Media buzz develops about a particle-accelerator advance
DOI: 10.1063/PT.5.8085
“Using one of the most powerful lasers in the world,” exclaims the “lede” in an 8 December press release from Lawrence Berkeley National Laboratory (LBNL), “researchers have accelerated subatomic particles to the highest energies ever recorded from a compact accelerator.” The release says the experiment illustrates the potential of “an emerging class of particle accelerators that physicists believe can shrink traditional, miles-long accelerators to machines that can fit on a table.” More than a smattering of press attention has ensued so far.
Wim Leemans and colleagues from LBNL and from the University of California, Berkeley, report the work in the paywalled 8 December Physical Review Letters paper “Multi-GeV electron beams from capillary-discharge-guided subpetawatt laser pulses in the self-trapping regime.” Many physicists and others, however, will want to see the “viewpoint” explanatory overview essay that the American Physical Society has posted online from Georg Korn of ELI Beamlines, Institute of Physics of the Academy of Sciences, Czech Republic. APS says that this kind of write-up is “aimed at the reader who wants to keep up with highlights of physics research with explanations that don’t rely on jargon and technical detail.” The venue is comparable to Nature’s News and Views pieces—a step or two down from the reading and interest level of the highly specialized, a step or five up from what’s in the lay press, with the added benefit of the commenting expert’s judgment about what it all means.
Korn’s lede exclaims, “A laser-driven particle accelerator, delivering a beam of electrons with a record-breaking energy of 4.2 giga-electron-volts, could lead to compact x-ray lasers or high-energy colliders.” He first summarizes disadvantages of large conventional accelerators and the prospect of future machines—kilometers long and costing billions. Then he describes the emergence in recent years of an alternative: laser wakefield acceleration (LWFA), in which short, intense laser pulses interact with a plasma to create accelerating electric fields of several hundred GeV per meter. With a 9-cm-long plasma waveguide, Korn writes, the Berkeley experimenters have demonstrated delivery of a high-quality electron beam with energy “in the same ballpark” as that of many large-scale accelerators. As Berkeley’s press release puts it, the result “corresponds to an energy gradient 1000 times greater than traditional particle accelerators.”
A technical illustration accompanying Korn’s write-up depicts the experiment. The caption narrates: “Laser pulses with 16 joules energy and 40 femtoseconds duration (corresponding to a peak power of about 0.3 petawatts) are focused into a 9-cm-long capillary waveguide in which a plasma was created by pulsing an electrical discharge through the capillary. Through laser-wakefield acceleration, electrons in the plasma are accelerated through the guiding structure up to an energy of 4.2 giga-electron-volts, as measured by monitoring their deviation in a magnetic spectrometer.”
Korn also summarizes the LWFA technique:
As the pulse propagates through the plasma, its electric field separates the plasma’s electrons from the ions. On the pulse trail, the displaced electrons feel an enormous electrostatic force pulling them back toward the heavier, barely moving, ions. This bubble of negative charges trails the laser pulse, moving through the plasma at about the speed of light and producing, in the laser pulse’s wake, a traveling longitudinal electric field that offers a steep acceleration gradient: for typical plasma electron densities (1018–1019 cm−3), the resulting accelerating structure is only only 10–100 μm long and can sustain fields of over 100 GV/m, outperforming conventional acceleration structures based on superconducting rf cavities by 2–3 orders of magnitude. Once electrons are injected in this plasma wakefield structure they can be accelerated to relativistic energies within very short distances.
He cites previous advances, outlines advances seen in this experiment, and declares that the work “can be regarded as a substantial advancement towards two grand goals of laser plasma acceleration sources.” One is a linear electron–positron collider that would involve multiple LWFA stages and that, with decades of further R&D, could rival the present vision for a huge International Linear Collider. The paragraph about the second grand goal, which Korn sees as “within closer reach"—and which media reports generally underplay or ignore—requires verbatim quoting:
The demonstrated high-energy electron beam could be used to feed an x-ray free-electron laser (XFEL) that would fit into a university laboratory. In [an] FEL, high-energy electrons zigzag through a periodic arrangement of magnets, called an undulator, organizing themselves into microbunches that emit coherent, laserlike radiation. A laser-driven XFEL would be a very compact alternative (less than 30 m long) to current kilometer-long XFELs like the LCLS or the European XFEL. Should tabletop XFELs become available, a vast number of applications, most notably the determination of the 3D structure of biomolecules, could be carried out by a much broader community of researchers, complementing large-scale facilities where beam time is expensive and scarce. Researchers have already demonstrated that laser-accelerated electrons, fed into undulators, can generate (incoherent) short pulse radiation at soft-x-ray wavelengths. If the electrons can be made to emit x rays coherently, the device would turn into [an] FEL. It is reasonable to think that such a compact laser-driven XFEL could become a reality within the next decade.
Korn ends by citing a few technological challenges, then declaring that “many of these obstacles will likely be overcome in coming years, in particular thanks to continuing advances in laser technology, which commercial and scientific applications drive at an extremely rapid pace.”
---
Steven T. Corneliussen, a media analyst for the American Institute of Physics, monitors three national newspapers, the weeklies Nature and Science, and occasionally other publications. He has published op-eds in the Washington Post and other newspapers, has written for NASA’s history program, and is a science writer at a particle-accelerator laboratory.