Sustainable Accelerators – The Case for Permanent Magnets and Fixed Fields (submitted)
Author: Adam Steinberg
Particle accelerators have a sustainability problem.
Accelerators have been applied to a wide range of cases, ranging from colliders for fundamental science to X-ray sources for radiotherapy. In total there are more than 50,000 accelerators globally [1], and their combined environmental impact is not insignificant. Recognising the dangers of anthropogenic climate change, there has been an increasing push towards ‘green accelerators’ in recent years [2], aiming towards reducing the lifetime emissions from particle accelerator facilities. These emissions have many origins, including well-known sources such as the carbon dioxide released by fossil fuel burning for electricity production and the carbon emissions when manufacturing concrete, to less widely recognised examples including leaks of sulphur hexafluoride [3] (an insulating gas used to prevent sparking in high voltage machines, with a global warming potential 23,000 times that of carbon dioxide). To meet ambitious net-zero goals, there must be an accounting of all
particle accelerator components, eliminating highly emitting subsystems where possible, or mitigating the lifetime emissions of parts that can’t be replaced.
Magnets for beam steering and focusing are an example of an essential accelerator component that can’t be replaced, although their emissions may be reduced. As the beam energy rises during acceleration, the magnetic fields used to bend the beam must also be increased: in synchrotrons, this is achieved by utilising normal conducting electromagnets (or superconducting magnets), and increasing the current through the magnet coils synchronously with the rising energy. Manufacturing these magnets will lead to some carbon emissions (such as for producing steel used in the magnet yoke), but over the lifetime of the accelerator this will be dwarfed by emissions from operating the magnets, namely due to the electricity required to produce the magnetic fields and for temperature regulation where this is required. In a case study for the sustainability of RUEDI [4], it was found that the annual emissions from magnet operations is more than five times larger than from their construction: this is an extreme case, but it demonstrates that the lifetime emissions for an accelerator facility can be much more than just from the construction requirements. Although decarbonisation of power networks continues apace, alternatives to current magnet technologies should be considered to ensure that accelerators can meet environmental goals.
Permanent magnets present a solution to the problem of excessive operational emissions, as they do not require a time-dependent power supply to run. Arrays of rare earth permanent magnets in accelerators are not novel, and they are already widely used for wigglers and undulators at synchrotron light sources, as well as in linear accelerators. Unlike with electromagnets, where the field can be dynamically adjusted by varying the current through the magnet coils, permanent magnet arrays produce a fixed magnetic field. In cases where a ‘drop-in’ alternative for electromagnets is required, the Zero-Power Tuneable Optics (ZEPTO) project [5] proposes a solution: by moving a carriage of magnetic material inside in a broader steel structure, redirecting the lines of magnetic flux around an alternative magnetic circuit. This enables variable field strength using permanent magnets, with minimal power requirements beyond the relatively small amount required to move the magnet carriages. This is not without some downsides: the time taken to switch the field is slower than with conventional technologies, and the maximum field strength is less than a superconducting magnet could provide. Despite these disadvantages, ZEPTO-style magnets may be used to decarbonise some existing or new accelerator facilities without severely impacting operations, in a win-win situation for both users and the environment.
Future accelerator facilities are not necessarily bound by the limitations of current utilised technologies. Permanent magnets also find use in a wholly different class of accelerator, wherein a constant magnetic field is used to steer and focus all beam energies, with each different energy following a unique closed-orbit trajectory. These Fixed Field Accelerators (FFAs) remove the requirement for time-varying magnetic fields, however factors such as the design complexity imposed by the orbit variation have prevented their widespread adoption. This being said, FFAs should not be viewed as a ‘risky’ technology, and several examples have been successfully operated in recent years: in Japan, FFAs have been used to provide proton beams for experiments for many years, including radiation hardness testing for electronics and research into accelerator-driven nuclear reactors [6]; in the UK, the EMMA [7] accelerator used electromagnets to demonstrate a novel acceleration scheme for rapid acceleration, made possible by the momentum dependent orbit variation; and in the US, CBETA [8] became the first FFA to
use permanent magnets for the bulk of the accelerator, showcasing greatly improved accelerator energy efficiency with an ‘energy recovery linac’, but requiring many custom-manufactured wedges of magnetic material. In all cases, the energy efficiency is superior to an equivalent synchrotron, made even more apparent by the permanent magnet case where the only required power for magnets is for small correctors and temperature regulation. If FFAs were Fixed Field Accelerators more widely adopted, particularly those using permanent magnets, the lifetime emissions of accelerator facilities could be significantly reduced.
To minimise the lifetime emissions of an accelerator using permanent magnets, it is vital to ensure that the magnetic material can be readily
repurposed for different projects: this is not trivial for Fixed Field Accelerators, as they often require highly complex and specialised magnets, which may not be useful in other accelerators. The ZEPTO project found an innovative solution, wherein the mobile magnet carriage comprises many individual magnet blocks housed in an aluminium lattice, such that individual blocks may be replaced in case of damage, or redeployed in another ZEPTO magnet with a different design. In an FFA, a similar idea may be used to produce the required magnetic fields, using many individual magnet blocks rather than large custom pieces of magnetic material. This is one of the new concepts we have been working on as part of the TURBO project at the University of Melbourne [9], which seeks to demonstrate potential advances in proton therapy for cancer treatment by building a small scale technology demonstrator beamline: it is expected that each of the twelve magnets required will comprise approximately fifty individual blocks of NdFeB or SmCo, housed in a 3D printed or milled-aluminium casing. Designing and constructing our magnets in this way – removing the need for custom magnetic material – enables rapid prototyping of new ideas for magnet designs, brings down the cost per magnet, and of course, reduces both the waste and effective lifetime emissions for the components of the TURBO project.
Permanent magnets do not present a panacea for the environmental impacts of accelerators. One issue relates to the procurement of raw materials for rare earth magnets: although producing the magnets themselves does not have a significantly different environmental impact compared with electromagnets, the local environment around the mine can be severely contaminated by toxic metal dust [10], harming local flora and fauna as it pollutes the air and water. Another relates to the lifetime of the magnets, which can be curtailed by the demagnetising effects of radiation produced by accelerators. Although rare earth magnets have many applications beyond accelerators, including electric
vehicles and renewable power generation, there are currently very few environmentally friendly and efficient methods to recycle the raw materials [11], although this is an area of extensive ongoing research. As such, the use of permanent magnets for accelerators as a method of reducing emissions has many benefits, but caveats relating to their manufacture and end-of-life considerations must be appreciated.
In all, Fixed Field Accelerators and permanent magnet arrays offer many potential environmental advantages over currently prevalent technologies. Though there are some drawbacks, including production concerns for the magnets and additional complexities imposed by the use of static fields, they should both be viewed as valuable tools in our arsenal of weapons to improve accelerator sustainability. When considering the lifetime emissions and overall environmental impact of a particle accelerator, the use of rare earth magnets combined with Fixed Field Accelerator techniques presents a significant potential improvement, and they should be considered according to their merit
and strengths compared to conventional accelerator technologies.
1. Doyle, B. L., et al. ‘The Future of Industrial Accelerators and Applications’. Reviews of Accelerator Science and Technology 10 (2019): 93–116.
2. for example, see the recent ‘Sustainable High Energy Physics (HEP) workshop’, https://indico.cern.ch/event/1355767/overview
3. Lichter, K. E. et al. ‘Tracking and Reducing SF6 Usage in Radiation Oncology: A Step Toward Net-Zero Health Care Emissions’. Practical Radiation Oncology 13, no. 6 (November 2023)
4. Shepherd, B. et al. ‘Sustainability for Particle Accelerators: RUEDI – A Case Study’, 2024
5. Bainbridge, A. et al. ‘Demonstration of “ZEPTO” Permanent Magnet Technology on Diamond Light Source’. Proceedings of the 12th International Particle Accelerator Conference
6. Ishi, Y. et al. ‘Present Status and Future of FFAGs at KURRI and the First ADSR Experiment’. Proceedings of the 1st International Particle Accelerator Conference
7. Barlow, R. et al. ‘EMMA—The World’s First Non-Scaling FFAG’. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 624, no. 1 (December 2010): 1–19.
8. Bartnik, A. et al. ‘CBETA: First Multipass Superconducting Linear Accelerator with Energy Recovery’. Physical Review Letters 125, no. 4 (23 July 2020): 044803.
9. Steinberg, A. F. et al. ‘Design of a Large Energy Acceptance Beamline Using Fixed Field Accelerator Optics’. Preprint on arXiv: http://arxiv.org/abs/2402.01120.
10. Balaram, V. ‘Rare Earth Elements: A Review of Applications, Occurrence, Exploration, Analysis, Recycling, and Environmental Impact’. Geoscience Frontiers 10, no. 4 (July 2019): 1285–1303.
11. Kumari, A. and Sahu, S. K. ‘A Comprehensive Review on Recycling of Critical Raw Materials from Spent Neodymium Iron Boron (NdFeB) Magnet’. Separation and Purification Technology 317 (July 2023): 123527.
