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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>
<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>
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>
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>
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>