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Saturday, July 19, 2008

Cern lab goes 'colder than space'

LHC tunnel (M. Brice/Cern)
Superconducting magnets are cooled down using liquid helium

A vast physics experiment built in a tunnel below the French-Swiss border is fast becoming one of the coolest places in the Universe.

The Large Hadron Collider is entering the final stages of being lowered to a temperature of 1.9 Kelvin (-271C; -456F) - colder than deep space.

The LHC has thousands of magnets which will be maintained in this frigid condition using liquid helium.

The magnets are arranged in a ring that runs for 27km through the giant tunnel.

Once the LHC is operational, two particle beams - usually consisting of protons accelerated to high energies - will be fired down pipes running through the magnets.

These beams will then travel in opposite directions around the main ring at close to the speed of light.

At allotted points along the tunnel, the beams will cross paths, smashing into one another with cataclysmic force. 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.

The most powerful physics experiment ever built, the LHC will re-create the conditions just after the Big Bang.

Currently, six out of the LHC's eight sectors are between 4.5 and 1.9 Kelvin, though all sectors of the machine have been down to 1.9 Kelvin at some stage over the last few months.

By comparison, the temperature in remote regions of outer space is about 2.7 Kelvin (-270C; -454F).

CMS detector at end of 2007 (M. Brice/Cern)
The CMS detector will search for the Higgs boson - the so-called "God particle"

Roberto Saban, the LHC's head of hardware commissioning, said that in order to obtain high magnetic fields without consuming too much power, the magnets were required to be "superconducting".

This is the property, exhibited by some materials at very low temperatures, to channel electrical current with zero resistance and very little power loss.

Helium exhibits spectacular properties at 2.2 Kelvin - becoming "superfluid". This allows it to conduct heat very rapidly, making it an extremely efficient refrigerant.

No particle physics facility on this scale has ever operated at such low temperatures. But, so far, the hardware was performing as predicted, Roberto Saban explained.

"We have a very systematic process for the commissioning of this machine, based on very carefully designed procedures prepared with experience we have gathered on prototypes."

He added: "Our motto is: no short cuts? exchanging a single component which today is cold, is like bringing it back from the Moon. It takes about three to four weeks to warm it up. Then it takes one or two weeks to exchange. Then it needs three to six weeks to cool down again.

"So, you see, it is three months if we make a mistake."

Two sectors of the LHC are currently not cold enough for testing to proceed. Electronics that control the cryogenic systems in these sectors are being moved to an area where they will be better shielded against particles that shoot out of the machine during collisions.

Closing the circle

One sector of the ring is being run as if the LHC was operational and carrying a beam. This is so that crews can de-bug software and hardware and gain experience of running operating cycles.

The LHC's magnets must also undergo electrical testing. Each sector of the machine contains about 200 electrical circuits. Each circuit may consist of as many as 154 magnets or as few as one.

They are being tested for their ability to handle very high currents - up to 12,000 Amps .

"We power each circuit, making sure it goes to its design current. But above all, we are verifying that all the protection systems around it - which are there to detect an eventual quench - are operating as expected," said Roberto Saban.

A quench occurs when some part of the magnet starts to heat up, becoming resistant to electrical current. Engineers have built in a recovery system to detect these quenches before they affect the magnetic field bending particles around the ring and shut off the circulating beams.

The machine's cool-down should take another two weeks to complete, provided no serious problems are found. Electrical testing of the magnets may take another couple of weeks.

Before the LHC is "switched on" for the first time, the proton beams have to be boosted to high energies in a chain of particle accelerators called the injectors.

Once the machine is cold, operators will inject beams into the main ring, threading them through each independent sector of the LHC until they close the circle.

A timing, or synchronisation, system is used to ensure each of these sectors behaves as if they were a single machine.

When the LHC is switched on it will operate at an energy of five trillion electron-volts. It will then be shut down for the winter, so that the magnets can be "trained" to handle a beam run at seven trillion electron-volts.

Original here

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