Beam dream

Over the past few years, materials scientists around the world have looked on with a mixture of awe and gratitude as physicists have constructed a series of immensely powerful and dazzling light sources that can be used to analyse the innermost secrets of everything from individual molecules to pieces of aircraft.

These light sources, or synchrotrons, typically send particles slaloming through a device known as a wiggler, causing them to give off photons that are conducted down experimental lines and used to analyse materials.

One of the most notable of these devices — certainly in terms of scale — is the Diamond synchrotron, the sports stadium sized-instrument that dominates the Rutherford Appleton Laboratory in Oxfordshire. But as Diamond, a so-called third-generation light source, nears completion (earlier this year, it generated its first beam of synchrotron light) a group at the CCLRC’s (Council for the Central Laboratory of the Research Councils) other laboratory in Daresbury, Cheshire, is finalising plans for a device that will leave Diamond standing in the shade.

If an estimated £200m funding is granted by next autumn, the so-called fourth-generation light source (4GLS) could be illuminating some of physics’ and engineering’s most enduring problems as soon as 2008.

Generating pulsed beams of light some 10 million times more intense than those produced by Diamond, the 4GLS will enable researchers to view chemical and biological reactions as they happen, study molecules in real-time and examine the spin of electrons. Its backers claim that as well as expanding the boundaries of physics, real-world applications could lead to improved cancer treatments, more efficient fuels and potentially spark a revolution in the design of electronic devices.

Prof Elaine Seddon, the project manager for 4GLS at Daresbury, explained that the powerful instrument will have at its core a relatively new type of particle accelerator known as a superconducting energy recovery linac (linear accelerator).

Using this system, photoinjectors generate high-quality beams of electrons, which are accelerated in the linear accelerator from about 10MeV to around 700MeV and transported round the system just once before it recovers their energy, dumps them and replaces them with new electrons.

This is a very different approach to third-generation light sources, such as Diamond, which generate sufficient energy for their experiments by sending blocks of electrons hurtling round a ring millions of times. The problem here is that these many cycles cause miniscule timing errors to creep up and effectively blur the packet of charge. This limits the synchrotron to having long pulses of light which are only really suitable for probing the structure of materials and not dynamic events.

While 4GLS will also have a ring, the block of charge will only go round once, where it will be wiggled to produce radiation that will be directed to specially-designed experimental stations, and then put through an accelerator which is run in reverse. Here the beam will give its energy back to the accelerator where it will be stored and used to help accelerate the next block of charge.

So, like a sort of atomic relay race, the energy will be passed on, baton-style, from one block of charge to the next, enabling 4GLS to achieve the same sort of energies as 3GLS but without the blurring effect. This means it could be used to generate much shorter and much brighter pulses of light down to less than 100 femtoseconds (fs) — about the time it takes for a bond to be made or broken in a chemical reaction.

But the capability of 4GLS will not stop here. As well as this improved version of the shorter pulse undulator source, the instrument will also be equipped with three free electron lasers (FELs) which produce light millions of times more intense than that produced by third-generation light sources.

Seddon stressed that the FEL capability of 4GLS will be very different from other FEL sources such as Hamburg’s XFEL particle accelerator (The Engineer, 13 March). She said that while XFEL is mainly geared towards producing hard x-rays for structural studies, the three FELs on 4GLS will cover the THz (far IR), VUV (Vacuum Ultraviolet) and soft X-ray regions, making it suitable for dynamics experiments.

‘The analogy is that you could take an X-ray of Sir Steve Redgrave and it would tell you which bone is joined to which bone, but if you want to know what really makes him a fantastic rower, you use longer wavelength light, take a video image of him, and that would tell you how everything works together.’

But the most impressive capability, and the unique thing about 4GLS, will be its capability to combine the light from all of these different sources — effectively creating a suite of investigative tools that hasn’t existed before. ‘You can actually pick out the best source or source combinations for your experiments, and this will be one of the big attractions of the facility,’ Seddon claimed.

This million-fold intensity gain, coupled with the ability to generate intense short pulses, could lead to a renaissance in the field of synchrotron research — enabling scientists to look at dynamic events going on inside matter so that they can observe chemical reactions as they occur.

One area where the instrument is expected to be particularly useful is in the development of new catalysts for fuels. Currently, a catalyst will be put into a reaction and will make it go much faster. But often, the reaction will go so much faster that it’s extremely hard to characterise the mechanisms of that reaction, and therefore exceptionally difficult to come up with improved catalysts.

According to Seddon, the short pulse durations offered by 4GLS will, for the first time, enable engineers to be able to get a handle on what some of the intermediate stages are in these processes, and therefore arm themselves with the skills to develop new catalysts.

Knowledge gleaned through the use of 4GLS could also underpin the next generation of electronic devices, thanks to an improved understanding of the spin of electrons. ‘The unique characteristics of the combined sources of 4GLS will help accelerate a revolution in microelectronics,’ claimed Seddon. She said that it will enable physicists to better understand what is required to integrate magnetism and semiconductor technologies that utilise both the charge and the spin of electrons. This, she said, could ultimately lead to more efficient logic circuits, designed using fewer components. In turn this could mean lighter, faster and more powerful portable computers with longer battery life and more sensitive magnetic sensors.

The instrument is also expected to be exceptionally useful for medical imaging applications. 4GLS will, for instance, be one of the world’s brightest terahertz sources and it’s thought that ultra-high intensity THz light could help in the fight against cancer.

A group at Liverpool Universityis investigating the use of linking a THz beam line from the 4GLS prototype (the Energy Recovery Linac Prototype — ERLP) to a tissue culture facility where skin cancer cells will be grown. Indications are that the THz light may be preferentially absorbed by cancer cells and the experiments will help scientists understand how to use this technology in future treatments for the disease in humans.

This will be the first time THz technology has been used on cancer cells and it will also be developed to characterise genetic material. THz has the capability of identifying mutations in DNA, which could help doctors identify pharmaceutical therapies that will be compatible with individual patients’ DNA information.

Work on the prototype is going well. Earlier this summer the DTI added £2m to £14.8m the group has already received, and last month, the team celebrated the emission of the first electrons from the photon injector.

Indeed, this scaled down version of 4GLS will itself represent a significant scientific milestone. When fully operational, it will be both the most powerful source of broadband terahertz available in Europe, and the first demonstration of energy recovery using superconducting linac technology. It will also be put to immediate practical use.

For instance, the Liverpool group is keen to use ERLP to investigate the effects of THz waves on live cells. With increasingly vocal calls for terahertz scanning machines at airports and other public places, this project could provide valuable information on any detrimental effects that THz light might have on people exposed to such radiation.

However, the challenges of going beyond the prototype to the full instrument are massive. ‘4GLS is a whole level again in terms of vision to anything else that has been done,’ said Seddon.

It may sound obvious, but one of the biggest challenges for such a sensitive and powerful instrument is ensuring reliability of operation. One of the keys to this is the work carried out by UK radiofrequency and sensing specialist E2V technologies of Chelmsford, Essex, which is developing an integrated amplifier containing the inductive output tubes (IOTs) that provide the RF to power the accelerations.

‘Synchrotrons are user tools — not the toys of science boffins that play with the origins of the universe,’ said E2V’s RF Systems manager, Paul Burley. ‘they’re there to service industry and they need to be highly available and when they’re being used, highly reliable.’

The system under development by E2V is thus built around concept of line replaceable units and, wherever possible, industrially available components with existing reliability data.

It is also equipped with a novel control system based on hardware with embedded computing power that will continually monitor 4GLS and hopefully avoid the need to shut down needlessly. ‘you can intelligently look at the data you’ve got and shut down only in an emergency to keep the machine running as much as possible so that owner can deliver to his customer the radiation that they’ve paid for,’ said Burley.

Another big challenge involves synchronisation. A unique feature of 4GLS is the vision for combining sources of very different wavelengths from different parts of the machine, as well as combining table-top lasers with the sources of 4GLS.

As many processes of interest occur on very short timescales the sources have to be synchronised, initially to hundreds of femtoseconds and ultimately to tens of femtoseconds. Achieving this over hundreds of metres is a considerable challenge and will require the development of electron beam diagnostic techniques, new methods of distributing timing signals over large distances (some of which are currently being developed on ERLP) and feedback systems to correct things when changes start to occur.

‘These are huge challenges,’ said Seddon, ‘but as long as you put sufficient money in you can solve it, and by the time 4GLS build starts I believe they will have been solved.’

With many of the key elements in place, and work on the prototype gathering momentum, the UK is in a good position to reap the benefits of a world-leading facility. but with similar projects being discussed by the French and at a number of US research institutes, Seddon warned that we must act now if we are to lead the way in the next generation of synchrotron research.

‘The importance of 4GLS cannot be overestimated,’ she insisted. ‘The UK has the opportunity to take a world lead. The technology is ready, the team is ready, the science vision is there — we need to act now.’