Measurements by system experts have located the fault to within 10 cm by injecting current locally and using the standard cold mass instrumentation, which includes voltage and current taps. Each dipole of the LHC has a diode stack situated in a box under the magnet. The diode provides a path for current in the event of a quench. The fault is located in the vertical tube that leads from the magnet enclosure to the diode box. The most probable scenario is that a small piece of metal has found its way into this tube and  made contact between the tube (earth) and one of the cables that leads to the diode.

Engineer Aline Piguiet carefully aligns equipment to take an X-ray of the shorted superconducting magnet in sector 3-4 of the LHC tunnel (Image: Maximilien Brice/CERN)

Engineer Markus Albert checks the angle of alignment of the X-ray equipment with the shorted superconducting magnet in sector 3-4 of the LHC tunnel (Image: Maximilien Brice/CERN)

To further understand the cause of the short circuit, engineers X-rayed the affected dipole.

X-rays of the area containing the fault are as yet inconclusive (Image: Maximilien Brice/CERN)

The operations team explored three main options to fix the short: inject a controlled pulse of current to try to melt the offending object; try to dislodge the object by altering the flow of helium in that region; partially warm up the sector and open the magnet interconnect concerned. Though the third option would allow direct access to the diode box, the warm-up, intervention, and subsequent cool-down would take around 6 weeks.

LHC restart back on track

Yesterday, the teams working on the Large Hadron Collider (LHC) resolved the problem that had been delaying the restart of the accelerator. A few days ago, a short circuit to ground occurred in one of the connections between a magnet and its diode. These diodes are part of the protection system for the LHC’s superconducting magnets: they divert the current into a parallel circuit in the event of a quench, i.e. when the magnet changes from a superconducting to a conducting state.

During the training of the magnets for a beam energy of 6.5 TeV, a metal fragment became stuck in the connection, creating a short circuit to ground and preventing the diode from operating correctly. After having located the fault and carried out precise measurements, the accelerator teams decided to melt the metal fragment, in a similar way to blowing a fuse. Yesterday they injected a current of almost 400 amps into the diode circuit for just a few milliseconds, in order to make the fragment disintegrate. And it worked! Measurements made today showed that the short circuit had disappeared.

A full assessment is on going, and a revised schedule for commissioning the whole machine will be announced as soon as it is known. Whatever the case, the impact on LHC operation will be minimal: 2015 is a year for fully understanding the performance of the upgraded machine with a view to full-scale physics running in 2016-2018.

“All the signs are good for a great run 2,” said CERN Director-General Rolf Heuer. “In the grand scheme of things, a few weeks delay in humankind’s quest to understand our universe is little more than the blink of an eye.”