Irradiating high-power lasers in the relativistic intensity level ionizes a material, thereby creating a high-energy density plasma. In such situations, the strong laser radiation pressure of giga-bar level pushes the irradiated plasma surface, where electron acceleration by lasers in the relativistic regime and heating of the high-temperature plasma occur simultaneously. It is an important issue in plasma physics to explore phenomena in such extreme conditions.
　Dr. Natsumi Iwata has been theoretically studying the dynamics of high-energy density plasma driven by high-power lasers. She proposed that the plasma induces a strong electric field in the picosecond regime at the irradiated plasma surface and thus stops pushing the surface due to the laser radiation pressure, and clarified that continuous irradiation of lasers gives rise to the blowout of the heated plasma, resulting in a strong acceleration of plasma particles. Furthermore, highlighting the fact that there is a transition in plasma behavior, under continuous laser irradiation, to a statistical regime where random scatterings due to fluctuating electromagnetic fields become dominant, Dr. Iwata conducted simulations for time evolution of the electron energy under statistical scatterings and presented the corresponding plasma expansion theory. In particular, she elucidated that a coupling to fluctuating electromagnetic fields changes the electron dynamics from ballistic to diffusive and thus suppresses the scattering of electrons, leading to a confinement of the high-energy density plasma. In a more recent study, she proposed a statistical approach to the multivariate analysis of experimental data using the Bayesian inference and obtained, for example, a scaling law in the ion acceleration driven by the strong laser light, thereby contributing to the development of data-analysis methods.
　These observations lead us to conclude that Dr. Iwata's scientific achievements deserve the Fumiko Yonezawa Memorial Prize of the Physical Society of Japan.