Finding fusion’s missing input energy

One technological constraint for inertial confinement fusion is that lasers lose energy when they pass through ionized plasma gas. In the past, Researchers have mainly numerically modeled one possible laser-plasma interaction, stimulated Raman scattering (SRS), for light scattered in the opposite direction of the laser.

Vilayphone et al. developed code that models SRS in multiple scattering directions and computes Raman spectra. Their code can be applied to large-scale fusion experiments like at the Laser Mégajoule in France, which can measure SRS in many directions.

“In order to get the gas capsule to the highest temperature possible to start the fusion reactions, you have to get all of your laser energy into the target,” said author Kévin Vilayphone. “So, each mechanism that makes you lose energy is something that we must understand, control, and minimize.”

For both backscattering and sidescattering, the researchers adapted numerical methods from the scientific literature to incorporate the propagation of laser light through plasma, SRS, and the amplification and refraction of scattered light through plasma. They compared their numerical results with theoretical predictions and validated their code on a natural fusion experiment test case.

Their code could differentiate spectra parts resulting from backscattering or sidescattering. In the future, the researchers plan to apply their code to complex fusion set-ups where backscattering alone cannot fully explain the measured spectra.

“This code will be a very practical tool to help the people working on fusion experiments to interpret or even predict the shape of the Raman spectra they can expect,” said Vilayphone.

Source: “Development of a ray-based linear gain post-processor to model stimulated Raman scattering in multiple emission directions,” by Kévin Vilayphone, Sylvie Depierreux, and Luc Bergé, Physics of Plasmas (2026). The article can be accessed at https://doi.org/10.1063/5.0350401 .