Friday, December 9, 2005

Quenchers, Part II

Monica, Andrew and I went down to Fermilab on Thursday of last week. Remember when I said I didn't think this was such a big deal? Well, it was a bigger deal than I thought.

Our friend Bob Mau, whom we crossed with our tardiness a year ago, was on vacation. Dan Johnson was filling in, and he was extremely helpful. He set up some interviews for us (!) and generally helped facilitate everything. I now know a little more about what a quench is.

Here's an overhead of the Tevatron:


As I've written before, this thing is 4 miles in circumference, and it's all underground (the circular "river" than runs above it is to help cool it off).

Here's what it looks like underground, inside the tunnel:


And this:

view of Tevatron in A sector of main ring tunnel

See that red part they're looking at, followed by the yellow part? That's the actual pipe (OK, so it's square) that the protons and anti-protons are running through. You can see that the tunnel is curving way back in the distance --- if you wanted to, you could put on your track suit and start jogging --- four miles later you'd be back in this same spot.

How do you get protons and anti-protons to go in a circle? You use magnets --- lots of very large, very powerful magnets --- spaced every few feet or so. These magnets give the protons and anti-protons a little nudge, guiding them in a circle. Imagine you're at a circular race track, and you've rigged the outer guard rail with a magnet every 6 inches, all the way around the track. You load up your rifle with a steel bullet, aim it along the guard rail and pull the trigger. If you've done your math correctly, each magnet will nudge the bullet as it passes, making it curve a little more, until it goes all the way around the circle. You'd have to jump out of the way or the bullet would hit you in the back. These magnets are the key to understanding what a quench is.

These magnets use electricity, and electricity tends to make things hot. In order to work properly, the magnets have to be really, really REALLY cold. This means the electricity works without resistance. Think of it like this: let's say you were riding in a car on a highway. Maybe you're not too bright, and you decide to open your car door and stick your hand down on the road. All that friction is going to burn up your hand in a hurry. That's what happens when electricity runs through things --- it meets resistance, and it heats up (that's why a light bulb gets so hot). Now, imagine that you are on a perfectly slick, icy road (ignore for the moment that your car would probably end up in a ditch). When you stick your hand down, it slides along without the slightest trouble. Your hand doesn't heat up at all. That's the basic premise --- that's how they keep the magnets working.

How do they keep the magnets so cool? Liquid helium, my friend, liquid helium.

Detector machinery

But liquid helium is tricky stuff. What happened last Monday is that a bit of liquid helium insulation failed and let some of the liquid helium heat up. When it gets a little warmer, it changes from liquid to gas (just like water does). The only problem is, a gas is a lot more volatile than a liquid. When it started turning into a gas, it expanded by a factor of about 700. Have you ever put a frozen dinner in the microwave and forgotten to poke a hole in the plastic wrap? It expands and pops because the water turns to steam. If it has no where to go, it busts through whatever is holding it. Ditto for liquid helium, but more so. Suddenly finding itself 700 times bigger, the helium busted through the pipes like the Incredible Hulk bursting out of his shirt. This caused that particular magnet to suddenly overheat and stop working, and the protons and anti-protons didn't get the nudge they needed and sprayed into the wall instead of going around the circle.

Alarms went off, people choked on their coffee, warning lights started blinking across the board, and the whole thing shut down. And I mean THE WHOLE THING shut down. Both detectors, everyone taking data, several hundred people did a collective "huh?" and all operations ground to a halt. No one was in the tunnel, of course, since there is too much radiation during operation, but I asked one of the engineers what someone would have seen if they had been standing right there.

"There would have been a loud bang," he said, "and insulation being blown apart like confetti." Later, once we had gotten inside the tunnel, one of the mechanics showed us the insulation --- it looked like that kind of high-tech tin foil you see on NASA satellites. He said that stuff was all over the floor.

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