[tt] NS: Large Hadron Collider: Extreme machine

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Large Hadron Collider: Extreme machine
http://technology.newscientist.com/article.ns?id=mg19926711.400&print=true

* 27 August 2008
* Anil Ananthaswamy

AT 27 KILOMETRES long, it is the largest machine in the world. It
will accelerate counterrotating beams of protons to within a whisker
of the speed of light and smash them head-on 600 million times a
second. The most distinctive feature of the Large Hadron Collider,
though, is its temperature. At 1.9 kelvin - a smidgen above absolute
zero - the LHC is the coldest ring in the universe, unless an alien
civilisation has built one that is colder.

Were it not for this searing cold, the LHC might have suffered the
same fate as the Superconducting Super Collider (SSC), which was on
its way to becoming the most powerful accelerator in the world
before the US government canned it in 1993. The SSC's partially
completed tunnel, near Waxahachie, Texas, now lies derelict - killed
by a ballooning budget and a deficit of innovation.

Keen to avoid a similar debacle, CERN, the European particle physics
lab near Geneva, Switzerland, took the momentous decision to cram
the LHC into an existing circular tunnel 100 metres underground,
which had been built in the 1980s for the Large Electron Positron
(LEP) collider. It was a decision that led to a carefully
choreographed dance of extreme engineering. Underground rivers were
frozen in mid flow, components shipped around the world and
superconductors pushed to their limits.

One of the tasks planned for the SSC ring was to search for the
Higgs boson, the particle through which the universe is thought to
get its mass. It would have meant smashing protons into each other
at energies of 40 teraelectronvolts (TeV). To get the particles
travelling 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 kilometres
round. The cost of building a machine this big doomed the SSC.

Anxious not to make the same mistake, LHC's engineers had to be
bold. By 1998, they had designed the LHC to fit into the old LEP
tunnel, which had been built for a collider with a peak collision
energy of 209 gigaelectronvolts. The LHC was aiming for 14 TeV,
nearly a 70-fold increase. It needed the next generation of
superconducting coils both for the supremely powerful magnets needed
to bend its high-energy proton beams around the tunnel's tight
curve, and for the radio-frequency cavities used to accelerate
protons. The RF cavities and the magnets had to be compact enough to
fit into the small-bore tunnel, while carrying extremely high
currents.

The designers went for coils made of niobium-titanium, the only ones
that could be made in the large quantities required by the LHC.
Generating the extra-strong magnetic fields for the machine meant
cooling the coils down to 1.9 K so that they could carry much more
current. This, however, came at a price. At that temperature, liquid
helium becomes superfluid, with weird quantum properties. 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 "at
least as good as those in a nuclear plant", says LHC project leader
Lyn Evans.

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.

ATLAS ended up with the world's largest superconducting magnet, by
volume. The cavern created to house this 12-storey-high behemoth had
to be 35 metres high. It's so big that the cavern's hydrostatic
pressure causes it to rise rather like a bubble in water, albeit
extremely slowly. "It moves about 0.2 millimetres upward every
year," says ATLAS spokesman Peter Jenni, so the floor had to be cast
5 metres thick to ensure that it doesn't warp as it rises.

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.

As protons collide inside the detectors, the aftermath can slightly
disrupt the path of other protons as they race around the ring. To
minimise this effect, the detectors have to be as far away from each
other as possible. So the CMS has been sited diametrically opposite
ATLAS, which puts it at the base of the Jura Mountains. This spelled
trouble. "It was the worst possible place from a civil engineering
point of view," says project engineer John Osborne. "The conditions
were terrible."

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.

Meanwhile, the CMS engineers were working on the world's most
powerful superconducting magnet - the detector's "pièce de
résistance", according to the experiment's spokesman Jim Virdee. 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 took eight
years to do that," says Virdee. "The coil was tested in 2006, and it
worked perfectly."

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. "It's like being in outer space," says Evans. "If
anything goes wrong, you can't just go in there and touch it."

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. Not surprisingly, "the quality control has been draconian on
this machine", says Evans.

Still, the engineers have done their bit. Now it's the scientists'
turn. "I'm really looking forward to the next few months, and to see
the physics coming out," says Evans.

The Large Hadron Collider - find out more about the world's biggest
experiment in our cutting-edge special report.

Related Articles

* Monsters of the universe 
http://technology.newscientist.com/article/mg18324625.700
* 28 August 2004
* The black hole
* http://technology.newscientist.com/article/mg18624961.800
* 23 April 2005
* Diverse career paths in physics: Lyn Evans
* http://technology.newscientist.com/article/mg19325942.400
* 10 March 2007
* LEP's long goodbye
* http://technology.newscientist.com/article/mg16822707.300
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Weblinks

* The particle adventure
* http://particleadventure.org/
* The safety of the LHC, CERN
* http://public.web.cern.ch/Public/en/LHC/Safety-en.html
* The Large Hadron Collider, CERN
* http://public.web.cern.ch/Public/en/LHC/LHC-en.html
* The Superconducting Supercollider Project
* http://www.hep.net/ssc/

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