A train for the bus(bars)

On 8 April, the first SMACC (Superconducting Magnets and Circuits Consolidation) teams began work in the LHC tunnel. They are responsible for opening the interconnects between the magnets, laying the groundwork for the series of operations that will be performed by the team riding the ‘consolidation train’.

A technician installs the machine tool that allows them to prepare the surface of the section of the bar where the shunt will be fixed.

The LHC’s 1,670 dipoles and quadrupoles are powered by power converters and connected by copper 'busbars’. The superconducting cables run through these bars, carrying a current of up to 11,850 amps. Six superconducting cables meet at each interconnect, where they are held together by a soldered (see box) electrical connection sandwiched between two pieces of copper, forming the splice between the busbars of the neighbouring magnets. The integrity of the electrical circuit is dependent on the quality of these solders; a weak solder can cause a discontinuity leading to an electric arc, which can have serious consequences.

In order to avoid this kind of problem, the SMACC project was launched in 2009. Its main objective is to install a shunt – a small copper plate 50 mm long, 15 mm wide and 3 mm thick – on each splice, straddling the main electrical connection and the busbars of the adjacent magnets. In this way, should a quench occur in the superconducting cable, the current will pass through the copper part, which must therefore provide an unbroken path.

“Several teams will be working on the chain, each occupying a car in the ‘consolidation train’,” explains Frédéric Savary from TE-MSC. “Each team comprises several technicians so that they can work on a number of interconnects in parallel. The team at the front of the train will open the lines containing the electrical connections by using special machines to cut the welds of the cylindrical sleeves that form the mechanical and hydraulic links between two magnets.” The team in the next car will then remove the electrical insulation that protects the circuit before using a machine tool developed by the MSC Group to prepare the surface of the section of the bar where the shunt will be fixed. In total, more than 27,000 shunts will have to be put in place, an average of one every three minutes.

The copper bars between two magnets at an interconnection.

The engineers have performed full-scale tests to verify the efficiency of the shunts. “We tested a set of two magnets connected in series at 4.5 K on a measurement bench in SM18,” Savary tells us. “We intentionally used poor connections between the busbars of the two magnets, leaving a gap of 8 mm between the copper parts. We put the shunts in place and then ran a current of 14,000 amps through the circuit - that’s much higher than the LHC's nominal current of 11,850 amps - and we caused quenches.” Everything went just as expected: the current successfully passed through the alternative route created by the shunt.

by CERN Bulletin