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        • SCISCIESCOPUS

          Extended scaling and Paschen law for micro-sized radiofrequency plasma breakdown

          Lee, Min Uk,Lee, Jimo,Lee, Jae Koo,Yun, Gunsu S IOP PUBLISHING 2017 PLASMA SOURCES SCIENCE AND TECHNOLOGY Vol.26 No.3

          <P>The single particle motion analysis and particle-in-cell merged with Monte Carlo collision (PIC/MCC) simulations are compared to explain substantial breakdown voltage reduction for helium microwave discharge above a critical frequency corresponding to the transition from the drift-dominant to the diffusion-dominant electron loss regime. The single particle analysis suggests that the transition frequency is proportional to the product of <img ALIGN='MIDDLE' ALT='${p}^{-{m}}$' SRC='http://ej.iop.org/images/0963-0252/26/3/034003/psstaa52a8ieqn1.gif'/> and <img ALIGN='MIDDLE' ALT='${d}^{-({m}+1)}$' SRC='http://ej.iop.org/images/0963-0252/26/3/034003/psstaa52a8ieqn2.gif'/> where <img ALIGN='MIDDLE' ALT='$p$' SRC='http://ej.iop.org/images/0963-0252/26/3/034003/psstaa52a8ieqn3.gif'/> is the neutral gas pressure, <img ALIGN='MIDDLE' ALT='$d$' SRC='http://ej.iop.org/images/0963-0252/26/3/034003/psstaa52a8ieqn4.gif'/> is the gap distance, and <I>m</I> is a numerical parameter, which is confirmed by the PIC simulation. In the low-frequency or drift-dominant regime, i.e., <img ALIGN='MIDDLE' ALT='$\gamma \mbox{-} {\rm{r}}{\rm{e}}{\rm{g}}{\rm{i}}{\rm{m}}{\rm{e}},$' SRC='http://ej.iop.org/images/0963-0252/26/3/034003/psstaa52a8ieqn5.gif'/> the secondary electron emission induced by ion drift motion is the key parameter for determining the breakdown voltage. The fluid analysis including the secondary emission coefficient, <img ALIGN='MIDDLE' ALT='$\gamma ,$' SRC='http://ej.iop.org/images/0963-0252/26/3/034003/psstaa52a8ieqn6.gif'/> induces the extended Paschen law that implies the breakdown voltage is determined by <img ALIGN='MIDDLE' ALT='$pd,$' SRC='http://ej.iop.org/images/0963-0252/26/3/034003/psstaa52a8ieqn7.gif'/> <img ALIGN='MIDDLE' ALT='$\,f/p,$' SRC='http://ej.iop.org/images/0963-0252/26/3/034003/psstaa52a8ieqn8.gif'/> <img ALIGN='MIDDLE' ALT='$\gamma ,$' SRC='http://ej.iop.org/images/0963-0252/26/3/034003/psstaa52a8ieqn9.gif'/> and<img ALIGN='MIDDLE' ALT='$\,d/R$' SRC='http://ej.iop.org/images/0963-0252/26/3/034003/psstaa52a8ieqn10.gif'/> where <I>f</I> is the frequency of the radio or microwave frequency source, and <I>R</I> is the diameter of electrode. The extended Paschen law reproduces the same scaling law for the transition frequency and is confirmed by the independent PIC and fluid simulations.</P>

        • SCISCIESCOPUS

          <i>In situ</i> impedance measurement of microwave atmospheric pressure plasma

          Lee, S T,Nam, W J,Lee, J K,Yun, G S IOP PUBLISHING 2017 PLASMA SOURCES SCIENCE AND TECHNOLOGY Vol.26 No.4

          <P>The impedance of atmospheric pressure argon plasma jets driven by microwave frequency is determined <I>in situ</I> by a novel ‘two frequency method’. In the conventional method of reflection coefficient <img ALIGN='MIDDLE' ALT='$({{S}}_{11})$' SRC='http://ej.iop.org/images/0963-0252/26/4/045004/psstaa5d26ieqn1.gif'/> measurement, the frequency of the driving microwave power is scanned, which inevitably affects the plasma characters and leads to uncertainty in the estimated plasma impedance. In our proposed method, the frequency-scanning signal additional to the driving power is used to measure <img ALIGN='MIDDLE' ALT='${{S}}_{11}$' SRC='http://ej.iop.org/images/0963-0252/26/4/045004/psstaa5d26ieqn2.gif'/> over a wide frequency range, which enables accurate determination of the plasma impedance based on an equivalent circuit model. The measured resistance and reactance of the plasma increase with the driving power in agreement with the transmission line theory. Based on this <I>in situ</I> measurement of the plasma impedance, the net power coupled to the plasma has been determined. The overall power efficiency remains approximately unchanged around 45% for different input power levels owing to the competing effects between the impedance mismatch and the volume change of the plasma.</P>

        • SCISCIESCOPUS

          Doping dependence of the vortex dynamics in single-crystal superconducting NaFe<img ALIGN='MIDDLE' ALT='${}_{1-x}$' SRC='http://ej.iop.org/images/0953-2048/30/10/105006/sustaa7ec3ieqn1.gif'>Co<sub> <i>x</i> </sub>As

          Ahmad, D,Choi, W J,Seo, Y I,Jung, S-G,Kim, Y C,Salem-Sugui Jr, S,Park, T,Kwon, Y S IOP Publishing Ltd 2017 Superconductor science & technology Vol.30 No.10

          <P>We investigate the doping dependence of flux pinning in superconducting NaFe<img ALIGN='MIDDLE' ALT='${}_{1-x}$' SRC='http://ej.iop.org/images/0953-2048/30/10/105006/sustaa7ec3ieqn3.gif'/>Co<SUB> <I>x</I> </SUB>As (<I>x</I> = 0.01, 0.03, 0.05 and 0.07) single crystals grown by the Bridgman method. The electronic specific heat displays a pronounced anomaly in a sample series at superconducting transition temperature, which hardly shows any residual part at low temperature. We found that Co doping plays an important role in signifying the secondary peak in the magnetic hysteresis of optimally doped (<I>x</I> = 0.03) and heavily doped (<I>x</I> = 0.05, 0.07) crystals. Furthermore, the dependence of the relaxation rate <I>S</I> = ∣d ln <I>M</I>/d ln <I>t</I>∣ on magnetic field and temperature exhibits a decreasing trend within a certain range corresponding to the secondary peak effect in the optimally and heavily doped samples. The magnetic relaxation rate combined with the Maley analysis of the current-dependent creep energy shows a single-vortex pinning in the lightly doped sample dominant at low applied fields, and plastic pinning at high applied fields, without showing a secondary peak. However, in the optimally and heavily doped samples, the magnetic relaxation rate and <I>U</I>(<I>J</I>) isothermal analysis show that the collective pinning that dominates below <I>H</I> <SUB>peak</SUB> crosses over to plastic pinning for fields above <I>H</I> <SUB>peak</SUB>.</P>

        • SCISCIESCOPUS

          An <i>H</i>–<i>T</i> diagram characterizing the activation barriers obtained from the magnetic relaxation of HgBa<sub>2</sub>Ca<sub>2</sub>Cu<sub>3</sub>O<sub>8+δ</sub> thin film

          Kim, Myoung-Hwan,Lee, Sung-Ik,Kim, Mun-Seog,Kang, Won Nam IOP Publishing Ltd 2005 Superconductor science & technology Vol.18 No.6

          <P>The magnetic relaxation of an epitaxial HgBa<SUB>2</SUB>Ca<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>8+δ</SUB> (Hg-1223) thin film was measured at several fields and temperatures. After the activation energy and the normalized relaxation rate had been analysed, the <I>U</I>–<I>j</I> characteristics were mapped on the <I>H</I>–<I>T</I> diagram. The power-law region with a creep exponent of <img SRC='http://ej.iop.org/images/0953-2048/18/6/008/sust197655ieqn1.gif' ALIGN='MIDDLE' ALT='\mu \simeq 7/9 '/>, which indicates large bundle creep, became a logarithmic region in the small μ limit as <I>j</I> went far below <I>j</I><SUB>c</SUB>. From this analysis, we found that a large bundle of flux acts like a single vortex, the size of which was temperature independent. This behaviour is quite different from that of other cuprate superconductors. Also, the crossover temperature, <I>T</I><SUB>b</SUB>, between the power-law and the logarithmic regions was inversely proportional to <I>H</I><SUP>β</SUP>. The fitting of <I>T</I><SUB>b</SUB>(β) on the <I>H</I>–<I>T</I> diagram is in good agreement with the field exponent of <img SRC='http://ej.iop.org/images/0953-2048/18/6/008/sust197655ieqn2.gif' ALIGN='MIDDLE' ALT='\beta \simeq 0.4 '/> that was obtained from the scaling of <I>U</I><SUB>eff</SUB>(<I>H</I>).</P>

        • SCISCIESCOPUS

          A new process to prepare thick bufferless Tl-1223/Ag high temperature superconducting coated conductors using electrodeposition

          Jeong, D Y,Kim, Y H,Shirage, P M,Kim, S Y,Horiuchi, S,Lee, J H IOP Publishing Ltd 2007 Superconductor science & technology Vol.20 No.12

          <P>The present study shows general characteristic features of processes such as electrodeposition, drying, and heat-treatment, involved in the preparation of bufferless Tl-1223/Ag superconducting coated conductors by using an electrodeposition method, the details of which have not been reported in the literature. The importance of the present study, however, lies in a new process to make a thick Tl-1223 superconducting layer by using electrodeposition. In the new method, a relatively thin Tl-1223 precursor film is firstly electrodeposited on an Ag sheet, dried, and then heat-treated for phase transformation to superconducting phases to assign conductivity to the film. Then Tl-1223 precursor film is electrodeposited again on the heat-treated superconducting film, dried, and heat-treated. The process of electrodeposition, drying, and heat-treatment can be repeated many times to prepare a very thick superconducting layer. In the present study, a 2–3 µm thick Tl-1223 layer was prepared on a <img SRC='http://ej.iop.org/images/0953-2048/20/12/026/sust252315ieqn1.gif' ALIGN='MIDDLE' ALT='\{113\}\langle 121\rangle '/> intensified Ag sheet by repeating the process once. Even though <I>J</I><SUB>c</SUB> values in thick Tl-1223/Ag were quite low mainly due to material loss during heat-treatment, the microstructure analysis indicated that Tl-1223 layers were epitaxially grown on the <img SRC='http://ej.iop.org/images/0953-2048/20/12/026/sust252315ieqn2.gif' ALIGN='MIDDLE' ALT='(113)\langle 121\rangle '/> intensified Ag sheets provided that the Ag surfaces were flat and clean. </P>

        • Proposal of New Measurement Method for Internal Quantum Efficiency of Light-Emitting Diodes

          Ma, Byungjin,Lee, Kwanhoon IOP Publishing 2013 Japanese journal of applied physics Vol.52 No.8

          <P>A new evaluation method for effective internal optical power (IOP) and internal quantum efficiency (IQE) of light-emitting diodes (LEDs) is demonstrated. This method is based on the optical and thermal properties of LEDs. By using this proposed method, the effective IOP and the IQE of LEDs could be directly extracted from the measurements of external optical power (EOP) and junction temperature of LEDs. This method needs no assumption of the injection efficiency of carriers in the LEDs and no measurement-condition limitation of low current-injection level. (C) 2013 The Japan Society of Applied Physics</P>

        • SCISCIESCOPUS

          MAGNETIZATION DEGREE AT THE JET BASE OF M87 DERIVED FROM THE EVENT HORIZON TELESCOPE DATA: TESTING THE MAGNETICALLY DRIVEN JET PARADIGM

          Kino, M.,Takahara, F.,Hada, K.,Akiyama, K.,Nagai, H.,Sohn, B. W. IOP Publishing 2015 The Astrophysical journal Vol.803 No.1

          <P>We explore the degree of magnetization at the jet base of M87 by using the observational data of the event horizon telescope (EHT) at 230 GHz obtained by Doeleman et al. By utilizing the method in Kino et al., we derive the energy densities of the magnetic fields (U-B) and electrons and positrons (U-+/-) in the compact region detected by EHT (the EHT region) with its FWHM size. 40 mu as. First, we assume that an optically thick region for synchrotron self-absorption (SSA) exists in the EHT. region. Then, we find that the SSA-thick region should not be too large, in order to. not. overproduce the Poynting power at the EHT. region. The allowed ranges of the angular size and the magnetic-field strength of the SSA-thick region are 21 mu as <= theta(thick) <= 26.3 mu as and 50 G <= B-tot <= 124 G, respectively. Correspondingly, U-B >> U-+/- is realized in this case. We further examine the composition of plasma and energy density of protons by utilizing the Faraday rotation measurement at 230 GHz obtained by Kuo et al. Then, we find that U-B >> U-+/- + U-p still holds in the SSA-thick region. Second, we examine the case when the EHT. region is fully SSA-thin. Then, we find that U-B >> U-+/- still holds unless protons are relativistic. Thus, we conclude that the magnetically driven jet scenario in M87 is viable in terms of energetics close to the Innermost Stable Circular Orbit scale unless the EHT. region is fully SSA-thin and relativistic protons dominated.</P>

        • SCISCIESCOPUS

          KINEMATICS OF THE ENVELOPE AND TWO BIPOLAR JETS IN THE CLASS 0 PROTOSTELLAR SYSTEM L1157

          Kwon, Woojin,Ferná,ndez-Ló,pez, Manuel,Stephens, Ian W.,Looney, Leslie W. IOP Publishing 2015 The Astrophysical journal Vol.814 No.1

          <P>A massive envelope and a strong bipolar outflow are the two main structures characterizing the youngest protostellar systems. In order to understand the physical properties of a bipolar outflow and the relationship with those of the envelope, we obtained a mosaic map covering the whole bipolar outflow of the youngest protostellar system L1157 with about 5 '' angular resolution in CO J = 2-1 using the Combined Array for Research in Millimeter-wave Astronomy. By utilizing these observations of the whole bipolar outflow, we estimate its physical properties and show that they are consistent with multiple jets. We also constrain a preferred precession direction. In addition, we observed the central envelope structure with 2' resolution in the lambda = 1.3 and 3 mm continua and various molecular lines: (CO)-O-17, (CO)-O-18, (13)CO3 CS, CN, N2H+, CH3OH, H2O, SO, and SO2. All of the CO isotopes and CS, CN, and N2H+ have been detected and imaged. We marginally detected the features that can be interpreted as a rotating inner envelope in (CO)-O-17 and (CO)-O-18 and as an infalling outer envelope in N2H+. We also estimated the envelope and central protostellar masses and found that the dust-opacity spectral index changes with radius.</P>

        • SCISCIESCOPUS

          MEASUREMENT OF HDCO/H<sub>2</sub>CO RATIOS IN THE ENVELOPES OF EXTREMELY COLD PROTOSTARS IN ORION

          Kang, Miju,Choi, Minho,Stutz, Amelia M.,Tatematsu, Ken’ichi IOP Publishing 2015 The Astrophysical journal Vol.814 No.1

          <P>We present observations of HDCO and H2CO emission toward a sample of 15 Class 0 protostars in the Orion A and B clouds. Of these, 11 protostars are Herschel-identified PACS Bright Red Sources (PBRSs) and 4 are previously identified protostars. Our observations revealed the chemical properties of the PBRS envelope for the first time. The column densities of HDCO and H2CO are derived from single-dish observations at an angular resolution of similar to 20 '' (similar to 8400 AU). The degree of deuteration in H2CO ([HDCO]/[H2CO]) was estimated to range from 0.03 to 0.31. The deuterium fractionation of most PBRSs (70%) is similar to that of the non-PBRS sources. Three PBRSs (30%) exhibit high deuterium fractionation, larger than 0.15. The large variation of the deuterium fractionation of H2CO in the whole PBRS sample may reflect the diversity in the initial conditions of star-forming cores. There is no clear correlation between the [HDCO]/[H2CO] ratio and the evolutionary sequence of protostars.</P>

        • PARTIAL REFLECTION AND TRAPPING OF A FAST-MODE WAVE IN SOLAR CORONAL ARCADE LOOPS

          Kumar, Pankaj,Innes, D. E. IOP Publishing 2015 ASTROPHYSICAL JOURNAL LETTERS - Vol.803 No.2

          <P>We report on the first direct observation of a fast-mode wave propagating along and perpendicular to cool (171 angstrom) arcade loops observed by the Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA). The wave was associated with an impulsive/compact flare near the edge of a sunspot. The EUV wavefront expanded radially outward from the flare center and decelerated in the corona from 1060 to 760 km s(-1) within similar to 3-4 minutes. Part of the EUV wave propagated along a large-scale arcade of cool loops and was partially reflected back to the flare site. The phase speed of the wave was about 1450 km s(-1), which is interpreted as a fast-mode wave. A second overlying loop arcade, orientated perpendicular to the cool arcade, is heated and becomes visible in the AIA hot channels. These hot loops sway in time with the EUV wave, as it propagated to and fro along the lower loop arcade. We suggest that an impulsive energy release at one of the footpoints of the arcade loops causes the onset of an EUV shock wave that propagates along and perpendicular to the magnetic field.</P>

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