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Table-top particle accelerators could be poised to move from fantasy to reality thanks to a novel optical lens system.
Accelerating particles to near light speed in order to smash them together to investigate the fine structure of the Universe has always been a large-scale operation. From the earliest particle accelerators to the current generation of accelerators, the motto has always been ‘bigger is better’. This is perfectly exemplified by the most powerful particle accelerator ever constructed — CERN’s Large Hadron Collider (LHC) — which runs for 17 miles under the border of France and Switzerland.
This may be about to change, however. Using a novel optics system and specially designed laser, researchers from the University of Rochester’s Laboratory for Laser Energetics (LLE) including John Palastro, Plasma Theory Group Leader, and Dustin Foula, a senior scientist, believe they could have found a way to significantly scale-down particle accelerators. This revolution could result in particle accelerators being present in laboratories across the globe, becoming more accessible to more and more scientists.
This development could grant more benefits than just a more widespread investigation into fundamental particle physics. Whilst there is no doubt that the LHC’s crowning achievement was the discovery of the Higgs Boson — the particle which grants other particles mass and has been, somewhat glibly, dubbed the ‘God Particle’ — particle accelerators have more down to Earth commercial uses. Amongst these uses is one that could take particular importance as a result of the COVID-19 pandemic crisis.
The sterilization of disposable medical equipment using an electron beam (E-Beam) was first introduced to the commercial market by Johnson&Johnson in 1956. It has since been adopted by a wide range of companies. The process uses an accelerator to create a powerful stream of electrons in a curtain that bombards medical supplies passing underneath it. The electrons can stream through packaging and containers thus making it an efficient way to sterilize small pieces of equipment quickly and efficiently.
With companies such as EBeam Services offering the medical industry quick and efficient sterilization, the cost benefits of a smaller system that requires less space to operate and less energy to power are clear. The question is, how would such a small system work?
Making Table-Top Acceleration Feasible
The team’s paper, published in the journal Physical Review Letters details a method by which intense laser light can be used to accelerate electrons to record energies, but over incredibly short distances in comparison to the near 17 miles of the LHC.
The system could be so effective that it could reduce a potential accelerator by a factor of 10,000. To put this into perspective that is equivalent to reducing an accelerator that would reach from one end of Rhode Island to the other to one that would fit on a modest dining-room table.
The key to the size reduction lies in a system of exotically shaped lenses — an axiparabola which resembles an amphitheatre with wavelength sized steps— which allow the production of “diffraction-free” beams to high-peak-power and broadband laser pulses to be extended over the distance of a few meters. The design of this axiparabola means that the light pulse that passes through it can have the peaks of its maximum intensity shifted to travel at varying velocities.
The lenses in question trump traditional lenses as, instead of being designed to focus light from a laser source to a single distance from them, these lenses focus each ring of light to a different distance. This creates a line of high-intensity light rather than a single point.
This optical setup eliminates many of the drawbacks and weaknesses of Laser Wakefield Accelerators (LWA)— first theorized over 40 years ago — which this study builds upon. LWAs use unstructured light pulses which spread more slowly than light speed. This means that electrons are able to outpace the wake of the light thus reducing the amount of acceleration achievable.
This new optical system allows an almost ‘sculpted’ light pulse in which the intensity peak travels so fast that electrons have no chance of outrunning it. Thus, the electrons can ride this wake and be almost continuously accelerated.
The lenses that will be used by the system were designed at the experimental and theoretical plasma physics groups and built by the Optics Manufacturing Group, all of which are located at LLE. The novel lens system is only part of the story when it comes to this potential avenue to table-top particle acceleration, however. The system also needs an incredibly powerful laser to provide the light pulse upon which the electrons can surf.
Fortunately, the team at LLE has plans for the most powerful laser in the world at its disposal. The laser with the provisional name EP-OPAL will provide the researchers with their sculpted laser pulses. The fact that researchers at LLE have access to theoretical developments and experimental plasma physics gives them a major lead in the development of the kind of technology that could lead to table-top accelerators.
With that said, as it stands, this work is almost purely theoretical. In addition to this, it is also worth noting that whilst the scaled-down accelerator discussed could theoretically match the power of the LHC in terms of accelerating electrons, it would be impossible for it to accelerate heavier particles such as protons. Thus, whilst these table-top accelerators could eventually provide an alternative to linear colliders , they are unlikely to unseat the LHC from its unique placement in this realm of technology.
Thankfully, the acceleration of electrons is all that is required for the efficient sterilization of medical supplies, and it is heartening to think that in the future, thanks to research such as that which this article has discussed, this could be done more cheaply and quickly.
References and Further Reading
Palastro. J.P. Shaw. J. L, Franke. P, et al, [2020], ‘Dephasingless Laser Wakefield Acceleration,’ Physical Review Letters, https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.134802
Smartsev. S, Caizergues. C, Oubrerie. K, et al, [2019], ‘Axiparabola: a long-focal-depth, high-resolution mirror for broadband high-intensity lasers,’ Optics Letters, https://doi.org/10.1364/OL.44.003414
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