Metaplectic geometrical optics for ray-based modeling of caustics: Theory and algorithms

The optimization of radiofrequency-wave (RF) systems for fusion experiments is often performed using ray-tracing codes, which rely on the geometrical-optics (GO) approximation. However, GO fails at caustics such as cutoffs and focal points, erroneously predicting the wave intensity to be infinite. This is a critical shortcoming of GO, since the caustic wave intensity is often the quantity of interest, e.g. RF heating. Full-wave modeling can be used instead, but the computational cost limits the speed at which such optimizations can be performed. We have developed a less expensive alternative called metaplectic geometrical optics (MGO). Instead of evolving waves in the usual $\textbf{x}$ (coordinate) or $\text{k}$ (spectral) representation, MGO uses a mixed $\textbf{X} \equiv \textsf{A}\textbf{x} + \textsf{B}\textbf{k}$ representation. By continuously adjusting the matrix coefficients $\textsf{A}$ and $\textsf{B}$ along the rays, one can ensure that GO remains valid in the $\textbf{X}$ coordinates without caustic singularities. The caustic-free result is then mapped back onto the original $\textbf{x}$ space using metaplectic transforms. Here, we overview the MGO theory and review algorithms that will aid the development of an MGO-based ray-tracing code. We show how using orthosymplectic transformations leads to considerable simplifications compared to previously published MGO formulas. We also prove explicitly that MGO exactly reproduces standard GO when evaluated far from caustics (an important property which until now has only been inferred from numerical simulations), and we relate MGO to other semiclassical caustic-removal schemes published in the literature. This discussion is then augmented by an explicit comparison of the computed spectrum for a wave bounded between two cutoffs.Comment: Invited paper for APS DPP 2021. 21 pages, 6 figures, 5 appendice

On the Correlation between the Magnetic Activity Levels, the Metallicities and the Radii of Low-Mass Stars

The recent burst in the number of radii measurements of very low-mass stars from eclipsing binaries and interferometry of single stars has opened more questions about what can be causing the discrepancy between the observed radii and the ones predicted by the models. The two main explanations being proposed are a correlation between the radius of the stars and their activity levels or their metallicities. This paper presents a study of such correlations using all the data published to date. The study also investigates correlations between the radii deviation from the models and the masses of the stars. There is no clear correlation between activity level and radii for the single stars in the sample. Those single stars are slow rotators with typical velocities v_rot sini < 3.0 km s^-1. A clear correlation however exists in the case of the faster rotating members of binaries. This result is based on the of X-ray emission levels of the stars. There also appears to be an increase in the deviation of the radii of single stars from the models as a function of metallicity, as previously indicated by Berger et al. (2006). The stars in binaries do not seem to follow the same trend. Finally, the Baraffe et al. (1998) models reproduce well the radius observations below 0.30-0.35Msun, where the stars become fully convective, although this result is preliminary since almost all the sample stars in that mass range are slow rotators and metallicities have not been measured for most of them. The results in this paper indicate that stellar activity and metallicity play an important role on the determination of the radius of very low-mass stars, at least above 0.35Msun.Comment: 22 pages, 4 figures. Accepted for publication on Ap

Investigation of the influence of CO2 cryogenic coolant application on tool wear

The use of cryogenic coolants has emerged as an environmentally conscious alternative to emulsion coolant options. Cryogenic media can be delivered with a variety of methods to the cutting edge and they can be used in combination with other traditional coolant options such as Minimum Quantity Lubrication (MQL) and compressed air cooling in order to aid dissipation of heat generated in the cutting zone and maximize the lubrication of the cutting edge and thus prolong tool life. This study focuses on the investigation of tool life when milling aerospace grade titanium (Ti-6Al-4 V) under different coolant delivery options. Tool wear progression was recorded for the following coolant options: cryogenic CO 2 , emulsion flood cooling, dry machining, cryogenic CO 2 combined with air or MQL as well as MQL alone

Overcoming electron transfer efficiency bottlenecks for hydrogen production in highly crystalline carbon nitride-based materials

The hydrogen evolution reaction (HER) is a complex reaction involving many interdependent physicochemical steps. Highly ordered carbon nitride-based materials, such as Na-PHI and K-PHI, display some of the highest activities for H2 evolution among the carbon nitride-based materials, due to their electronic properties, but also the presence of cyanimide terminations, which favors the charge transfer for the Pt cocatalyst nanoparticles (NPs). For such highly optimized semiconductor structures, the necessity to control and improve other steps of the photocatalytic process becomes essential, in particular the poor electron transfer from the Pt NPs to the protons in solution over the Helmholtz or Stern layer. Taking highly ordered Na-PHI as a test material, the influence of water-dissolved alkali cations on the HER is systematically studied and it is experimentally verified that the electron transfer from the Pt NPs to the protons in solution limits the efficiency of heterogeneous carbon nitride-based catalysts. This paper explains how hydrated alkali cations influence electron transfer and are able to boost the H2 evolution rate of the same Na-PHI from 2401 up to 5330 µmol h-1 g-1 with an apparent quantum yield of 13% at 420 nm

Past distribution of Tilia-feeding Phyllonorycter micromoth (Lepidoptera: Gracillariidae) in the Russian Far East based on survey of historical herbarium

Distribution data of Tilia-feeding Phyllonorycter in the Russian Far East have been retrieved from a century-old Tilia herbarium stored in Vladivostok. Overall, 280 typical mines of Phyllonorycter, some with larvae and pupae, were found on 61 out of 799 herbarized specimens of Tilia spp. collected in Khabarovskii krai and Primorskii krai. For the first time, the presence of Tilia-feeding Phyllonorycter has been documented in Amurskaya oblast and Jewish Autonomous oblast. High densities of the leafminer have been recorded on Tilia amurensis sampled in Khabarovskii krai and Primorskii krai between 1937 and 2005 suggesting a population dynamics with recurrent outbreaks. Our results confirm the importance of historical herbarium collections in studying trophic interactions and invasion ecology of folivore organisms