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The regeneration cavity (RC) is a critical component of the Any Light Particle Search II (ALPS II) experiment. It increases the signal from possible axions and axion-like particles in the experiment by nearly four orders of magnitude. The total round-trip optical losses of the power circulating in the cavity must be minimized in order to maximize the resonant enhancement of the cavity, which is an important figure of merit for ALPS II. Lower optical losses also increase the cavity storage time, and with the 123 meter long ALPS II RC we have demonstrated the longest storage time of a two-mirror optical cavity. We measured a storage time of 7.17±0.01ms, equivalent to a linewidth of 44.4 Hz and a finesse of 27,500 at a wavelength of 1064 nm. 

Abstract Ferromanganese concretions commonly occur in shallow‐water coastal regions worldwide. In the Baltic Sea, they can record information about past and present underwater environments and could be a potential source for critical raw materials. We report on their microstructural characteristics and magnetic properties and link them to their formation mechanisms and environmental significance. Microstructural investigations from nano‐ and micro‐computed tomography, electron microscopy, and micro‐X‐ray fluorescence elemental mapping reveal diverse growth patterns within concretions of different morphologies. Alternating Fe‐ and Mn‐rich growth bands indicate fluctuating redox conditions during formation. Bullet‐shaped magnetofossils, produced by magnetotactic bacteria, are present, which suggests the influence of bacterial activity on concretion formation. Spheroidal concretions, which occur in deeper and more tranquil environments, have enhanced microbial biomineralization and magnetofossil preservation. Conversely, crusts and discoidal concretions from shallower and more energetic environments contain fewer magnetofossils and have a greater detrital content. Our results provide insights into concretion formation mechanisms and highlight the importance of diagenetic processes, oxygen availability, and bacterial activity in the Baltic Sea. 

An underwater acoustic (UWA) channel model with high validity and re-usability is widely demanded. In this paper, we propose a variational auto-encoder (VAE)-based deep generative model which learns an abstract representation of the UWA channel impulse responses (CIRs) and can generate CIR samples with similar features. A customized training process is proposed to avoid the model collapse and being trapped in a gradient pit. The proposed deep generative model is validated using field experimental data sets. 

Foveation and (de)focus are two important visual factors in designing near eye displays. Foveation can reduce computational load by lowering display details towards the visual periphery, while focal cues can reduce vergence-accommodation conflict thereby lessening visual discomfort in using near eye displays. We performed two psychophysical experiments to investigate the relationship between foveation and focus cues. The first study measured blur discrimination sensitivity as a function of visual eccentricity, where we found discrimination thresholds significantly lower than previously reported. The second study measured depth discrimination threshold where we found a clear dependency on visual eccentricity. We discuss the study results and suggest further investigation.