S J Lee

Publications (11)8.49 Total impact

• Article: Quench Simulation and Detection in KSTAR PF Magnet System

  Y. Chu, S.H. Park, H. Yonekawa, Y.O. Kim, H.J. Lee, K.P. Kim, S.J. Lee, K.R. Park, Y.K. Oh, H.K. Na
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  ABSTRACT: To detect quenches in the Poloidal Field (PF) magnet system is more difficult than the Toroidal Field (TF) magnet system due to excessively high inductive voltages generated by PF pulse-currents and plasma currents. According to reference scenarios being considered so far, the maximum voltage across the PF coils is inductively generated up to about 3.5 kV during the start of plasma (SoP) stage in a very short time period. The voltage measured by compensation of the inductive voltage should be below a certain level which is called as the quench voltage threshold. However, the compensated voltage might be higher than the threshold even with the well-designed compensation schemes. Accordingly, the quench voltage threshold and the quench protection delay time should be properly determined for the quench detection not to take a false action which could cause the fast energy discharge. From the quench simulation using the calculation of hot spot temperature and the resistive voltage growth as a function of time, the proper values of the quench detection parameters of the PF magnet system were derived for the maximum hot temperature rise to be limited within 150 K.
  IEEE Transactions on Appiled Superconductivity 07/2010; · 1.04 Impact Factor
• Article: Quench Detection Based on Voltage Measurement for the KSTAR Superconducting Coils

  Yong Chu, H. Yonekawa, Y.O. Kim, K.R. Park, Hyun Jung Lee, M.K. Park, Y.M. Park, S.J. Lee, T.H. Ha, Y.K. Oh, J.S. Bak
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  ABSTRACT: To protect the KSTAR (Korea Superconducting Tokamak Advanced Research) superconducting coils against a quench, the quench detection system based on voltage measurement was fabricated. It used a detect/dump scheme which detects the presence of non-recovering normal zones and activates a dump circuit that transfers most of the stored energy into a dump resistor. Even though it is desirable to discharge the coil energy as fast as possible after quench detection, a time delay between the quench detection and the complete dump circuit actuation will be necessary in order that the quench voltage is to be distinguished from various noises. The quench threshold voltage and the delay time should be set before operation for quench voltage detection so that the maximum hot-spot temperature could be limited to 150 K. This paper describes the hardware techniques to prevent the malfunction of the quench detection due to voltage noises arising from the KSTAR operating scenarios. During the first operation of the KSTAR machine, the inductive voltages and other voltage noises were measured and effectively compensated below voltage thresholds. A quench did not occur and the quench detection system was well operated without any false activation.
  IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
• Article: Etching of TiN-based gates for advanced complementary metal-oxide-semiconductor devices

  V. Bliznetsov, N. Singh, R. Kumar, N. Balasubramanian, P. Guo, S. J. Lee, Y. Cai
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  ABSTRACT: This article presents results of study and optimization of plasma etching of TiN and TiN-TaN gates for sub-45 mm CMOS technology. By design of experiment in decoupled plasma source using HBr/Cl 2 plasma, etching parameters were established providing smooth post-etch surface.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 08/2008; · 1.34 Impact Factor
• Source

  Available from: ac.kr

  Article: AC Loss Characteristics of the KSTAR CSMC Estimated by Pulse Test

  S. Lee, Y. Chu, W.H. Chung, S.J. Lee, S.M. Choi, S.H. Park, H. Yonekawa, S.H. Baek, J.S. Kim, K.W. Cho, K.R. Park, B.S. Lim, Y.K. Oh, K. Kim, J.S. Bak, G.S. Lee
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  ABSTRACT: The KSTAR Central Solenoid Model Coils (CSMC), which are in the form of split coils with same dimension, have been tested. The CSMC were successfully charged up to 20 kA and down to zero with different ramp rates. Various pulse waveforms were applied to the CSMC to analyse the AC-loss characteristics of the coils. The measurement method was a gas-flow calorimetry. In this work, two types of waveforms, the DC-biased sinusoidal wave (I_dc=2, 4 kA; DeltaI_ac=1 kA; f=0.08~0.67 Hz) and a single triangular pulse (I_max=6~10 kA; dI/dt=0.5~2 kA/s), are selected and analysed. From the measured data at the one outlet where helium channel 3 and 4 are merged, the AC loss parameters, related with the hysteresis loss and the coupling loss, are calculated. The B-field strength differs depending on the position within the cooling channels. The spatial field variation and the ramp rate were 0.048~1.9 T and 8.1~ 71 mT/s for the DC-biased sinusoidal waves, while 0.29~ 4.3 T and 0.024~ 0.86 T/s for the triangular pulses. The measured AC losses are compared with estimated values and the behavior agrees well. The coupling time constant (ntau) varies with the field strength. In case of the triangular pulses, ntau increases by increasing the field amplitude and the maximum value of ntau is 41 ms with I_max=10 kA
  IEEE Transactions on Appiled Superconductivity 07/2006; · 1.04 Impact Factor
• Source

  Available from: kaist.ac.kr

  Article: AC loss characteristics of the KSTAR CSMC estimated by pulse test

  S Lee, Y Chu, W H Chung, S J Lee, S M Choi, S H Park, H Yonekawa, S H Baek, J S Kim, K W Cho, K R Park, B S Lim, Y K Oh, K Kim, J S Bak, G S Lee
  [show abstract] [hide abstract]
  ABSTRACT: The KSTAR Central Solenoid Model Coils (CSMC), which are in the form of split coils with same dimension, have been tested. The CSMC were successfully charged up to 20 kA and down to zero with different ramp rates. Various pulse waveforms were applied to the CSMC to analyse the AC-loss characteristics of the coils. The measurement method was a gas-flow calorimetry. In this work, two types of waveforms, the DC-biased sinusoidal wave (= 2, 4 kA; = 1 kA; = 0 08 0 67 Hz) and a single triangular pulse (= 6 10 kA; = 0 5 2 kA s), are selected and analysed. From the measured data at the one outlet where helium channel 3 and 4 are merged, the AC loss parameters, related with the hysteresis loss and the coupling loss, are calculated. The B-field strength differs depending on the position within the cooling channels. The spatial field variation and the ramp rate were 0.048 1.9 T and 8.1 71 mT/s for the DC-biased sinusoidal waves, while 0.29 4.3 T and 0.024 0.86 T/s for the triangular pulses. The measured AC losses are compared with estimated values and the behavior agrees well. The coupling time constant () varies with the field strength. In case of the triangular pulses, increases by increasing the field amplitude and the maximum value of is 41 ms with = 10 kA.
  IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY. 01/2006; 16.
• Article: Status of the KSTAR PF6 and PF7 coil development

  K. Park, W. Chung, S. Baek, B. Lim, S.J. Lee, H. Park, Y. Chu, S. Lee, K.P. Kim, J. Joo, K. Lee, D. Lee, S. Ahn, Y.K. Oh, K. Kim, J.S. Bak, G.S. Lee
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  ABSTRACT: The KSTAR superconducting magnet system consists of 16 TF (Toroidal Field) and 14 PF (Poloidal Field) coils. Both of the TF and PF coil systems use internally-cooled Cable-In-Conduit Conductors (CICC). The PF coil system, which consists of 8 coils in the CS (Central Solenoid) coil system and 6 outer PF coils, provides 17 V-s and sustains the plasma current of 2 MA for 20 seconds inductively. PF 1-5 coils use Nb₃Sn CICC in an Incoloy 908 conduit and PF 6-7 coils use NbTi CICC in a modified stainless steel 316LN(316LN+). The fabrication of PF7L/U & PF6L/U coils is completed at present. The engineering issues and fabrication process, which are related with the KSTAR PF 6-7 coil development, are presented and discussed in this paper. TF and PF coils are in the fabrication stage for KSTAR completion in the year 2006.
  IEEE Transactions on Appiled Superconductivity 07/2005; · 1.04 Impact Factor
• Article: Heat transfer from a flat plate to a fully developed axisymmetric impinging jet

  D. Lee, S.J. Lee, J.H. Lee
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  ABSTRACT: Impinging jets have been used in a number of applications to enhance heat and mass transfer; this includes cooling steel plates, the tempering of glass, drying papers and films, cooling turbine blades, cooling electronic components, etc. Studies have included the effects of Reynolds number, nozzle-to-plate distance, nozzle geometry, jet temperature, orientation, multiple jets, crossflow, and impinging surface shape on the resulting flow and heat transfer. Critical reviews of impinging jet heat transfer studies have been published. Martin, Jambunathan et al., and Viskanta reviewed the heat transfer data for single jets and for arrays of axisymmetric and planar jets. Heat transfer measurements for various nozzle geometries and flow conditions have been made and studied the heat transfer characteristics for a turbulent air jet issuing from an elliptical nozzle.
  Journal of Heat Transfer. 07/1995; 117(3).
• Article: Electronic structure of DO3-ordered Fe3Co and Co3Fe studied by spectroscopic ellipsometry

  Kwang Joo Kim, S.J. Lee, J.M. Park
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  ABSTRACT: Optical properties of DO3-ordered Fe3Co and Co3Fe compounds were investigated by spectroscopic ellipsometry in the 1.5–5.2 eV photon energy region. A broad interband absorption structure exists around 2 eV in the optical conductivity spectra of the compounds. The absorption structure of Co3Fe is located at lower energies than that of Fe3Co by about 0.5 eV. The calculated spin-polarized band structures of the compounds by the linear-muffin-tin orbitals method revealed that the Fe- and Co-like d states disperse in the same energy region because of hybridization. The theoretically calculated optical absorption structure of Fe3Co shows a good agreement with the experimental result while that of Co3Fe is located at higher energies than the experimental result by about 0.3 eV. Such discrepancy is interpreted as due to the self-energy shift of the interband transitions involving correlated Co-like d electrons.
  Journal of Magnetism and Magnetic Materials 241(1):6-10. · 1.78 Impact Factor
• Article: Electron density dependence of the effective g-factor in a two-dimensional electron gas at strong magnetic fields

  J.H. Oh, K.J. Chang, G. Ihm, S.J. Lee
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  ABSTRACT: The dependence of the effective g-factor (g*) on the electron density and the Landau level filling factor v are studied for a two-dimensional electron gas in strong magnetic fields. The trial many-body wavefunctions are constructed by multiplying the Jastrow correlation factor to the Laughlin-like wavefunctions, then, the ground state energy are calculated using a variational Monte Carlo method for 1 < v < 2. For a dimensionless parameter rs = 1.33, g* is estimated to be about 11 at v = 1.0, while the measured value is 7.3. Considering the level broadening due to impurity and phonon scatterings, a better agreement of g* = 9 is found.
  Surface Science.
• Article: Soluble canonical ensemble models of fragmentation processes

  S.J. Lee, A.Z. Mekjian
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  ABSTRACT: Soluble models of fragmentation are developed. These models give closed form expressions for the distribution of fragment sizes and for correlations and fluctuations. Simple equations are found which show modes of fragmentation from evaporation to total fragmentation.
  Physics Letters A. 149(1):7-11.
• Source

  Available from: 144.206.159.178

  Article: Aspects of operational conditions of a double gap prototype RPC for the CMS/LHC experiment

  S.H. Ahn, R. Amirikas, S.Y. Bahk, K.M. Baik, V.A. Gapienko, B. Hong, S.J. Hong, J.Y. Kim, Y.J. Kim, Y.U. Kim, D.G. Koo, K.S. Lee, S.J. Lee, I.T. Lim, S.K. Nam, M.Y. Pac, S. Park, J.T. Rhee, S.W. Seo, K.S. Sim
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  ABSTRACT: This paper presents the results from cosmic ray tests performed in the framework of effort on the forward Resistive Plate Chamber (RPC) for CMS. A double gap RPC of dimensions with 2 mm gas gaps was tested. We report data on the use of tetrafluorethane-based gas mixture, with additions of water vapour and SF6. Efficiency, time resolution and cross-talk behaviour of the detector were investigated.
  Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 465:447-454. · 1.21 Impact Factor