Page last updated at 18:04 GMT, Tuesday, 23 September 2008 19:04 UK

Collider halted until next year

By Paul Rincon
Science reporter, BBC News

LHC dipole magnets (Cern/M. Brice)
The incident was probably caused by a faulty connection between magnets

The Large Hadron Collider near Geneva will be shut off until spring 2009 while engineers probe a magnet failure.

The incident on 19 September caused a tonne of liquid helium to leak out into the experiment's 27km-long tunnel.

Officials said the time required to fully investigate the problem precluded a re-start before the lab's winter maintenance period.

The collider is built to smash protons together at huge speeds, recreating conditions moments after the Big Bang.

Scientists hope it will shed light on fundamental questions in physics.

"Coming immediately after the very successful start of LHC operation on 10 September, this is undoubtedly a psychological blow," said Robert Aymar, director-general of the European Organization for Nuclear Research (Cern), in a statement.

But he praised the skill and preparation of the teams involved in building the particle accelerator.

High priority

A spokesman for Cern told BBC News it was unclear at this stage when the collider could re-start operations after the lab's regular winter shut-down - which is partly done to save money on electricity during this period of peak demand.

A number of factors could affect when the lab re-opened, including prolonged cold weather.

Infographic (BBC)
Problem thought to have been caused by a quench in the connection between two magnets
A stretch of cabling called the bus bar was probably affected
The quench may have melted a connection in the bus bar, causing liquid helium to leak

"It's usually around late March or early April that we start re-commissioning the whole accelerator chain. The LHC being at the end of that chain," said James Gillies, Cern's director of communications.

"It will take us a while to get beams injected into the LHC, but I think it's fair to say this will be the priority for next year's start-up."

The accelerator chain prepares the beams of protons to be fired through the machine to make possible the collisions that physicists will use to study the make-up of our Universe.

The problem occurred last weekend, when a failure, known as a quench, caused around 100 of the LHC's super-cooled magnets to heat up by as much as 100 degrees.

The fire brigade were called out after a tonne of liquid helium leaked into the tunnel, which straddles the French-Swiss border.

Helium spill

The machine has more than 1,200 "dipole" magnets arranged end-to-end in the 27km-long, ring-shaped tunnel that houses the LHC.

These magnets carry and steer beams of protons which will whizz around the machine at close to the speed of light.

At allotted points around the "ring", these beams cross paths, smashing together near four massive "detectors" that monitor the collisions for interesting events.

Cern said the most likely cause of the equipment failure was a faulty electrical connection between two of the accelerator's magnets.

This connection melted during testing of the machine and caused a huge leak of super-cool helium.

This helium is used to chill the magnets to a temperature of 1.9 kelvin (-271C; -456F) - which is colder than deep space.

This makes the magnets "superconducting", allowing them to generate the large magnetic fields required to steer the beams while at the same time consuming relatively little power.

A quench occurs when part of a superconducting magnet heats up and causes superconducting properties to be lost.

Hot spot

Cern has procedures in place to deal with quenches before they damage equipment, but in this instance a hot spot in the machine got out of control.

"It does seem that all the systems that are supposed to protect the machine in cases like this worked as far as we can tell. But obviously something went wrong," said Mr Gillies.

"The engineers have decided that in order to find out what really happened, they are going to have to go into the machine."

One of the LHC's eight sectors will now have to be warmed up so an inspection can be carried out.

Mr Gillies told BBC News that this was likely to take a week, and that engineers would then have a much better idea of how to fix the fault.

Each particle accelerator is a unique machine, so Cern says that teething troubles were to be expected with such a complex machine at the cutting edge of technology.

"Events occur from time to time that temporarily stop operations, for shorter or longer periods, especially during the early phases," said Cern physicist Peter Limon.

David Shukman heads deep underground to take a look at the LHC's tunnel

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