Like hams in a smokehouse

Hall SM18 is now fitted out for testing magnet prototypes developed for the successors of the LHC. Testing is conducted vertically, for greater convenience.


A Nb3Sn magnet (the aluminium-coloured cylinder in the image) suspended from one of the Block 4 cryostat lids, ready for testing.

A few months ago, a new work area started to take shape in hall SM18. Intended for studies on magnet prototypes being developed for future upgrades of the LHC, the work area has three vertical cryostats, recovered from the former test area on the Prévessin site known as “Block 4”. What we are doing here is research and development, unlike the horizontal test benches, which check magnets that are already operational before they are installed in the LHC tunnel,explains Marta Bajko, head of the TF section of the Magnets, Superconductors and Cryostats group (TE department). Our work is very different, because we have to come up with a protocol of customised tests for the prototypes, some of which are being tested for the first time. As there are no benchmarks for the tests, new ways of interpreting the measurements and graphs we obtain will have to be devised.”

The MSC team will soon get a fourth vertical cryostat. For the time being, a large hole in the floor marks its future position; this is because the vertical cryostats are installed in pits. This one measures 2.1 m in diameter, and will be able to hold magnets up to 2.5 m long. The three cryostats already in place allow for the testing of magnets up to 3.8 m long. The way it works is that the magnet is attached to and hoisted by one of the cryostat lids, in preparation for insertion,” explains Marta Bajko. “All of the connections – including sensor wiring, electrical power, liquid helium connections and magnetic measurements – pass through the lid. From here, measurement data are sent to the control console.

One of the cryostats already in place in SM18’s new Block 4. The cryostat is half-buried, leaving only its upper part – which is used for connections – visible.

So why is it necessary to suspend the magnets vertically, like hams in a smoke-house? This is because horizontal testing is only possible on a fully assembled magnet, i.e. one that has its own helium cooling system and cryostat, which would add greatly to the complexity and cost. The advantage of vertical cryostats is that they are readily adapted to accept just the bare magnet of whatever type, i.e. without cooling system and its own cryostat, for study purposes. This solution is also effective in terms of energy consumption. Indeed, the magnets are suspended from the cryostat lids by thin steel rods, which limits the heat exchange between the outside air and the liquid helium.

Currently the MSC members are working on niobium-tin (Nb3Sn) magnets being developed for the HL-LHC project. Next March they should be ready to accept the first 11 T Nb3Sn prototype, which is being developed in collaboration with Fermilab (see the article on this subject in Bulletin 35-36/2012). If the tests are successful, the magnets could one day replace the niobium-titanium (NbTi) magnets currently used in the LHC.

by Anaïs Schaeffer