Current estimates suggest that nearly half of all accelerators worldwide are used for biomedical applications. Other applications of particle accelerators include advanced light sources that use the acceleration of electrons to generate high-energy photons to perform spectroscopy and imaging for materials science and fundamental research.
Image Credit: Thomas Hecker/Shutterstock.com
A hospital seems like an unlikely setting for a particle accelerator, but many medical treatments rely on the use of such accelerators. In hospital environments, particle accelerators are used for both diagnosis and treatment. Particle accelerators can be used to prepare radioisotopes for diagnosis and also to prepare beams of charged particles for treatment with methods such as proton beam therapy.
Probably the most famous particle accelerators are those at CERN, where a number of accelerators work together to accelerate particles to some of the highest energies on Earth.
While designs and scales of particle accelerators vary greatly, nearly all of them rely on the use of electromagnetic fields to control the trajectory of particles and accelerate them to ever greater speeds. Most accelerators are made of a particle source to generate the charged species to be accelerated, and then a series of electromagnets around a beam pipe that transports the charged species.
With advances in electromagnets, it is now possible to carefully tune and control the fields that the charged particles are exposed to. By carefully controlling the charged particle orbits, it is possible to create unusual and complex pulses of light with extremely short temporal durations.
Electromagnetic control of charged particles can be used to precisely control dosing and irradiation during proton beam therapy for focusing the beams to tight spots to limit damage to surrounding tissue and achieve the correct penetration depth.
Hardware Challenges
Although the applications of particle accelerators range from fundamental high energy physics to practical medical treatments, one of the issues that all applications face is often the scale of the hardware. Synchrotrons and other cyclotron-based designs use circular orbits for particles to keep orbital distances large without requiring very long linear footprints. One of the world’s largest linear accelerators now exceeds over 3 km in length for just the acceleration region.
Generally, longer acceleration lengths and times make it possible to achieve higher particle energies, but this rapidly becomes problematic from an engineering and space perspective. While not all applications require particles as energetic as those used for collision experiments at CERN, hospital position electron tomography (PET) scanners still require bulky infrastructure and are highly inflexible in how patients have to be imaged.
Accelerator on a chip
How can we miniaturize a 3 km long accelerator? A team of scientists, led by Prof. Dr. Peter Hommelhoff from the Friedrich-Alexander-Universität Erlangen-Nürnberg, think they may have the answer. By using a technology called dielectric laser acceleration.
Dielectric laser acceleration uses a combination of ultrafast laser pulses and field-free drift regions to focus the electron beam. One of the challenges with controlling charged particle beams in three dimensions is dealing with the space-charge effects that occur when particles of like charges repel each other. This causes the particle beam to spread out and defocus.
The advantage of the dielectric laser acceleration is that it can achieve significantly higher acceleration gradients than is possible with most other accelerator technologies. This is due to the double grating structure in the device that, when combined with the longitudinally oscillating field of an incident laser beam, can support speed-of-light acceleration.
INFN Particle accelerator. Image Credit: GinkgoBiloba/Shutterstock.com
This cannot be achieved with single grating structures, and the laser beam alone has a phase velocity that is not equal to the speed of light, so cannot accelerate the particles for any more than short distances.
Over just 80 micrometers, the team was able to demonstrate that dielectric laser acceleration had 35 times the acceleration potential over other currently used accelerator technologies. Current accelerators compensate for only being able to accelerate electrons at the speed of light for short distances by having many accelerating units joined together, which ultimately results in the huge sizes of many accelerators.
The new, compact acceleration chips could lead to a remarkable scaling down of accelerator technologies. The challenge is designing and building the chips so they can achieve the required alternating phase focusing for the height and width of the beam in subsequent steps of the design. It is not possible to focus the beam in both dimensions simultaneously though with most complex alternating phase focusing schemes more exotic beam configurations could also be created.
Now the team wishes to push to smaller devices creating a full ‘accelerator on a chip’. Some of the first proof-of-principle experiments were carried out on dielectric laser acceleration just over ten years ago and the technology is already achieving excellent results, which may mean a linear accelerator in the laboratory is not so far away.
References and Further Reading
Silari, M. (2011). Applications of particle accelerators in medicine. Radiation Protection Dosimetry, 146(4), 440–450. https://doi.org/10.1093/rpd/ncr243
Mirian, N. S., Di Fraia, M., Spampinati, S., Sottocorona, F., Allaria, E., Badano, L., Danailov, M. B., Demidovich, A., De Ninno, G., Di Mitri, S., Penco, G., Rebernik Ribič, P., Spezzani, C., Gaio, G., Trovó, M., Mahne, N., Manfredda, M., Raimondi, L., Zangrando, M., … Giannessi, L. (2021). Generation and measurement of intense few-femtosecond superradiant extreme-ultraviolet free-electron laser pulses. Nature Photonics, 15(7), 523–529. https://doi.org/10.1038/s41566-021-00815-w
Decking, W., & Weise, H. (2017). Commissioning of the European XFEL Accelerator. Proceedings of the 8th International Particle Accelerator Conference (IPAC2017), 8, 1–6. http://jacow.org/ipac2017/doi/JACoW-IPAC2017-MOXAA1.html
Shiloh, R., Illmer, J., Chlouba, T., Yousefi, P., Schönenberger, N., Niedermayer, U., Mittelbach, A., & Hommelhoff, P. (2021). Electron phase-space control in photonic chip-based particle acceleration. Nature, 597(7877), 498–502. https://doi.org/10.1038/s41586-021-03812-
Breuer, J., & Hommelhoff, P. (2013). Laser-based acceleration of nonrelativistic electrons at a dielectric structure. Physical Review Letters, 111(13), 1–5. https://doi.org/10.1103/PhysRevLett.111.134803
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