Question: what is a superconducting RF cavitie

Matt Maddock answered on 15 Mar 2012:

OK – first things first, it might help if you’ve read my answer here – /measurementm12-zone/2012/03/what-is-a-particle-eccelerator/comment-page-1#comment-217

To summarize, I’ll talk about the particle accelerator where I work. We make electrons go around in a big circle very, very fast with a LOT of energy. We do this, because the electrons give out energy in the form of light called synchrotron radiation which is really useful. Now, when they give out that energy, it has to come from somewhere – it comes from the speed and energy that we’ve given the electrons, so as they go around the particle accelerator, those electrons tend to lose energy and slow down until they’re not a lot of use.

A superconducting RF cavity is a piece of the machine we use that gives the electrons an electrical kick every time they go around the ring, to get them back up to speed! Without them, the electrons going around the ring would only be useful for a few hundredths of a second, but we can keep them working for weeks at a time!

A superconducting RF cavity, if you could see inside its box, looks a bit like two bells welded together at the bottom, with a tube running in and out of the ends so that the electrons can pass through. This is then put inside a bath of liquid helium to keep it at -269 degrees C – that’s about the same temperature as outer-space – VERY cold. So cold, in fact, that we have to keep that bath inside another bath full of liquid Nitrogen at -196 degreed C!

We have to keep them so cold, because we pump a huge amount of power into them – 200 kiloWatts – that’s like 300 microwave ovens at full power! All that power makes what we call a voltage gradient inside the cavity – when the electrons move through that voltage gradient, they gain a lot of energy so that they can go around again! Think of it like rolling down a hill – by the time they reach the bottom, they’re going really fast!

That might all be a bit complicated…feel free to ask more question in the comments and I’ll do my best to clear things up!