Researchers from the University of Osaka accelerate protons to record energies using ultrathin graphene, opening new possibilities for next-generation ion sources

Osaka, Japan – Imagine a proton catching a wave and surfing it to gain speed. While the imagery may seem wild, the premise of laser-driven ion acceleration has promise as an alternative to conventional accelerators. However, the ultrathin targets used to drive this increase in ion energy are vulnerable to the weak prepulse that precedes the main high-intensity laser pulse, meaning an adaptation in the process is necessary.

Nevertheless, researchers from the University of Osaka have now used lasers to create a moving electric field that accelerates protons to very high energies. This adapted approach could ultimately contribute to the development of next-generation particle accelerators, with fewer concerns about target vulnerabilities. The findings have recently been published in Progress of Theoretical and Experimental Physics.

The team used a long-pulse laser and ultrathin graphene targets to accelerate protons to 132 MeV, nearly half the speed of light. The experiment demonstrated improved capabilities for long-pulse laser-driven ion acceleration, while simulations showed that protons gained energy from a moving electric field over an extended period.

“By using ultrathin graphene layers and a relatively long laser pulse, we are able to accelerate protons for an extended period and reach a record energy of 132 MeV,” says lead author, Takumi Minami. “Our results show that long-duration acceleration can push proton energies beyond those typically achieved with shorter laser pulses.”

The key to this approach involved the properties of graphene, which has an unusual combination of extreme thinness and durability. The nanometer-thick graphene targets withstood the laser’s initial prepulse, allowing them to remain intact until the main pulse arrived. Simulations indicated that the laser then generated a propagating electrostatic wave that moved through the plasma, creating an accelerating field that carried energetic protons forward for several picoseconds.

“The challenge is not only to produce these rare high-energy protons, but also to reliably identify them,” says senior author, Yasuhiro Kuramitsu. “We need to search millions of detector images for signals left by individual ions and distinguish the highest-energy protons from background noise.”

High-energy protons are rare, meaning the signals they leave in the detector are difficult to identify. To overcome the arduous challenge of examining millions of images, the researchers used a convolutional neural network to search for proton signals. The neural network achieved 99.2% precision in one high-energy measurement and allowed the team to confirm the presence of proton signals reaching 132 MeV.

The findings highlight the advantages of both long-pulse laser acceleration and AI-based detection for exploring higher-energy ions, with further developments being made in real-time online ion detectors. Combining these technologies could eventually allow laser experiments to analyze their results and optimize themselves, bringing the field closer to autonomous laser systems and exciting new possibilities for future research.
###
The article, “Proton surfing acceleration via propagating electrostatic waves induced by intense laser irradiation on large-area suspended graphene,” was published in Progress of Theoretical and Experimental Physics at DOI: https://doi.org/10.1093/ptep/ptag130

About The University of Osaka
The University of Osaka was founded in 1931 as one of the seven imperial universities of Japan and is now one of Japan's leading comprehensive universities with a broad disciplinary spectrum. This strength is coupled with a singular drive for innovation that extends throughout the scientific process, from fundamental research to the creation of applied technology with positive economic impacts. Its commitment to innovation has been recognized in Japan and around the world. Now, The University of Osaka is leveraging its role as a Designated National University Corporation selected by the Ministry of Education, Culture, Sports, Science and Technology to contribute to innovation for human welfare, sustainable development of society, and social transformation.
Website: https://resou.osaka-u.ac.jp/en