synchrotron radiation
/ SIN-kruh-tron /
Whenever you force a charged particle to change direction, it gives off light. This is a basic rule of electromagnetism: accelerating a charge — and turning a corner counts as accelerating, because the direction of motion is changing — makes it radiate electromagnetic waves. When the charge is moving close to the speed of light and is being bent around a circle, the light it throws off is intense and sharply focused, and it has a special name: synchrotron radiation, because it was first noticed glowing in synchrotrons.
For a circular accelerator this radiation is a double-edged sword. On one hand it is a loss: every photon of synchrotron light carries away energy the machine just spent pushing the beam, so the cavities must constantly top the beam back up. The loss grows extremely fast for light particles — an electron of a given energy radiates far more than a proton of the same energy, because the effect depends steeply on how light the particle is. This is the deep reason the highest-energy circular colliders use heavy protons, and why a very-high-energy circular electron machine becomes impractical; you would spend more and more power just replacing radiated energy.
On the other hand, that 'wasted' light is a fantastic tool. The radiation comes out as an extraordinarily bright, finely tunable beam of X-rays and ultraviolet, far brighter than any laboratory X-ray tube. So physicists turned the nuisance into an instrument: dedicated machines called synchrotron light sources are built precisely to generate this radiation and aim it at samples. They let chemists, biologists, and materials scientists image the atomic structure of proteins, batteries, and countless other materials — one of the most widely used spin-offs of accelerator physics.
LEP, an electron-positron ring in the same tunnel later used by the LHC, lost so much energy to synchrotron radiation that it could not be pushed much past about 100 GeV per beam — a wall that protons would not hit until far higher energies.
A loss for circular electron machines, a brilliant light source for everyone else.
Synchrotron radiation is not nuclear or dangerous in the way radioactivity is — it is ordinary electromagnetic light (mostly X-rays and ultraviolet) emitted because a fast charge is being bent; its importance is that it drains energy from circular machines and serves as an intense research light source.