‘Subtle defect’ worry at LHC

By Hamish Johnston

Something to worry about: brown bars are the copper stabilizers

A few weeks ago Matthew Chalmers reported that Higgs-hunting particle physicists will have to wait until 2013 before the LHC reaches its maximum energy.

The nuts and bolts of why are described nicely in a new paper called Superconductivity: its role, its success and its setbacks in the Large Hadron Collider of CERN, which has just been published in Superconductor Science and Technology.

The paper — by CERN’s Lucio Rossi — includes a section called “Incident in sector 3-4” that describes in great detail the accident that took down the LHC in 2008. You can read exactly how an electrical connector failed — vaporizing a length of superconducting cable, heating the surrounding helium, which surged along the beam line wreaking havoc.

Could a similar disaster strike again? Rossi appears to say yes — “…a more subtle defect, related to a lack of continuity in the copper stabilizer, is now evident and is worrying since it is diffused around the machine”.

These copper stabilizers are part of an electrical circuit used to divert current from the superconducting cable if a fault occurs. Rossi is worried because at some connectors there is insufficient electrical contact between the cable and stabilizers — and between adjacent stabilizers. In the diagram above this trouble spot is labelled “Gaps with lack of Sn-Ag filler”.

If such a splice between cables fails — as one did in 2008 — the adjacent cable is heated and is no longer superconducting. If the cable is bare and there is a gap between stabilizers, current is forced to flow through the cable causing it to melt in a matter of seconds.

While the LHC has implemented a new system for detecting bad splices before they can cause damage, Rossi says that the gaps at the connectors could themselves be a problem. Warm helium from a minor problem elsewhere could, for example, heat the connector — triggering a similar disaster as occurred in 2008.

LHC scientists have devised a way of finding such gaps — but it works best when the LHC is warm. Half the accelerator was warmed up in 2008 for repairs and gaps were found and fixed. Most of the other half, however, was kept at 80 K and could not be thoroughly tested and repaired. As a result, Rossi believes that several gap defects could remain in the accelerator.

The upshot is that CERN will run the LHC in 2010 at the lower energy of 7 TeV, hoping that the connectors will hold. Then the accelerator will be shut down in 2012 for a year so all 10,000 connectors can be replaced. Finally, in 2013 protons in the LHC will collide at 14 TeV.

But the paper is not all bad news — it also describes how the LHC is at the pinnacle of superconducting technology. Here are a few superlatives:

– The accelerator has nearly 10,000 superconducting magnets

– The magnets are cooled by 130 tonnes of helium held at 1.9 and 4.2 K

– The accelerator contains about 15,000 MJ of magnetic energy

– 1200 tonnes of Nb-Ti superconducting cables were used to wind the magnets

– There is a 0.01% variation in field quality among the 1232 main dipole magnets