Radiation from Wavepackets

We provide experimental evidence for the absence of a magnetic moment in bulk RuO₂, a candidate altermagnetic material, by using a combination of Mössbauer spectroscopy, nuclear forward scattering, inelastic X-ray and neutron scattering, and density functional theory calculations. Using complementary Mössbauer and nuclear forward scattering, we determine the Ru magnetic hyperfine splitting to be negligible. Inelastic X-ray and neutron scattering-derived lattice dynamics of RuO₂ are compared to density functional theory calculations of varying flavors. Comparisons among theory with experiments indicate that electronic correlations, rather than magnetic order, are key in describing the lattice dynamics.

Polymer foams play a critical role in contemporary inertial fusion energy (IFE) target designs by enhancing energy yield and optimizing implosion dynamics. However, the lack of high-resolution characterization of the nanostructure of these foams restricts progress in fusion science. In this work, we demonstrate the first high-resolution three-dimensional (3D) reconstruction of a low-density, Si-doped polymer foam fabricated via two-photon polymerization, using ptychographic x-ray computed tomography (PXCT) at an x-ray free electron laser (XFEL). This imaging method reconstructs two-dimensional (2D) attenuation and phase information at multiple sample angles that are combined into a 3D density map used to extract local mass density and determine structural dimensions. We achieve a 2D spatial resolution of 19 ± 3 nm on a high-contrast Ronchi pattern target and 78.7 ± 3 nm for low-contrast polymer foams, marking a significant advancement for XFEL-based ptychography of low-density materials. Furthermore, our experimental results reveal an average foam strut thickness of 1.17 ± 0.4 μm, consistent with fabrication expectations, and a reconstructed average mass density of 0.35 g/cc, aligning closely with the predicted density of 0.29 g/cc. These findings provide important insights for improving foam design and refining radiation hydrodynamics modeling in future IFE experiments. Our study establishes PXCT at an XFEL as a powerful tool for high-resolution characterization of fusion-relevant materials, paving the way for enhanced target performance in IFE research.

Atmospheric turbulence causes fluctuations in the angle-of-arrival (AOA) of sound waves. These fluctuations adversely affect the performance of sensor arrays used for source detection, ranging, and recognition. This article examines, from a theoretical perspective, the variance of the AOA fluctuations measured with two microphones. The AOA variance is expressed in terms of the propagation range, transverse distance between two microphones, acoustic frequency, and effective spectrum of quasi-homogeneous and isotropic turbulence, with parameters dependent upon the height above the ground. The effective spectrum is modeled with the von Kármán and Kolmogorov spectral models. In the latter case, the results simplify significantly, and the variance depends on the path-averaged effective structure-function parameter, which characterizes the intensity of temperature and wind velocity fluctuations in the inertial subrange of turbulence. The standard deviation of the AOA fluctuations is studied numerically for typical meteorological regimes of the daytime atmospheric boundary layer. For the cases considered, the standard deviation varies from a fraction of degree to around 1°–2°, and increases with increasing friction velocity and surface heat flux.

Time reversal (TR) is a process that can be used to generate high amplitude focusing of sound. It has been previously shown that high amplitude focused sound using TR in reverberant environments exhibits multiple nonlinear features including waveform steepening and a nonlinear increase in peak compression pressures [Patchett and Anderson, J. Acoust. Soc. Am. 151(6), 3603–3614 (2022)]. The present study investigates the removal of one possible cause for these phenomena: free-space Mach stems. By constraining the focusing in the system to one-dimensional (1-D) waves, the potential formation of Mach stems is eliminated so that remaining nonlinear effects can be observed. A system of pipes is used to restrict the focused waves to be planar in a 1-D reverberant environment. Results show that waveform steepening effects remain, as expected, but that the nonlinear increase in compression amplitudes that appears in TR focusing of three-dimensional (3-D), finite-amplitude sound in rooms disappears here because Mach stems cannot form in a 1-D system. These experiments do not prove that Mach stems cause the nonlinear increase observed for focusing in a 3-D environment, but they do support the Mach stem explanation.

This Letter presents an analysis of near-field acoustic data collected on Space Launch System's Mobile Launcher tower during the Artemis I mission. Twelve pressure sensors located two and four effective nozzle diameters ( De) from the vehicle centerline recorded maximum overall sound pressure levels ranging from ∼ 162 dB to more than 170 dB, originating ∼ 10  De downstream of the nozzle exit plane. Frequency-dependent characteristics are also discussed. The peak noise is radiated over a broader frequency range than in the far field. Low-frequency noise locations match other rockets, but high-frequency locations diverge, falling between prior measurements of undeflected and deflected plumes.

Transfer learning (TL) is used to predict source-receiver range in a laboratory tank with varying water temperature. The input data are single-hydrophone spectral levels from linear chirps over the 50–100 kHz band recorded at different ranges. Data measured in room temperature water are used to train one-dimensional convolutional neural networks. When the trained models are applied to data measured in warmer water, a bias is introduced. TL with a small dataset improves the generalization results at the new temperature, demonstrating the potential of TL to improve performance under variable environmental conditions.