CERN Accelerating science

A revolutionary mini-accelerator
by Panos Charitos (CERN)

A glimpse in the accelerator structures of the world’s smallest accelerator (Credit: CERN)

CERN is the home of the 27-kilometre Large Hadron Collider (LHC) that searches for new discoveries by colliding protons at extraordinarily high energies. The unprecedented energy levels led to the discovery of the Higgs boson, the last missing piece in the Standard Model, and now open a new chapter in fundamental physics. The development of such complex machines is based on the advancement of novel technologies and invaluable know-how, which can be capitalised in other fields outside particle physics.

Sometimes working for the largest accelerators gives ideas on how to build the smallest ones; the construction of the world’s smallest Radio Frequency Quadrupole (RFQ) for proton acceleration that was completed in September provides one of the most successful examples. This miniature machine is a linear accelerator (linac) consisting of four sections of only 130 mm diameter, operating at a frequency of 750 MHz, for a total length of 2 metres. It can accelerate low-intensity proton beams of a few hundreds of microA up to the energy of 5 MeV.

It should be noted that the mini RFQ cannot be used for the large colliders needed for fundamental research, since it cannot achieve high peak currents. The small size and low current is however what makes this design ideal for a wide range of medical and industrial applications.

Maurizio Vretenar (CERN), head of the LINAC4 project and coordinator of the design and construction of the mini accelerator, said: “The challenge to develop this miniature accelerator came from a spin-off company that aims to take advantage of the knowledge and infrastructure of CERN in building new accelerators. The main idea was that a mini-RFQ is a much more efficient injector than a cyclotron to a compact proton linac for particle therapy. The linac-based facility under development will permit a more precise 3D scanning of tumours than what is possible with other proton therapy machines or conventional radiotherapy.”

Vretenar explained: “Reaching high frequencies is particularly challenging, but it is the only way to build compact accelerators. For proton linacs at CERN, we started with the 200 MHz LINAC2 at the end of the 1970s and since then we have almost doubled the frequency to 350 MHz for the recently commissioned LINAC4. With the new LINAC4 we will be able to double the beam intensity in the LHC injectors, thus significantly contributing to an increase of the LHC luminosity,” and continues: “the idea of constructing a smaller accelerator that could produce low-intensity beams for medical purposes has been a long-standing technological challenge. It dates back to the 1990s when it seemed almost impossible to build such a small RFQ.”

The rich experience that the CERN team has gained from the design and development of LINAC4 made a new miniature RFQ accelerator seem more plausible. The main challenge was to double the operating frequency, resulting in more accelerating cells and a shorter length, but at the same time leading to a very challenging beam optics design and RF resonator. With the high frequency RFQ, we have more than doubled the accelerating capabilities (2.5 MeV/metre in place of 1 for the LINAC4 RFQ) and reduced by a factor 2 the construction cost per metre.

The way to the higher frequencies was opened by a new beam dynamics approach developed by Alessandra Lombardi, who now follows the testing and commissioning of the RFQ in ADAM’s premises. The next challenges to address were the tuning of RFQs that are long with respect to the wavelength and the machining and brazing of RFQ parts of unprecedented small size.

The design and construction of the RFQ relied on a sophisticated mechanical approach defined by Serge Mathot and on a detailed definition of the resonator properties and tuning strategy by Alexej Grudiev (BE).

Thanks to the collaborative spirit and the passionate work of CERN’s people who worked in this project, the team recently completed the brand-new mini accelerator. The four modules that make up the final accelerator have been entirely constructed in CERN's workshops within less than two years through the effort of a small but enthusiastic team. The fact that what they were building could help treating thousands of patients gave extra motivation to everyone involved in the project. In addition, Serge Mathot explains: “the construction was a very delicate procedure, given the need for high precision and the geometry of each module. Thanks to the experience and the skills we have gained from our previous works on the cavities for LINAC4, we successfully met the challenges of this project”.

Serge Mathot in front of one of the four modules (Credit: CERN)

The technological breakthrough achieved by the team behind the mini-accelerator has attracted interest from the industry, in first instance from A.D.A.M. SA, which stands for Applications of Detectors and Accelerators to Mediciane, a Geneva-based spin-off company from CERN, and from its parent company Advanced Oncotherapy in the United Kingdom. "Behind every innovative aspect of this accelerator, there is unique CERN intellectual property and know-how", says David Mazur from CERN's Knowledge Transfer Group, "and we have concluded a license agreement with A.D.A.M. SA which enables them to commercialize such accelerators in the field of proton therapy, based on our IP".

The mini accelerator was delivered to the ADAM test facility last September and is presently being commissioned. It is more modular, more compact and cheaper than its “big brothers”. Its small size and light weight mean that the mini-RFQ could become the key element of proton therapy systems but also of systems able to produce radioactive isotopes on-site in hospitals.

The mini accelerator (RFQ) installed in the ADAM test stand (Credit: ADAM)

The team that developed the mini-RFQ foresees many other potential medical applications, such as acceleration of alpha particles for advanced radiotherapy techniques that may be the new frontier in the treatment of cancer or industrial applications, where a mini accelerator could analyse the quality of surfaces or trace aerosol pollution for example.

Also, the small size of the new accelerator means that it can be easily transported, which would be particularly useful for the surface analysis of archaeological materials or artworks presently exhibited in museums around the world, using proton-induced x-ray emission (PIXE) analytical technique. Indeed a new generation of mini accelerators have great potential and could find numerous applications in many fields. The mini-RFQ offers another example of the societal benefits stemming from fundamental research.