Matter and Radiation at Extremes

Injection induced by coaxial laser interference in laser wakefield accelerators

Wang Jia, Zeng Ming, Li Dazhang, Wang Xiaoning, Lu Wei, Gao Jie

2022, 7(5) doi: 10.1063/5.0101098

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Abstract:
We propose a new injection scheme that can generate electron beams with simultaneously a few permille energy spread, submillimeter milliradian emittance, and more than a 100 pC charge in laser wakefield accelerators. In this scheme, a relatively loosely focused laser pulse drives the plasma wakefield, and a tightly focused laser pulse with similar intensity triggers an interference ring pattern that creates onion-like multisheaths in the plasma wakefield. Owing to the change in wavefront curvature after the focal position of the tightly focused laser, the innermost sheath of the wakefield expands, which slows down the effective phase velocity of the wakefield and triggers injection of plasma electrons. Both quasicylindrical and fully three-dimensional particle-in-cell simulations confirm the generation of beams with the above mentioned properties.

High order modes of intense second harmonic light produced from a plasma aperture

Bacon E. F. J., King M., Wilson R., Frazer T. P., Gray R. J., McKenna P.

2022, 7(5) doi: 10.1063/5.0097585

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Abstract:
Because of their ability to sustain extremely high-amplitude electromagnetic fields and transient density and field profiles, plasma optical components are being developed to amplify, compress, and condition high-power laser pulses. We recently demonstrated the potential to use a relativistic plasma aperture—produced during the interaction of a high-power laser pulse with an ultrathin foil target—to tailor the spatiotemporal properties of the intense fundamental and second-harmonic light generated [Duff et al., Sci. Rep. 10 , 105 (2020)]. Herein, we explore numerically the interaction of an intense laser pulse with a preformed aperture target to generate second-harmonic laser light with higher-order spatial modes. The maximum generation efficiency is found for an aperture diameter close to the full width at half maximum of the laser focus and for a micrometer-scale target thickness. The spatial mode generated is shown to depend strongly on the polarization of the drive laser pulse, which enables changing between a linearly polarized TEM₀₁ mode and a circularly polarized Laguerre–Gaussian LG₀₁ mode. This demonstrates the use of a plasma aperture to generate intense higher-frequency light with selectable spatial mode structure.

Reflection and transmission properties of a finite-length electron plasma grating

Lehmann G., Spatschek K. H.

2022, 7(5) doi: 10.1063/5.0096386

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Abstract:
Considered here is a plasma grating generated by two counterpropagating short laser pulses. Because of the shortness of the laser pulses, the plasma dynamics are determined by only electrons, which respond to the ponderomotive pressure generated by the interacting laser fields. An electron grating cannot exist for longer than the inverse ion plasma frequency, and so because of the limited time of the ponderomotive pressure, both the life time and spatial extent of an electron grating are finite. When one of the short laser pulses is circularly polarized (propagating in the x direction with electric field vectors in the yz plane) and the other is linearly y-polarized, the electron grating is produced by the y components. Meanwhile, the z component is partially reflected, and only a fraction of it is transmitted. Thus, the finite plasma grating can either alter the polarization of the yz-polarized pulse or act as a pulse splitter. The present paper is focused on the reflection and transmission rates. The action of the density grating on the z component cannot be explained by the Bloch wave theory for infinite crystals, and instead a theory is developed based on four-wave mixing, which explains the transmission and reflection of the z component when interacting with a grating of finite extent.

Dream fusion in octahedral spherical hohlraum

Lan Ke

2022, 7(5) doi: 10.1063/5.0103362

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Abstract:
The octahedral spherical hohlraum provides an ideal and practical approach for indirect-drive toward a dream fusion with predictable and reproducible gain and opens a route to the development of a laser drive system for multiple laser fusion schemes. This paper addresses a number of issues that have arisen with regard to octahedral spherical hohlraums, such as how to naturally generate a highly symmetric radiation drive at all times and for all spectra without the use of symmetry tuning technology, how to determine the three-dimensional, temporal, and spectral characteristics of the real radiation drive on a capsule in experiments, and the relative energy efficiency of an octahedral spherical hohlraum compared with a cylindrical hohlraum. A design island for an octahedral spherical hohlraum is presented. Finally, the challenges and future tasks for the path forward are presented.

Explosion dynamics of thin flat foils at high current density

Shelkovenko T. A., Tilikin I. N., Pikuz S. A., Mingaleev A. R., Romanova V. M., Atoyan L., Hammer D. A.

2022, 7(5) doi: 10.1063/5.0098333

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Abstract:
This paper presents characteristic features of the explosion of thin flat foils for currents and pulse risetimes ranging from 8 kA at 350 ns to 1000 kA at ∼100 ns. Foils made of aluminum, copper, nickel, and titanium with thicknesses of 1–100 µm are tested. Various diagnostics in the optical, UV, and x-ray spectral ranges are used to image the exploding foils from initial breakdown to complete destruction or pinching. It is shown that foil explosion is a complex process that depends on many factors, but features common to all foils are found that do not depend on the parameters of the generators or, accordingly, on the energy deposited in the foil: for example, the breakdown of flat foils under different conditions occurs at the edges of the foil. For the first time, the formation of a precursor over the central part of the foil is shown, which significantly changes the dynamics of the foil explosion.

Mitigation of the ablative Rayleigh–Taylor instability by nonlocal electron heat transport

Li Jun, Yan Rui, Zhao Bin, Zheng Jian, Zhang Huasen, Lu Xiyun

2022, 7(5) doi: 10.1063/5.0088058

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Abstract:
The effects of electron nonlocal heat transport (NLHT) on the two-dimensional single-mode ablative Rayleigh–Taylor instability (ARTI) up to the highly nonlinear phase are reported for the first time through numerical simulations with a multigroup diffusion model. It is found that as well as its role in the linear stabilization of ARTI growth, NLHT can also mitigate ARTI bubble nonlinear growth after the first saturation to the classical terminal velocity, compared with what is predicted by the local Spitzer–Härm model. The key factor affecting the reduction in the linear growth rate is the enhancement of the ablation velocity V_a by preheating. It is found that NLHT mitigates nonlinear bubble growth through a mechanism involving reduction of vorticity generation. NLHT enhances ablation near the spike tip and slows down the spike, leading to weaker vortex generation as the pump of bubble reacceleration in the nonlinear stage. NLHT more effectively reduces the nonlinear growth of shorter-wavelength ARTI modes seeded by the laser imprinting phase in direct-drive laser fusion.

Clear evidence against superconductivity in hydrides under high pressure

Hirsch J. E., Marsiglio F.

2022, 7(5) doi: 10.1063/5.0091404

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Abstract:
The Meissner effect, magnetic field expulsion, is a hallmark of superconductivity. Associated with it, superconductors exclude applied magnetic fields. Recently, Minkov et al. [Nat. Commun. 13 , 3194 (2022)] presented experimental results reportedly showing “definitive evidence of the Meissner effect” in sulfur hydride and lanthanum hydride under high pressure, and Eremets et al. [J. Supercond. Nov. Magn. 35 , 965 (2022)] argued that “the arguments against superconductivity (in hydrides) can be either refuted or explained.” Instead, we show here that the evidence presented in those papers does not support the case for superconductivity in these materials. Together with experimental evidence discussed in earlier papers, we argue that this strongly suggests that hydrides under pressure are not high-temperature superconductors.

First-principles study on the electronic structure transition of β-UH₃ under high pressure

Wu Juefei, Yue-Chao Wang, Liu Yu, Sun Bo, Zhao Yanhong, Xian Jiawei, Gao Xingyu, Liu Haifeng, Song Haifeng

2022, 7(5) doi: 10.1063/5.0091969

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Abstract:
We investigate the electronic properties of stable β-UH₃ under high pressure up to 75 GPa within the first-principles DFT + U formalism with pressure-dependent U in a self-consistent calculation, and we find an electronic structure transition at about 20 GPa due to the quantum process of localization and itinerancy for partially filled uranium 5f electrons. The electronic structure transition is examined from four perspectives: magnetization, band structure, density of states, and 5f electron energy. On the basis of the density of states of 5f electrons, we propose an order parameter, namely, the 5f electron energy, to quantify the electronic structure transition under pressure. Analogously to the isostructural transition in 3d systems, β-UH₃ retains its magnetic order after the electronic structure transition; however, this is not accompanied by volume collapse at the transition point. Our calculation is helpful for understanding the electronic properties of β-UH₃ under high pressure.

Universal Fermi velocity in highly compressed hydride superconductors

Talantsev Evgeny F.

2022, 7(5) doi: 10.1063/5.0091446

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Abstract:
The Fermi velocity v_F is one of the primary characteristics of any conductor, including any superconductor. For conductors at ambient pressure, several experimental techniques have been developed to measure v_F, and, for instance, Zhou et al. [Nature 423 , 398 (2003)] reported that high-T_c cuprates exhibited a universal nodal Fermi velocity vF,univ=2.7±0.5×105 m/s. However, there have been no measurements of v_F in highly compressed near-room-temperature superconductors (NRTS), owing to experimental challenges. Here, to answer the question of the existence of a universal Fermi velocity in NRTS materials, we analyze the full inventory of data on the ground-state upper critical field B_c2(0) for these materials and find that this class of superconductors exhibits a universal Fermi velocity vF,univ=1/1.3×2Δ0/kBTc×105 m/s, where Δ(0) is the ground-state amplitude of the energy gap. The ratio 2Δ0/kBTc varies within a narrow range 3.2≤2Δ0/kBTc≤5, and so v_F,univ in NRTS materials lies in the range 2.5 × 10⁵ m/s ≤ v_F,univ ≤ 3.8 × 10⁵ m/s, which is similar to the range of values found for the high-T_c cuprate counterparts of these materials.

Corrigendum to: “Incompatibility of published ac magnetic susceptibility of a room temperature superconductor with measured raw data” [Matter Radiat. Extremes 7, 048401 (2022)]

Hirsch J. E., van der Marel D.

2022, 7(5) doi: 10.1063/5.0108683

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Abstract:

2022 Vol. 7, No. 5

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