. To fix the problem, they knew that they might have to go through a weeks-long process of carefully rewarming and then recooling one section.

The short circuit was caused by a fragment of metal caught between the magnet and the diode. After locating the fragment and examining it via X-ray, engineers and technicians decided to try to melt it. They could do this in a way similar to blowing a fuse. Importantly, the technique would not require them to warm up the magnets.

They injected almost 400 amps of current into the diode circuit for a few milliseconds. Measurements made on March 31 showed the short circuit had disappeared.

Now the teams must conduct further tweaks and tests and restart the final commissioning of the accelerator. The LHC could see beams as early as the week of April 6.

Image: Maximilien Brice, CERN

Image: Maximilien Brice, CERN