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Superconducting Required to Obtain High Magnetic Fields without Consuming too Much Power
:: 21 July, 2008
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).
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.
About Large Hadron Collider
The Large Hadron Collider (LHC) is a particle accelerator complex intended to collide opposing beams of 7 TeV protons. Its main purpose is to explore the validity and limitations of the standard model, the current theoretical picture for particle physics. This model is known to break down at a certain high energy level.
The LHC is being built by the European Organization for Nuclear Research (CERN), and lies under the Franco-Swiss border near Geneva, Switzerland. The LHC will become the world's largest and highest-energy particle accelerator. It is funded and built in collaboration with over two thousand physicists from thirty-four countries as well as hundreds of universities and laboratories.
The collider is currently undergoing commissioning while being cooled down to its final operating temperature of approximately 1.9 K (−271.25 °C). The first particle beams are due for injection in August 2008, with the first collisions planned to take place about two months later.
When activated, it is theorized that the collider will produce the elusive Higgs boson, the observation of which could confirm the predictions and "missing links" in the Standard Model of physics and could explain how other elementary particles acquire properties such as mass. The verification of the existence of the Higgs boson would be a significant step in the search for a Grand Unified Theory, which seeks to unify three of the four known fundamental forces: electromagnetism, the strong nuclear force and the weak nuclear force, leaving out only gravity. The Higgs boson may also help to explain why gravitation is so weak compared to the other three forces. In addition to the Higgs boson, other theorized novel particles that might be produced, and for which searches are planned, include strangelets, micro black holes, magnetic monopoles and supersymmetric particles.
The collider is contained in a circular tunnel with a circumference of 27 kilometres (17 mi) at a depth ranging from 50 to 175 metres underground. The 3.8 metre diameter, concrete-lined tunnel, constructed between 1983 and 1988, was formerly used to house the LEP, an electron-positron collider. It crosses the border between Switzerland and France at four points, although most of it is in France. Surface buildings hold ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants.
The collider tunnel contains two adjacent beam pipes, each containing a proton beam (a proton is one type of hadron). The two beams travel in opposite directions around the ring. Some 1232 bending magnets keep the beams on their circular path, while an additional 392 focusing magnets are used to keep the beams focused, in order to maximize the chances of interaction between the particles in the four intersection points, where the two beams will cross. In total, over 1600 superconducting magnets are installed, with most weighing over 27 tonnes. Approximately 96 tonnes of liquid helium is needed to keep the magnets at the operating temperature, making the LHC the largest cryogenic facility in the world at liquid helium temperature.
About Superfluidity
Superfluidity is a phase of matter or description of heat capacity in which unusual effects are observed when liquids, typically of helium-4 or helium-3, overcome friction by surface interaction when at a stage, known as the "lambda point" for helium-4, at which the liquid's viscosity becomes zero. Also known as a major facet in the study of quantum hydrodynamics, it was discovered by Pyotr Kapitsa, John F. Allen, and Don Misener in 1937 and has been described through phenomenological and microscopic theories. In the 1950s Hall and Vinen performed experiments establishing the existence of quantized vortex lines. In the 1960s, Rayfield and Reif established the existence of quantized vortex rings. Packard has observed vortex rings directly, and Avenel and Varoquaux have studied the Josephson effect, in superfluid 4He.
Recently in the field of chemistry, superfluid helium-4 has been successfully used in spectroscopic techniques, as a quantum solvent. Referred to as Superfluid Helium Droplet Spectroscopy (SHeDS), it is of great interest in studies of gas molecules, as a single molecule solvated in a superfluid medium allows a molecule to have effective rotational freedom - allowing it to behave exactly as it would in the "gas" phase.
Superfluids are also used in high-precision devices, such as gyroscopes, which allow the measurement of some theoretically predicted gravitational effects.
Recently, superfluids have been used to trap light and slow its speed. In an experiment, performed by Lene Hau, light was passed through a superfluid and found to be slowed to 17 metres per second from its normal speed of 299,792,458 metres per second in vacuum. This does not change the absolute value of c, nor is it completely new: any medium other than vacuum, such as water or glass, also slows down the propagation of light in a certain fraction.
The Infrared Astronomical Satellite (IRAS), launched in January 1983 to gather infrared data was cooled by 720 litres of superfluid helium, maintaining a temperature of 1.6K (-271.4 °C).
In figure 1, Superconducting magnets are cooled down using liquid helium
In figure 2, The CMS detector will search for the Higgs boson - the so-called "God particle"
In figure 3, The Alice experiment recreates the conditions just after the Big Bang
Tags: Cern lab goes 'colder than space' , Large Hadron Collider , superfluid ,
