How does a synchrotron work?

Synchrotron light, also called synchrotron radiation, is produced when high-energy electrons circulating in a storage ring are deflected by magnetic fields. The first synchrotron radiation beam was observed in 1947. Since then, spectacular progress has been made not only in accelerator physics, electronics, and computing, but also in magnet and vacuum technologies. As a result, today’s synchrotron light sources have been conceived and optimized to produce brilliant X-rays for which there is a widespread and fast-growing demand.

The linear accelerator (or "linac")

Electrons emitted by an electron gun are packed into "bunches" at the start of the linac and then gradually accelerated (200 million electron-volts) using electric fields until they are travelling very close to the speed of light.

The booster synchrotron

The electrons then enter the booster synchrotron, a ring with a circumference of 300 metres. The electrons travel round this ring several thousand times, gaining a little more energy with every lap. Once they reach their final energy of 6 billion electron-volts (6 GeV), they are sent into the storage ring. Every 50 milliseconds, the booster can send a bunch of 6 GeV into the storage ring.

The storage ring

The storage ring has a circumference of 844 metres. The electrons travel for hours around the storage ring at the speed of light inside a tube under ultra-high vacuum conditions (around 10^-9 mbar). As they travel round, they pass through different types of magnets, such as bending magnets, undulators and focusing magnets. Each time they pass through certain magnets, the electrons lose energy in the form of electromagnetic radiation, known as "synchrotron radiation", which includes X-rays.

• The bending magnets are essential because they force the electrons to change direction. They are also a source of synchrotron light, which is emitted tangentially to the curved path of the electron beam and is directed towards the beamlines.
• The undulators are magnetic structures made up of a series of small magnets with alternating polarity. The beams of X-rays they produce are a million times more intense than those generated by the bending magnets and have brightness and coherence properties that are close to lasers.
• The focusing magnets, also known as magnetic lenses, are used to focus the electron beam so that the beam is as narrow as possible.

The beamlines

In a synchrotron, the experiments take place in beamlines. These highly specialised laboratories are equipped with state-of-the-art instruments and each has its own dedicated support team. The ESRF has 44 such beamlines, designed for use with a specific technique and for a specific type of research.