Surge snubber design for high power-density DC-DC converters in HVDC power distribution systems

In an isolated DC-DC converter utilized in the HVDC power distribution system, a large surge voltage occurs across the secondary-side diodes due to the transformer\u27s leakage inductance, and then the diodes with a large withstand voltage are required. However, those diodes cause a lot of power loss. In this paper, a simple surge snubber with prominent surge suppression capability is examined, and the surge-voltage evaluation through the analysis using the high-frequency equivalent circuits, which is followed by the experimental confirmation. Furthermore, the the optimum design of its surge-snubber capacitor is presented.1st International Conference on Renewable Energy Research and Applications, ICRERA 2012; Nagasaki; Japan; 11 November 2012 ～ 14 November 201

Social cost of mining-related lead (Pb) pollution in Kabwe, Zambia, and potential remediation measures

Lead (Pb) pollution has been one of the major environmental problems of worldwide significance. It is a latent factor for several fatal illnesses, whereas the exposure to lead in early childhood causes a lifetime IQ loss. The social cost is the concept to aggregate various adverse effects in a single monetary unit, which is useful in describing the pollution problem and provides foundation for the design of interventions. However, the assessment of the social cost is scarce for developing countries. In this study, we focus on the lead pollution problem of a former mining town, Kabwe, Zambia, where mining wastes abandoned near residential areas has caused a critical pollution problem. We first investigated the social cost of lead pollution that future generations born in 2025–2049 would incur in their lifetime. As the channels of the social cost, we considered the lost income from the IQ loss and the lost lives from lead-related mortality. The results showed that the social cost would amount to 224–593 million USD (discounted to the present value). Our results can be considered conservative, lower bound estimates because we focused only on well-identified effects of lead, but the social cost was still substantial. Then we examined several engineering remediation measures. The results showed that the social cost can be reduced (the benefits of remediations) more than the costs of implementing remediation measures. This study is the first to investigate the social cost of mining-related lead pollution problem in developing countries. Our interdisciplinary approach utilises the micro-level economic, health and pollution data and integrates the techniques in economics, toxicology and engineering.publishedVersio

Evidence for Spin–Orbit Alignment in the TRAPPIST-1 System

In an effort to measure the Rossiter–McLaughlin effect for the TRAPPIST-1 system, we performed high-resolution spectroscopy during transits of planets e, f, and b. The spectra were obtained with the InfraRed Doppler spectrograph on the Subaru 8.2 m telescope, and were supplemented with simultaneous photometry obtained with a 1 m telescope of the Las Cumbres Observatory Global Telescope. By analyzing the anomalous radial velocities, we found the projected stellar obliquity to be λ = 1 ± 28° under the assumption that the three planets have coplanar orbits, although we caution that the radial-velocity data show correlated noise of unknown origin. We also sought evidence for the expected deformations of the stellar absorption lines, and thereby detected the "Doppler shadow" of planet b with a false-alarm probability of 1.7%. The joint analysis of the observed residual cross-correlation map including the three transits gave λ = 19_(-15)^(+13)°. These results indicate that the the TRAPPIST-1 star is not strongly misaligned with the common orbital plane of the planets, although further observations are encouraged to verify this conclusion

Limits on the Spin-Orbit Angle and Atmospheric Escape for the 22 Myr-old Planet AU Mic b

We obtained spectra of the pre-main sequence star AU Microscopii during a transit of its Neptune-sized planet to investigate its orbit and atmosphere. We used the high-dispersion near-infrared spectrograph IRD on the Subaru telescope to detect the Doppler "shadow" from the planet and constrain the projected stellar obliquity. Modeling of the observed planetary Doppler shadow suggests a spin-orbit alignment of the system ($\lambda=-4.7_{-6.4}^{+6.8}$ degrees), but additional observations are needed to confirm this finding. We use both the IRD data and spectra obtained with NIRSPEC on Keck-II to search for absorption in the 1083 nm line of metastable triplet He I by the planet's atmosphere and place an upper limit for the equivalent width of 3.7 m\AA at 99 $\%$ confidence. With this limit and a Parker wind model we constrain the escape rate from the atmosphere to $<0.15-0.45\, M_{\oplus}$ Gyr$^{-1}$, comparable to the rates predicted by an XUV energy-limited escape calculation and hydrodynamic models, but refinement of the planet mass is needed for rigorous tests.Comment: 10 pages, 5 figures, accepted for publication in ApJ Letter

Direct Imaging Explorations for Companions around Mid-Late M Stars from the Subaru/IRD Strategic Program

The Subaru telescope is currently performing a strategic program (SSP) using the high-precision near-infrared (NIR) spectrometer IRD to search for exoplanets around nearby mid/late-M~dwarfs via radial velocity (RV) monitoring. As part of the observing strategy for the exoplanet survey, signatures of massive companions such as RV trends are used to reduce the priority of those stars. However, this RV information remains useful for studying the stellar multiplicity of nearby M~dwarfs. To search for companions around such ``deprioritized" M~dwarfs, we observed 14 IRD-SSP targets using Keck/NIRC2 observations with pyramid wavefront sensing at NIR wavelengths, leading to high sensitivity to substellar-mass companions within a few arcseconds. We detected two new companions (LSPM~J1002+1459~B and LSPM~J2204+1505~B) and two new candidates that are likely companions (LSPM~J0825+6902~B and LSPM~J1645+0444~B) as well as one known companion. Including two known companions resolved by the IRD fiber injection module camera, we detected seven (four new) companions at projected separations between $\sim2-20$~au in total. A comparison of the colors with the spectral library suggests that LSPM~J2204+1505~B and LSPM~J0825+6902~B are located at the boundary between late-M and early-L spectral types. Our deep high-contrast imaging for targets where no bright companions were resolved did not reveal any additional companion candidates. The NIRC2 detection limits could constrain potential substellar-mass companions ($\sim10-75\ M_{\rm Jup}$) at 10~au or further. The failure with Keck/NIRC2 around the IRD-SSP stars having significant RV trends makes these objects promising targets for further RV monitoring or deeper imaging with JWST to search for smaller-mass companions below the NIRC2 detection limits.Comment: 16 pages, 8 figures, accepted for publication in A