From electron gun to beamline

The idea in one paragraph

Electrons are tiny charged particles, and when they are forced to change direction they give off light. At the ESRF, where they move at almost the speed of light, that light is mostly X-rays. A synchrotron exploits this: it accelerates electrons, keeps them going round a ring for hours and forces them to bend at precisely chosen points, so that beams of X-rays shoot out in exactly the directions where scientists have set up their experiments.

1. A first push in a straight line

It starts with an electron gun. The electrons it releases are grouped into bunches and sent into a linear accelerator, the linac, which brings them to 200 million electronvolts. At that energy they are already moving close to the speed of light.

2. The booster ring

Next comes the booster, a ring 300 metres round. The electrons circle it several thousand times and gain a little energy on every lap, until they reach 6 billion electronvolts (6 GeV). The booster then passes a bunch to the main ring, and it can do so every 50 milliseconds.

An electronvolt is the energy an electron gains when it crosses a voltage of one volt. Six billion of them is a great deal of energy for a single particle, which is why the ESRF calls itself a high-energy synchrotron.

3. The storage ring

The storage ring is 844 metres round. The electrons stay in it for hours, inside a pipe kept at an ultra-high vacuum of about 10⁻⁹ millibar, so that they almost never hit a stray gas molecule. The ring is built from 32 repeating sections called cells. Focusing magnets, which work like lenses, keep the electron beam as narrow as possible.

Since 2020 the ring has used a new magnet layout, the Hybrid Multi-Bend Achromat, invented by Pantaleo Raimondi. It squeezes more than 1,000 magnets, nearly twice as many as before, into the same tunnel. The result is a much thinner electron beam: the ESRF gives its size as about 2 micrometres high and 20 micrometres wide. This new layout is what made the ESRF’s X-ray beams 100 times better in brilliance and coherence.

4. Where the light comes from

Electrons give off radiation whenever they are forced to change direction. In bending magnets, which steer the beam round the ring, the light leaves at a tangent, straight towards an experimental station. Each cell also has a straight section about five metres long for special magnet arrays called insertion devices.

The most important of these are undulators: rows of small magnets with alternating poles that make the electrons wiggle. The ESRF says undulators produce X-rays a million times more intense than bending magnets, with a brightness and coherence that approach those of a laser.

5. The beamline

Each beam of X-rays ends in a beamline, a specialised laboratory with its own instruments and support team. The ESRF has 44 of them, each built for a particular kind of measurement: from the structure of proteins to materials under extreme pressure.

What a beamline can do depends on its design. Across the facility, the ESRF offers spatial resolution from millimetres down to the atomic scale and time resolution below a microsecond, and samples can be studied in situ, for example under real manufacturing or operating conditions. Because many of these measurements are non-destructive, they also suit fossils and works of art.

Generations of synchrotrons

The ESRF’s own history shows how the technology has moved on. When it was founded in 1988 it was, in its words, the world’s first third-generation synchrotron. In 2020 the Extremely Brilliant Source made it, again in its words, the first high-energy fourth-generation synchrotron, with X-ray beams 100 times better in brilliance and coherence. The new ring was built inside the old tunnel during a 20-month shutdown.

Why brightness matters

A brighter beam is not just a nicer picture. The ESRF points out that very intense X-rays suit very small samples and fast processes: the more light arrives each second, the quicker a measurement can be made. What X-rays can and cannot see is the subject of Two ways of seeing atoms; the machine itself is described on The European Synchrotron.