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EMMA & ALICE in Wonderland
   

The "girls from Daresbury" ALICE and EMMA have had a busy June. With construction of EMMA now complete, the end of the month marked a successful first stage of commissioning of a world's first in compact accelerators.

Smaller, cheaper accelerators

EMMA (Electron Model for Many Applications) is a proof-of-principle machine to see if a technology known as Fixed Field Alternating Gradient (FFAG) could herald a future of cheaper, more compact particle accelerators.

Scaling FFAG accelerators were first studied in detail in the 1950s. They use fixed magnetic fields to both focus and bend particle beams. This configuration results in small magnets, similar to a synchrotron, but with fixed fields like a cyclotron.

What makes EMMA unique is it is a non-scaling FFAG (ns-FFAG), a technology that emerged in the 1990s. This allows for even smaller magnets and the use of accelerating cavities of higher frequencies than scaling FFAGs. These novel features show promise for a variety of applications including neutrino factories and proton cancer therapy.

     


The Daresbury Laboratory of STFC, located close to Manchester in the UK. Image courtesy of Google Maps. Thumbnail image main page panel from the Alice Window courtesy of All Saints Church, Daresbury.
 
 
 
 

EMMA has been built at the STFC Daresbury Laboratory in the UK to demonstrate that the technology works and to understand the effects in detail before exploiting the potential. It uses the existing ALICE accelerator as an injector.

The other ALICE

Not to be confused with Alice in Wonderland or the ALICE experiment at CERN, Daresbury's ALICE is short for Accelerators and Lasers In Combined Experiments. It is a unique facility to investigate future accelerators, in particular a next generation of light sources.

The first of its kind in Europe, ALICE has at its heart an Energy Recovery Linac (ERL) accelerator. This produces extremely high quality bunches of electrons by intense light pulses from a laser bombarding a semiconductor surface. This electron beam is accelerated to 35 MeV using superconducting radio frequency (RF) cavities. These bunches are then compressed, to stimulate the production of intense, short pulses of light. In energy recovery mode, the bunches are decelerated and the energy that is released is used to accelerate the next set of bunches.

For EMMA, ALICE is used to produce a single bunch of electrons with an energy between 10 and 20 MeV. This is transported to the EMMA ring through a purpose built injection line (see images below).

 



The ALICE and EMMA accelerators at the Daresbury Laboratory Image courtesy of Rob Edgecock, STFC
 

EMMA's vital statistics

To demonstrate and study in detail all the features of non-scaling FFAGs, while also keeping costs to a minimum, it was decided that EMMA should accelerate electrons from 10 to 20 MeV. In addition, the machine is designed to be very flexible, with many diagnostic devices to make detailed measurements of the beam parameters.

 

It is built from 42 magnetic cells, each about 40cm long, making the circumference 16.5m. Each of these cells has two ring magnets and almost every other cell has an RF cavity. The cells in between have vacuum pumps or diagnostic devices. Because further diagnostics would significantly degrade the quality of the beam when making a measurement, these will be mounted in an external diagnostics beam line.

Within EuCARD there are two main tasks relating to EMMA. The first is the design, construction and testing of the external beam diagnostics devices and the diagnostics beam line. The second is the commissioning of EMMA and the execution of an experimental programme to evaluate the use non-scaling machines for a variety of applications.

The start of commissioning

  



Photograph of the EMMA ring and the injection line, taken on 18th June 2010. Image courtesy of Rob Edgecock, STFC
 
 

EMMA construction was from April 2007 to June 2010. The injection line was completed in March 2010 and commissioning of this has already started. The ring is built on 7 girders, the last of which was installed mid-June. Now commissioning has begun with the first successful steering of electron beams through a major fraction of the accelerator circumference (For more details, see the ASTeC news item). The diagnostics beam line will follow later in the year.

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

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