Daejeon, South Korea
Daejeon, South Korea

Time filter

Source Type

Choi J.,National Fusion Research Institute | Cha H.,Chungnam National University | Han B.-M.,Myongji University
IEEE Transactions on Power Electronics | Year: 2010

In this paper, a three-phase interleaved dc-dc converter with an active clamp and three-phase pulsewidth modulation (PWM) method are proposed. The proposed converter has a three-phase interleaved boost converter, which consists of three input inductors, three active clamp switches, three-phase high frequency transformer, and diode rectifiers. Interleaved operation of a three-phase boost converter reduces overall ripple current, which is imposed into fuel cells and realizes smaller sized filter components, increasing effective operating frequency, and leading to higher power density. Each output current of the three-phase boost converter is combined by the three-phase transformer and flows in the continuous conduction mode, which leads to a reduction of rms current through switches and transformer windings. A peak efficiency of above 95% is achieved by a reduction in the conduction losses resulted by continuous current conduction and switching losses reduced by the action of active clamp branches, as well. The proposed converter and three-phase PWM strategy are analyzed, simulated, and implemented in hardware. Experimental results are presented to verify the feasibility of the proposed converter. © 2006 IEEE.

Seol J.,National Fusion Research Institute | Shaing K.C.,National Cheng Kung University
Physics of Plasmas | Year: 2012

In a tokamak H-mode, a strong E × B flow shear is generated during the L-H transition. Turbulence in a pedestal is suppressed significantly by this E × B flow shear. In this case, neoclassical transport may become important. The neoclassical fluxes are calculated in the plateau regime with the parallel plasma flow using their kinetic definitions. In an axisymmetric tokamak, the neoclassical particles fluxes can be decomposed into the banana-plateau flux and the Pfirsch-Schlüter flux. The banana-plateau particle flux is driven by the parallel viscous force and the Pfirsch-Schlüter flux by the poloidal variation of the friction force. The combined quantity of the radial electric field and the parallel flow is determined by the flux surface averaged parallel momentum balance equation rather than requiring the ambipolarity of the total particle fluxes. In this process, the Pfirsch-Schlüter flux does not appear in the flux surface averaged parallel momentum equation. Only the banana-plateau flux is used to determine the parallel flow in the form of the flux surface averaged parallel viscosity. The heat flux, obtained using the solution of the parallel momentum balance equation, decreases exponentially in the presence of sonic M p without any enhancement over that in the standard neoclassical theory. Here, M p is a combination of the poloidal E × B flow and the parallel mass flow. The neoclassical bootstrap current in the plateau regime is presented. It indicates that the neoclassical bootstrap current also is related only to the banana-plateau fluxes. Finally, transport fluxes are calculated when M p is large enough to make the parallel electron viscosity comparable with the parallel ion viscosity. It is found that the bootstrap current has a finite value regardless of the magnitude of M p. © 2012 American Institute of Physics.

Lesur M.,National Fusion Research Institute | Lesur M.,Kyushu University | Idomura Y.,Japan Atomic Energy Agency
Nuclear Fusion | Year: 2012

The Berk-Breizman (BB) extension of the bump-on-tail instability includes a finite, fixed wave damping (γ d), and a collision operator with drag (ν f) and diffusion (ν d). The BB model is applied to a one-dimensional plasma, to investigate the kinetic nonlinearities, which arise from the resonance of a single electrostatic wave with an energetic-particle beam. For a fixed value of the linear drive normalized to the linear frequency, γ L0/ω 0=0.1, the long-time nonlinear evolution is systematically categorized as damped, steady-state, periodic, chaotic and chirping. The chirping regime is sub-categorized as periodic, chaotic, bursty and intermittent. Up-down asymmetry and hooked chirping branches are also categorized. For large drag, holes with quasi-constant velocity are observed, in which case the solution is categorized into steady, wavering and oscillating holes. Two complementary parameter spaces are considered: (1) the (γ d, ν d) space for fixed ν d/ν f ratios; (2) the (ν f, ν d) space for fixed γ d/γ L0 ratios, close to and far from marginal stability. The presence of drag and diffusion (instead of a Krook model) qualitatively modifies the nonlinear bifurcations. The bifurcations between steady-state, periodic and steady-hole solutions agree with analytic theory. Moreover, the boundary between steady and periodic solutions agrees with analytic theory. Nonlinear instabilities are found in both subcritical and barely unstable regimes. Quasi-periodic chirping is shown to be a special case of bursty chirping, limited to a region relatively far from marginal stability. © 2012 IAEA, Vienna.

Xi P.W.,Beijing University of Technology | Xi P.W.,Lawrence Livermore National Laboratory | Xu X.Q.,Lawrence Livermore National Laboratory | Diamond P.H.,National Fusion Research Institute | Diamond P.H.,University of California at San Diego
Physical Review Letters | Year: 2014

We derive a new nonlinear criterion for the occurrence of fast relaxation (crash) events at the edge of high-confinement-mode plasmas. These fast relaxation events called ELMs (edge-localized modes) evolve from ideal magnetohydrodynamics (MHD) instabilities, but the crash is not due only to linear physics. We show that for an ELM crash to occur, the coherence time of the relative phase between potential and pressure perturbations must be long enough to allow growth to large amplitude. This phase coherence time is determined by both linear and nonlinear dynamics. An ELM crash requires that the instability growth rate exceed a critical value, i.e., γ>γc, where γc is set by 1/τc and τc is the phase coherence time. For 0<γ<γc, MHD turbulence develops and drives enhanced turbulent transport. The results indicate that the shape of the growth rate spectrum γ(n) is important to whether the result is a crash or turbulence. We demonstrate that ELMs can be mitigated by reducing the phase coherence time without changing linear instability. These findings also offer an explanation of the occurrence of ELM-free H-mode regimes. © 2014 American Physical Society.

Bae C.,National Fusion Research Institute
Proceedings - Symposium on Fusion Engineering | Year: 2016

Investigation of the localized effects of ICRH (Ion Cyclotron Resonance Heating), LHCD (Lower Hybrid Current Drive), and ECRH (Electron Cyclotron Resonance Heating) to plasma rotation and momentum transport in magnetically confined plasmas poses inherent difficulties due to the lack of understanding on the actual torque amount being locally deposited. There are no proven analytic models to simulate such effects in fusion plasmas and diagnostics must provide a sufficient accuracy in the affected local area measurements as well. Given reliable velocity profile measurements, there are possibilities of investigating their localized effects to plasma rotation and momentum transport. In this work, a forward and a backward calculation schemes to predict the localized torque density are presented and discussed. © 2015 IEEE.

Terzolo L.,National Fusion Research Institute
Journal of the Korean Physical Society | Year: 2014

For flexible control of the plasma pressure and the current profiles, which are essential for a high performance plasma with long pulse operation, KSTAR is going to implement several heating and current systems, which include Neutral Beam Injection (NBI), Ion Cyclotron Resonant Heting (ICRH)/Fast Wave Current Drive (FWCD), Lower Hybrid Current Drive (LHCD), and Eclectron Cyclotron Heating (ECH)/Electron Cyclotron Current Drive (ECCD). Here, the NBI system is typically used for the central heating and current drive. For the time being, only one NBI device (composed of 3 sources) is available in KSTAR. The first two sources were successfully commissioned in 2010 and 2013. The last source will be installed in 2014. In this work, we present a simulation study of the heating and current drive of the first NBI system (3 sources) during the ramp-up phase. We consider two different NBI configurations (low and high beam energy). The simulation is performed with NUBEAM, a well-recognized Monte-Carlo code. Several different types of KSTAR target equilibria (scan from lower to higher plasma density) are used for the calculation of the current drive, the heating and the different NB losses (shinethrough, charge exchange and bad orbit). The study shows the dependency of those quantities on the plasma density, the position of the NB source and the beam energy. It also shows that because of the shinethrough loss is too high, each NB source cannot be used when the plasma density is under a certain threshold. This study can be used to determine the starting time of the different NB sources during the KSTAR ramp-up phase. © 2014 The Korean Physical Society.

Lee K.Y.,National Fusion Research Institute
Journal of the Korean Physical Society | Year: 2014

A high-resolution Thomson scattering system is presently being developed to measure the electron temperature and density profile during plasma interaction with molten salt. The system uses a 20-Hz Nd:YAG laser operating at the second harmonic (532 nm). The collection lens, having a 1:10 magnification ratio, measures 63 points along the 10-cm profile. The scattered light is transmitted by using an optical-fiber bundle, and is analyzed with a triple-grating spectrometer to further reduce stray light. Its spectral resolution is expected to be 0.03 nm. An intensified charge-coupled device (ICCD) camera consisting of a gated image intensifier coupled to the CCD camera is used to record the spectral distribution of the scattered light. An additional feature of operating the ICCD camera at 40-Hz to record the background signal is incorporated. © 2014, The Korean Physical Society.

Miki K.,National Fusion Research Institute | Diamond P.H.,National Fusion Research Institute | Diamond P.H.,University of California at San Diego
Nuclear Fusion | Year: 2011

Recent experiments have noted the coexistence of multiple shearing fields in edge turbulence, and have observed that the shearing population ratios evolve as the L-H transition is approached. A novel model including zonal flows (ZFs), geodesic acoustic modes (GAMs) and turbulence as a zero-dimensional self-consistent two predator-one prey system with multiple frequency shearings is proposed. ZF with finite frequency (i.e. GAM) can have different shearing dynamics from that with zero frequency, because of the finite shearing field autocorrelation times. Decomposing the broadband ZF spectrum into the two populations enables us to assign different shearing weights to the components of the shearing field. We define states with no ZF and GAM as an L-mode-like state, that with ZF and without GAM as an ZF-only state, with GAM and without ZF as a GAM-only state and both with ZF and GAM as the coexistence state. To resolve the origins of multiple shear coexistence, mode-competition effects are introduced. These originate from higher order perturbation of wave populations. The model exhibits a sequence of transitions between various states as the net driving flux increases. For some parameters, bistability of ZF and GAM is evident, which predicts hysteretic behaviour in the turbulence intensity field during power ramp up/down studies. The presence of noise due to ambient turbulence offers a mechanism to explain the bursts and pulsations observed in the turbulence field prior to the L-H transition. © 2011 IAEA, Vienna.

Uhm H.S.,Kwangwoon University | Hong Y.C.,National Fusion Research Institute
Thin Solid Films | Year: 2011

Various microplasma jets including coplanar-electrodes device, hollow-electrode device, twin injection-needle device, jet from a flexible tube, and pencil-type electrode will be discussed. Argon plasma jets penetrate deep into ambient air and create a path for oxygen radicals to sterilize microbes including spores. A sterilization experiment with bacterial endospores indicates that an argon-oxygen plasma jet very effectively kills endospores of Bacillus atrophaeus (ATCC 9372), thereby demonstrating its capability to clean surfaces and its usefulness for reinstating contaminated equipment as free from toxic biological agents. The key element of the sterilization is the oxygen radicals. The pencil-type configuration produces a long cold plasma jet capable of reaching 3.5 cm and having various excited plasma species shown through optical emission spectrum. By introducing an appropriate gas flow rate, striated discharge patterns in the plasma jet from the pencil-type configuration are produced through ionization wave propagation. 2 W operation of an air plasma jet in pencil-type electrode provides an excellent opportunity for sterilization of microbes. Finally, the twin plasma columns in the twin injection-needle device will be introduced. © 2011 Elsevier B.V.

Lee H.J.,National Fusion Research Institute
IEEE Transactions on Applied Superconductivity | Year: 2011

Superconducting magnets of the Korea Superconducting Tokamak Advanced Research (KSTAR) are cooled by supercritical helium with 4.5 K, which was supplied and recovered by the 9 kW of the Helium Refrigerator System (HRS). While current is being charged, the supercritical helium expands to both side of the helium inlet and the outlet of the magnets due to the generated AC losses. To maintain the pressure gradient, both the supply and the return pressures of the HRS are increased at the same time and the differential pressure of the HRS was reduced after the event. However, the pressure rising in the magnets may block the helium flow or create reversal flow of the helium. During unipolar experiment of PF1 magnet up to 2 kA with 1 kA/s of ramp-up rate, the mass flow rate was decreased at the PF1 cooling tube (manifold) in the helium distribution system (HDS), whereas the pressure is increased and the temperature is to be increased or decreased according to compression and expansion of the heated helium in the magnets. For the bipolar experiment of PF1 up to ±2 kA with 1 kA/s ramp-up rate and 2 kA/s ramp-down rate, the conditions in the helium flow were drastically changed, especially the mass flow rate was measured to be maintained at zero for a few second (more than 4 s). This behavior could decisively affect the cryogenic stabilities in the magnet, and may impose a major limit on the long pulse operation of KSTAR. In this paper, we investigated this behavior and analysed by using 1-dimentional thermo-hydraulic code, GANDALF. © 2011 IEEE.

Loading National Fusion Research Institute collaborators
Loading National Fusion Research Institute collaborators