The miniature accelerator
The image that most people have of CERN is of its enormous accelerators and their capacity to accelerate particles to extremely high energies. But thanks to some cutting-edge studies on beam dynamics and radiofrequency technology, along with innovative construction techniques, teams at CERN have now created the first module of a brand-new accelerator, which will be just 2 metres long. The potential uses of this miniature accelerator will include deployment in hospitals for the production of medical isotopes and the treatment of cancer. It’s a real David-and-Goliath story.
Serge Mathot, in charge of the construction of the "mini-RFQ", pictured with the first of the four modules that will make up the miniature accelerator.
The miniature accelerator consists of a radiofrequency quadrupole (RFQ), a component found at the start of all proton accelerator chains around the world, from the smallest to the largest. The LHC is designed to produce very high-intensity beams at a very high energy, but its little brother is content to produce beams at low speeds, containing particles which, after travelling two metres, have an energy of 5 MeV. “When we took up the challenge of creating the first high-frequency compact RFQ accelerator, with the support of the Office for CERN Medical Applications, we knew that the technology was within our reach after all the years we had spent developing innovative solutions for Linac4," explains Maurizio Vretenar, head of the Linac4 project and coordinator of the small accelerator project.
From the start, the small accelerator was designed to be modular, compact and cheaper than its big brothers. “We carried out feasibility and beam dynamics studies for several months,” adds Alessandra Lombardi from the BE department, who is in charge of the design of the RFQ. “The starting point was the idea of increasing the operating frequency by a factor of 2 compared to the most recent RFQs, which would allow us to reduce the dimensions proportionally. But such a frequency had never been achieved before and posed a number of new challenges. At first it seemed impossible, but eventually, thanks to new beam dynamics and innovative ideas for the radiofrequency and mechanical aspects, we came up with an accelerator design that was much better adapted to the practical requirements of medical applications.”
The RFQs used for physics are designed to produce high-intensity beams, but this “mini-RFQ” is able to produce low-intensity beams of just a few microamps that are stable beams (no significant losses) and are grouped at a frequency of 750 MHz. These specifications make the “mini-RFQ” a perfect injector for the new generation of high-frequency compact linear accelerators used for the treatment of cancer with particles (hadron therapy). In addition, its small size belies its remarkable power: the “mini-RFQ” accelerates beams to an energy of 2.5 MeV per metre, compared to less than one MeV per metre for a classic RFQ.
The construction of the first of the four 50 cm long modules that will make up the final accelerator has been successfully completed in CERN's workshops and in a few months’ time the teams will be able to test all of the modules together. “With this first module, we have validated all of the stages of construction and the concept in general,” explains Serge Mathot of the EN department, who is in charge of the construction of the “mini-RFQ”. “At first, several stages in the construction process seemed very tricky, but thanks to the experience we gained from the brazing of the cavities for Linac4 and to the skills of CERN’s technical teams, we were eventually able to obtain excellent results, even when faced with a new technological challenge.”
The applications of this high-tech miniature accelerator go far beyond its use as an injector for hadron therapy. Thanks to its small size and light weight, the “mini-RFQ” could become the key element of a system able to produce radioactive isotopes on site in hospitals for use in medical imaging. This could avoid complications relating to the transportation of radioactive material and could also widen the range of isotopes produced for this purpose. Small but powerful and with many potential uses, the “mini-RFQ” will also be capable of accelerating alpha particles for advanced radiotherapy techniques, which many consider to be the new frontier in the treatment of cancer. And, to finish on a lighter note, its small size means that in principle it can be fairly easily transported, which would be particularly useful for the analysis of archaeological materials and objects.
Assembly of the four modules is planned for the start of next year.
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