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Daejeon, South Korea

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. Source

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. Source

Lee K.C.,National Fusion Research Institute
Journal of the Korean Physical Society | Year: 2013

Debye shielding is found not to be effective for the sudden change of ion trajectories induced by charge-exchange reactions with neutrals in typical tokamak plasmas. When the number of charge-exchange occurrences in Debye shielding is greater than unity, electrons are simultaneously engaged in the screening process for multiple point charges, which makes the shielding ineffective. The ion polarization drift is also found not to be applicable for the charge-exchange reactions because the individual particles experience an electric field that changes faster than particle's cyclotron frequency and because the electron polarization drift is slower than the ion velocity responsible for the gyro-center shift current when the dimensionless number λ D/r x is greater than unity, which results in a violation of quasi-neutrality. © 2013 The Korean Physical Society. Source

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. Source

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. Source

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