From issue 2671 of New Scientist magazine
27 August 2008, page 31-33
Find out what the LHC could discover
To get the particles traveling at such energies in a ring requires steering so precise that it can only be provided by intense magnetic fields created by superconducting magnets. These operate without loss of power when chilled below a critical temperature. The SSC had not pushed the technological boundaries, though, opting for superconducting magnets cooled by liquid helium to a relatively tepid 4.5 K, which were already being used in other accelerators. This proved to be its undoing.
For a given radius of tunnel, the more energetic the particles, the more powerful the magnets need to be. Since the SSC's magnets were not powerful enough, the tunnel had to be 87 kilometers round. The cost of building a machine this big doomed the SSC.
The RF cavities and the magnets had to be compact enough to fit into the small-bore tunnel, while carrying extremely high currents.
This means it has zero viscosity and can slip through microscopic cracks. So the thousands upon thousands of welds in the plumbing had to be,
By the late 1990s, the most pressing concern was the massive cavern needed to house ATLAS, the 7000-tonne detector that will track the particles that fly out from the collisions. Among the most important of these are muons, heavier versions of electrons, and the way to measure their momentum is to bend their paths in a magnetic field.
Because of the LHC's power, these muons will be more energetic and faster moving than anything seen in previous colliders, so the magnetic fields have to be very strong. The stronger the field, the more the particles bend, and the more precisely their properties can be measured.
It's so big that the cavern's hydrostatic pressure causes it to rise rather like a bubble in water, albeit extremely slowly.
To cross-check the findings from ATLAS, a second catch-all experiment called the Compact Muon Solenoid (CMS) will hunt for the same particles using different technology. It has thrown up its own share of challenges.
This spelled trouble.
First, the engineers had to dig two 60-metre-deep shafts, one for elevators and one to lower the detector.
When they got there, they found that the area consisted of loose, gravelly moraine that was permeated by two aquifers, so they borrowed a technique from the mining industry known as ground-freezing. Miners rarely have to contend with the fast-moving water found at the CMS site, though. The workers drilled holes along the periphery of the shafts into which they sank 60-metre pipes.
For six months, they circulated brine at -5 °C through them. Then, for a month, they filled the pipes with liquid nitrogen at -196 °C. This created a 3-metre-thick retaining wall of ice that kept the groundwater at bay, while the workers dug the dry earth within and constructed the shafts.
The CMS team decided to outdo ATLAS when it came to the strength of its magnetic field and built one twice as strong. Key to its 10,000-tonne magnet are superconducting coils designed to withstand an outward force of 60 atmospheres generated by the magnet's 4-tesla field - about 100,000 times stronger than Earth's magnetic field. However, the technology was so advanced and varied that no one company, or country, could do the job.
So the magnet's coils were shunted around Europe on a journey that started in Finland and took in Switzerland, France and Italy en route to CERN.
It is crucial that the entire LHC and its detectors work from the word go, as repairing them once the system is up and running will be far from trivial.
To repair the LHC, for instance, it would have to be allowed to warm back up to room temperature, which takes about five weeks.
Afterwards, its 40,000 tonnes of magnets would need to be cooled back to 1.9 K, a process that takes another five weeks and requires nearly 10,000 tonnes of liquid nitrogen and 130 tonnes of superfluid helium.
Still, the engineers have done their bit. Now it's the scientists' turn.
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