Monday, 11 April 2011

Exciting news about EMMA: A world first in the UK!

This post is a week or so late as I've been at a conference in Glasgow, but we recently had some very exciting news: the announcement of a major milestone for one of the projects I'm involved in. We had first acceleration in EMMA!

EMMA is the "electron model for many applications" and is the first accelerator if its kind anywhere in the world. It is a prototype or proof-of-principle machine for a technology which could pave the way to new accelerators for cancer treatment, energy production and other applications. I've been involved in the project since its design stages so for me, this is something to celebrate! 

The type of accelerator is called a non-scaling FFAG, which stands for fixed-field alternating-gradient accelerator. The name, while complicated, doesn't do justice to the beauty or the simplicity of this machine. I like to think of it as a clever combination of two existing types of accelerator, the cyclotron and the synchrotron.

One of the first kinds of circular particle accelerator was the 'cyclotron', where the beam spirals outward as it gains energy, and the beam is confined by a constant magnetic field. The magnets of these machines become unwieldly for high energies (I'm talking 700 tons and up!) making the magnets expensive, particularly in terms of the cost of iron for such a big magnet! They have other issues reaching high energies too.

The other type of circular accelerator we usually use is the synchrotron. This is the type of machine most people think of nowadays as an accelerator. Not least because the LHC is a synchrotron, and so are two of the major scientific facilities in the UK; Diamond (the synchrotron light source) and ISIS spallation neutron source. 

A synchrotron uses a principle called 'strong focusing' where you have focusing and defocusing magnets alternating around the ring. Also known as alternating gradient focusing, this gives you stronger focusing than in the cyclotron. (We have to have the alternating gradient as the magnets focus in one plane and defocus in the other!) 

The beam doesn't spiral out in this machine as the magnet strengths are ramped so that the beam stays at the same radius. That means that you can have a ring of small aperture magnets, so it looks rather different to the cyclotron. At the same time, the particles get faster as they gain energy, so the accelerating 'cavities' that give the beam an energy kick each time it goes around, have to be timed perfectly with the magnet ramp and the energy gain. It has to be 'synchronous', hence the term 'synchrotron'.

The FFAG is almost a combination of the two. We take the strong focusing of the synchrotron, meaning that the machine is ring-like. But we also take the fixed magnetic field. But now we increase the field with the radius of the machine. What it means it that the particles spiral out a little bit during acceleration, but nowhere near as much as in a cyclotron. FFAGs have been built before, but the magnetic fields were complicated and followed a very exact 'scaling law' that their original designers in the 1950's and '60s calculated would keep the particle beam stable.

So what is different about EMMA? In the 1990's it was realised that it might be possible to break this 'scaling law' and dramatically simplify the FFAG. This is why the machine is called 'non-scaling'! EMMA uses very simple magnets (just quadrupoles, like the focusing/defocusing magnets in a synchrotron) to provide both the bending and focusing. It is a ring like a synchrotron, but can be made much smaller because the same magnets are used for both bending and focusing.

In a way, the FFAG is the ultimate particle accelerator. It opens up the continuum of designs between the cyclotron and the synchrotron, and can access the benefits of both types of machines in terms of energy reach, beam intensity, machine size etc...

There are some really new ideas in this machine. We are attempting to build an accelerator which relies on at least two things which have never been done (intentionally, at least!) in other accelerators. First, we intentionally make the beam cross through resonances, which are usually strictly and painstakingly avoided in other machines! The idea here is that if you cross through a resonance quickly enough it doesn't have time to build up and the beam will be fine. We've also had to use a completely new type of acceleration (called serpentine acceleration) to get the electrons in EMMA from 10 to 20 MeV.

Understandably, given the challenges in this machine and the number of world firsts, it wasn't going to be easy to get it working! But after many late-night shifts at STFC Daresbury Laboratory, we are finally starting to see the fruit of our efforts. 

Personally, the first time I saw on the oscilloscope that little beam moving outward during acceleration I had a little flutter of excitement. It wasn't the big "eureka" that is so often portrayed, it was the result of painstaking work, hours and hours of late night shifts and detailed calculations of what exactly was happening to the beam. Even then, what if we were seeing something else? What if we weren't REALLY accelerating? Our scientific training leads us to naturally doubt our own results, even as we produced them!

Thankfully, we're now convinced of our own results! The official announcement is below and work in commissioning the machine is ongoing. 

I have to say this is an amazing leap forward and I only hope that this type of accelerator really takes hold in the future.

The official announcement reads:
"During the last three to four days, the EMMA accelerator physics team have been tuning the accelerator settings as part of the systematic studies towards first acceleration within EMMA. The team have studied the interaction of the RF field with the beam, on more than one occasion, with different accelerator settings in order to tune and understand the acceleration process. Detailed analysis of one of the data sets from an early run in this period has just confirmed acceleration. This analysis indicates that the injected EMMA beam energy has been increased by a few MeV. The acceleration is clearly detected by the high resolution, turn by turn, beam position monitoring system.
This is a very significant achievement for the scientists and technologists who designed and assembled this unique accelerator before embarking on the present programme which has rigorously studied, commissioned and developed the systems and processes required to take this step forward. The team includes STFC staff, members of the Cockcroft and John Adams accelerator institutes, university colleagues and a number of international experts from the FFAG community who have supported the project with their expertise from conception. 

The next steps will be to move towards acceleration over the full range, from 10 to 20 MeV and commence the detailed characterisation of the EMMA accelerator and its novel acceleration scheme.
The picture shows an image of the accelerated, extracted EMMA beam on the first screen of the diagnostics line taken on 17:53 on Tuesday 29th March."