An Engineer in Wonderland – You can’t just turn off CERN

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I have just read an article by those folk at the US Fermilab in their magazine Symmetry. They are collaborating with European physicists at the Large Hadron Collider (LHC) in CERN, Switzerland, and some of the numbers they reveal are mind boggling.

Apparently, when everyone goes home at night, they can’t just turn the LHC off as it has two proton beams travelling around in opposite directions, each with as much energy as a train doing 120 miles an hour.

Letting them go off at a tangent would result in a hole “tens of metres long in any material”

Instead, two sets of magnetic points direct the beams through spreading magnets, which turn the beams into cones.

Some distance away, where energy density has been divided by 100,000, the proton cones hit cylinders of graphite composite, each 1m in diameter, 8m long, and encased in concrete.

The energy density in the cones is not constant, but where it peaks the graphite heats up to 700°C in 80µs.

If the beams have a lot of energy – 362MJ – the associated magnets have a colossal amount of energy – 10GJ – mostly in 1232 superconducting dipole magnets.

Whenever a patch on a superconducting magnet gets warm enough to become resistive its energy has to be dumped – if for example LHC is cranked up to high and a few particles get loose and hit it. According to the article, around 500 billion particles escape from the beams and collisions every 10 hour operating session.  

If I understand correctly, when this happens the beams are immediately diverted into their graphite blocks and power to the affected 35 tonne magnet is cut. Then heaters raise the temperature of the whole magnet to dissipate its power, presumably, evenly.

Every magnet is apparently connected to 153 neighbours, so they also need to be shut down which involves switching their energy into an eight tonne resistor which heats to 300°C over the two minutes this shut down takes.

This really is big science.

By the way, re-start takes from 30 minutes to five hours.

‘Alice’

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