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CERN | Geneva | Switzerland

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Sun, waves and wind

If you wanted to sum up the attractions of Donostia-San Sebastián on the northern coast of Spain in late summer, then “sun, waves and wind” would do a good job. There at La Zurriola beach, the wind and waves combine to make the conditions perfect for surfers, with the sunshine an added bonus. But the words work just as well to sum up many of the attractions at the meeting of more than 1200 accelerator experts that has just taken place (on 5-9 September) in the large modern conference centre right on the edge of the beach.

Nowadays, with all the interest there is in the LHC and the search for the Higgs bosons, supersymmetry, dark matter and other curious beasts, big meetings of particle physicists can make the news headlines. So it seems a shame that when the people who design and build particle accelerators get together, no one else takes much notice. These are the machines that make possible our journeys deep into matter and back in time towards the Big Bang, and now they are increasingly opening up new territory in biology, medicine, chemistry and the science of materials. This all goes to make IPAC, the new International Particle Accelerator Conference, THE occasion to find out who is building what, where and why, as well as what exciting new ideas are on the horizon.

Take “light sources”, for example. These are machines in which electrons are accelerated to shine light, admittedly not quite as the sun does, but in the form of intense beams at the short wavelengths of ultraviolet light and X rays, or in some cases, the longer infrared. According to the website lightsources.org, there are more than 50 such facilities worldwide. They range from large-scale international facilities, akin to the LHC, to quite small machines that allow individual countries to support front-line home-grown research across a broad range of science. It’s an impressive example of how the effort to push one frontier – towards higher energies – can open up others.

Note the word wavelength. It would be impossible to talk about particle accelerators without mentioning waves of one kind or another and in particular the radio-frequency (rf) waves that give the kicks in energy to the particles in all accelerators. The principle of using rf waves has been around since the first accelerator designs of more than 80 years ago: the waves provide the electric field that accelerates the particles. Now the state-of-the art is intimately linked to the goal to reach the highest particle beam energies and the highest intensities. Waves set up in beautifully engineered “cavities” of exotic mixtures of metals are making it possible to accelerate electrons over shorter and shorter distances, in order to reach a given energy. This is not only crucial for a linear collider that would complement the LHC, but it also has implications for making less energetic machines that are more compact and therefore ultimately more practical.

Higher energies need higher electric fields, but eventually nature says “enough” as the field rips electrons apart from atoms in the material of the cavity, so that it sparks in a localized lightning display. To overcome this limit, one ingenious idea is to use a material that is already broken down – a plasma of electrons and ions. Here very high electric fields can occur in the wake that forms when a particle or laser beam passes through (note a further allusion to water!). It seems that such “advanced accelerator concepts”, which have long seemed far-fetched, are fast becoming a real possibility. The principle has already been shown to work – for example, by taking the electron beam emerging from the 3 km machine at SLAC and doubling its energy in a plasma tube only 85 cm long – and there is now a significant effort being made towards realistic machines.

And what of the wind? It can’t be used to accelerate particles directly, but as one of the talks explained it can be used to provide some of the electricity required to run a more sustainable accelerator facility.

Sun, waves and wind. It was a fascinating conference and an excellent reminder that particle physics advances not only through new theoretical ideas but also through innovation in experimental techniques, which in turn adapt and spread into other sciences, making the field not only truly international but also remarkably diverse.

Christine Sutton

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