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ATLAS Week April 12, 2008

Posted by gordonwatts in ATLAS, LHC, physics.

ATLAS has just finished one of its large collaboration meeting. One of the nice things about these meetings is we get to hear a fairly detailed report on the machine status – something I don’t always hear except in rumors. In this case it was filling in some of the blanks that were in the recent press release explaining that the start up of the LHC would be at the reduced energy of 10 TeV instead of 14 TeV.

The problem is some of the dipole magnets. They have to be trained to run at full field. Full field for most magnets is 8 Tesla, which is about 133333 times stronger than the earth’s magnetic field. They have to be that strong in order to bend the very high energy 7 TeV beams of protons (magnets are to charged particles like protons what lenses are to light). The power requirements are stupendous (scientific term). In fact, they would probably melt if they were made out of regular copper wire. Instead they make them out a special wire that is superconducting when it is very cold. About -270 degrees Celsius.

The beauty about superconducting wire is that it doesn’t dissipate any energy of the current it is carrying. You know how an overloaded plug socket gets warm? That is because some of the current is converted into heat instead of being used to run your computer – a waste. When dealing with the currents in these magnets – well, it would be so hot that it would melt the magnet.

These magnets have a tendency to quench. Which is a problem. Lets say you have a bundle of wires all at -270 degrees carrying a huge amount of current. Lets say a flaw in one part of one wire causes it suddenly to loose its superconducting property. As a result the current flowing through that bit of wire starts to generate heat. That heat, of course, warms up all the wire around it, which causes it to “go normal” as well. This process rapidly cascades until the whole magnet ceases to be superconducting. This is called a quench. If not handled correctly this can be disastrous – you could melt the whole thing (and these things are expensive!). Part of the magnet design is quench protection.

Now, here is the cool thing. To get to their full field strength you have to train the magnets. This is particularly true when you are pushing the envelope of what the technology can do. You do this by slowly increasing the current in the magnet until it quenches. Once it has, you cool it down again and try again. And repeat.

This process is what will prevent the LHC from being ready to run at 14 TeV this year. The retraining of some magnets is taking too long (all the magnets were trained to full strength before they were installed – so some have become “untrained”). So their plans are to retain these magnets that are not properly trained over the first shutdown in the winter of 2008-2009.

And that, right there, tells us how long we will be running at the reduced energy of 10 TeV. If we are very lucky we will see beam in August and that will be our first run. So, probably a few months. Now, if I’m allowed to put on my old-guy hat, I’m going to guess that we won’t really get collisions until later than that and then the data coming out of our detector won’t make much sense until just around the shutdown. So it could well be this initial 10 TeV run gets almost no useful physics out – but is exactly what we need to get our brand spanking new detector into shape for the first real 14 TeV run.

BTW, I should say that the LHC has not told the experiments at what energy it will actually run yet. People think it will probably be 10 TeV, but the official word has not come from the machine division yet. Next week that should happen.

There were several other things of general note at the meeting (actually, there was a lot, but…). One thing is if you watched Peter Jenni’s talk – he gave out a few links you can go for status info. One has the current cooling status of the accelerator. I don’t think it is meant for everyone to look at, so I won’t post the link. But if you are member of ATLAS you can just look at Peter’s talk on the agenda server. The graphic is cool! I want to make it the background on my computer!

The other thing that, as a member of ATLAS, really makes this time exciting is the detection of cosmic rays. More and more detectors are getting turned on – and the first thing that is done with them is to look for cosmic rays. A few months ago people talked about the first cosmic ray having been seen. Now everyone in ATLAS is showing these things. Maybe this thing will work after all… 🙂



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