By Paul Rincon
Science reporter, BBC News
Cern has spent about 40m Swiss Francs (г24m) on repairs to the LHC
Engineers hope an early warning system being installed at the
Large Hadron Collider
could prevent incidents of the kind which shut the machine last year.
The helium leak last September, which resulted from a "faulty splice" between magnets, has delayed the start of science operations by more than a year.
Officials aim to re-start the collider, known as the LHC, in mid-November.
The vast physics lab is built inside a 27km-long circular tunnel straddling the French-Swiss border near Geneva.
The LHC will send two beams of particles crashing into each other at close to the speed of light. Scientists hope to see new particles in the debris of these collisions, revealing fundamental new insights into the nature of the cosmos and how it came into being.
This LHC "ring" is split into eight distinct parts, or sectors. Six of these are now at or close to their operating temperature of 1.9 kelvin (-271C; -456F).
Engineers are powering up magnets in three sectors to prepare for the injection of proton beams into the ring.
A low-intensity beam could be injected into the LHC in the second half of October, officials told the BBC.
This beam test would involve only parts of the collider, rather than the whole "ring". If all goes to plan, the first beam collisions could occur before the end of the year.
collider has been shut down since 19 September 2008
, when a magnet problem called a "quench" caused a tonne of liquid helium to leak into the LHC tunnel.
An investigation carried out for the European Organization for Nuclear Research (Cern), confirmed the cause of the accident was an electrical fault in one of the splices, or "interconnects", linking two of the 1,200 "superconducting" magnets that accelerate particles around the LHC.
Superconductivity is the property, exhibited by some materials at very low temperatures, to channel electrical current with zero resistance and very little power loss. A quench occurs when part of a magnet heats up, causing its superconducting properties to be lost.
Engineers have been making major upgrades to the system designed to protect hardware against these events.
They have had to install hundreds of new detectors around the machine.
Among other things, the upgraded quench protection system is expected to improve monitoring of the interconnects between magnets.
Gianluigi Arduini, deputy head of hardware commissioning for the LHC, told BBC News: "It will allow us to constantly monitor the status of the interconnections. If there is any deterioration detected by the system, the powering of the magnets will be automatically stopped, preventing any damage."
James Gillies, Cern's director of communications, said this would prevent the kind of damage which occurred in 2008. He told BBC News: "Last year, we didn't see this thing coming."
With the improved warning system, he added: "We would be looking at downtime of a matter of weeks, rather than a year... we're in a much better place than we were 12 months ago."
Mr Arduini said the upgraded system was currently undergoing testing in one of the LHC's sectors.
Cern has spent about 40m Swiss Francs (£24m) on repairs to the LHC, including upgrades to the quench protection system.
When the collider finally re-starts at the end of this year, it is expected to do so at about half its intended energy.
The LHC was designed to run at energies of seven trillion electron volts. But the machine will clash together protons at energies of just 3.5 trillion electron volts in its first few months of operation.
Two issues stand in the way of Cern achieving its goal of seven trillion electron volts in the immediate future.
Firstly, engineers have discovered the collider has hundreds to thousands of faulty electrical splices between magnets.
"This limits the amount of current you can safely put in," said Mr Gillies.
Officials have decided to put off repairing all the faulty splices. Instead, when the collider re-starts in November, engineers have been told not to exceed a maximum "safe" limit of five trillion electron volts.
Secondly, tests uncovered concerns about a number of the magnets themselves.
All LHC magnets undergo a "training" process, in which engineers crank up the electrical current until the magnets quench.
After a few iterations, a stable configuration is reached where the magnet can reliably operate at the current it was designed for.
The magnets had been trained to their operating current before being lowered into the LHC tunnel. But once connected underground, some of them were found to have "lost" their training.
"We couldn't get up to the current necessary for operating at seven [trillion electron volts] at the first try," said Mr Gillies.
When the collider re-starts in November, engineers will hope to get the LHC up to five trillion electron volts before the machine goes into its planned downtime again in November 2010.
They will then subsequently attempt to re-train magnets and possibly replace a proportion of the splices to ready the LHC for a push towards seven trillion electron volts.