About Diamond | How do synchrotrons work?

What is an electron beam?

An electron beam is a beam of tiny charged particles, produced by an electron gun, accelerated by the booster synchrotron to very high speeds before injection into the storage ring.

 Watch an animation of the electron's journey from the electron gun to the storage ring.

How is an electron beam produced?

Electrons are produced in the electron gun. A high voltage cathode is heated under vacuum, giving the electrons in the material sufficient thermal energy to "evaporate" from the surface and escape (thermionic emission). These liberated electrons are then accelerated to produce a stream of electrons with an energy of ninety thousand electron volts (90keV). The stream of electrons is the accelerated by a sequence of particle accelerators until it becomes a stable beam in the storage ring.

See Generating the electron beam.

What are the dimensions of the electron beam?

On average the beam is about about the thickness of a business card wide (1/4 mm) and about the thickness of cling film in height (1/60 mm). However, the exact dimensions of the beam vary as it travels around the storage ring, from 87 (horizontal) x 58 (vertical) µm (1 µm = 1/1000 mm) in a bending magnet up to 419 x 30 µm in a long straight section.

Why must the electron beam be contained in an Ultra High Vacuum?

If the electrons were to travel through air as they orbited the storage ring they would quickly collide with air molecules and be lost from the electron beam. To minimise these losses, the electrons circulate around the storage ring in a vacuum chamber in which the pressure has been reduced to approximately one million million times lower than atmospheric pressure. At this pressure the useful lifetime of the electron beam is about 20 hours.

What is the Linac?

The linac, or linear accelerator, is the first of the three particle accelerators that make up the machine. It is used to accelerate the electrons to an extreme relativistic energy of a hundred million electron volts (100 MeV) using radio frequency (RF) cavities.

What is the booster synchrotron?

The booster synchrotron is the second particle accelerator. Electrons enter the booster from the linac, where they follow an "athletics track" shaped trajectory with two straight sections joined together with semicircular curves. Thirty six dipole bending magnets are used to curve the electrons around the bends, and a radio-frequency (RF) voltage source is used to accelerate the electrons in the straight sections. Here they reach an energy of 3 GeV before they are transferred into the storage ring.

What is the storage ring?

Diamond’s storage ring consists of 24 straight sections angled together to form a closed loop 562m in circumference. 48 large electromagnets (called dipole magnets, or bending magnets) are used to curve the electron beam between adjacent straight sections. As the electrons follow this curved path they generate synchrotron light.

The entire storage ring is maintained under vacuum conditions to minimise electrons scattering off air molecules. Electrons with complete the circuit in approximately two millionths of a second - that’s nearly seven and a half times around the Earth’s equator in a single second!

How long do electrons remain in the storage ring? What happens to them?

Electrons remain in the storage ring for around 20 hours – this is known as the lifetime of the beam. As the electrons circulate the storage ring they collide with the few particles that remain in the vacuum and are lost. New electrons are added to the ring about twice a day. In the long term, Diamond will operate in "top-up" mode, where the electron beam is regularly topped up throughout operation. This will mean the beam is more stable and capable of producing light with higher average brightness.

What does the RF system do and why is it required?

Synchrotron light is emitted when the electrons travel round the storage ring. However, energy cannot come from nowhere and the energy that the electrons give out as synchrotron light is lost from their mass. The lighter electrons start to take a different path through the storage ring until they eventually hit a wall and are lost. However, the RF system provides a ‘kick’ to the electrons every time they complete a turn of the storage ring, replacing the energy – and the mass – that they lost last time around so that they follow the correct path.

What causes synchrotron light?

Synchrotron light is emitted when a beam of electrons moving close to the speed of light is bent by a powerful magnetic field. The light that is produced spans the electromagnetic spectrum from infrared, through visible and ultra-violet light to X-rays.

What is a beamline?

Beamlines are placed at tangents to the storage ring to guide narrow beams of light to experimental stations where the light is focused for use in experiments. Each beamline is optimised for a particular type of experiment, but all beamlines have three main sections – the optics hutch, where the X-ray beam is filtered and focussed, the experimental hutch, where the light interacts with the sample being studied, and the control cabin, where users control the experiment and collect data.

Beamlines typically have three hutches, l-r control cabin, experimental hutch, optics hutch.

 Watch an animation of a beamline.

How is the light removed from the storage ring for use in the beamlines?

Beamlines use two different sources of synchrotron light – bending magnets and insertion devices. As the electron beam passes through a bending magnet, it emits a wide fan of synchrotron light, which is channelled into a beamline. Beamlines with the prefix B use bending magnets as the source. Insertion device beamlines use special arrays of magnets inserted into the straight sections of the storage ring. These cause the electron beam to follow a wiggling, or undulating path. Beamlines with the prefix I use insertion devices as the source, whereas those prefixed with B use bending magnets.

How many beamlines are there currently, and how many can Diamond accommodate?

Diamond’s construction has been divided into phases. In January 2007 the first phase, comprising seven beamlines, went into operation. As part of the second phase, approximately four beamlines are going into operation every year, so that by 2011 there will be 22 operational beamlines. In total Diamond could support around 40 beamlines. See the Beamline availability diagram for the current number of operational beamlines. There is scope for more beamlines to be proposed as part of Phase III.

What experiments do the current beamlines support?

Most beamlines support a range of experiments. The experiments can be broken down into three main techniques: diffraction, imaging and spectroscopy. View the Applications pages of each beamline to learn more about the types of science it supports.

What is a bending magnet?

Bending magnets are dipole magnets which are used to curve the electron beam around the storage ring. Bending magnets also produce synchrotron light, and Diamond currently has four beamlines using bending magnets, as the source. Bending magnets produce a very stable beam over a broad spectrum including infrared, and are the workhorses of spectroscopy experiments on synchrotrons.

What is an insertion device?

An insertion device is an array of magnets which can be inserted into the straight sections of the storage ring to produce more intense, tuneable light. Insertion devices come in two main types: wigglers and undulators.

Insertion devices consist of magnet arrays which cause the electron beam to follow a wiggling, or undulating path.

What is a wiggler?

A wiggler is a type of insertion device. It consists of an array of magnets which cause the electron beam to follow a "wiggling" path. This causes the light to be produced in a wide cone, spanning a broad spectrum of X-rays. Wigglers are used in beamlines where the priority is for very high energy X-rays – for example the Extreme Conditions beamline, which has a superconducting wiggler. Here, high energy X-rays up to 100 keV are required to penetrate into a diamond anvil cell.

What is an undulator?

Undulators, the more common type of insertion device at Diamond, produce very bright light in a very narrow beam. By varying the separation of the magnet arrays, it is possible to tune the undulator and choose the energy that is generated. They can be used to produce very bright X-rays over a continuous frequency range – essential for a range of experiments, particularly in protein crystallography and microfocus spectroscropy.

<< How was Diamond built? | What is Synchrotron Light used for? >>