Matter and Radiation at Extremes

Observation of a highly conductive warm dense state of water with ultrafast pump–probe free-electron-laser measurements

Chen Z., Na X., Curry C. B., Liang S., French M., Descamps A., DePonte D. P., Koralek J. D., Kim J. B., Lebovitz S., Nakatsutsumi M., Ofori-Okai B. K., Redmer R., Roedel C., Schörner M., Skruszewicz S., Sperling P., Toleikis S., Mo M. Z., Glenzer S. H.

2021, 6(5) doi: 10.1063/5.0043726

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Abstract:
The electrical conductivity of water under extreme temperatures and densities plays a central role in modeling planetary magnetic fields. Experimental data are vital to test theories of high-energy-density water and assess the possible development and presence of extraterrestrial life. These states are also important in biology and chemistry studies when specimens in water are confined and excited using ultrafast optical or free-electron lasers (FELs). Here we utilize femtosecond optical lasers to measure the transient reflection and transmission of ultrathin water sheet samples uniformly heated by a 13.6 nm FEL approaching a highly conducting state at electron temperatures exceeding 20 000 K. The experiment probes the trajectory of water through the high-energy-density phase space and provides insights into changes in the index of refraction, charge carrier densities, and AC electrical conductivity at optical frequencies. At excitation energy densities exceeding 10 MJ/kg, the index of refraction falls to n = 0.7, and the thermally excited free-carrier density reaches n_e = 5 × 10²⁷ m⁻³, which is over an order of magnitude higher than that of the electron carriers produced by direct photoionization. Significant specular reflection is observed owing to critical electron density shielding of electromagnetic waves. The measured optical conductivity reaches 2 × 10⁴ S/m, a value that is one to two orders of magnitude lower than those of simple metals in a liquid state. At electron temperatures below 15 000 K, the experimental results agree well with the theoretical calculations using density-functional theory/molecular-dynamics simulations. With increasing temperature, the electron density increases and the system approaches a Fermi distribution. In this regime, the conductivities agree better with predictions from the Ziman theory of liquid metals.

Generating optical supercontinuum and frequency comb in tenuous plasmas

Qu Kenan, Fisch Nathaniel J.

2021, 6(5) doi: 10.1063/5.0052829

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Abstract:
There are several mechanisms by which the frequency spectrum of a laser broadens when it propagates at near-relativistic intensity in a tenuous plasma. Focusing on one-dimensional effects, we identify two strong optical nonlinearities, namely, four-wave mixing (FWM) and forward Raman scattering (FRS), for creating octave-wide spectra. FWM dominates the interaction when the laser pulse is short and intense, and its combination with phase modulation produces a symmetrically broadened supercontinuum. FRS dominates when the laser pulse is long and relatively weak, and it broadens the laser spectrum mainly toward lower frequencies and produces a frequency comb. The frequency chirping combined with group velocity dispersion compresses the laser pulse, causing higher peak intensity.

In situ observation of the Rayleigh–Taylor instability of liquid Fe and Fe–Si alloys under extreme conditions: Implications for planetary core formation

Terasaki Hidenori, Sakaiya Tatsuhiro, Shigemori Keisuke, Akimoto Kosaku, Kato Hiroki, Hironaka Yoichiro, Kondo Tadashi

2021, 6(5) doi: 10.1063/5.0029448

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Abstract:
Rayleigh–Taylor (RT) instability, which occurs when a heavy fluid overlies a light fluid in a gravitational field, is an important scenario for planetary core formation, especially beneath the planetary magma ocean. This process has been discussed based on numerical simulations and experiments using analog materials. However, experiments on the RT instability using the core-forming melt have not been performed at high pressures. In this study, we perform in situ observation of the RT instability of liquid Fe and Fe–Si (Si = 10 and 20 wt. %) alloys under high pressure using a high-power laser-shock technique. The observed perturbation on the Fe–Si surface grows exponentially with time, while there is no obvious growth of perturbations on the Fe in the measured time range. Therefore, the growth rate of the RT instability increases with Si content. The timescale of the initial growth of the RT instability in planetary interiors is likely to be much faster (by more than two orders of magnitude) than the 30–40 × 10⁶ year timescale of planetary core formation.

Scattering of ultrashort laser pulses on plasmons in a Maxwellian plasma

Astapenko V. A., Rosmej F. B., Khramov E. S.

2021, 6(5) doi: 10.1063/5.0065835

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Abstract:
On the basis of equations obtained in the framework of second-order quantum-mechanical perturbation theory, the standard approach to the calculation of scattering radiation probability is extended to the case of ultrashort laser pulses. We investigate the mechanism of the appearance of plasmon peaks in the spectrum of the plasma form factor for different parameters of the problem. For the case in which scattering on plasmons dominates over scattering on electron density fluctuations caused by chaotic thermal motion, we derive analytical expressions describing the scattering probability of ultrashort laser pulses on plasmons. Together with this, we obtain a simple expression connecting the frequency of scattered radiation and the energy transmitted from the incident pulse to plasmon, and vice versa. In considering the scattering probability, our emphasis is on the dependence on the pulse duration. We assess in detail the trends of this dependence for various relations between pulse carrier frequency and plasmon energy.

Analyzing and relieving the thermal issues caused by fabrication details of a deuterium cryogenic target

Yang Hong, Gao Shasha, Jiang Baibin, Xie Jun, Liang Juxi, Qi Xiaobo, Wang Kai, Tao Chaoyou, Dai Fei, Lin Wei, Zhang Juan

2021, 6(5) doi: 10.1063/5.0039131

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Abstract:
In inertial confinement fusion experiments, fuel quality is determined mainly by the thermal environment of the capsule in the layering procedure. Owing to the absence of a radial thermal gradient, formed deuterium–deuterium (DD) ice shells in the capsule are thermally instable. To obtain a solid DD layer with good quality and long lifetime, stringent demands must be placed on the thermal performance of cryogenic targets. In DD cryogenic target preparation, two issues arise, even after the capsule’s temperature uniformity has been improved by the use of thick aluminized films. The first is the inconsistent ice shape, which is related to the capsule’s thermal field. In this article, some typical fabrication details are investigated, including adhesive penetration during assembly, the presence of the fill tube, the optical properties of the hohlraum and film surfaces, the jacket–hohlraum connection, deviations in capsule location, and asymmetrical contact at the arm–jacket interfaces. Detailed comparisons of the thermal effects of these factors provide guidance for target optimization. The second issue is the instability of seeding crystals in the fill tube due to unsteadiness of the direction of the thermal gradient in the fill tube assembly. An additional thermal controller is proposed, analyzed, and optimized to provide robust controllability of tube temperature. The analysis results and optimization methods presented in this article should not only help in dealing with thermal issues associated with DD cryogenic targets, but also provide important references for engineering design of other cryogenic targets.

Mitigating parametric instabilities in plasmas by sunlight-like lasers

Ma H. H., Li X. F., Weng S. M., Yew S. H., Kawata S., Gibbon P., Sheng Z. M., Zhang J.

2021, 6(5) doi: 10.1063/5.0054653

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Abstract:
Sunlight-like lasers that have a continuous broad frequency spectrum, random phase spectrum, and random polarization are formulated theoretically. With a sunlight-like laser beam consisting of a sequence of temporal speckles, the resonant three-wave coupling that underlies parametric instabilities in laser–plasma interactions can be greatly degraded owing to the limited duration of each speckle and the frequency shift between two adjacent speckles. The wave coupling can be further weakened by the random polarization of such beams. Numerical simulations demonstrate that the intensity threshold of stimulated Raman scattering in homogeneous plasmas can be doubled by using a sunlight-like laser beam with a relative bandwidth of ∼1% as compared with a monochromatic laser beam. Consequently, the hot-electron generation harmful to inertial confinement fusion can be effectively controlled by using sunlight-like laser drivers. Such drivers may be realized in the next generation of broadband lasers by combining two or more broadband beams with independent phase spectra or by applying polarization smoothing to a single broadband beam.

Enhanced ion acceleration using the high-energy petawatt PETAL laser

Raffestin D., Lecherbourg L., Lantuéjoul I., Vauzour B., Masson-Laborde P. E., Davoine X., Blanchot N., Dubois J. L., Vaisseau X., d’Humières E., Gremillet L., Duval A., Reverdin Ch., Rosse B., Boutoux G., Ducret J. E., Rousseaux Ch., Tikhonchuk V., Batani D.

2021, 6(5) doi: 10.1063/5.0046679

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Abstract:
The high-energy petawatt PETAL laser system was commissioned at CEA’s Laser Mégajoule facility during the 2017–2018 period. This paper reports in detail on the first experimental results obtained at PETAL on energetic particle and photon generation from solid foil targets, with special emphasis on proton acceleration. Despite a moderately relativistic (<10¹⁹ W/cm²) laser intensity, proton energies as high as 51 MeV have been measured significantly above those expected from preliminary numerical simulations using idealized interaction conditions. Multidimensional hydrodynamic and kinetic simulations, taking into account the actual laser parameters, show the importance of the energetic electron production in the extended low-density preplasma created by the laser pedestal. This hot-electron generation occurs through two main pathways: (i) stimulated backscattering of the incoming laser light, triggering stochastic electron heating in the resulting counterpropagating laser beams; (ii) laser filamentation, leading to local intensifications of the laser field and plasma channeling, both of which tend to boost the electron acceleration. Moreover, owing to the large (∼100 μm) waist and picosecond duration of the PETAL beam, the hot electrons can sustain a high electrostatic field at the target rear side for an extended period, thus enabling efficient target normal sheath acceleration of the rear-side protons. The particle distributions predicted by our numerical simulations are consistent with the measurements.

Pressure-induced robust emission in a zero-dimensional hybrid metal halide (C₉NH₂₀)₆Pb₃Br₁₂

Chen Mengting, Guo Songhao, Bu Kejun, Lee Sujin, Luo Hui, Wang Yiming, Liu Bingyan, Yan Zhipeng, Dong Hongliang, Yang Wenge, Ma Biwu, Lü Xujie

2021, 6(5) doi: 10.1063/5.0058821

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Abstract:
Zero-dimensional (0D) hybrid metal halides are under intensive investigation owing to their unique physical properties, such as the broadband emission from highly localized excitons that is promising for white-emitting lighting. However, fundamental understanding of emission variations and structure–property relationships is still limited. Here, by using pressure processing, we obtain robust exciton emission in 0D (C₉NH₂₀)₆Pb₃Br₁₂ at room temperature that can survive to 80 GPa, the recorded highest value among all the hybrid metal halides. In situ experimental characterization and first-principles calculations reveal that the pressure-induced emission is mainly caused by the largely suppressed phonon-assisted nonradiative pathway. Lattice compression leads to phonon hardening, which considerably weakens the exciton–phonon interaction and thus enhances the emission. The robust emission is attributed to the unique structure of separated spring-like [Pb₃Br₁₂]⁶⁻ trimers, which leads to the outstanding stability of the optically active inorganic units. Our findings not only reveal abnormally robust emission in a 0D metal halide, but also provide new insight into the design and optimization of local structures of trimers and oligomers in low-dimensional hybrid materials.

2021 Vol. 6, No. 5

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