Matter and Radiation at Extremes

Dielectronic recombination in non-LTE plasmas

Rosmej F. B., Astapenko V. A., Lisitsa V. S., Vainshtein L. A.

2020, 5(6) doi: 10.1063/5.0014158

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Abstract:
Novel phenomena and methods related to dielectronic capture and dielectronic recombination are studied for non-local thermodynamic equilibrium (LTE) plasmas and for applications to non-LTE ionization balance. It is demonstrated that multichannel autoionization and radiative decay strongly suppress higher-order contributions to the total dielectronic recombination rates, which are overestimated by standard approaches by orders of magnitude. Excited-state coupling of dielectronic capture is shown to be much more important than ground-state contributions, and electron collisional excitation is also identified as a mechanism driving effective dielectronic recombination. A theoretical description of the effect of angular-momentum-changing collisions on dielectronic recombination is developed from an atomic kinetic point of view and is visualized with a simple analytical model. The perturbation of the autoionizing states due to electric fields is discussed with respect to ionization potential depression and perturbation of symmetry properties of autoionization matrix elements. The first steps in the development of statistical methods are presented and are realized in the framework of a local plasma frequency approach. Finally, the impact of collisional–radiative processes and atomic population kinetics on dielectronic recombination is critically discussed, and simple analytical formulas are presented.

Statistical and quantum photoionization cross sections in plasmas: Analytical approaches for any configurations including inner shells

Rosmej F. B., Vainshtein L. A., Astapenko V. A., Lisitsa V. S.

2020, 5(6) doi: 10.1063/5.0022751

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Abstract:
Statistical models combined with the local plasma frequency approach applied to the atomic electron density are employed to study the photoionization cross-section for complex atoms. It is demonstrated that the Thomas–Fermi atom provides surprisingly good overall agreement even for complex outer-shell configurations, where quantum mechanical approaches that include electron correlations are exceedingly difficult. Quantum mechanical photoionization calculations are studied with respect to energy and nl quantum number for hydrogen-like and non-hydrogen-like atoms and ions. A generalized scaled photoionization model (GSPM) based on the simultaneous introduction of effective charges for non-H-like energies and scaling charges for the reduced energy scale allows the development of analytical formulas for all states nl. Explicit expressions for nl = 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, and 5s are obtained. Application to H-like and non-H-like atoms and ions and to neutral atoms demonstrates the universality of the scaled analytical approach including inner-shell photoionization. Likewise, GSPM describes the near-threshold behavior and high-energy asymptotes well. Finally, we discuss the various models and the correspondence principle along with experimental data and with respect to a good compromise between generality and precision. The results are also relevant to large-scale integrated light–matter interaction simulations, e.g., X-ray free-electron laser interactions with matter or photoionization driven by a broadband radiation field such as Planckian radiation.

Region-of-interest micro-focus computed tomography based on an all-optical inverse Compton scattering source

Ma Yue, Hua Jianfei, Liu Dexiang, He Yunxiao, Zhang Tianliang, Chen Jiucheng, Yang Fan, Ning Xiaonan, Yang Zhongshan, Zhang Jie, Pai Chih-Hao, Gu Yuqiu, Lu Wei

2020, 5(6) doi: 10.1063/5.0016034

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Abstract:
Micro-focus computed tomography (CT), which allows the hyperfine structure within objects to be reconstructed, is a powerful nondestructive testing tool in many fields. However, current x-ray sources for micro-focus CT are typically limited by their relatively low photon energy and low flux. An all-optical inverse Compton scattering source (AOCS) based on laser wakefield acceleration can generate intense quasi-monoenergetic x/gamma-ray pulses in the kilo- to megaelectronvolt range with micrometer-level source size, and its potential application for micro-focus CT has become very attractive in recent years because of the rapid progress made in laser wakefield acceleration. Reported here is a successful experimental demonstration of high-fidelity micro-focus CT using an AOCS (∼70 keV) by imaging and reconstructing a test object with complex inner structures. A region-of-interest CT method is adopted to utilize the relatively small field of view of the AOCS to ensure high spatial resolution. This demonstration of AOCS-based region-of-interest micro-focus CT is a key step toward its application in the field of hyperfine nondestructive testing.

Proton beams from intense laser-solid interaction: Effects of the target materials

Geng Y. X., Wu D., Yu W., Sheng Z. M., Fritzsche S., Liao Q., Wu M. J., Xu X. H., Li D. Y., Ma W. J., Lu H. Y., Zhao Y. Y., He X. T., Chen J. E., Lin C., Yan X. Q.

2020, 5(6) doi: 10.1063/5.0014854

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Abstract:
We report systematic studies of laser-driven proton beams produced with micrometer-thick solid targets made of aluminum and plastic, respectively. Distinct effects of the target materials are found on the total charge, cutoff energy, and beam spot of protons in the experiments, and these are described well by two-dimensional particle-in-cell simulations incorporating intrinsic material properties. It is found that with a laser intensity of 8 × 10¹⁹ W/cm², target normal sheath acceleration is the dominant mechanism for both types of target. For a plastic target, the higher charge and cutoff energy of the protons are due to the greater energy coupling efficiencies from the intense laser beams, and the larger divergence angle of the protons is due to the deflection of hot electrons during transport in the targets. We also find that the energy loss of hot electrons in targets of different thickness has a significant effect on the proton cutoff energy. The consistent results obtained here further narrow the gap between simulations and experiments.

Two-temperature warm dense hydrogen as a test of quantum protons driven by orbital-free density functional theory electronic forces

Kang Dongdong, Luo Kai, Runge Keith, Trickey S. B.

2020, 5(6) doi: 10.1063/5.0025164

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Abstract:
We consider a steady-state (but transient) situation in which a warm dense aggregate is a two-temperature system with equilibrium electrons at temperature T_e, ions at T_i, and T_e ≠ T_i. Such states are achievable by pump–probe experiments. For warm dense hydrogen in such a two-temperature situation, we investigate nuclear quantum effects (NQEs) on structure and thermodynamic properties, thereby delineating the limitations of ordinary ab initio molecular dynamics. We use path integral molecular dynamics (PIMD) simulations driven by orbital-free density functional theory (OFDFT) calculations with state-of-the-art noninteracting free-energy and exchange-correlation functionals for the explicit temperature dependence. We calibrate the OFDFT calculations against conventional (explicit orbitals) Kohn–Sham DFT. We find that when the ratio of the ionic thermal de Broglie wavelength to the mean interionic distance is larger than about 0.30, the ionic radial distribution function is meaningfully affected by the inclusion of NQEs. Moreover, NQEs induce a substantial increase in both the ionic and electronic pressures. This confirms the importance of NQEs for highly accurate equation-of-state data on highly driven hydrogen. For T_e > 20 kK, increasing T_e in the warm dense hydrogen has slight effects on the ionic radial distribution function and equation of state in the range of densities considered. In addition, we confirm that compared with thermostatted ring-polymer molecular dynamics, the primitive PIMD algorithm overestimates electronic pressures, a consequence of the overly localized ionic description from the primitive scheme.

Development of low-coherence high-power laser drivers for inertial confinement fusion

Gao Yanqi, Cui Yong, Ji Lailin, Rao Daxing, Zhao Xiaohui, Li Fujian, Liu Dong, Feng Wei, Xia Lan, Liu Jiani, Shi Haitao, Du Pengyuan, Liu Jia, Li Xiaoli, Wang Tao, Zhang Tianxiong, Shan Chong, Hua Yilin, Ma Weixin, Sun Xun, Chen Xianfeng, Huang Xiuguang, Zhu Jian, Pei Wenbing, Sui Zhan, Fu Sizu

2020, 5(6) doi: 10.1063/5.0009319

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Abstract:
The use of low-coherence light is expected to be one of the effective ways to suppress or even eliminate the laser–plasma instabilities that arise in attempts to achieve inertial confinement fusion. In this paper, a review of low-coherence high-power laser drivers and related key techniques is first presented. Work at typical low-coherence laser facilities, including Gekko XII, PHEBUS, Pharos III, and Kanal-2 is described. The many key techniques that are used in the research and development of low-coherence laser drivers are described and analyzed, including low-coherence source generation, amplification, harmonic conversion, and beam smoothing of low-coherence light. Then, recent progress achieved by our group in research on a broadband low-coherence laser driver is presented. During the development of our low-coherence high-power laser facility, we have proposed and implemented many key techniques for working with low-coherence light, including source generation, efficient amplification and propagation, harmonic conversion, beam smoothing, and precise beam control. Based on a series of technological breakthroughs, a kilojoule low-coherence laser driver named Kunwu with a coherence time of only 300 fs has been built, and the first round of physical experiments has been completed. This high-power laser facility provides not only a demonstration and verification platform for key techniques and system integration of a low-coherence laser driver, but also a new type of experimental platform for research into, for example, high-energy-density physics and, in particular, laser–plasma interactions.

Theory-orientated discovery of high-temperature superconductors in superhydrides stabilized under high pressure

Lv Jian, Sun Ying, Liu Hanyu, Ma Yanming

2020, 5(6) doi: 10.1063/5.0033232

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Exploring high-temperature superconductivity in hard matter close to structural instability

Chen Xiao-Jia

2020, 5(6) doi: 10.1063/5.0033143

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This paper discusses where and how high-temperature superconductors can be found.

Diamond gets harder, tougher, and more deformable

Xu Bo, Tian Yongjun

2020, 5(6) doi: 10.1063/5.0029519

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Abstract:
Diamonds may not be forever, but research interest in diamond has never ebbed. Owing to its highly symmetric crystal structure and strong covalent C–C bonds, diamond possesses an exceptional combination of physical properties. Its hardness and thermal conductivity are the highest among covalent materials. It also has a large bandgap and electric breakdown field, as well as optical transparency over a wide range of wavelengths. All of these are essential for a wide range of applications in both industrial and scientific areas. Despite these outstanding advantages, however, diamond is extremely brittle, with inferior toughness and poor deformability. These shortcomings have caused undesired tool breakage and have imposed severe constraints on technological innovations. To surmount these intrinsic deficiencies, tremendous research effort has been dedicated to developing advanced diamond products, with great progress being achieved in the past few years.

Exploring nanomechanics with high-pressure techniques

Chen Bin

2020, 5(6) doi: 10.1063/5.0032600

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Abstract:
For around three decades, high-pressure techniques have been used to study nanomaterials. In most studies, especially the early ones, x-ray diffraction and Raman and infrared spectroscopy were used to investigate the structural transition and equation of state. In recent years, the exploration has been extended to the plastic deformation of nanomaterials by using radial diamond-anvil-cell x-ray diffraction and transmission electron microscopy. Compared with the traditional techniques, high-pressure techniques are more advantageous in applying mechanical loads to nanosized samples and characterizing the structural and mechanical properties either in situ or ex situ, which could help to unveil the mysteries of mechanics at the nanoscale. With such knowledge, more-advanced materials could be fabricated for wider and specialized applications. This paper provides a brief review of recent progress.

2020 Vol. 5, No. 6

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