Ultra-intense Laser Achieves Efficient Direct Ion Acceleration

Osaka University researchers, in collaboration with the National Institutes for Quantum Science and Technology (QST), Kobe University, and National Central University in Taiwan, have reported direct energetic ion acceleration by irradiating what they claim to be the world’s thinnest and strongest graphene target with the ultra-intense J-KAREN laser at Kansai Photon Science Institute at QST in Japan.

Thin targets are necessary for higher ion energy in laser ion acceleration. However, it’s been difficult to directly accelerate ions with an extremely thin target regime because the noise components of an intense laser tend to destroy the targets before the main peak of the laser pulse. To mitigate this, plasma mirrors are deployed to remove the noise components in order to realize efficient ion acceleration with an intense laser.

(a) Schematics of experiment. By irradiating a large-area suspended graphene target (LSG) with the ultra-intense J-KAREN laser, energetic ions are generated; (b) and (c) show the Raman spectrum and microscope image of graphene, respectively; (d) and (e) show the schematic drawing of stack detector using solid-state path trackers and Thomson parabola spectrometer (TPS), respectively; (g) and (f) show the typical data from TPS and stack, respectively. Courtesy of Y. Kuramitsu et al. Robustness of large-area suspended graphene under interaction with intense laser. Courtesy of Scientific Reports.

The researchers therefore developed large-area suspended graphene (LSG) as a target of laser ion acceleration.

“Atomically thin graphene is transparent, highly electrically and thermally conductive, and lightweight, while also being the strongest material,” said study author Wei-Yen Woon of National Central University. “To date, graphene has seen a variety of applications, including those in transportation, medicine, electronics, and energy. We demonstrate another disruptive application of graphene in the field of laser-ion acceleration, in which the unique features of graphene play an indispensable role.”

Direct irradiation of the LSG targets generate MeV protons and carbons from subrelativistic to relativistic laser intensities from low-contrast to high-contrast conditions without a plasma mirror, demonstrating the durability of graphene.

“The outcomes of this research are applicable to the development of compact and efficient laser-driven ion accelerators for cancer therapy, laser nuclear fusion, high-energy physics, and laboratory astrophysics,” said lead author Yasuhiro Kuramitsu of Osaka University.

The direct acceleration of energetic ions without the use of a plasma mirror, he continued, demonstrates the utility of LSG. The researchers plan to use the atomically thin LSG as target mount to accelerate other materials that can’t stand on their own.

They also plan to show the energetic ion acceleration at nonrelativistic intensity, which would enable the investigation of laser ion acceleration at relatively small laser facilities.

“Furthermore, even without a plasma mirror at the extremely thin target regime, energetic ion acceleration is realized,” Kuramitsu said. “This opens up a new regime of laser-driven ion acceleration.”

The research was published in Scientific Reports (www.doi.org/10.1038/s41598-022-06055-4).