Muon g-2: What's that thing for, anyway?

A natural question to ask in light of my recent post is what exactly the enormous Muon g-2 (remember, it's pronounced gee minus two) electromagnet is good for. But first, a point of clarification: the electromagnet wasn't on while it was being transported; as an electromagnet, it has to be plugged in before it becomes magnetic. And before we can really plug it in, we need to cool it down. A lot. Along with the superconducting coils, the cryostat for the experiment also couldn't be safely disassembled, so the two were transported together. With the aid of the cryostat and liquid helium, the temperature of the coils will be reduced to just 5 Kelvin (around -450 degrees Fahrenheit), a hair above absolute zero, which chills them enough to be able to superconduct; that is, conduct electricity with exactly zero resistance. It's a fascinating physical phenomenon that falls squarely in the realm of "post some other day," but suffice it to say that once we cool this thing down and plug it in (slightly more complicated than your standard wall socket, but the same general idea), it generates a pretty uniform magnetic field of around 1.5 T (tesla) inside the storage area for the muons. As far as magnetic fields go, 1.5 T is pretty strong - the magnetic field of the Earth is on the order of a few dozen millionths of a tesla at the surface, and a standard refrigerator magnet has a field strength of a few thousandths of a tesla. That said, it is fairly similar to the magnet in an MRI, which tends to generate a magnetic field of one to three tesla. This is moderately unsurprising, as both MRIs and the g-2 ring make use of superconducting coils to produce their magnetic fields. (This, by the way, is why it is more expensive to turn an MRI off overnight than to leave it running - the coils just keep conducting in any case, and the costs and danger of releasing all that liquid helium in gas form are pretty high.)

Okay, we now have a magnetic field...so what? We rely on the fact that a magnetic field bends the path of a charged particle. In this experiment, we inject a very pure muon beam into this ring, and if we've calibrated the magnetic field just right, it bends these muons into a circular path so that they whiz around the ring in the region we want them to. Once they're injected, we essentially leave the muons alone - we're not pushing them to higher and higher energies like they do in the enormous ring at the Large Hadron Collider at CERN. Instead, we just let the muons circle around a few thousand times in just a few millionths of a second and wait for it to decay. Thus this particular ring is classified as a storage ring rather than a cyclotron, synchrotron, or synchrocyclotron. That's the general story behind what our giant electromagnet will be used for - I'll post more about the science behind it and some of the fascinating techniques used in the next few days.

If you want to check out other posts on Muon g-2, they can be found here.