Notes – read this first
I have spent some time thinking about FFAGs and rather more time looking at the Basic
Technology criteria, including the successful cases.
The boxes to tick are
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Opening up exciting new opportunities
Widespread applicability across a range of science
Being interdisciplinary
Being a really top rank group
There are no requirements (at least, in my view, but it’s arguable) on
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Industrial links
International links – the successful bids look very UK parochial
We must avoid at all costs
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Looking like the project belongs to one research council
Being worthy-but-boring
Looking at the last (unsuccessful) bid, I (with all deference to a valiant effort) reckon it
failed because
1. It really stressed the medical applications, so it looked like a front for an
MRC bid
2. The language was at times severely over-technical: I couldn’t understand
the terms they used
3. There was a loose use of flowery language (‘paradigm shift’ and ‘toolkit’)
which were unjustified and obviously just in there for effect
4. An accelerator which doesn’t accelerate is not really exciting.
I have tried to do better in what follow
Developing the FFAG Accelerator
In conventional synchrotron accelerators the magnetic field increases to match the
particle energy, so that a pulse of particles follows the same trajectory turn by turn. This
severely limits the repetition rate and duty cycle of the accelerator, as magnet currents
cannot be ramped up and down again faster than ~1Hz. When such accelerators were
only used by particle physicists this did not pose a problem, as such a rate fitted the
detector apparatus. But as their use has branched out across the fields of application they
cover today, in medicine, engineering, physics and chemistry, this is a severe limitation.
The FFAG (‘Fixed Field - Alternating Gradient’) design overcomes this by using an
alternative technique: it keeps the field constant and allows the bunch trajectory to grow
in a spiral-type orbit. The magnet configuration is a mixture of dipoles and quadrupoles
that combines fixed field magnets with the alternating gradient field variation technique,
which provides the strong focusing without which the bunch would diverge and be lost.
The basic science is not new – it was known in the 1960s. However accelerator
technology did not go down this route. Most textbooks state that ‘the synchrocyclotron
evolved into the synchrotron’ without mentioning that this evolution could have taken
another path. This happened partly because the needs of the particle physicists, who
sought high energy rather than high power, were met by the synchrotron design, and
partly because of limitations of existing technology: the FFAG requires complicated
magnetic field configurations which have to be very precisely attained, and the variation
in path length requires a matching change in the frequency of the RF used to supply the
acceleration.
Today there is a new interest in the FFAG design. Advances in RF materials and design,
and in magnetic materials and computer-aided magnet design have meant that these
limitations can be overcome.
The advantages of the FFAG are:
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The repetition rate for pulses can be much higher, as it is much easier to cycle the
frequency of the RF driving circuit than to cycle a magnet. Repetition rates in the
KHz region are attainable, which translates directly into 3 orders of magnitude
increase in the delivered beam.
The combination of bending and focussing in the design means that the FFAGs
are significantly smaller.
The FFAG has a much higher dynamic aperture, which means it will accept a
much wider spread in energies and angles/position of particles and deliver them to
the output. This means that sources can be used far more efficiently.
These advantages increase the effectiveness of accelerator applications across a broad
range of areas, and open new possibilities for accelerator use.
Applications
There is widespread interest in the FFAG principle from applications that need compact,
efficient sources of intermediate-energy charged particles
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The Cancer Therapy community would welcome a design that could produce
protons and heavy ions for cancer therapy. An FFAG in the region of interest
(hundreds of MeV per nucleon) could be built to fit inside a (large) room,
enabling such a machine to be installed at a hospital near the patients and medical
staff, rather than at a separate accelerator establishment.
The Medical community are also interested in lower energy (few MeV) electron
designs usable for X-ray radiotherapy and CT scans. Such an FFAG could be
really small (tens of centimeters) so that it could be mounted at the end of a gantry
and maneuvered round the patient directly, rather than rotating them. Such a small
source of X rays could also have industrial applications, for inspection or
sterilisation.
The ability of the FFAG design to handle sources efficiently also makes it
attractive for the production of radioisotopes used in PET scanning, and other
radioactive tracer techniques.
The particle physicists are interested in FFAGs as intermediate-energy boosters
for higher energy machines, particularly for new developments such as the muon
factory and high intensity neutrino beams
A high intensity proton FFAG could make practicable the transmutation of
nuclear waste through proton bombardment, and also the Thorium-fuelled
‘Energy Amplifier’
The solid state physicists and chemists are interested in FFAGs as a source of
muons for muon spin resonance studies.
Although FFAGs have been proposed in the above areas, none are actually being
constructed, apart from a couple of prototypes in Japan. This is because the technology is
still too unknown a risk to be depended on for a specific application, and because the big
accelerator labs are fully committed to conventional design studies and projects.
We therefore propose to build a proof-of-principle prototype which would open the door
to these applications. We would show that it was possible to build the machine, and learn
how it is to be done. This would be an electron machine (easier than protons, as they are
always relativistic and so whatever their energy, their velocity is essentially c)
accelerating from 10 to 20 MeV. It would be 16m in circumference, sited at the
Daresbury laboratory, where the existing ERLP machine will provide an appropriate
injector.
There are still many opportunities to explore in the layout of the lattice. Existing FFAGs
have been built with a ‘scaling’ design in which the fields match, and the trajectory
shape, and the transverse oscillations about it, are the same for each turn. It turns out that
relaxing this assumption (‘non-scaling FFAG’) makes a more compact design, and this is
the type we propose to construct.
The proposal
We are a group of particle physicists from Liverpool and Manchester, theoretical
physicists from Lancaster, RF engineers from Lancaster, and Accelerator Scientists and
technologists from ASTeC, Daresbury, <and the Adams Institute and RAL>. <Mention
some specific people: Rob Edgecock, Mike Poole, Lancaster RF group….>. We would
proceed through the appointment of a full-time project manager, assisted by two
postdoctoral RAs specific to the project, and using the expertise in magnet, RF and
general accelerator design and construction available at the Cockcroft Institute, the
Adams Institute, and CCLRC.
We cost the programme at around £3M, as follows.
Staff- new hires
Project manager
Two Postdoctoral RAs
Staff – Effort from CCLRC
Magnet design
RF design
Design and commissioning
Engineering support
Construction - technical
Operation – technical
Materials
Injection line
Magnets
RF
Vacuum, instrumentation etc
Total
3x50K
2x3x40K
150
240
0.5 SY
0.5 SY
1.0 SY
1.0 SY
2.0 SY
0.5 SY
40
40
80
80
160
40
200
500
1000
500
£3.03M
The programme would run for 3 years, with (roughly) the first year being spent in
finalizing the design, the second in building the accelerator, and the third in
commissioning and characterizing it.
Thereafter we envisage that the FFAG programme would continue, utilizing the
prototype as a research accelerator for studying the behaviour of a basic FFAG
accelerator that we knew well, increasing our understanding and finding ways of
enhancing performance.
Demonstration of a successful non-scaling FFAG will open a whole new landscape for
accelerator physics applications. We have many contacts in the medical and industrial
areas mentioned above, and would spread to them what we learn about FFAG
technology, and work with them on future projects to bring our design to their areas.
These FFAGs will be of many different types and sizes, but the basic principles of
technology we propose to explore will be common to them all.