A research group including Assistant Professor Shogo Isayama of the Faculty of Engineering Sciences, Kyushu University, Senior Principal Researcher Yuji Fukuda of the Kansai Institute for Photon Science, National Institutes for Quantum Science and Technology (QST), and Professor Yasuhiro Kuramitsu of the Graduate School of Engineering, Osaka University, has demonstrated ion wakefield acceleration for the first time in the world using QST Kansai's high-intensity laser facility, J-KAREN-P. By irradiating a foam target with a high-intensity laser, the researchers controlled the velocity of plasma waves propagating alongside the laser pulse and successfully accelerated ions by trapping them within the waves. The achievement represents the first laboratory reproduction of physical processes common to the acceleration of high-energy particles in the universe. The technique is expected to find applications in areas such as particle beam cancer therapy. The findings were published in Communications Physics on August 3.
(B) Analogous waves are excited when a laser propagates through plasma. In low-density plasma, the wave travels nearly at the speed of light, so mainly light electrons can be trapped. In an intermediate-density foam target, the wave propagation speed is reduced, allowing heavy protons to be trapped and accelerated at the leading bow-wake.
Credit: Shogo Isayama, Kyushu University
In accelerator science and medicine, a key challenge is the development of more compact and capable high-energy ion accelerators. Conventional accelerators, such as the Large Hadron Collider, become increasingly large as higher particle energies are sought. In astrophysical environments, ions are believed to undergo a form of "surfing" acceleration. Direct acceleration by wakefields with strong electric fields has therefore attracted attention as a potential mechanism for generating ultra-high-energy cosmic rays.
Just as a ship creates waves in its wake as it moves through water, an intense laser propagating through plasma can generate a wakefield. These wakefields contain strong electric fields capable of accelerating charged particles. While lightweight electrons can readily be trapped and accelerated by such waves, the much heavier protons and ions have proved far more difficult to accelerate efficiently.
In the present study, the researchers used a foam target capable of producing denser plasma than conventional gas targets, allowing them to control the propagation speed of the laser pulse. By reducing the phase velocity of the plasma wave travelling with the laser pulse, they created conditions under which even nearly stationary protons could be trapped. The protons were accelerated to energies of up to 60 MeV.
Once trapped by the wakefield's electric field, the protons are accelerated as they ride the plasma wave propagating with the laser pulse. This acceleration mechanism is known as bow-wake acceleration.
The team aims to extend the technique to a two-stage acceleration scheme in which ions are first accelerated by one laser and then injected into a wakefield generated by a second laser for further acceleration. By effectively giving the heavier ions a "running start," researchers expect to inject them into faster-moving wakefields and accelerate them to even higher energies.
Ultimately, the goal is to develop a multi-stage acceleration system capable of generating higher-energy ion beams, paving the way for next-generation laser-driven ion accelerators.
Isayama said, "In this study, we reproduced in the laboratory a mechanism that shares the same underlying physics as high-energy particle acceleration in the universe and demonstrated the principle of accelerating protons by trapping them in plasma waves. We hope this work will contribute both to a better understanding of how cosmic rays attain extremely high energies and to the realization of compact next-generation laser-driven ion accelerators."
Journal Information
Publication: Communications Physics
Title: Bow-wake acceleration of ions toward a relativistic regime
DOI: 10.1038/s42005-026-02749-7
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.