In June
CERN director general Robert Aymar announced that the $7 billion Large Hadron Collider
would start up in May 2008, eight months later than planned. The delay was no surprise to accelerator
and particle physicists, and it was generally blamed on a highly publicized failure related to
magnets made at the US's Fermilab. Actually, the magnets only added to other complications at the
LHC.
"We've had to adjust the schedule to
take into account the problems we've had," says LHC project leader Lyn Evans. The current plan is
to begin engineering trials at 900 GeV next May and then ramp up to 14 TeV by mid-July.
Even with the delays, Aymar says that for physics experiments, "the startup date remains exactly
the same—July 2008."
The LHC was first proposed
in 1984, and installation began in 2000 after an earlier accelerator, the Large Electron–
Positron (LEP) Collider, was shut down. To save costs the LHC uses the same tunnel 85 meters below
the surface and some of the same equipment as the dismantled LEP.
The LHC will be able to study
the inner structure of matter on the smallest scale yet, says CERN theoretical physicist John Ellis,
and should help answer questions such as why quarks, leptons, gauge bosons, and neutrinos have
mass, and also explain the asymmetry between matter and antimatter.
Industrial contracts
for the LHC were first awarded in 1994. "Thirteen years is not long for a project of this magnitude,"
says Evans, "but it is a long time for industry, and we've had to face issues such as companies going
bankrupt and litigation, issues that have given us problems in four or five instances. I'm very
happy now to have all of the hardware at CERN, and it's now in our hands."
In the LHC, protons will be accelerated
in two counterrotating beams. At four points along the ring, the protons collide; one of four detectors—ATLAS,
the Compact Muon Solenoid (CMS), LHCb, or ALICE (see page 90 of this issue)—sits at
each collision point. The LHC uses an unprecedented 6000-plus superconducting magnets to position
and focus the beams. "We did a lot of prototyping work to validate our design principles before building
the machine," says Evans.
The LHC delays were initially
blamed on the 24 focusing magnets provided by Fermilab that were designed to squeeze the proton
beams together to create collisions. In March, during a pressure test while cooling a magnet in
vacuum to 1.9 K with liquid helium, "there was an unbalanced longitudinal force that was
not taken account of in the design that was the equivalent of about 15 tons at the peak of the pressure
test. The structure supporting the magnet in the cryostat broke and the magnet lurched forward,"
says Fermilab's Peter Limon, who helped commission the magnets. The pressure test, he adds, "is
at a higher pressure than we would see operationally."
Engineers at first thought
the magnets had to be removed from the LHC tunnel and brought to the surface for repairs, which would
have meant significant delays to the project. Instead, says Limon, the support beams are being
fixed in place. Four metal cartridges are being fitted to each of the affected magnets. The extra
bracing transmits the force to the floor and should stabilize the magnets when the LHC is operational,
he explains. (See PHYSICS TODAY's website for a video demonstration.) The cost of the repairs and
what Fermilab and CERN will each pay are still being negotiated.
While fixing the Fermilab
magnet supports, workers discovered that some of the beam supports around the ring-shaped tunnel
were not up to safety specifications. Extra braces are being installed. "The only remaining safety
hazards in the tunnel are the bicycles," says Limon. "They are too quiet and can sneak up on you."
A leaky schedule
Among the other things CERN has had to
fix was a leak in a vacuum pipe. As part of the final preparations for start-up, sections of the vacuum
pipe are being baked to remove foreign materials. Cooling a section of pipe to near absolute zero
in vacuum and then bringing it back to room temperature can take more than six weeks. In a recent section
test, a leak required time-consuming repairs. CERN has also experienced problems with regulating
the compressors that cool the machine to 1.9 K. Says Evans, "Some sections have now been cold
for three or four months and are working perfectly, except for a few of these teething problems."
In addition, researchers
working on the CMS experiment discovered a noise problem in the photomultipliers when they observed
cosmic rays as part of a test. "Fortunately, [the noise problem] disappears when we reach our operating
standards, although we have no idea what is causing it," says Pawel de Barbaro of the University
of Rochester. The delayed schedule is proving advantageous to the experiment teams, especially
ATLAS and the CMS, says Evans.
But perhaps the LHC's weakest
link will prove to be the initial injector, which fires particles into the ring and is a relic of LEP
and earlier machines. The injector is old, says Evans. "It's very flaky and needs a major investment
and refurbishment or it needs to be replaced." Aymar agrees and says that his concern is the lack
of spare parts for the machine. Construction of a new $150 million injector will start in
2012 when CERN finishes paying off the loans it took out to build the LHC (see PHYSICS TODAY, December
2001, page 21). Says Aymar, "Once the LHC is up and running, building a replacement injector will
be a high priority."
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