Norimasa Ozaki

We report direct measurements of magnetically inhibited heat transport in an underdense laser-produced plasma subjected to externally applied magnetic fields of up to 20 T. Spatially and temporally resolved Thomson-scattering data show that the peak electron temperature increases by a factor ∼ 1.4 relative to the unmagnetized case, as cross-field thermal conduction is maximally suppressed. This state is observed for B ≥ 10 T and is sustained over ∼ 0.5 ns . The subsequent temperature decrease is primarily driven by a reduction in inverse-bremsstrahlung heating as the electron density cavitates. Our experimental results are consistent with simple analytical estimates and are corroborated by Vlasov-Fokker-Planck simulations, suggesting that the plasma reaches the maximum temperature gain for an electron Hall parameter χ Hall ≳ 50 ( T e ∼ 800 eV , n e ∼ 3 × 10 19 cm − 3 ). These findings demonstrate that effective magnetothermal insulation can occur in magnetized inertial confinement fusion schemes without the need for extremely large magnetic fields.

Extreme pressures and temperatures create conditions that allow even hard and brittle materials to flow plastically. Despite extensive research, the limits of flow strength under such conditions remain uncertain, and the mechanisms driving deformation at the relevant stresses are a subject of debate. Using femtosecond in situ x-ray diffraction experiments and large-scale molecular dynamics simulations, we demonstrate that stacking fault-mediated strengthening enables shock-compressed nanopolycrystalline diamond to achieve a peak flow strength of 92 ± 3 GPa at a stress of 212 ± 6 GPa. Our findings show that extreme conditions can unlock ultrahigh strength via a complex array of competing deformation mechanisms and thermodynamic effects.

The phase stability, crystal structure, and melting of Nb have been examined under high pressure shock compression to 365 GPa using ultrafast x-ray diffraction measurements on two x-ray free electron laser facilities. On compression, Nb remains stable in the bcc phase up to 220 GPa, with coexistence of bcc-Nb and liquid from 249 to 298 GPa, and complete melting at 301 GPa, with melt identified by diffuse liquid diffraction. Melting initiates at higher pressure than expected based on theoretical predictions, and the data consistently excludes the presence of other suggested phases of Nb at these pressures, including and hcp, thereby resolving the long-standing structural discrepancy. Singh-type strength analysis provides experimental evidence for a change in sign of the elastic anisotropy parameter at 121 GPa, consistent with the previously reported heat-induced hardening to heat-induced softening transition in Nb.

Perovskite-structured silicates are key minerals of Earth’s lower mantle and have also been identified in shocked meteorites as the (Mg,Fe)SiO₃-perovskite phase, bridgmanite. In contrast, CaSiO₃-perovskite (davemaoite) has never been found in meteorites and is known only from rare inclusions in deep-mantle diamonds. This contrast highlights a fundamental gap: while natural samples provide indirect evidence for silicate perovskites under shock conditions, laboratory dynamic compression experiments have yet to directly capture perovskite transitions in either MgSiO₃ or CaSiO₃, likely due to kinetic limitations imposed by nanosecond timescales. Here we present the in situ, atomic-scale observations of CaSiO₃-perovskite formation under dynamic compression. Using nanosecond laser-driven shocks and ultrafast X-ray diffraction (XRD) at SACLA, we tracked the transformation of CaSiO₃ from low-pressure phases to an amorphous state, and ultimately to davemaoite above ~91(10) GPa, suggesting a disorder–order transformation pathway toward reconstructive crystallization on nanosecond timescales. Upon release, davemaoite undergoes amorphization via a transient low-symmetry phase. These observations provide direct structural evidence for silicate perovskite formation under dynamic loading, resolving a long-standing experimental problem and establishing dynamic compression as an effective route to probe deep-mantle minerals relevant to Earth and Ca-rich exoplanet interiors.

Serpentine is a major hydrous mineral present in hydrous asteroids that have evolved through planetary impacts. Understanding its dynamic behavior is essential to elucidate the redistribution and retention of water during high‐velocity impact processes. The shock response of antigorite was examined at pressures up to 106 GPa using laser‐driven shock compression combined with ultrafast time‐resolved X‐ray diffraction measurements. Results show that shock‐compressed antigorite remained crystalline up to 44 GPa (impact velocity ∼4.7 km/s), whereas it transformed to an amorphous state above 66 GPa (∼6.4 km/s). This transformation was completed within a few nanoseconds during compression, and recrystallization from the amorphous state did not occur during subsequent decompression, indicating that shock‐induced amorphization is the dominant structural response of antigorite. Adiabatic release calculations further indicate that the decompression path of shocked antigorite from a peak shock pressure of 60 GPa intersects the stability fields of several high‐pressure nominally anhydrous minerals (NAMs). On the other hand, the decompression path from peak pressures of 80 to 100 GPa is predicted to remain above the liquidus until ambient pressure is reached. These findings have significant implications for the impact thermal histories of serpentine‐rich hydrous asteroids, such as Ryugu and Bennu.

Oxygen and other light elements comprise up to 5 wt% of the Earth’s outer-core, and may significantly influence its physical properties and the operation of the geodynamo. Here we report in situ X-ray diffraction measurements of Fe, Fe + 4.5 FeO (atomic proportion), and Fe ₂ O ₃ melts at 177-440 GPa, achieved using laser-driven shock compression at an x-ray free-electron laser. The melts exhibit Fe-O coordination numbers between 4.0(0.4) and 4.5(0.4), indicating predominantly four-fold coordination environments. These coordination states are significantly smaller than those of Fe-bearing lower-mantle phases such as bridgmanite and ferropericlase. Shorter Fe-Fe interatomic distances in compressed iron oxide melts drive the denser packing relative to ambient melts, while the structural differences between Fe + 4.5 FeO and Fe ₂ O ₃ melts under shock indicate that the oxidation state modulates oxygen solubility in liquid Fe. At 177 GPa ( ~ 380 km below the core-mantle boundary) and 3800 K, Fe ₂ O ₃ melts exhibit higher Fe-O coordination, suggesting that local variations in oxygen content could contribute to the stratification in the uppermost outer-core inferred from seismological and geomagnetic observations.

Plagioclase records characteristic shock features that constrain meteorite impact conditions, but its phase evolution along the Hugoniot remains unresolved. We investigated shock‐induced phase transition in albite using in situ X‐ray diffraction during laser‐driven shock at 14–66 GPa. Albite retained its crystal structure up to at least 14 GPa and underwent complete amorphization above 28 GPa, at pressures lower than those reported in shock‐recovery experiments. No hollandite‐type phase, jadeite, or Ca‐ferrite‐type NaAlSiO ₄ was observed during shock loading, indicating that the high‐pressure region of the albite Hugoniot is dominated by amorphization. Upon decompression, crystalline peaks reappeared, indicating structural recovery, which may contribute to the wide range of amorphization pressures in the recovery experiments. The absence of decomposition products indicates strong kinetic suppression under nanosecond shock durations, defining the short‐timescale limit of albite decomposition during impact loading. These results refine interpretations of shock metamorphism in plagioclase under planetary impact conditions.