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A world first for EMMA

3 May 2011

At the end of March, an electron beam was steered round the ring of a new type of particle accelerator and successfully accelerated to 18 MeV for the first time. EMMA (Electron Model for Many Applications) is a proof-of-principle prototype built at the UK Science and Technology Facilities Council’s Daresbury Laboratory to test the concept of the non-scaling fixed-field alternating gradient accelerator (FFAG). The technique should allow the construction of a new generation of more powerful, yet more compact and economical accelerators.

The successful acceleration – a “world first” – confirms not only that the design of the most technically demanding aspects of EMMA is sound but it also demonstrates the feasibility of the technology used. The next steps will be to move towards full acceleration, from 10 to 20 MeV, and commence the detailed characterization of the accelerator.

The basic concept underlying EMMA is that of the FFAG, in which a ring of fixed-field magnets simultaneously steers and focuses the electron beam round the machine (CERN Courier September 2008 p21). The focusing is as strong as in an alternating-gradient synchrotron but the beam spirals outwards while it is accelerated, as in a cyclotron. However, with sufficiently strong magnetic focusing the displacement of the beam as it accelerates and spirals can be kept much smaller than in other types of accelerator. This makes the FFAG concept attractive for a range of applications, from treating cancer to powering safer nuclear reactors that produce less hazardous waste.

The design of EMMA’s magnet ring presented several challenges. The focusing magnets have a standard quadrupole geometry but they are used to steer the beam by offsetting it horizontally. The magnets are short, so “end effects” become important, and pairs of magnets are closely spaced around the ring, so the interaction between magnets is non-trivial.

• EMMA is a major part of the British Accelerator Science and Radiation Oncology Consortium CONFORM project and is funded by the Research Councils UK (RCUK) Basic Technology programme.

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