A research group that included Professor Tomonao Hosokai from SANKEN (Institute of Scientific and Industrial Research) at the University of Osaka (also Team Leader at RIKEN), Director Masaki Kando, of the Kansai Institute for Photon Science (KPSI), National Institutes for Quantum Science and Technology (QST); and Professor Emeritus Shigeru Yamamoto from the Institute of Materials Structure Science (IMSS), KEK has succeeded in achieving free-electron laser (FEL) generation in the extreme ultraviolet (XUV) range using electron beams produced by laser wakefield acceleration (LWFA). Hosokai commented: "I have been involved in this research since 1998; plasma control technology has at last been established, and we have finally obtained a beam that rivals those from accelerators." Their findings were published in Physical Review Research.
An intense laser pulse generated by the upstream laser system is focused onto a supersonic gas-jet target, producing a plasma. Electrons are trapped and accelerated by the plasma wave (laser wakefield) excited in the plasma, resulting in the generation of a high-energy electron beam. The electron beam is transported through an electron beam transport line to a downstream undulator, where it undergoes transverse oscillations in the periodic magnetic field, leading to the emission of a free-electron laser in the XUV region.
Provided by the University of Osaka
The X-ray free-electron laser (XFEL) is an exceptionally powerful light source capable of generating ultrabright, coherent X-rays in femtosecond-order ultrashort pulses with a brightness approximately ten billion times that of the sun. Because it enables atomic and molecular structural changes to be observed as if they are being videoed, it has become an indispensable foundation for cutting-edge research worldwide, including observation of atomic arrangements, microstructural analysis of next-generation semiconductor materials, and ultrafast dynamics measurements of chemical reactions and biological molecules. However, the use of an XFEL requires high-energy, high-quality electron beams, and thus a large-scale facility equipped with a high-energy accelerator several hundred meters in length.
This is where LWFA has attracted attention. This is a technique in which a high-intensity laser pulse is used to create plasma, and electrons are accelerated within that plasma. While LWFA holds the potential for dramatically miniaturizing accelerators, plasma control is difficult and the quality of the electron beams it generates is highly variable. These disadvantages had prevented LWFA from achieving the stable, high-quality electron beams required for FEL generation, let alone XFEL generation.
The research group suppressed wave-front distortion of the laser pulse and improved the stability of the supersonic gas jet target used as a plasma source. They further developed techniques for precisely controlling its internal structure, and succeeded in raising the quality and stability of the electron beam to a level that enabled FEL radiation in the XUV range.
They resolved many technical challenges. For example, the peripheral portion of the laser pulse was removed using a spatial filter, and only the highly flat, central portion was used, improving the focusing accuracy. This stabilized plasma generation and, as a result, greatly reduced fluctuations in the electron beam generation point, improving both the stability and monochromaticity of the electron beam.
Principal Researcher Nobuhiko Nakanii at QST, remarked: "This was discovered by chance. We originally placed a small mask on the equipment with the intention of reducing the beam size for a different purpose, and found dramatic improvement. We also discovered that moving the mask allowed us to control the beam, so we incorporated it."
In addition, the group designed an internal flow-rectification structure to homogenize the gas flow, realizing a supersonic gas jet target with higher stability and reproducibility than previous approaches. They also developed a new technique for stably forming a sharp, step-shaped density structure inside the gas target using a shock wave, and succeeded in generating a highly monochromatic electron beam required for FEL generation.
By passing the high-quality, high-stability LWFA electron beams obtained through these improvements through an undulator, the researchers confirmed that the intensity of the radiation in the XUV range (wavelengths of 27-50 nm) was amplified by up to approximately 20 times relative to spontaneous emission, meaning that FEL radiation was achieved. Furthermore, by adding repelling magnets to cancel the strong magnetic attraction between the magnets, the undulator was made significantly more compact and lightweight, achieving miniaturization of the FEL generation section. This clearly demonstrated the feasibility of a future desktop FEL system.
The results obtained in this study demonstrate FEL generation driven by LWFA electron beams and open the path to realizing compact, coherent X-ray and XUV light sources that could be deployed even at the university or laboratory level. This is also an extremely significant milestone demonstrating that LWFA is approaching the level of a high-quality electron beam accelerator suitable for practical use. In the future, desktop XFELs may be realized, and the day may come when they are available in many laboratories.
Journal Information
Publication: Physical Review Research
Title: Optimized laser wakefield acceleration: Generating stable, high-energy, monoenergetic electron beams and demonstrating extreme-ultraviolet free-electron lasers
DOI: 10.1103/qvg7-ng8n
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.