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World's Largest Supercooled Magnet Activated
An anonymous reader writes to mention a C|Net article about the activation of the world's largest superconducting electromagnet. Switched on today at Geneva's CERN lab, the experiment is part of the Large Hadron Collider (LHC) project. The magnet, called ATLAS, worked on its first start up. From the article: "In use, the magnet will be used to bend the paths of particles formed from the collision of protons or lead ions accelerated to near light speeds in 27km diameter subterranean contra-rotating circular beams. The ATLAS experiment is one of five in the LHC, and engages 1,800 scientists from 165 universities and laboratories in 35 countries."
IANAPP (particle physicist), but bending the particles' path is often done to determine mass: heavier particles will be pulled off their course less than lighter particles, so they'll impact the detector in a different place.
The magnet needs supercooling because a huge magnetic field is easier to achieve with a superconductor than with a conventional magnet.
This was once featured on slashdot and for those confused, this is just a part of the world largest (longest) particle accelerator thing and one of the purposes of this huge facility is to generate small blackholes.
/ Spotlight/SpotlightATLAS-en.html
http://public.web.cern.ch/Public/Content/Chapters
We need to bend the particles path so we can measure its momentum. A charged particle in an magnetic field will have a radius of curvature inversely proportional to the magnetic field and proportional to its momentum, with opposite charged particles curving in different ways. The radius of curvature decreases as the magnetic field increases and increases as the momentum of the particle increases. So for very high momentum particles, the radius of curvature is very large so the particle travels in almost a straight line which makes it very difficult to measure the radius of curvature. Hence you increase the magnetic field to force the particle to "bend" more and make it easier to measure the amount of "bending". So you want as big as magnetic field as possible and at the moment superconducting magnets give the most powerfull fields.
Here, have a look at this picture of a particle physics event (not from ATLAS but CDF at the Tevatron but the idea is the same). Lines in the circle are particle tracks, the two pink ones are very high momentum charged particles (in this case electrons). Notice how they are straight. As such we dont have a very good measurement of their momentum. The other grey lines are low momentum particles as they bend a lot since the radius of curvature is small.
Why do we want to measure the momentum of a particle? Well the Higgs boson (if it exists) will decay to 4 muons (basically heavy electrons) (nb: the Higgs can decay to other stuff but for a heavy higgs this is the cleanest signature and will be how its discovered). You want to measure the momentum of these muons and from that you can measure the mass of the particle that produced them. If you get a lot of events at a certain mass above what you expect from background, you've just discovered a new particle, likely to be the Higgs.
> why do we need to bend the particles path
/ AskAnExpert/LHC-en.html
/ AskAnExpert/LHC-en.html
r
I am not a particule physicist, but the particules need be accelerated and are 'pushed' by the magnets before being collided, so they need to circulate many times around the accelerator in order to get sufficient speed.
"A beam might circulate for 10 hours, travelling more that 10 billion kilometres, enough to get to the planet Neptune and back again. At near light-speed, a proton in the LHC will make 11 245 circuits every second."
What is the LHC power consumption?
It is around 120 MW which corresponds more or less to the power consumption for households in the Canton (State) of Geneva."
http://public.web.cern.ch/Public/Content/Chapters
> why does the magnet need to be super cooled?
To magnets are used also to maintain the beam within its path, and the requires huge amount of energy to create a magnetic field that is strong enough to prevent the beam to escape. These magnets are using a massive amount of power, and must be cooled down (a lot) do reduce their electrical resistance down to supraconductivity.
"In order to cool the magnets down to -193.16 C (pre-cooling), 10 080 tonnes of liquid nitrogen will be used. Afterwards, the refrigerators turbines will bring the helium temperature down to -268.7 C and fill the magnets with almost 60 tonnes of liquid helium. Once the magnets are filled, the refrigeration units will bring the temperature down to -271.3 C by lowering the saturation pressure - and therefore the temperature - of the liquid helium in a heat exchanger in contact with the static pressurized helium of the magnets' cold masses."
http://public.web.cern.ch/Public/Content/Chapters
http://en.wikipedia.org/wiki/Large_Hadron_Collide
For reference, the LHC will also use a massive computing Grid: http://www.cern.ch/LCG/
Romain.
It might have been an inherrent property of all particles (except the massless photon and gluon), but it turns out that the nature of the weak force (normally known from beta decays of nuclei) conflicts with this.
The real understanding of this problem requires knowledge of Quantum Field Theory, but the gist of the problem is as follows:
All known matter particles (fermions) as well as the particles that mediates the weak force (the W and Z) behaves in experiments as if they have masses. However, if they actually do have masses the theory breaks down (it becomes non-renormalizable, and gives non-sensical answers such as "that decay have a branching ratio of 500%". It becomes a bit like sports-commentators, I guess).
The proposed solution to this conundrum, and the one the LHC and ATLAS will try to verify, sounds kind of like a lawyer finding a legal loophole when you first hear it. In essence it is: "All particles are really massless, but some of them behaves as if they have mass". The way to accomplish this is by the so-called Higgs Mechanism, in which particles acquire masses the same way that a light-weight guy walking in a waist-high pool will feel as much or more difficulty walking as a really fat guy walking on dry ground: All particles move around in a soup of Higgs particles and thus acquire the appearance of being massive due to their interactions with this Higgs-soup.
I thought it was kind of cheesy back when I first heard about it, but later I realised that similar effects already are known to happen elsewhere in nature, which kind of makes it more acceptible (for instance, those familiar with the Meissner effect for superconductors might recall how the otherwise massless photon acquires the appearance of mass inside superconductors due to the presence of a soup of electronic cooper-pairs).
But we will have to see when the LHC starts!
ps. I am actually a member of the ATLAS collaboration. Go magnets!
You seem to suggest the ATLAS magnet is used to contain the particles within the accelerator itself, which is not the case.
The accelerator does use magnets to contain the particles, just not this one.
The ATLAS experiment is one of the detectors which use the output from the accelerator.
The electromagnet not used to hold anything together. The energy is just "stored" in the coils and when you remove the power supply the field dissipates. Now, should you short-circuit the coils - that would be interesting.
There is about 1 GJ of energy stored in the magnet when it is at full strength. I don't remember my TNT converstions, but admitedly that is a lot. The energy is disapated through resistors and that heat is dumped into a LOT of mass all while actively cooling everything. Here is a pretty picture of the current as a function of time during the test (notice how fast it was shut down) http://jenni.web.cern.ch/jenni/BT.9Nov06.jpg/ The axis are in amps and minutes by the way. And yes, that is ~20,000 amps. As another intresting LHC note, the magnets in the accerator store ~11GJ of energy which is disapated into something like 50 tonns of steel. This is (breaking out the obscure unit conversions) the energy of something like 40 bullet trains traveling at full speed, or a nuclear aircraft carrier traveling at full speed. The energy stored in the actual beam of protons is also not anywhere near negligible and systems had to be designed to dump all of this energy as well.
Actually No, particle accelerators such as this are called cyclotrons and their ring shape is such that any particle placed within them can orbit a central point until the deisered velocity can be reached by proceeding through the same series of magnets over and over again, often close the speed of light. This is opposed to a cheaper and simpler liniar accelerator, which shoots particles down a long and stright tunnel.
This is because we already know the mass of the particles (such as molecular and atomic ions and hardrons) being used in the accelerator, and other means are used to dertermine the mass of the particles resulting from the collisions such measirng the radius of curves made in bubble-chambers close to a collison point.
This is also why it is called a Hadron Super Collider.
No, not in this case (good try but you're thinking clasically). This may be true at low speeds but the particles detected in these experiments that are of interest are relativistic and are moving at signficant fractions of the speed of light (about 0.999c or there abouts) and as such for all intents and purposes have zero mass. These particles are often the decay products of more massive particles. The reason we want to bend them is to measure their momentum (or spectifically their transverse momentum) NOT their mass. We can then add together the momentum of these particles to obtain the momentum of the orginal particle from which we can get its mass using special relativity.
Just your friendly neighbourhood particle physicist.
Actually, no. The LHA is not a cyclotron. In a cyclotron, the particles travel in a spiral, in an area sandwiched between two huge electromagnets. The size of the magnet limits the size of the cyclotron.
The LHA consists of a tube running through a series of magnets, a bit like a linear accelerator. The tube is bent into a circle so you can have the particles do multiple laps around the accelerator to increase their energy.
How's this for size? ATLAS calorimeter, the tunnel, one of the tubes, the "crab", the hole, and the cavern. Bonus: They do have retina scanners!
http://atlas.ch/webcams.html With images from when they began.
That explains it! There I was, walking around in my suit of armor, when suddenly, WHAM! Stuck against the wall! And now, every time I pass the kitchen, the silverware shoots out at me!
Who would win this election: Andrew Weiner vs Andrew Weiner's weiner.
I was wondering what the magnetic field strength of this magnet would be, but the FA is a light on details. But there's a pamphlet!
Peak field strength for the barrel toroid magnet is 3.9 Tesla. And apparently it will take 30 days to cool the thing down with liquid helium to operating temperature.
I received email from Dr. James Gillies of CERN on this:
Dear Gregory,
I believe that the field of the ATLAS magnet is around 2 tesla, but the volume is vastly larger than an MIR magnet. Another experiment at CERN, which has a smaller volume than ATLAS has a field that reaches 4 tesla. In the LHC particle accelerator itself, the field in the magnets is around 9 tesla.
Regards, James Gillies
This gets modded informative??
You are completely missing the point. This magnet is not part of the accelerator, it is part of a *detector*. And inside it, the radius of the curve a particle describes (together with timing data) is used to determine its mass.
Superconductivity is needed to be able to achieve the huge currents used to build up the magnetic field in limited space - and to keep energy losses (and heating!) due to resistance inside the detector as low as possible. The energy stored in the magnetic field is enormous, a breakdown of superconductivity (which hopefully will never happen) would melt "fuses" consisting of massive copper blocks.
And yes, IAAPP at CERN, working on the CMS, another detecor there.
I believe the wires mentioned in step 1 lead to a small header coil that heats a very small section of magnet wire just above the critical temperature. The resistive load is then the section of magnet wire itself - which now is resistive. However, you wouldn't want to use a "large" resistive load. Standard NMRs run hundreds of amps so you don't want much resistance at all.
If you generate too much heat the situation goes exponentially out of control as the entire magnet rapidly heats up (as each portion of the wire heats it becomes resistive and generates even more heat). Almost instantly you vaporize all the coolant, and a hundred gallons of liquid He displaces a LOT of air when this happens. Basically you have to run for the door and hit the emergency ventilators fast, or you will collapse in a few 10s of seconds.
Haven't seen a quench firsthand but I know folks who have and it is quite an impressive site. I did walk past an NMR lab after a weekend quench and all the computer monitors had psychedelic color schemes.