radio-frequency cavity

If a magnet is what steers a beam, a radio-frequency cavity is what speeds it up. It is the engine of the accelerator. Picture a hollow metal chamber, often shaped a bit like a bell or a stack of doughnuts, with a hole through the middle for the beam to pass. Inside, an electric field is made to slosh back and forth at radio frequencies, and every time a bunch of particles flies through at just the right moment, the field gives it a forward kick.

The physics is like pushing a child on a swing. A swing has a natural rhythm, and if you push exactly in time with it, small pushes add up to a big swing. A radio-frequency cavity has a natural electromagnetic rhythm too, and the beam must arrive in time with that rhythm so each pass gets a push in the forward direction rather than a shove backward. The cavity stores oscillating electromagnetic energy and hands a slice of it to the beam on every crossing. String several cavities together, or send the beam through the same ones many times in a ring, and the energy mounts up.

Radio-frequency cavities are the heart of both linacs and synchrotrons. The most advanced ones are made of superconducting metal so they can sustain very strong accelerating fields without wasting energy as heat, which is essential for efficient high-gradient machines. Their performance — how big a push per metre, called the accelerating gradient — is one of the main things that decides how long, and how expensive, a future accelerator must be to reach a target energy.