Inside the Machine

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

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 through a process called thermionic emission. These liberated electrons are accelerated to produce a stream of electrons with an energy of ninety thousand electron volts (90keV). The stream of electrons is then accelerated by a sequence of particle accelerators until it becomes a stable beam in the storage ring.
 

On average the beam is about the thickness of a business card in width (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.

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. That’s only 10 times greater than the pressure on the moon.

The linac, or linear accelerator, is the first of the three particle accelerators that make up the machine. The electrons are fired from the electron gun down the tunnel. In this time, they accelerate to an extreme relativistic energy of a hundred million electron volts (100 MeV) using radio frequency (RF) cavities.

The booster synchrotron is the second particle accelerator. Electrons enter the booster from the linac, where they follow an "athletics track" shaped trajectory. 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 section. Here they reach an energy of 3 GeV before they are transferred into 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 complete the circuit in approximately two millionths of a second, equivalent to travelling around the earth 7.5 times in a single second.

As the electrons circulate in the storage ring they collide with each other, and with the few gas molecules that remain in the vacuum, and are lost. To make up for this loss, new electrons are added to the ring every 10 minutes in a process known as “top-up”. The advantage of top-up, compared to the previous method of injecting electrons only twice per day, is that the light beams delivered to the beamlines are more stable and remain at maximum intensity at all times

Without the RF system, the electrons would only spiral into the walls. Synchrotron light is emitted when the electrons travel round the storage ring. However, energy doesn’t come from nowhere, and the electrons lose energy and momentum as they give out synchrotron light. 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 boost to the electrons every time they complete a turn of the storage ring, replacing the energy and the momentum that they lost last time around so that they follow the correct path.

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.

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.

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

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.

Diamond’s construction has been divided into phases. In January 2007 the first phase, comprising of seven beamlines, went into operation. The second phase saw approximately four beamlines going into operation every year. By the end of 2012, Phase II was complete and there were 22 operational beamlines. Further funding for a Phase III of construction was then granted and the total number of operational beamlines is now 32.

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

Bending magnets are dipole magnets which are used to curve the electron beam around the storage ring. Bending magnets also produce synchrotron light, and produce a very stable beam over a broad spectrum including infrared. They are the workhorses of spectroscopy experiments on synchrotrons.

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. They cause the electron beam to wiggle, making the light even brighter.

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

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-ray; 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.

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; this is essential for a range of experiments, particularly in protein crystallography and microfocus spectroscropy.