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IGRINS SPECTROSCOPY OF CLASS I SOURCES: IRAS 03445+3242 AND IRAS 04239+2436
Lee, Seokho,Lee, Jeong-Eun,Park, Sunkyung,Lee, Jae-Joon,Kidder, Benjamin,Mace, Gregory N.,Jaffe, Daniel T. American Astronomical Society 2016 The Astrophysical journal Vol.826 No.2
<P>We have detected molecular and atomic line emission from the hot and warm disks of two Class I sources, IRAS 03445+3242 and IRAS 04239+2436, using the high-resolution Immersion GRating INfrared Spectrograph (IGRINS). CO overtone band transitions and near-IR lines of Na I and Ca I, all in emission, trace the hot inner disk, while CO rovibrational absorption spectra of the first overtone transition trace the warm gas within the inner few AU of the disk. The emission-line profiles for both sources show evidence for Keplerian disks. A thin Keplerian disk with power-law temperature and column density profiles with a projected rotational velocity of similar to 60-75 km s(-1) and a gas temperature of similar to 3500 K at the innermost annulus can reproduce the CO overtone band emission. Na I and Ca I emission lines also arise from this disk, but they show complicated line features possibly affected by photospheric absorption lines. Multi-epoch observations show asymmetric variations of the line profiles on one-year (CO overtone bandhead and atomic lines for IRAS 03445+3242) or on one-day (atomic lines for IRAS 04239+2436) timescales, implying non-axisymmetric features in disks. The narrow CO rovibrational absorption spectra (v = 0 -> 2) indicate that both warm (>150 K) and cold (similar to 20-30 K) CO gas are present along the line of sight to the inner disk. This study demonstrates the power of IGRINS as a tool for studies of the sub-AU-scale hot and AU-scale warm protoplanetary disks with its simultaneous coverage of the full H and K bands with high spectral resolution (R = 45,000) allowing many aspects of the sources to be investigated at once.</P>
Principal Component Regression by Principal Component Selection
Lee, Hosung,Park, Yun Mi,Lee, Seokho The Korean Statistical Society 2015 Communications for statistical applications and me Vol.22 No.2
We propose a selection procedure of principal components in principal component regression. Our method selects principal components using variable selection procedures instead of a small subset of major principal components in principal component regression. Our procedure consists of two steps to improve estimation and prediction. First, we reduce the number of principal components using the conventional principal component regression to yield the set of candidate principal components and then select principal components among the candidate set using sparse regression techniques. The performance of our proposals is demonstrated numerically and compared with the typical dimension reduction approaches (including principal component regression and partial least square regression) using synthetic and real datasets.
Lee, Woo Seung,Nam, Seokho,Kim, Jinsub,Lee, Jeyull,Ko, Tae Kuk IEEE 2015 IEEE transactions on applied superconductivity Vol.25 No.3
<P>Most superconducting devices are designed for a fixed operating temperature range, however, temperature varia- tions are inevitable in some situations. For example, the temperature of a resistive-type superconducting fault current limiter becomes higher during fault cycle operation. Therefore, knowledge of the temperature dependence of the current-to-voltage characteristic is necessary for accurate simulation or design processes. This paper explores the temperature dependence of the n-index, which describes sharpness of superconductivity to normal transition, using a second-generation high-temperature superconducting tape sample under a liquid nitrogen surrounded environment. The voltage across the sample according to given current and temperature is measured by establishing a measurement system with a protection system. The average of the data from multiple measurements is calculated and compared with three different n-index models. Constant, inversely proportional, and modified inversely proportional models are analyzed. The modified inversely proportional model showed the least error when compared to the measurement data.</P>
Lee, Hongseok,Mo, Young Kyu,Lee, Onyou,Kim, Junil,Bang, Seungmin,Kang, Jong O,Nam, Seokho,Kang, Hyoungku The Korea Institute of Applied Superconductivity a 2015 한국초전도저온공학회논문지 Vol.17 No.1
High voltage superconducting apparatuses have been developed presently around the world under AC and DC sources. In order to improve electrical reliability of superconducting apparatuses with AC and DC networks, a study on the DC as well as the AC electrical breakdown characteristics of cryogenic insulations should be conducted for developing a high voltage superconducting apparatus. Recently, a sub-cooled liquid nitrogen cooling system is known to be promising method for developing a high voltage superconducting apparatus. A sub-cooled liquid nitrogen cooling system uses gaseous nitrogen to control the pressure and enhance the dielectric characteristics. However, the dielectric characteristics of gaseous nitrogen are not enough to satisfy the grade of insulation for a high voltage superconducting apparatus. In this case, the application of solid insulating barriers is regarded as an effective method to reinforce the dielectric characteristics of a high voltage superconducting apparatus. In this paper, it is dealt with a barrier effect on the DC and AC dielectric characteristics of gaseous nitrogen with respect to the position and number of solid insulating barriers. As results, the DC and AC electrical breakdown characteristics by various barrier effects is verified.
Seokho Nam,Woo Seung Lee,Jeyull Lee,Haeryoung Jeon,Seunghak Han,Yojong Choi,Jiho Lee,Tae Kuk Ko Institute of Electrical and Electronics Engineers 2017 IEEE transactions on applied superconductivity Vol.27 No.4
<P>A current lead is a connecting device between the superconducting magnet and the power supply. A large amount of heat transfer ocurrs through the current lead because the cold-end in a superconducting system is thermally linked with the warm-end part. Therefore, high-Tc superconductor (HTS) is used in the current lead for a tokamak system. However, it is hard to consider the dependance of HTS's critical current with respect to temperature because the conventional HTS current leads consist of the HTS tapes with the same width and thickness, and their critical currents can differ from their temperature. For this reason, cold-end of the HTS section in current lead is designed excessively than necessary specifications. This generates the additional heat transfer between cold-end and warm-end within the HTS current lead. In this paper, a study on the HTS current leads with asymmetric structure is conducted to reduce the heat transfer by reducing the cross-sectional area of the HTS current lead. Asymmetric current leads are fabricated considering the design parameters about heat transfer within the HTS current lead. Current transporting tests on the conventional, symmetrical, and asymmetrical HTS current leads are performed to confirm the feasibility of the suggested design. And then, heat load of the asymmetric current lead is compared with the symmetric current lead. From this study, it is found that heat load of the current lead is reduced by using the asymmetric HTS current lead.</P>