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Saturday, March 10, 2007

synchrotron light sources

I suppose there's a time when you should do something with what you've learned. You can't be a student your whole life! (Well, of course you can, and I fully plan to, but I should also do something useful.)

So. What am I doing? Well, I'm part of a group designing a third generation synchrotron light source: the NSLS-II.

Ok, so what is a synchrotron light source?

Check out this photo of a machine called the NSLS VUV ring:
See the brownish tube in the center going all the way around? (Just above the dark blue things, and with some yellow, orange and light blue things here and there.) Its a copper pipe, called the beam pipe and for this machine its 51 meters around. Electrons travel down that tube at close to the speed of light.

All right. Nice. Three questions:
1) How'd the electrons get there in the first place?
2) Why do they stay in the pipe and not slam into the walls?
3) Why on earth would we want to do this?

Question 1 is why this is really a branch of the field of accelerator physics. We need a particle accelerator to get the electrons going fast enough. The electrons in this machine have an energy of 800 thousand electron volts. That's the energy one electron would have after going through 800 thousand 1 volt batteries.

By the way, in case you forgot what an electron was... we're mainly made of them. They are negatively charged particles that orbit the nucleus in atoms.

The answer to question 2 involves the yellow, orange, and light blue things surrounding the beam pipe. They are magnets. The light blue one is a dipole magnet, meaning it has two poles, north and south. The magnetic field bends the charged electrons. So that's how we can get the bunch of electrons going in a circular shape. The problem is that with just the bending, the beam would be unstable- if it was just a bit off the "perfect" trajectory, it would very quickly die a copper death. So the yellow magnets are there. They are called quadrupole magnets, four poles. They are like lenses for the particles. The orange magnets are sextupole magnets, six poles. They're also necessary, but its a bit technical as to why.

Ok. So, on to question 3. Why?

Well, for those of you who have taken an electricity and magnetism class, you might remember that charged particles create electric and magnetic fields. And remember that light is just an oscillating electric and magnetic field? Look around you. Yep. That's what's hitting your eyes: electric and magnetic fields. Basically, whenever a charged particle changes state, it gives off light. So, the electrons running around the ring here are having their path bent by the dipole magnets and thus giving off light. Because these machines are also called "synchrotrons", the light is called synchrotron radiation.

So that's it. Get some electrons stored in a big circle and whenever they bend, they give off light. That ought to be good for something.

Actually, these machines, synchrotrons, were originally designed to smash particles together. The higher the energy the particle, the more interesting things that might pop out from the collisions. Unfortunately, the higher the energy, the more of this synchrotron radiation gets produced which saps energy from the system. So, originally synchrotron radiation was seen as a big nuisance! Until some biophysicist (and probably some others) came along and said: hey, we'll take the light! So they started running the particle smashing synchrotrons parasitically, using the synchrotron light for whatever they needed it for. It turned out that this source of light was so useful that it was worth building these machines solely to get the light out! Second generation light sources were redesigned so that the electron beam was more suited for radiation purposes rather than particle smashing purposes. Then, third generation light sources were designed where the beam was really allowed to bend. In fact these devices were put in that caused the beam to wiggle back and forth. They were called wigglers. They are also sometimes called undulators. Here's a picture of one of these beasts:

So that's what I've been up to. The work I do, mainly relates to Question 2 above.
In particular, now that we add these crazy undulators and wigglers to the ring, we have to really be sure that the beam will be stable. So we need to write some computer code that tracks the electrons around to see what happens. To do this, we need to understand Hamiltonian dynamics, a formulation of classical mechanics. In my own skewed world, this is the reason I got into this field, and I still actually find this aspect quite interesting! Remember the craze surrounding chaos theory? One of these days, I need to try to really understand the KAM theorem. Well, this one of the fields where some of the math was developed, or at least was a major recipient. There are still plenty of open problems here! As for whether quantum mechanics is relevant for these beams of electrons in these machines, the answer is that it is and it isn't (and it might be). But that's a story for another day.

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