CERN
power and cooling grids, all the systems
are operated from a single, purpose-built
control centre. It is a haven of blue-
carpeted calm where cataclysmic forces
are marshalled and focused. ‘We can have
50 or 60 people in here and it still feels
quiet,’ said Paul Collier, one of the LHC
engineers.
20
The beams run for most of their
circuit through two separate pipes about
20cm across, which are under a high
vacuum of 10 -13 atm. At regular intervals,
the beams pass through radiofrequency
cavities, which pump power in to acceler-
ate the particles, and a series of magnets
that generate a field of 8.3T (for compari-
son, the Earth’s magnetic field is about
40mT at the surface), steering the protons
around their near-circular orbit.
The amount of current needed to
maintain the field is huge, some 11,700A, so
the magnets are superconducting; they are
made from strands of a niobium/titanium
alloy. When cooled to below 10K, this
material conducts electricity with zero
resistance. In the LHC magnets, the cables
are cooled by liquid helium at 2.7K. At this
temperature, helium is in a state known
as a superfluid, which allows it to
conduct a large amount of heat and
therefore makes it an extremely efficient
refrigerant.
The superconductivity is essential
to reduce the amount of electricity
needed to run the magnets. ‘Even so,
we use about 30 per cent of the electric-
ity demand of the Canton of Geneva,’
‘We come from different schools of physics,
with different ideas, but both groups believe
they have the right answer’ Marzio Nessi, ATLAS
said LHC physicist Mike Lamont.
The total power consumption of the
LHC alone is 120MW, with CERN as a
whole consuming 230MW. ‘That’s one
of the advantages of being in both
France and Switzerland. The Swiss
Grid alone can’t supply all the electric-
ity we need; we actually get most of it
from France. Even then, glitches in the
power supply cause major problems —
the system can take weeks to settle
down properly after a glitch. We have a
large amount of power conditioning to
make sure the supply is smooth, but we
can still have problems.’
There are several types of magnet
used on the LHC. Most of them are
dipoles, but immediately before the
beams enter the caverns housing the
detectors, they run through a more
complex type of magnet called a
quadrupole. These squeeze the
diameter of the proton bunch down
from about a millimetre to about 16μm
and aim it so that it will collide with
the beam travelling in the opposite
the EnGIneeR 19 MAY–1 JUNE 2008
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