Will Rogers

The Diamond Light Source Accelerators

Diamond Light Source is a particle accelerator. As part of my work there in the Control Systems group, I need to understand the fundamentals of accelerator physics.

Introduction

Diamond is a synchrotron light source, which means that it is a special type of particle accelerator designed for producing high-energy electromagnetic radiation. To do this, it accelerates electrons, first using a linear accelerator or linac, then in two circluar accelerators: the booster synchrotron and the storage ring. As the electrons turn through the bending magnets, high-energy photons (particles of light) are emitted, directed through special holes in the wall of the synchrotron and into so-called 'beamlines' that use those photons for scientific experiments.

A simplified view of the Diamond Light Source accelerators

Diamond is tucked away in the Oxfordshire countryside, contrasting with the ancient Ridgeway and Icknield Way paths, both of which run past the Harwell Campus on which Diamond is located. The campus itself was established on a World War II air base for the purposes of nuclear research just after the war.

The striking circular building that houses the Diamond accelerators

## Synchrotron Light Sources vs Colliders

There are many different types of particle accelerator, but the ones that get the most attention are the huge ones at CERN: in the 1990s the Large Electron-Positron Collider (LEP), and now the Large Hadron Collider (LHC). The purpose of these machines are to accelerate particles to the highest possible energies in opposite directions, then smash them together to look at what comes out. This was the process that finally revealed the Higgs Boson in 2012.

Diamond is different. It's much smaller than the LHC, although it's still quite large with an accelerator circumference of 561.6 m. The electrons travel in only one direction, and we keep them going as long as we can. As they circulate, they continuously generate the electromagnetic radiation (loosely X-rays) that we need for scientific research.

The emission of synchrotron radiation was a hindrance in early accelerators, and it limited the maximum particle energy of (LEP) at CERN, but subsequently it was realised that the emitted radiation could be useful and it is the primary purpose of accelerators like Diamond.

Although this type of accelerator is not widely known about, there are dozens of facilities similar to Diamond around the world.

The Linac

See also Where do the electrons come from?.

Charged particles such as electrons naturally accelerate if you place them in an electric field, much like a ball running down a hill. The lowest energy acceleration in particle accelerators uses a static electric field, but there is a natural limit to the magnitude of static electric field that can be maintained. If you try and raise it higher, it will be unstable and an arc between the terminals will collapse the field.

To raise the energy of the particles higher, you can use oscillating electric fields. If the particles pass through the field at the correct time, they will be boosted to a higher energy in the correct direction. This also means that if they arrive at the wrong time they will lose energy and be lost. This effect naturally forms the electrons into bunches that pass through the oscillating electric fields together.

Diamond's linac uses these oscillating fields to get the energy of the electrons up to 100 MeV.

The Diamond linac. Photo © Andy Davis (cyberdavis on Flickr).

The Booster Synchrotron

There is also a natural limit to the acceleration you can do with a linac, simply because of its length: the higher the energy you want, the longer the linac has to be. This is solved by accelerating particles in a circle. The particles circulate indefinitely, gaining a small amount of energy each time, until they reach the target energy for the accelerator.

At Diamond, the electrons are guided from the linac into the booster synchrotron, where they are accelerated from 100 MeV to the Diamond design energy of 3 GeV in a tenth of a second. The synchronisation implied by the name 'synchrotron' comes from the fact that the strength of the magnets in the booster synchrotron must increase at the same rate that the electrons gain energy.

The Storage Ring

Once the electrons have reached 3 GeV, they are guided from the booster ring into the storage ring, so-called because the elctrons are kept there for as long as possible. As they travel around the storage ring, guided by magnets, they naturally emit the light used for the experiments at Diamond. As they do so, they lose some of their energy; this is restored by more of the oscillating electric fields in certain locations around the ring. These oscillations are at a frequency of about 500 MHz, and since this is a similar frequency to that used in radio, these devices are known as radio frequency or (more commonly) RF Cavities.

Insertion Devices

Although the electrons emit radiation as they go through the bending magnets in the storage ring, that radiation tends to have a fairly wide range of energy. It was subsequently discovered that you could produce more tightly-controlled and hiegher-energy photons by placing special gadgets in the straight sections of the accelerator. These are called insertion devices (I would have thought that an insertion device inserted something, but apparently they are called insertion devices because they themselves are inserted into the ring), and are arrays of magnets designed to cause the electrons to deviate from their straight path in carefully-controlled ways, emitting the desired radiation as they do so.

Science

The role of the accelerators at Diamond is simply to produce the photons for science. Once those photons pass through the wall to the beamlines, the work of the accelerators is done. In many ways, this part is less cutting-edge than what the scientists do with those photons, which covers a huge range of science and engineering. However, if the accelerator stops none of that science can take place.

Hidden behind the few paragraphs above are a huge amount of details and the expertise of a large number of experts. It is a tribute to them that Diamond operates at all.