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A word about EMMA: a world first

A new type of particle accelerator has been built and commissioned in the north-west of England. EMMA (Electron Model with Many Applications), at STFC Daresbury Laboratory, is the first non-scaling FFAG (Fixed Field Alternating Gradient) accelerator to be built anywhere in the world.

Catching up with EMMA

The EuCARD newsletter last profiled EMMA in June 2010 when commissioning had just begun. "Back in June 2010, there was beam in 4 of the 7 sectors of the EMMA ring" explains Rob Edgecock of STFC, "by August 2010 the ring was complete and the first full turns were observed".


Complete EMMA ring
EMMA ring with injection line (top right) and extraction line (top left). Image courtesy of Rob Edgecock, STFC.

"While many turns were being observed it was initially difficult to prove the beam was being accelerated. Either tunes or orbit changes needed to be measured to prove this" says Edgecock. "During last winter an extraction line was put in and the magnets were realigned to solve some of the orbit problems. Then in February 2011 acceleration was confirmed and by the end of March 2011 the commissioning was declared complete."

"Next, an experimental programme will start to investigate the performance of this machine more carefully. Eight different sets of magnet settings have been proposed and all of these will be tried to see how well they work", continues Edgecock. "The phase space and resonances will also be investigated in more depth. It is important to understand the operation well so that the computer models of the machine and the actual performance agree. This will mean that future machines can be designed more accurately."

Serpentine acceleration

EMMA is a Fixed Field Alternating Gradient (FFAG) accelerator. In FFAGs, the "fixed field" refers to the magnets that bend and focus the beam. These magnets have a fixed field like in a cyclotron instead of a varying field like in a synchrotron (see types of accelerator). The "alternating gradient" refers to the magnets focusing in alternate horizontal and vertical planes to give a net focusing in both planes (see magnets).

In a scaling FFAG the parameters describing the beam's orbit will scale with the energy. As a result, the tune remains constant. In a non-scaling machine, such as EMMA, the tune does not have to remain constant as the energy is increased. This removes a significant constraint on the machine design, allowing a lot of flexibility. It does however raise the possibility of beam control problems due to resonance conditions being crossed during acceleration.

Unlike a synchrotron, the RF frequency does not vary during acceleration. The beam in EMMA is accelerated from 10 MeV to 20 MeV in 8 turns, and while the acceleration is happening the phase of the beam with respect to the RF varies. This is known as serpentine acceleration. While the phase varies it never varies so much that the beam sees the decelerating phase of the RF. The beam crosses around 4 integer resonances while being accelerated, but they are passed sufficiently quickly that no significant beam loss occurs. During commissioning, there was also no observed degradation in the quality of the beam.

Future applications

EMMA was designed and built to prove the principle of ns-FFAGs, but numerous possibilities exist for these types of machines.

Besides medical applications there is the original use that was conceived to accelerate muons so that neutrinos could be produced for neutrino factory experiments (see WP3).

There is also the potential use in Accelerator Driven Subcritical Reactors (ADSRs), which are a type of nuclear reactor that uses Thorium instead of Uranium.


EMMA commissioning


EMMA success
Above: EMMA commissioning taking place
Below: EMMA team members celebrate success.
Images courtesy of Rob Edgecock, STFC.

Thorium, unlike Uranium, does not give enough neutrons to produce a chain reaction required to produce nuclear power. The extra neutrons could be provided by an accelerator. The advantage of this is that if the accelerator is switched off the nuclear chain reaction will instantly stop, thus making nuclear power much safer.

The next stage in this project (as yet unfunded) will be to build a version of this machine that accelerates protons and carbon ions. The design for this machine, which will be called PAMELA (Particle Accelerator for MEdicaL Applications), already exists and such a machine can be used for radiotherapy. Both EMMA and PAMELA are part of the CONFORM project.

Further work that would be required to make such a machine operational includes prototyping of magnets up to decapoles (EMMA only uses quadrupoles, obtaining a dipole component by passing the beam off-axis) and of an RF cavity. Since the protons would be non-relativistic, the RF frequency would have to be varied as the protons were accelerated and, as the aim is to cycle the machine at 500 Hz to 1000 Hz, the RF cavity design and manufacture is a significant challenge. The injection and extraction systems for such a machine will also need prototyping. The researchers are currently seeking funding to make these ideas a reality.

- Naomi Wyles and Kate Kahle, CERN, EuCARD-DCO (WP2); Rob Edgecock, STFC, EuCARD-ANAC (WP11).

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