Shining bright: inside the Diamond Light Source

Diamond from the sky
The target lifetime for the facility when it was first approved was 30 years. Individual components might need updating, but I imagine that this will be surpassed for the facility as a whole.
Dr Emily Longhi
What produces light 100 billion times brighter than the sun and can be used to help save the Mary Rose, uncover the therapeutic properties of turmeric, and find greener ways of producing women’s stockings? Content Editor Katy Edgington recently journeyed south to find out…

The Diamond Light Source is the only facility of its kind in the UK. The synchrotron, which has its home on the Harwell Oxford science and innovation campus in a quiet corner of Oxfordshire, accelerates electrons almost to the speed of light and stores them in an ultra-high vacuum in a ring more than half a kilometre in circumference.

A third generation synchrotron, this light source was built to replace its predecessor at the Science and Technology Facilities Council’s Daresbury Laboratory – the Synchrotron Radiation Source (SRS) – which was closed in August 2008 after almost 30 years in operation.

The beams of light generated by the synchrotron, ranging from infrared to X-rays, find applications in all areas of scientific and technological experimentation. Despite still being in its infancy, Diamond has already begun to demonstrate the contribution it can make to STEM in the UK. The recently opened UK Catalysis Hub, for example, which is also based at Harwell Oxford, is making use of the facility to conduct research in a sector that contributes upwards of £50bn annually to the British economy.


How does it work?
 

Electrons are pulled from the cathode of an electron gun with an electric field. The electrons then enter the linear accelerator (or linac), where they pass through a series of rapidly alternating electric fields. The field just ahead of them always has a positive charge, so that they proceed towards it at ever-increasing speed. By the time the negatively charged particles emerge from the linac into the booster synchrotron – a racetrack-shaped ring within the larger storage ring – they are travelling at almost the speed of light.

Diamond storage ring
The storage ring, showing dipole (green), quadrupole (red) and hexapole (yellow) magnets
As its name indicates, the storage ring, which has a circumference of more than 560m, is used to store the electrons for as long as possible. To maintain the beam this must be done in an ultra-high vacuum – since air molecules would get in the way and reduce the energy of the electrons – which is kept at as close as possible to 3 GeV (gigaelectron volts). The electron beam can be ‘topped-up’ by small, frequent injections of electrons into the system – a process which accelerator physicist Dr Emily Longhi pointed out was being monitored in the relatively spartan control room.

"The lifetime of the electrons is typically about 15 hours," she related. "We would naturally lose about half of them in that time, but a small amount of current is injected every ten minutes to keep the average current stable to +/-0.5 per cent."

Rather than being a perfect circle, the storage ring is actually made up of 24 straight segments which house dipole, quadrupole, and hexapole magnets to bend the electron beam into a closed loop. As Dr Longhi explained, the powerful water cooling system for these magnets is responsible for a large proportion of the facility’s energy consumption. 

Insertion devices (IDs) – arrays of magnets exerting a magnetic field – are used to cause oscillations in the beam. Depending on the change in path which is produced, the two classes of ID are known as ‘undulators’ and ‘wigglers’. The ID causes the beam of electrons to emit electromagnetic waves in the form of X-rays; this forward-directed X-ray beam is then channelled into a beamline for experimental use.



Bending magnets, by contrast, emit a wide fan of synchrotron light spanning the electromagnetic spectrum: from infrared through visible and ultraviolet light to X-rays. 


Built for precision
 

The success and physical accuracy of the facility is a direct result of the careful planning and execution of its construction. Misalignment of the beam is relatively rare, because the foundations were designed such that changes in temperature and the water table are offset; the floor supporting the storage ring and most of the experimental stations is connected directly to the bedrock by 1,500 concrete columns.

"The columns were drilled and then poured to stick out of the ground," Dr Longhi explained. "A concrete floor was poured on the ground with holes for the columns to poke through without being in contact. A layer of cardboard was then laid with the tips of the columns sticking through, with the final floor poured on top of the cardboard and on the top of the columns. When the concrete had set, the cardboard was collapsed by running pressurised water between the two layers of concrete floor."

Despite these solid foundations, the synchrotron can be affected by earthquakes and the like. For instance, a tiny earthquake in Russia recently caused the beam to misalign.

"Occasional beam loss is the main area of concern for the technical division," said Dr Longhi. "There is a framework for finding and addressing the causes of faults. The individual beamlines have their own issues which are dealt with more locally."

Emily Longhi insertion device
Dr Emily Longhi and, in the background, an insertion device
The facility is currently in the third phase of development. Phase II added 15 beamlines to the initial seven, and the phase III funding provides for a further 10, which means that Diamond should house 32 fully functional experimental beamlines by 2017. In this way, it is hoped that the light source will maximise returns on the original investment, utilising the ring’s capacity for just shy of 40 beamlines.

Although the beam is operational 24 hours a day while running, periodic shutdowns allow for the installation of new equipment and, occasionally, maintenance.

"The target run time for beamlines is 5000 hours per year, which we are currently delivering," stated Dr Longhi. "Once installation of new equipment has tailed off this might be adjusted. The length of an individual shutdown is adapted to the work that needs to be accomplished – and vice versa to some extent – which is planned six to 12 months ahead.

"The target lifetime for the facility when it was first approved was 30 years. Individual components might need updating, but I imagine that this will be surpassed for the facility as a whole."


Free at point of access
 

Researchers travel from all over the world, not just from within the UK, to use the experimental facilities available at the synchrotron. There is also an exchange of expertise in the form of active collaboration agreements between the Diamond Light Source and other facilities across Europe, including Elettra in Italy, the ESRF in France, ANKA in Germany, and the Swiss Light Source.

A call for proposals is issued every six months, with the decisions on which applicants will gain beamtime being taken by an external review panel. As long as the resulting work is in the public domain, use of the facility is free at the point of access

Since the facility derives its funding in part from the government via the STFC and in part from The Wellcome Trust, use is free at the point of access. However, Diamond is keen to promote the fact that paying industry users are also welcome. While the upper limit for the proportion of industrial use is 10 per cent, the current level is around five per cent. Awareness perhaps need to be raised of just what the light source can offer.

If you would like the chance to visit the synchrotron for yourself in the company of an expert member of staff, keep a weather eye on the website for updates. Diamond’s next open day is scheduled to take place in November...

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