H K Park

Publications (80)115.58 Total impact

• Article: Appearance and Dynamics of Helical Flux Tubes under Electron Cyclotron Resonance Heating in the Core of KSTAR Plasmas.

  G S Yun, H K Park, W Lee, M J Choi, G H Choe, S Park, Y S Bae, K D Lee, S W Yoon, Y M Jeon, C W Domier, N C Luhmann, B Tobias, A J H Donné
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  ABSTRACT: Dual (or sometimes multiple) flux tubes (DFTs) have been observed in the core of sawtoothing KSTAR tokamak plasmas with electron cyclotron resonance heating. The time evolution of the flux tubes visualized by a 2D electron cyclotron emission imaging diagnostic typically consists of four distinctive phases: (1) growth of one flux tube out of multiple small flux tubes during the initial buildup period following a sawtooth crash, resulting in a single dominant flux tube along the m/n=1/1 helical magnetic field lines, (2) sudden rapid growth of another flux tube via a fast heat transfer from the first one, resulting in approximately identical DFTs, (3) coalescence of the two flux tubes into a single m/n=1/1 flux tube resembling the internal kink mode in the normal sawteeth, which is explained by a model of two current-carrying wires confined on a flux surface, and (4) fast localized crash of the merged flux tube similar to the standard sawtooth crash. The dynamics of the DFTs implies that the internal kink mode is not a unique prerequisite to the sawtooth crash, providing a new insight on the control of the sawtooth.
  Physical Review Letters 10/2012; 109(14):145003. · 7.37 Impact Factor
• Article: Design of the reflective optics for Tore Supra ECEI system.

  Y Nam, W Lee, G S Yun, H K Park, D Elbeze, J L Segui, R Sabot, J P Chenevoix
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  ABSTRACT: A 2D electron cyclotron emission (ECE) imaging system for Tore Supra is under design for studying the MHD physics of the magnetically confined plasma such as sawteeth, tearing modes, and turbulent fluctuations. Complex beam path due to the tight access in Tore Supra led to the design of reflective optics made of 6 or more large cylindrical∕flat mirrors. The total path length of the ECE beam is about 11 m, including almost 4 m inside the vacuum vessel. The imaging property of the optics has been estimated using the Gaussian beam simulation and ray transfer analysis. The possible setups for the optical alignment of the diagnostic and the operation scenarios with single- or dual-array measurement system are discussed.
  The Review of scientific instruments 10/2012; 83(10):10E318. · 1.52 Impact Factor
• Article: Investigation of the hair of patients with scalp psoriasis using atomic force microscopy.

  M K Shin, K S Kim, J-J Ahn, N I Kim, H-K Park, C-R Haw
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  ABSTRACT: Psoriasis affects not only the soft keratin of the skin, but also hard keratin, such as nails and hair. However, few studies have described the changes induced in the hair of patients with psoriasis. Using atomic force microscopy (AFM), we investigated the morphological property of hair samples taken from the scalp of patients with psoriasis. Lesional and nonlesional hairs taken from 15 patients with scalp psoriasis were investigated. Hairs from 15 healthy adults were also examined as controls. Using AFM, surface images were taken of an area of 20 × 20 μm(2), with 512 × 512 pixels and a scan speed of 0.8 lines/s. results: Pits were frequently seen in the hair shafts of patients with psoriasis, similar to those seen in their nail plates. Macropit number, scale thickness and surface roughness were all significantly increased in lesional hairs compared with both nonlesional and control hairs, and macropits and scale thickness were also increased in nonlesional hairs compared with control hairs. The hair shafts of patients with scalp psoriasis exhibited the same macropits seen in their nails. Both lesional and nonlesional hairs had similar changes in morphological structure compared with controls. This supports the generalized nature of psoriasis, with changes in hair structure being analogous to the changes seen in skin and nails.
  Clinical and Experimental Dermatology 03/2012; 37(2):156-63. · 1.20 Impact Factor
• Article: Two-dimensional visualization of growth and burst of the edge-localized filaments in KSTAR H-mode plasmas.

  G S Yun, W Lee, M J Choi, J Lee, H K Park, B Tobias, C W Domier, N C Luhmann, A J H Donné, J H Lee
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  ABSTRACT: The filamentary nature and dynamics of edge-localized modes (ELMs) in the KSTAR high-confinement mode plasmas have been visualized in 2D via electron cyclotron emission imaging. The ELM filaments rotating with a net poloidal velocity are observed to evolve in three distinctive stages: initial linear growth, interim quasisteady state, and final crash. The crash is initiated by a narrow fingerlike perturbation growing radially from a poloidally elongated filament. The filament bursts through this finger, leading to fast and collective heat convection from the edge region into the scrape-off layer, i.e., ELM crash.
  Physical Review Letters 07/2011; 107(4):045004. · 7.37 Impact Factor
• Article: Characterization and interpretation of the Edge Snake in between type-­‐I edge localized modes at ASDEX Upgrade

  F. Sommer, S. Günter, A. Kallenbach, M. Maraschek, J. Boom, R. Fischer, N. Hicks, N. C. L. Jr, H. K. Park, B. Reiter, R. Wenninger, E. Wolfrum, ASDEX Upgrade Team
  Plasma Physics and Controlled Fusion 06/2011; · 2.42 Impact Factor
• Article: Imaging Techniques for Microwave Diagnostics

  B. Tobias, A.J.H. Donné, H.K. Park, J.E. Boom, M.J. Choi, I.G.J. Classen, C.W. Domier, X. Kong, W. Lee, T. Liang, N.C. Luhmann Jr, T. Munsat, L. Yu, G.S. Yun
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  ABSTRACT: Imaging diagnostics, such as Electron Cyclotron Emission Imaging (ECEI) and Microwave Imaging Reflectometry (MIR), exhibit unique characteristics that make them particularly well suited to the validation of theoretical models for plasma instabilities and turbulent fluctuations. A 2-D picture of plasma phenomena is provided unambiguously, from localized, time-resolved measurements. After more than a decade of development and successful demonstrations on RTP [1,2] and TEXTOR [3, 4, 5, 6], ECEI has come into maturity as an electron temperature diagnostic technique, and systems at ASDEX-UG [7] and DIII-D [8] are making regular contributions to plasma physics. The next generation ECEI diagnostic is currently being installed on KSTAR [9, 10]. MIR is a radar reflectometric density fluctuation diagnostic, and hence the perfect complement to ECEI when realized to simultaneously image the same plasma volume. Experiments with MIR at TEXTOR have guided a recent surge in analysis and laboratory experiments aimed at resolving remaining issues [11, 12]. Both techniques are discussed in this tutorial with brief examples of data which illustrate the capabilities of these techniques and motivate future development for application on ITER and burning plasma experiments to come (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  Beiträge aus der Plasmaphysik 03/2011; 51(2‐3):111 - 118.
• Source

  Available from: pppl.gov

  Article: Suppression of electron temperature gradient turbulence via negative magnetic shear in NSTX.

  H Y Yuh, S M Kaye, F M Levinton, E Mazzucato, D R Mikkelsen, D R Smith, R E Bell, J C Hosea, B P LeBlanc, J L Peterson, H K Park, W Lee
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  ABSTRACT: Negative magnetic shear is found to suppress electron turbulence and improve electron thermal transport for plasmas in the National Spherical Torus Experiment (NSTX). Sufficiently negative magnetic shear results in a transition out of a stiff profile regime. Density fluctuation measurements from high-k microwave scattering are verified to be the electron temperature gradient (ETG) mode by matching measured rest frequency and linear growth rate to gyrokinetic calculations. Fluctuation suppression under negligible E×B shear conditions confirm that negative magnetic shear alone is sufficient for ETG suppression. Measured electron temperature gradients can significantly exceed ETG critical gradients with ETG mode activity reduced to intermittent bursts, while electron thermal diffusivity improves to below 0.1 electron gyro-Bohms.
  Physical Review Letters 02/2011; 106(5):055003. · 7.37 Impact Factor
• Article: Fast ion induced shearing of 2D Alfvén eigenmodes measured by electron cyclotron emission imaging.

  B J Tobias, I G J Classen, C W Domier, W W Heidbrink, N C Luhmann, R Nazikian, H K Park, D A Spong, M A Van Zeeland
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  ABSTRACT: Two-dimensional images of electron temperature perturbations are obtained with electron cyclotron emission imaging (ECEI) on the DIII-D tokamak and compared to Alfvén eigenmode structures obtained by numerical modeling using both ideal MHD and hybrid MHD-gyrofluid codes. While many features of the observations are found to be in excellent agreement with simulations using an ideal MHD code (NOVA), other characteristics distinctly reveal the influence of fast ions on the mode structures. These features are found to be well described by the nonperturbative hybrid MHD-gyrofluid model TAEFL.
  Physical Review Letters 02/2011; 106(7):075003. · 7.37 Impact Factor
• Article: FinFET SRAM Optimization With Fin Thickness and Surface Orientation

  Mingu Kang, S.C. Song, S.H. Woo, H.K. Park, M.H. Abu-Rahma, L. Ge, B.M. Han, J. Wang, G. Yeap, S.O. Jung
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  ABSTRACT: In this paper, the design space, including fin thickness (T_fin), fin height (H_fin), fin ratio of bit-cell transistors, and surface orientation, is researched to optimize the stability, leakage current, array dynamic energy, and read/write delay of the FinFET SRAM under layout area constraints. The simulation results, which consider the variations of both T_fin and threshold voltage (V_th), show that most FinFET SRAM configurations achieve a superior read/write noise margin when compared with planar SRAMs. However, when two fins are used as pass gate transistors (PG) in FinFET SRAMs, enormous array dynamic energy is required due to the increased effective gate and drain capacitance. On the other hand, a FinFET SRAM with a one-fin PG in the (110) plane shows a smaller write noise margin than the planar SRAM. Thus, the one-fin PG in the (100) plane is suitable for FinFET SRAM design. The one-fin PG FinFET SRAM with T_fin = 10 nm and H_fin = 40 nm in the (100) plane achieves a three times larger noise margin when compared with the planar SRAM and consumes a 17% smaller bit-line toggling array energy at a cost of a 22% larger word-line toggling energy. It also achieves a 2.3 times smaller read delay and a 30% smaller write delay when compared with the planar SRAM.
  IEEE Transactions on Electron Devices 12/2010; · 2.32 Impact Factor
• Article: Commissioning of electron cyclotron emission imaging instrument on the DIII-D tokamak and first data

  B. Tobias, C. W. Domier, T. Liang, X. Kong, L. Yu, G. S. Yun, H. K. Park, I. G. J Classen, J. E. Boom, A. J. H. Donné, T. Munsat, R. Nazikian, M. Van Zeeland, R. L. Boivin, N. C. Luhmann
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  ABSTRACT: A new electron cyclotron emission imaging diagnostic has been commissioned on the DIII-D tokamak. Dual detector arrays provide simultaneous two-dimensional images of Te fluctuations over radially distinct and reconfigurable regions, each with both vertical and radial zoom capability. A total of 320 (20 vertical×16 radial) channels are available. First data from this diagnostic demonstrate the acquisition of coherent electron temperature fluctuations as low as 0.1% with excellent clarity and spatial resolution. Details of the diagnostic features and capabilities are presented.
  Review of Scientific Instruments 10/2010; 81(10):10D928-10D928-4. · 1.37 Impact Factor
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  Available from: A. J. H. Donne

  Conference Proceeding: Visualization of fusion plasma physics via millimeter wave imaging techniques

  H.K. Park, M. Choi, I. Hong, M. Kim, J.C. Kim, W. Lee, G.S. Yun, B. Tobias, C.W. Domier, X. Kong, T. Liang, N.C. Luhmann, I. Classen, J.E. Boom, A.J.H. Donné
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  ABSTRACT: Advances in microwave technology and innovative ideas have enabled visualization of complex physics of the high temperature plasmas in magnetic fusion devices. ECE Imaging system becomes a powerful tool for MHD physics in various devices and the potential of the MIR system has been reassessed and a system design for KSTAR is in progress.
  Infrared Millimeter and Terahertz Waves (IRMMW-THz), 2010 35th International Conference on; 10/2010
• Article: Relatively scaled ECE temperature profiles of KSTAR plasmas.

  M J Choi, G S Yun, H K Park, Y M Jeon, S H Jeong
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  ABSTRACT: A scheme to obtain relatively scaled profiles of electron cyclotron emission (ECE) temperature directly from uncalibrated raw radiometer data is proposed and has been tested for the 2009 campaign KSTAR plasmas. The proposed method utilizes a position controlled system to move the plasma adiabatically and compares ECE radiometer channels at the same relative radial positions assuming the profile consistency during the adiabatic change. This scaling method is an alternative solution when an absolute calibration is unreliable or too time consuming. The application to the two dimensional ECE imaging data, wherein calibration is extremely difficult, may also prove to be useful.
  The Review of scientific instruments 10/2010; 81(10):10D934. · 1.52 Impact Factor
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  Available from: R. J. E. Jaspers

  Article: 2D electron cyclotron emission imaging at ASDEX Upgrade (invited).

  I G J Classen, J E Boom, W Suttrop, E Schmid, B Tobias, C W Domier, N C Luhmann, A J H Donné, R J E Jaspers, P C de Vries, H K Park, T Munsat, M García-Muñoz, P A Schneider
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  ABSTRACT: The newly installed electron cyclotron emission imaging diagnostic on ASDEX Upgrade provides measurements of the 2D electron temperature dynamics with high spatial and temporal resolution. An overview of the technical and experimental properties of the system is presented. These properties are illustrated by the measurements of the edge localized mode and the reversed shear Alfvén eigenmode, showing both the advantage of having a two-dimensional (2D) measurement, as well as some of the limitations of electron cyclotron emission measurements. Furthermore, the application of singular value decomposition as a powerful tool for analyzing and filtering 2D data is presented.
  The Review of scientific instruments 10/2010; 81(10):10D929. · 1.52 Impact Factor
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  Available from: 144.206.159.178

  Article: Comparative study between the reflective optics and lens based system for microwave imaging system on KSTAR.

  W Lee, G S Yun, Y Nam, I Hong, J B Kim, H K Park, B Tobias, T Liang, C W Domier, N C Luhmann
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  ABSTRACT: Recently, two-dimensional microwave imaging diagnostics such as the electron cyclotron emission imaging (ECEI) system and microwave imaging reflectometry (MIR) have been developed to study magnetohydrodynamics instabilities and turbulence in magnetically confined plasmas. These imaging systems utilize large optics to collect passive emission or reflected radiation. The design of this optics can be classified into two different types: reflective or refractive optical systems. For instance, an ECEI/MIR system on the TEXTOR tokamak [Park et al., Rev. Sci. Instrum. 75, 3787 (2004)] employed the reflective optics which consisted of two large mirrors, while the TEXTOR ECEI upgrade [B. Tobias et al., Rev. Sci. Instrum. 80, 093502 (2009)] and systems on DIII-D, ASDEX-U, and KSTAR adopted refractive systems. Each system has advantages and disadvantages in the standing wave problem and optical aberrations. In this paper, a comparative study between the two optical systems has been performed in order to design a MIR system for KSTAR.
  The Review of scientific instruments 10/2010; 81(10):10D932. · 1.52 Impact Factor
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  Available from: pppl.gov

  Article: NSTX far infrared tangential interferometer/polarimeter electronics upgrade.

  W C Tsai, C W Domier, K C Lee, N C Luhmann, R Kaita, H K Park
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  ABSTRACT: New electronics for the multichannel far infrared tangential interferometer/polarimeter system employed on the National Spherical Torus Experiment (NSTX) have greatly extended its capability to monitor high frequency density fluctuations. Such measurements are essential in understanding transport physics issues in NSTX as well as for the coming ITER device. The electronics, which were previously limited to ∼250 kHz, have been upgraded with a video bandwidth that extends to 4 MHz when operating as an interferometry-only configuration, and to ∼500 kHz when operating as a simultaneous interferometer/polarimeter system. Experimental details and test results of the new electronics are presented.
  The Review of scientific instruments 10/2010; 81(10):10D526. · 1.52 Impact Factor
• Article: Innovations in optical coupling of the KSTAR electron cyclotron emission imaging diagnostic.

  T Liang, B Tobias, X Kong, C W Domier, N C Luhmann, W Lee, G S Yun, H K Park
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  ABSTRACT: The installation of a new electron cyclotron emission imaging diagnostic for the Korea Superconducting Tokamak Advanced Research (KSTAR) is underway, making use of a unique optical port cassette design, which allows placement of refractive elements inside the cryostat region without adverse effects. The result is unprecedented window access for the implementation of a state of the art imaging diagnostic. A dual-array optical design has been developed, capable of simultaneously imaging the high and low field sides of the plasma with independent features of focal plane translation, vertical zoom, and radial channel spacing. The number of translating optics has been minimized by making use of a zoom lens triplet and parabolic plasma facing lens for maximum channel uniformity over a continuous vertical zoom range of 3:1. The simulated performance of this design is presented along with preliminary laboratory characterization data.
  The Review of scientific instruments 10/2010; 81(10):10D909. · 1.52 Impact Factor
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  Available from: 144.206.159.178

  Article: A synthetic diagnostic for the evaluation of new microwave imaging reflectometry diagnostics for DIII-D and KSTAR.

  L Lei, B Tobias, C W Domier, N C Luhmann, G J Kramer, E J Valeo, W Lee, G S Yun, H K Park
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  ABSTRACT: The first microwave imaging reflectometry (MIR) system for characterization of fluctuating plasma density has been implemented for the TEXTOR tokamak [H. Park et al., Rev. Sci. Instrum. 75, 3787 (2004)]; an improved MIR system will be installed on DIII-D and KSTAR. The central issue remains in preserving phase information by addressing antenna coupling between the reflection layer and the detector array in the presence of plasma turbulence. A synthetic diagnostic making use of coupled full-wave diffractive codes has been developed in geometries and applied to a variety of optical arrangements. The effectiveness of each scheme is quantitatively compared with respect to the fluctuation levels accessible in the simulation.
  The Review of scientific instruments 10/2010; 81(10):10D904. · 1.52 Impact Factor
• Article: Data acquisition and processing system of the electron cyclotron emission imaging system of the KSTAR tokamak.

  J B Kim, W Lee, G S Yun, H K Park, C W Domier, N C Luhmann
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  ABSTRACT: A new innovative electron cyclotron emission imaging (ECEI) diagnostic system for the Korean Superconducting Tokamak Advanced Research (KSTAR) produces a large amount of data. The design of the data acquisition and processing system of the ECEI diagnostic system should consider covering the large data production and flow. The system design is based on the layered structure scalable to the future extension to accommodate increasing data demands. Software architecture that allows a web-based monitoring of the operation status, remote experiment, and data analysis is discussed. The operating software will help machine operators and users validate the acquired data promptly, prepare next discharge, and enhance the experiment performance and data analysis in a distributed environment.
  The Review of scientific instruments 10/2010; 81(10):10D931. · 1.52 Impact Factor
• Article: Microwave imaging reflectometry studies for turbulence diagnostics on KSTAR.

  H K Park, I Hong, M Kim, G S Yun, W Lee, J Kim, B Tobias, C W Domier, N C Luhmann, K W Kim
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  ABSTRACT: The first prototype microwave imaging reflectometry (MIR) system [H. Park et al., Rev. Sci. Instrum. 74, 4239 (2004)] clearly demonstrated the shortcomings of conventional reflectometry when the probe beam encountered a large amplitude and/or high fluctuation wavenumber at the reflection layer in laboratory tests, the distinctive advantages shown in these tests were not fully realized in the plasma operation. To understand the discrepancies, the MIR system performance has been thoroughly investigated at POSTECH. In this paper, a possible cause of the MIR performance degradation on TEXTOR will be presented together with a concept of multifrequency MIR system design that will be developed for KSTAR.
  The Review of scientific instruments 10/2010; 81(10):10D933. · 1.52 Impact Factor
• Article: Development of KSTAR ECE imaging system for measurement of temperature fluctuations and edge density fluctuations.

  G S Yun, W Lee, M J Choi, J B Kim, H K Park, C W Domier, B Tobias, T Liang, X Kong, N C Luhmann, A J H Donné
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  ABSTRACT: The ECE imaging (ECEI) diagnostic tested on the TEXTOR tokamak revealed the sawtooth reconnection physics in unprecedented detail, including the first observation of high-field-side crash and collective heat transport [H. K. Park, N. C. Luhmann, Jr., A. J. H. Donné et al., Phys. Rev. Lett. 96, 195003 (2006)]. An improved ECEI system capable of visualizing both high- and low-field sides simultaneously with considerably better spatial coverage has been developed for the KSTAR tokamak in order to capture the full picture of core MHD dynamics. Direct 2D imaging of other MHD phenomena such as tearing modes, edge localized modes, and even Alfvén eigenmodes is expected to be feasible. Use of ECE images of the optically thin edge region to recover 2D electron density changes during L/H mode transitions is also envisioned, providing powerful information about the underlying physics. The influence of density fluctuations on optically thin ECE is discussed.
  The Review of scientific instruments 10/2010; 81(10):10D930. · 1.52 Impact Factor