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  • 1
    Electronic Resource
    Electronic Resource
    Experimental test of the radial force balance equation in the compact helical system (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 1-4 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental test of the radial force balance equation was done in the Compact Helical System Heliotron/Torsatron [S. Okamura et al. Nucl. Fusion 39, 1337 (1999)]. A radial electric field is measured with a heavy ion beam probe, while plasma rotation and drift velocity of fully ionized carbon are measured with charge exchange spectroscopy. The two measurements agree with each other to within 10% of the radial electric field in a wide range of electron densities of 0.3–2.0×1019 m−3. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1331097
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  • 2
    Electronic Resource
    Electronic Resource
    Improved plasma performance on Large Helical Device : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2002-2008 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Since the start of the Large Helical Device (LHD) experiment, various attempts have been made to achieve improved plasma performance in LHD [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. Recently, an inward-shifted configuration with a magnetic axis position Rax of 3.6 m has been found to exhibit much better plasma performance than the standard configuration with Rax of 3.75 m. A factor of 1.6 enhancement of energy confinement time was achieved over the International Stellarator Scaling 95. This configuration has been predicted to have unfavorable magnetohydrodynamic (MHD) properties, based on linear theory, even though it has significantly better particle-orbit properties, and hence lower neoclassical transport loss. However, no serious confinement degradation due to the MHD activities was observed, resolving favorably the potential conflict between stability and confinement at least up to the realized volume-averaged beta 〈β〉 of 2.4%. An improved radial profile of electron temperature was also achieved in the configuration with magnetic islands, minimized by an external perturbation coil system for the Local Island Divertor (LID). The LID has been proposed for remarkable improvement of plasma confinement like the high (H) mode in tokamaks, and the LID function was suggested in limiter experiments. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1344561
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  • 3
    Electronic Resource
    Electronic Resource
    Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4152-4183 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The bifurcation nature of the electrostatic structure is studied in the toroidal helical plasma of the Compact Helical System (CHS) [K. Matsuoka et al., Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411]. Observation of bifurcation-related phenomena is introduced, such as characteristic patterns of discrete potential profiles, and various patterns of self-sustained oscillations termed electric pulsation. Some patterns of the electrostatic structure are found to be quite important for fusion application owing to their association with transport barrier formation. It is confirmed, as is shown in several tokamak experiments, that the thermal transport barrier is linked with electrostatic structure through the radial electric field shear that can reduce the fluctuation resulting in anomalous transport. This article describes in detail spatio-temporal evolution during self-sustained oscillation, together with correlation between the radial electric field and other plasma parameters. An experimental survey to find dependence of the temporal and spatial patterns on plasma parameters is performed in order to understand systematically the bifurcation property of the toroidal helical plasma. The experimental results are compared with the neoclassical bifurcation property that is believed to explain the observed bifurcation property of the CHS plasmas. The present results show that the electrostatic property plays an essential role in the structural formation of toroidal helical plasmas, and demonstrate that toroidal plasma is an open system with a strong nonlinearity to provide a new attractive problem to be studied. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1290483
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  • 4
    Electronic Resource
    Electronic Resource
    Inward turbulent particle transport in a helical plasma heated by neutral beam injection (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4035-4041 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In neutral-beam-injection (NBI) heated plasmas of the Compact Helical System (CHS) heliotron/torsatron [K. Matsuoka et al., Plasma Physics and Controlled Nuclear Fusion Research, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 441], inward turbulent particle flux is observed at a particular radial location near the plasma edge. The observed reversal of the particle flux is connected to the change in the cross-phase between electron density and poloidal electric field fluctuations. This phenomenon is observed in the parameter regime of low collisionality at the electron temperature Te〉15 eV, electron density ne〈3×1018 m−3, which approximately corresponds to the effective collision frequency normalized by the transit frequency of a circulating particle ν*(approximately-less-than)1. The large inward particle flux ((approximately-greater-than)1×1020 m−2 s−1) correlates with the positive shear of the radial electric field Er′ ((approximately-greater-than)1×106 V m−2), but does not have clear correlation with the radial electric field Er and the curvature Er″. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1387267
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  • 5
    Electronic Resource
    Electronic Resource
    Thermal transport barrier in heliotron-type devices (Large Helical Device and Compact Helical System) : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1802-1808 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In the discharges of the Large Helical Device [O. Motojima et al., Proceedings of the 16th Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 3, p. 437], a significant enhancement of the energy confinement has been achieved with an edge thermal transport barrier, which exhibits a sharp gradient at the edge. Key features associated with the barrier are quite different from those seen in tokamaks (i) almost no change in particle (including impurity) transport, (ii) a gradual formation of the barrier, (iii) a very high ratio of the edge temperature to the average temperature, (iv) no edge relaxation phenomenon. In the electron cyclotron heating (ECH) heated discharges in the Compact Helical System [K. Matsuoka et al., in Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, France, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411], the internal electron transport barrier has been observed, which enhances the central electron temperature significantly. High shear of the radial electric field appears to suppress the turbulence in the core region and enhance the electron confinement there. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874001
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  • 6
    Electronic Resource
    Electronic Resource
    Compatibility between high energy particle confinement and magnetohydrodynamic stability in the inward-shifted plasmas of the Large Helical Device : Review,Tutorial and Invited Papers from the 43rd Annual Meeting of the APS Division of Plasma Physics (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 2020-2026 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The experimentally optimized magnetic field configuration of the Large Helical Device [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)], where the magnetic axis is shifted inward by 15 cm from the early theoretical prediction, reveals 50% better global energy confinement than the prediction of the scaling law. This configuration has been investigated further from the viewpoints of high energy particle confinement and magnetohydrodynamic (MHD) stability. The confinement of high energy ions is improved as expected. The minority heating of ion cyclotron range of frequency was successful and the heating efficiency was improved by the inward shift. The confinement of passing particles by neutral beam injection was also improved under low magnetic field strength, and there could be obtained an almost steady high beta discharge up to 3% in volume average. This was a surprising result because the observed pressure gradient exceeded the Mercier unstable limit. The observed MHD activities became as high as beta but they did not grow enough to deteriorate the confinement of high energy ions or the performance of the bulk plasma, which was still 50% better than the scaling. According to these favorable results, better performance would be expected by increasing the heating power because the neoclassical transport can also be improved there. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1448345
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  • 7
    Electronic Resource
    Electronic Resource
    Charge exchange neutral particle analysis with natural diamond detectors on LHD heliotron : Papers from the thirteenth topical conference on high temperature plasma diagnostics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 611-614 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Semiconductor detectors based on natural diamonds have been installed on the Large Helical Device (LHD) heliotron to measure the energy distribution of charge exchange fast neutral particles from different viewing angles. Advantages of a natural diamond detector (NDD) are (1) very compact size, (2) relatively easy handling, and (3) high energy resolution. Although NDDs are sensitive to visible light, vacuum ultraviolet, and soft x rays, unfavorable pulses produced by such radiation were greatly reduced by choosing an appropriate stainless steel shield in this experiment. In LHD, the time-resolved energy distribution of counter-going beam ions and ion cyclotron range of frequency-produced energetic ions have been successfully obtained by means of an NDD. The performance of NDDs as a neutral particle analyzer and its good suitability to LHD plasmas were demonstrated throughout this work. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1318250
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  • 8
    Electronic Resource
    Electronic Resource
    Escaping fast ion diagnostics in compact helical system heliotron/torsatron : Proceedings of the 12th topical conference on high temperature plasma diagnostics (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 827-830 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Escaping energetic ions have been measured with a scintillator-based probe which can provide both information of pitch angle and gyroradius of detected ions in compact helical system. The measurement was started in February 1997. It was confirmed that output signals are due to neutral beam-injected fast ions. A Lorentz orbit code was developed and fast ion orbits reaching the probe were computed to understand the loss process of detected ions. Birthplace and initial pitch angle of fast ions were calculated by a Monte Carlo beam deposition code HFREYA. From the comparison between pitch angle variation of computed fast ion orbits and pitch angle distribution of birth fast ions, it turns out that fast ions whose birthplace is in the core region cannot reach the detection point directly. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149404
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  • 9
    Electronic Resource
    Electronic Resource
    Neutron diagnostics in compact helical system : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 532-535 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An efficient neutron diagnostic system has been established and applied to deuterium experiments on compact helical system. A neutron counter was installed at the center of the torus, and the detection efficiency was about two orders of magnitude higher than the values of most toroidal fusion devices. The effect of the stray magnetic field on the operation of a proportional counter was experimentally confirmed to be negligibly small. The ion temperature, as low as 400 eV, was obtained from neutron measurement with a time resolution of 1 ms. The decay time of neutron emission rate after the termination of a 1% deuterium-doped beam was measured and the confinement time of neutral beam-injected fast ions was estimated. The fast ion confinement was clearly improved by strengthening the toroidal magnetic field. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147622
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  • 10
    Electronic Resource
    Electronic Resource
    Fusion Neutronic Source deuterium–tritium neutron spectrum measurements using natural diamond detectors (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 1720-1724 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Two natural diamond detectors (NDDs) operating at room temperature were used for Fusion Neutronics Source (FNS) deuterium–tritium (DT) neutron spectra measurements at different points around the tritium target and for different deuteron beam energies. Energy resolution of both NDDs were measured, with values 1.95% and 2.8%. Due to the higher energy resolution of one of the two NDDs studied it was possible to measure the shape of the DT neutron energy distribution and its broadening due to deuteron scattering inside the target. The influence of pulse pileup on the energy resolution of the combined system (NDD+electronics) at count rates up to 3.8×105 counts/s was investigated. A 3.58% energy resolution for the spectrometric system based on NDD and a 0.25 μs shaping time amplifier has been measured at a count rate of 5.7×105 counts/s. It is shown that special development of a fast pulse signal processor is necessary for NDD based spectrometry at count rates of approximately 106 counts/s. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1148001
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