Publications (6)4.09 Total impact
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Article: Simultaneous realization of a high density edge transport barrier and an improved L-mode on CHS
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ABSTRACT: An edge transport barrier (ETB) formation and an improved L-mode (IL mode) have been simultaneously realized in the high density region ( ) on the Compact Helical System (CHS). When the ETB is formed during the IL mode, the density reduction in the edge region is suppressed by the barrier formation. As a result of the continuous increase in the temperature by the IL mode, the stored energy during the combined mode increased up to the maximum stored energy (Wp ~ 9.4 kJ) recorded in the CHS experiments. The plasma pressure in the peripheral region increases up to three times compared with the L-mode, and the large edge plasma pressure gradient is formed accompanying the pedestal structure. This is caused by the anomalous transport reduction that is confirmed by the sharp drop in the density fluctuation in the edge region. The neutral particle density reduction in the peripheral region and the metallic impurity accumulation in the core plasma are simultaneously observed during the high density ETB formation.Nuclear Fusion 07/2009; 49(8):085018. · 4.09 Impact Factor -
Article: Electron Bernstein Wave Heating through Slow X-B Mode Conversion in CHS
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ABSTRACT: Evident increases in plasma stored energy have been observed in the Compact Helical System (CHS) by injecting nearly X-mode-polarized 54.5 GHz electron cyclotron (EC) waves from the high-field side. An addi- tional plane mirror that enabled the high-field side injection of the EC waves was installed. The centrally peaked and increased electron temperature distributions with the X-mode wave directions which are not aimed at the fundamental resonance layer at the plasma core region strongly suggest that the heating effects occurred due to the excitation of the electron Bernstein waves via mode conversion from the X-mode waves injected from the high-field side. -
Article: O-X-B Heating of Overdense Plasmas by 54.5 GHz Electron Cyclotron Waves in CHS
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ABSTRACT: An evident increase in the plasma stored energy by applying 54.5 GHz electron cyclotron (EC) waves has been observed in plasmas sustained by neutral beam injection (NBI) with line-averaged electron density of above 3.8 × 1019 m−3, that is, the cut-off density of the 54.5 GHz waves, in the Compact Helical System (CHS). The heating effect can be seen even for high-density over 8 × 1019 m−3: more than twice the cut-off density. The 54.5GHz EC-wave beam is obliquely injected to high-density plasmas. The scanning of the EC-wave beam direction gives the evident increase in the stored energy at the beam direction where the leakage EC-wave power from the CHS vacuum vessel becomes almost the minimum. The most possible cause for this heating mechanism is an electron Bernstein wave heating via an Ordinary – eXtraordinary – Bernstein (O-X-B) mode conversion process. -
Article: Determination of the Major Impurity Radiators in the Reheat Mode Discharges in the Compact Helical System
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ABSTRACT: Radiation brightness and impurity behaviors have been studied for reheat mode discharges in the Compact Helical System (CHS) by three different types of impurity diagnostics. Total radiation power measured by a pyroelectric detector significantly reduces after entering the reheat mode, whereas the line-averaged radiation brightness measured by an absolute extreme ultraviolet (AXUV) photodiode array increases especially for a center viewing chord due to the impurity accumulation in the plasma core. One possible reason for this opposite behavior between the two bolometric detectors is the reduced sensitivity of the AXUV photodiode for lower energy photons in vacuum ultraviolet (VUV) region. This speculation is supported by temporal evolutions of VUV spectra measured by a grazing incidence spectrometer. These results demonstrate that the comparison of three impurity diagnostics would be beneficial to the determination of the major impurity radiators and a comprehensive understanding of impurity behaviors in the reheat mode discharges. -
Article: Experimental Conditions for Electron Bernstein Wave Heating Using EC Waves Injected from High-Field Side in CHS
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ABSTRACT: In the compact helical system (CHS), electron heating by electron Bernstein waves (EBWs) was experimentally investigated to study a technique for high-density plasma heating over cutoff density. The EBWs are excited through a mode conversion process by X-mode waves injected to plasmas from the high-field side. In the experiment, within the range of an oblique EC-wave beam injection angle, evident heating effect was observed. The dependences of the heating effect on the wave's toroidal injection direction and polarization show that the absorption of the mode-converted EBWs should be the cause of the plasma heating effect. -
Article: A Simultaneous Spectroscopic Measurement of the Global and Edge Local Structures in the Ion Temperature and Plasma Rotation Profiles in the Compact Helical System
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ABSTRACT: In charge exchange spectroscopy (CXS), a simultaneous observation in different plasma toroidal cross sections and/or viewing ports is required to investigate radial distributions of ion temperatures Ti (r), and poloidal rotation velocities Vp (r) in magnetically confined toroidal plasmas. In recent studies of the edge transport barrier (ETB) in the Compact Helical System (CHS), a simultaneous viewing of the vertically elongated and the horizontally elongated plasma cross sections is used to improve the spatial resolution at the edge region. The 90 fibers used for this purpose are connected to one spectrometer, and a 256 ×243 pixel sampling CCD is used to detect the diffraction image. It is found that there is a localized edge ion temperature pedestal region with ΔTi 〜 100 eV and Δr/a 〜 0.1, where r and a are flux surface averaged minor radii of measured surfaces and the outermost flux surface, respectively. The negative radial electric field at the edge is increased in the high confinement phase because of the increased ion pressure.
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- Nuclear Fusion (1)
Institutions
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2009
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National Institute for Fusion Science
Toki, Gifu-ken, Japan
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