QST achieves laser acceleration of carbon ions to 1 gigaelectron volt: Expectations for significant downsizing of cancer treatment equipment

A research team led by Senior Principal Researcher Yuji Fukuda at QST Kansai Institute for Photon Science, Professor Yasuhiro Kuramitsu from the Graduate School of Engineering at the University of Osaka, and Associate Professor Masato Kanasaki from Kobe University achieved laser acceleration of carbon ions to 1 gigaelectron volt using J-KAREN-P, Japan's largest ultra-short pulse, ultra-high intensity laser. This was achieved through the development and introduction of a laser beam image-relay system to increase laser intensity, and by using graphene with gold deposited on it as the target. The 1 gigaelectron volt represents the world's highest energy for carbon ion acceleration using ultra-short pulse lasers. This breakthrough will not only lead to significant downsizing of heavy particle beam cancer treatment equipment but also offers the possibility of experimentally reproducing plasma states from space that are difficult to observe directly.

Fukuda stated: "The radiation pressure of J-KAREN-P is equivalent to the pressure at the center of the Sun, making it possible to reproduce and verify plasma similar to the ultra-high temperature, high-density plasma that exists in space within the laboratory. For example, we can now engage in new research such as simulating plasma states near supernova remnants and reproducing the previously difficult-to-observe process by which plasma magnetic energy is converted into the kinetic energy of accelerated ions."

Among laser-based ion acceleration technologies, thin-film target acceleration has been theoretically shown to accelerate ions to higher energies by increasing laser energy while making the laser-irradiated target thinner. However, this requires both laser intensification and nanometer-precision control of target thickness and composition optimized for laser conditions, which have been difficult to achieve simultaneously.

The laser development team at QST Kansai developed and introduced an image-relay system into the laser pulse transmission section of J-KAREN-P, successfully transmitting the laser beam to the focusing mirror in front of the target without degrading the laser intensity distribution at the laser device exit. This achieved significant improvements in focusing conditions and realized high-intensity laser irradiation with transmissible laser pulse energy increased to 150% compared with conventional levels. The image-relay system involves installing a focusing mirror at the laser device exit and can stabilize the laser beam intensity distribution. The introduction of large, high-precision lenses was the key to success.

When developing the target, the researchers established a technique for depositing gold on the surface by stacking multiple sheets of graphene, a sheet-like material with large area and atomic-level thickness, successfully achieving free control of thickness and composition with nanometer precision for the first time. Gold, with its high atomic number and electron content, increases electron density in plasma and enhances the electric field strength in laser-plasma acceleration, enabling high-energy ion acceleration. The key was Kuramitsu's technique for fixing thin targets. In these experiments, gold with a thickness of 30 nanometers was deposited on four sheets of graphene. Kuramitsu noted: "Simulations showed 8-fold acceleration compared with graphene alone, but we achieved 10-fold acceleration in the experiment."

Through the synergistic effect of laser intensification and improved target fabrication technology, they successfully achieved 1 gigaelectron volt carbon ion acceleration, representing the world's highest energy for ion acceleration using ultra-short pulse lasers. This significantly surpasses the previous record of 0.6 gigaelectron volts (South Korea, 2019). Using the target fabrication technology developed in this study, even higher energy carbon ion acceleration is expected as J-KAREN-P continues to be upgraded.