120 results on '"Nagashima, Y."'
Search Results
2. Modal polarization analysis using Fourier-rectangular function transform in a cylindrical plasma.
- Author
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Nishimura, D., Fujisawa, A., Nagashima, Y., Moon, C., Yamasaki, K., Kobayashi, T.-K., Inagaki, S., Yamada, T., Kawachi, Y., Kasuya, N., Kosuga, Y., and Sasaki, M.
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CYLINDRICAL plasmas ,MODAL analysis ,PLASMA dynamics - Abstract
Fourier-rectangular function (FRF) transform [K. Yamasaki et al., J. Appl. Phys. 126, 043304 (2019)] is used to analyze the structure and dynamics of plasma with cylindrical symmetry. In this article, a new technique based on FRF transform is proposed to elucidate the polarization characteristics of azimuthal modes and fluctuations. The application results are shown for a two-dimensional tomography image of linear cylindrical plasma. The method successfully resolves the polarization characteristics of azimuthal modes and attributes the intermittent behavior of the m = 1 mode to its polarization characteristics. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
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3. Fourier-rectangular function analysis for cylindrical plasma images.
- Author
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Yamasaki, K., Fujisawa, A., Nagashima, Y., Moon, C., Inagaki, S., Kasuya, N., Kosuga, Y., Sasaki, M., and Yamada, T.
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CYLINDRICAL plasmas ,RADIAL basis functions ,GEOMETRIC tomography ,FOURIER series ,IMAGING systems - Abstract
An analysis method is developed for imaging measurement systems, such as the tomography system or fast camera imaging, in order to extract the spatial structure properties of fluctuation. In this analysis method, rectangular functions and Fourier series are adopted for the basis functions of the radial and azimuthal coordinates, respectively. The examples of this analysis method show that this method is useful for quantifying the spatial structure of the 2D image such as the radial profile of the power and the rotation angle of each azimuthal mode. The method proposed in this article allows us to elucidate the features of the 2D spatial structure of phenomena in magnetized plasma. [ABSTRACT FROM AUTHOR]
- Published
- 2019
- Full Text
- View/download PDF
4. Conceptual design of a heavy ion beam probe for the QUEST spherical tokamak.
- Author
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Ido, T., Hasegawa, M., Ikezoe, R., Onchi, T., Hanada, K., Idei, H., Kuroda, K., and Nagashima, Y.
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POLOIDAL magnetic fields ,ION beams ,HEAVY ions ,CONCEPTUAL design ,TOKAMAKS ,PLASMA currents - Abstract
A heavy ion beam probe (HIBP) has been designed for the QUEST spherical tokamak to measure plasma turbulence and the profiles of electric potential profiles. Using a cesium ion beam with an energy of several 10 keV, the observable region covers most of the upper half of the plasma. Although the probe beam is deflected by the poloidal magnetic field produced by plasma current and poloidal coil currents, it can be detected under plasma current up to 150 kA by modifying the trajectories with two electrostatic sweepers. According to the numerical estimation of the intensity of the detected beam, sufficient signal intensity for measuring plasma turbulence can be obtained over almost the measurable area when the electron density is up to 1 × 10
19 m−3 , which is larger than the cut-off density of electron cyclotron heating in QUEST. The performance of the designed HIBP is sufficient to explore the mechanisms of heat and particle transport in magnetically confined plasmas, including the influence of plasma wall interactions, which is a goal of the QUEST project. [ABSTRACT FROM AUTHOR]- Published
- 2022
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- View/download PDF
5. Dynamic interaction between fluctuations with different origins in a linear magnetized plasma.
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Kawachi, Y., Inagaki, S., Sasaki, M., Kosuga, Y., Yamasaki, K., Kobayashi, T., Kin, F., Yamada, T., Arakawa, H., Nagashima, Y., Moon, C., Kasuya, N., and Fujisawa, A.
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WAVENUMBER ,WAVE energy ,LANGMUIR probes ,ENERGY transfer - Abstract
This paper presents the experimental identification of the dynamic interaction between three fluctuations excited by different thermodynamical forces in a linear plasma. The observed fluctuations are characterized as an axial symmetric mode, a drift wave, and an azimuthally symmetric potential fluctuation from the spatiotemporal structures measured with Langmuir probe arrays. The intermittent burst of the axial symmetric mode is observed by the instantaneous wave number spectrum. The energy transfer analysis revealed that the axially symmetric mode gains energy from the other mode during the burst while the drift wave loses energy. The intermittent burst synchronizes with the azimuthal symmetric potential fluctuation. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
6. Conceptual design of heavy ion beam probes on the PLATO tokamak.
- Author
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Ido, T., Fujisawa, A., Takemura, K., Kobayashi, T.-K., Nishimura, D., Kasuya, N., Fukuyama, A., Moon, C., Yamasaki, K., Inagaki, S., Nagashima, Y., and Yamada, T.
- Subjects
TOKAMAKS ,HEAVY ions ,CONCEPTUAL design ,ELECTRON density ,ION beams ,PLASMA turbulence - Abstract
Heavy ion beam probe (HIBP) systems have been designed for the new tokamak, PLATO [A. Fujisawa, AIP Conf. Proc. 1993, 020011 (2018)]. The designs have been completed, and the installations are in progress. Two HIBPs are being installed in toroidal sections 180° apart to investigate long-range correlations in the toroidal direction. Each HIBP consists of an injection beamline and a detection beamline as usual. Yet, one of the HIBPs is equipped with an additional detection beamline; the measurement positions of its two detection beamlines can be placed on almost the same magnetic surface yet at poloidal angles that differ by ∼180°. The use of three detection beamlines allows us to investigate spatial asymmetry and long-range correlations in both the toroidal and poloidal directions, simultaneously. The detected beam intensity is expected to be enough for turbulence measurements in almost the entire plasma region when the electron density is up to 1 × 10
19 m−3 by selecting appropriate ion species for the probe beam. Each detector has three channels 10 mm apart, allowing measurement of local structures of micro-scale turbulence. Therefore, using the HIBPs on the PLATO tokamak will enable both local and global properties of plasma turbulence to be investigated, simultaneously. [ABSTRACT FROM AUTHOR]- Published
- 2021
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7. Ion-induced Processes in Polymer Composite Materials: Positron Annihilation Spectroscopy in Combination with UV-Vis Absorption and Raman Spectroscopy.
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Kavetskyy, T., Kuczumow, A., Iida, K., Nagashima, Y., Liedke, M. O., Butterling, M., Wagner, A., Krause-Rehberg, R., Šauša, O., Petkova, T., Boev, V., Kiv, A., and Stepanov, A. L.
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RAMAN spectroscopy ,COMPOSITE materials ,ULTRAVIOLET-visible spectroscopy ,POSITRON annihilation ,POSITRON beams ,COMPOSITE numbers - Abstract
Application of positron annihilation spectroscopy using a variable-energy slow positron beam (VESPB) as a key experimental tool to understand ion-induced processes and defect structures in a number of polymer composite materials important for a practical use is demonstrated. The results concerning 40 keV B
+ implanted polymethylmethacrylate (B:PMMA) and 30 keV Ag+ implanted PMMA (Ag:PMMA), and organic-inorganic ureasil composite (Ag:ureasil) are discussed. Utilized VESPB techniques allow to confirm carbonization of ion-irradiated B:PMMA, formation of carbon-shell Ag-core nanoparticles in Ag:PMMA and evolution in size of Ag nanoparticles in Ag:ureasil, which was revealed by means of UV-Vis absorption and Raman spectroscopy measurements. [ABSTRACT FROM AUTHOR]- Published
- 2019
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8. Roles of solitary eddy and splash in drift wave–zonal flow system in a linear magnetized plasma.
- Author
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Arakawa, H., Sasaki, M., Inagaki, S., Kosuga, Y., Kobayashi, T., Kasuya, N., Yamada, T., Nagashima, Y., Kin, F., Fujisawa, A., Itoh, K., and Itoh, S.-I.
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LINEAR systems ,EDDIES ,NONLINEAR waves ,GLACIAL drift ,MOMENTUM transfer - Abstract
We experimentally investigate the roles of drift wave type fluctuations interacting with zonal flow. The drift wave type fluctuations examined in this paper are characterized by nonlinear solitary wave, splash, and solitary eddy [Arakawa et al., Sci. Rep. 6, 33371 (2016)]. Compared to the nonlinear solitary wave, splash has a short lifetime, while solitary eddy has a long lifetime. Excitation/damping of the splash and the solitary eddy are synchronized with zonal perturbation. The roles of the splash and the solitary eddy in transport processes are also discussed. Solitary eddy contributes to momentum transport and accelerates zonal flow. The particle flux driven by the solitary eddy and the splash is in the inward and outward directions, respectively, with similar magnitudes. [ABSTRACT FROM AUTHOR]
- Published
- 2019
- Full Text
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9. Observations of radially elongated particle flux induced by streamer in a linear magnetized plasma.
- Author
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Kin, F., Fujisawa, A., Itoh, K., Kosuga, Y., Sasaki, M., Inagaki, S., Nagashima, Y., Yamada, T., Kasuya, N., Yamasaki, K., Hasamada, K., Zhang, B. Y., Kawachi, Y., Arakawa, H., Kobayashi, T., and Itoh, S.-I.
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PLASMA turbulence ,CYLINDRICAL plasmas ,BALLISTIC conduction ,GAUSSIAN distribution ,FLUX (Energy) ,PLASMA boundary layers - Abstract
The streamer-driven particle flux is evaluated experimentally in a linear cylindrical plasma. The streamers, a radially elongated and azimuthally localized structure created in turbulent plasma, really induce ballistic transport, and directly connect the plasma core to the edge within one order faster time scale than that of turbulence. The occurrence of large transport deviates from a Gaussian distribution and is likely to obey a power law. The fundamental plasma experiment clearly shows the significant and essential contributions of streamers to cross-field transport and structural formation of turbulent plasmas. [ABSTRACT FROM AUTHOR]
- Published
- 2019
- Full Text
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10. A high-quality and energy-tunable positronium beam system employing a trap-based positron beam.
- Author
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Michishio, K., Chiari, L., Tanaka, F., Oshima, N., and Nagashima, Y.
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POSITRON beams ,POSITRONIUM ,ULTRAHIGH vacuum ,PHOTODETACHMENT ,ANIONS ,RADIOISOTOPES ,THIN films ,MAGNETIC lenses - Abstract
We constructed a new apparatus, built upon a trap-based slow positron beam, for the production of a collimated, energy-tunable positronium beam under ultra-high vacuum conditions employing the photodetachment of positronium negative ions. A slow positron generator consisting of a
22 Na radioisotope (20 mCi) combined with a buffer-gas positron trap is employed to generate high-quality, nano-second positron bursts with a repetition rate of 1 Hz–1 kHz. The positron bursts are focused onto an efficient positron-to-positronium negative ion converter, a Na-coated W thin film in a transmission geometry, using a magnetic lens system. The ions emitted from the opposite surface of the film are electrostatically accelerated to a given energy and photodetached by a pulsed infrared laser to form a mono-energetic positronium beam with kinetic energies of 0.2 keV–3.3 keV. The achieved detection rate of Ps atoms is 23 cps at the energy of 3.3 keV with a signal-to-background ratio as high as 300. The energy spread of the beam was evaluated by comparing the result of the time-of-flight measurements and particle-tracking simulations. With the use of a collimator of 1 mm diameter, a coherent beam with an angular divergence of less than 0.3° is obtained. The obtained Ps beam, having a much higher quality than those reported hitherto, will open up a new field of experimental investigations, such as Ps interacting with a variety of materials and fundamental studies on Ps spectroscopy. [ABSTRACT FROM AUTHOR]- Published
- 2019
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11. Combined methods of moment vectors and Stokes parameters to analyze tomographic image of plasma turbulence.
- Author
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Fujisawa, A., Nagashima, Y., Yamasaki, K., Inagaki, S., Yamada, T., and Sasaki, M.
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STOKES parameters , *PLASMA turbulence , *CYLINDRICAL plasmas , *POLARIZATION (Nuclear physics) , *VECTORS (Calculus) - Abstract
Plasma turbulence tomography is now working to show a two dimensional structure and dynamics of plasma turbulence in a cylindrical plasma device, PANTA. A new method, the Stokes parameter analysis on moment vectors, is being developed to treat the image of plasma turbulence and extract hidden characteristics of polarization or rotation of spatial patterns created in turbulent plasmas. The article presents the explanation of the method and the application results obtained from a PANTA plasma. [ABSTRACT FROM AUTHOR]
- Published
- 2019
- Full Text
- View/download PDF
12. Tomography system for two-dimensional observation of fluctuation in magnetized plasma.
- Author
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Yamasaki, K., Fujisawa, A., Nagashima, Y., Moon, C., Inagaki, S., and Yamada, T.
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CYLINDRICAL plasmas ,LARMOR radius ,TOMOGRAPHY - Abstract
A tomography system is developed for observing local fluctuations, pursuing higher spatial resolution, over the entire plasma of a linear cylindrical plasma. The system is equipped with totally 126 lines-of-sight that are distributed equally to six sets of light guide arrays placed around the plasma. The system has succeeded in measuring the two-dimensional structure of plasma emission and resolving coherent fluctuation structures, with spatial resolution comparable to the ion Larmor radius. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
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13. Extraction of nonlinear waveform in turbulent plasma.
- Author
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Kin, F., Hasamada, K., Itoh, K., Kobayashi, T., Fujisawa, A., Kosuga, Y., Sasaki, M., Inagaki, S., Nagashima, Y., Kasuya, N., Itoh, S.-I., Yamada, T., Yamasaki, K., and Arakawa, H.
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PLASMA turbulence ,WAVE analysis ,NONLINEAR systems ,ELECTRICAL harmonics ,ELECTRIC discharges ,SOLITONS - Abstract
Streamers and their mediator have been known to exist in linear cylindrical plasmas [Yamada
et al. , Nat. Phys.4 , 721 (2008)]. Conditional averaging is applied to extract the nonlinear characteristics of a mediator, which has been simply treated as a linear wave. This paper reports that a mediator should have higher harmonic components generated by self-couplings, and the envelope of a streamer should be generated with not only fundamental but also higher harmonic components of the mediator. Moreover, both the mediator and the envelope of the streamer have common features with solitary waves, i.e., the height should increase inversely as the square of their localization width. [ABSTRACT FROM AUTHOR]- Published
- 2018
- Full Text
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14. Topological bifurcation of helical flows in magnetized plasmas with density gradient and parallel flow shear.
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Sasaki, M., Kasuya, N., Itoh, K., Toda, S., Yamada, T., Kosuga, Y., Nagashima, Y., Kobayashi, T., Arakawa, H., Yamasaki, K., Fujisawa, A., Inagaki, S., and Itoh, S. I.
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FREE energy (Thermodynamics) ,THERMODYNAMIC state variables ,HELICAL gears ,DENSITY gradient centrifugation ,SEDIMENTATION analysis - Abstract
The topological bifurcation of the flow in non-equilibrium magnetized plasmas is demonstrated by a turbulence simulation. A system with two generic sources of turbulence, the gradients of density and parallel flow, is considered. Topological index of the flow is introduced, in order to indicate the chirality of flow pattern. We here report that the turbulence-driven flow forms the structure of co-axial helixes with opposite chirality. By changing the source of plasma particles, which modifies the density gradient, the transition between three turbulent states is obtained. In addition to the two turbulent states, which are dominated by the drift wave and the D'Angelo mode, respectively, the new state is found. In this third state, fluctuations are driven by both of the free energy sources simultaneously and compete with the others. The result illustrates the generic feature of turbulence flow generation in non-equilibrium magnetized plasmas. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
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15. A proposal of Fourier-Bessel expansion with optimized ensembles of bases to analyse two dimensional image.
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Yamasaki, K., Fujisawa, A., and Nagashima, Y.
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INDUCTIVELY coupled plasma atomic emission spectrometry ,ATOMIC emission spectroscopy ,ORTHONORMAL basis ,VECTOR algebra ,TOMOGRAPHY - Abstract
It is a critical issue to find the best set of fitting function bases in mode structural analysis of two dimensional images like plasma emission profiles. The paper proposes a method to optimize a set of the bases in the case of Fourier-Bessel function series, using their orthonormal property, for more efficient and precise analysis. The method is applied on a tomography image of plasma emission obtained with the Maximum-likelihood expectation maximization method in a linear cylindrical device. The result demonstrates the excellency of the method that realizes the smaller residual error and minimum Akaike information criterion using smaller number of fitting function bases. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
- View/download PDF
16. Measurement of thickness of film deposited on the plasma-facing wall in the QUEST tokamak by colorimetry.
- Author
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Wang, Z., Hanada, K., Yoshida, N., Shimoji, T., Miyamoto, M., Oya, Y., Zushi, H., Idei, H., Nakamura, K., Fujisawa, A., Nagashima, Y., Hasegawa, M., Kawasaki, S., Higashijima, A., Nakashima, H., Nagata, T., Kawaguchi, A., Fujiwara, T., Araki, K., and Mitarai, O.
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TOKAMAKS ,FUSION reactors ,PLASMA confinement devices ,COLORIMETRY ,OPTICAL measurements - Abstract
After several experimental campaigns in the Kyushu University Experiment with Steady-state Spherical Tokamak (QUEST), the originally stainless steel plasma-facing wall (PFW) becomes completely covered with a deposited film composed of mixture materials, such as iron, chromium, carbon, and tungsten. In this work, an innovative colorimetry-based method was developed to measure the thickness of the deposited film on the actual QUEST wall. Because the optical constants of the deposited film on the PFW were position-dependent and the extinction coefficient k
1 was about 1.0–2.0, which made the probing light not penetrate through some thick deposited films, the colorimetry method developed can only provide a rough value range of thickness of the metal-containing film deposited on the actual PFW in QUEST. However, the use of colorimetry is of great benefit to large-area inspections and to radioactive materials in future fusion devices that will be strictly prohibited from being taken out of the limited area. [ABSTRACT FROM AUTHOR]- Published
- 2017
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17. Measurements of edge plasma parameters during internal reconnection events in the TST-2 spherical tokamak.
- Author
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Furui, H., Ejiri, A., Nagashima, Y., Takase, Y, Sonehara, M., Tsujii, N., Roidl, B., Shinya, T., Togashi, H., Homma, H., Nakamura, K., Takeuchi, T., Yajima, S., Yoshida, Y., Toida, K., Takahashi, W., and Yamazaki, H.
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PLASMA boundary layers ,CURRENT density (Electromagnetism) ,ELECTRON density ,ELECTRON temperature measurement ,POLOIDAL magnetic fields - Abstract
Measurements of edge plasma parameters such as current density, electron density, and electron temperature were performed during internal reconnection events in TST-2 Ohmic plasmas. The measured current density consists of two components: a slowly varying component and a spiky bipolar component. The magnitude of the slowly varying component is comparable to the mean current density averaged over the poloidal cross section, and it seems to represent the global transport from the core to the edge. The spiky bipolar component is about an order of magnitude larger than the slowly varying component, but the spatial structure seems to be localized and its effect on plasma confinement is not catastrophic. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
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18. Structure formation in parallel ion flow and density profiles by cross-ferroic turbulent transport in linear magnetized plasma.
- Author
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Kobayashi, T., Inagaki, S., Kosuga, Y., Sasaki, M., Nagashima, Y., Yamada, T., Arakawa, H., Kasuya, N., Fujisawa, A., Itoh, S.-I., and Itoh, K.
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PLASMA turbulence ,ION flow dynamics ,FERROELECTRIC materials ,REYNOLDS stress ,PLASMA instabilities - Abstract
In this paper, we show the direct observation of the parallel flow structure and the parallel Reynolds stress in a linear magnetized plasma, in which a cross-ferroic turbulence system is formed [Inagaki et al., Sci. Rep. 6, 22189 (2016)]. It is shown that the parallel Reynolds stress induced by the density gradient driven drift wave is the source of the parallel flow structure. Moreover, the generated parallel flow shear by the parallel Reynolds stress is found to drive the parallel flow shear driven instability D’Angelo mode, which coexists with the original drift wave. The excited D’Angelo mode induces the inward particle flux, which seems to help in maintaining the peaked density profile. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
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19. Results of lower hybrid wave experiments using a dielectric loaded waveguide array antenna on TST-2
- Author
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Wakatsuki, T., primary, Ejiri, A., additional, Shinya, T., additional, Takase, Y., additional, Furui, H., additional, Hiratsuka, J., additional, Imamura, K., additional, Inada, T., additional, Kakuda, H., additional, Kasahara, H., additional, Nagashima, Y., additional, Nakamura, K., additional, Nakanishi, A., additional, Oosako, T., additional, Saito, K., additional, Seki, T., additional, Shimpo, F., additional, Sonehara, M., additional, Togashi, H., additional, Tsuda, S., additional, Tsujii, N., additional, and Yamaguchi, T., additional
- Published
- 2014
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20. Memory Effect of GaAs Thin-Film Diode.
- Author
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Takahashi, K., Moriizumi, T., and Nagashima, Y.
- Published
- 1971
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21. Observation of an edge coherent mode and poloidal flow in the electron cyclotron wave induced high βp plasma in QUEST.
- Author
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Banerjee, Santanu, Zushi, H., Nishino, N., Mishra, K., Mahira, Y., Tashima, S., Ejiri, A., Yamaguchi, T., Onchi, T., Nagashima, Y., Hanada, K., Nakamura, K., Idei, H., Hasegawa, M., Fujisawa, A., Kuzmin, A., and Matsuoka, K.
- Subjects
CYCLOTRON resonance ,PLASMA gases ,COHERENCE (Nuclear physics) ,OHMIC contacts ,TOKAMAKS - Abstract
Fluctuations are measured in the edge and scrape-off layer (SOL) of QUEST using fast visible imaging diagnostic. Electron cyclotron wave injection in the Ohmic plasma features excitation of low frequency coherent fluctuations near the separatrix and enhanced cross-field transport. Plasma shifts from initial high field side limiter bound (inboard limited, IL) towards inboard poloidal null (IPN) configuration with steepening of the density profile at the edge. This may have facilitated the increased edge and SOL fluctuation activities. Observation of the coherent mode, associated plasma flow, and particle out-flux, for the first time in the IPN plasma configuration in a spherical tokamak may provide further impetus to the edge and SOL turbulence studies in tokamaks. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
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22. A model of plasma current through a hole of Rogowski probe including sheath effects.
- Author
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Furui, H., Ejiri, A., Nagashima, Y., Takase, Y., Sonehara, M., Tsujii, N., Yamaguchi, T., Shinya, T., Togashi, H., Homma, H., Nakamura, K., Takeuchi, T., Yajima, S., Yoshida, Y., Toida, K., Takahashi, W., and Yamazaki, H.
- Subjects
PLASMA currents ,PLASMA probes ,PLASMA sheaths ,ELECTROMAGNETIC fields ,ELECTRIC currents - Abstract
In TST-2 Ohmic discharges, local current is measured using a Rogowski probe by changing the angle between the local magnetic field and the direction of the hole of the Rogowski probe. The angular dependence shows a peak when the direction of the hole is almost parallel to the local magnetic field. The obtained width of the peak was broader than that of the theoretical curve expected from the probe geometry. In order to explain this disagreement, we consider the effect of sheath in the vicinity of the Rogowski probe. A sheath model was constructed and electron orbits were numerically calculated. From the calculation, it was found that the electron orbit is affected by E × B drift due to the sheath electric field. Such orbit causes the broadening of the peak in the angular dependence and the dependence agrees with the experimental results. The dependence of the broadening on various plasma parameters was studied numerically and explained qualitatively by a simplified analytical model. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
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23. Azimuthal inhomogeneity of turbulence structure and its impact on intermittent particle transport in linear magnetized plasmas.
- Author
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Kobayashi, T., Inagaki, S., Sasaki, M., Kosuga, Y., Arakawa, H., Yamada, T., Nagashima, Y., Miwa, Y., Kasuya, N., Fujisawa, A., Itoh, S. -I., and Itoh, K.
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TURBULENCE ,PARTICLE dynamics analysis ,MAGNETIZATION ,SPATIAL distribution (Quantum optics) ,NONLINEAR systems - Abstract
Fluctuation component in the turbulence regime is found to be azimuthally localized at a phase of the global coherent modes in a linear magnetized plasma PANTA. Spatial distribution of squared bicoherence is given in the azimuthal cross section as an indicator of nonlinear energy transfer function from the global coherent mode to the turbulence. Squared bicoherence is strong at a phase where the turbulence amplitude is large. As a result of the turbulence localization, time evolution of radial particle flux becomes bursty. Statistical features such as skewness and kurtosis are strongly modified by the localized turbulence component, although contribution to mean particle flux profile is small. [ABSTRACT FROM AUTHOR]
- Published
- 2015
- Full Text
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24. Heat flux and plasma flow in the far scrape-off layer of the inboard poloidal field null configuration in QUEST.
- Author
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Onchi, T., Zushi, H., Mishra, K., Mahira, Y., Nagaoka, K., Hanada, K., Idei, H., Hasegawa, M., Nakamura, K., Fujisawa, A., Nagashima, Y., Matsuoka, K., Tashima, S., Banerjee, S., Kuzmin, A., Kawasaki, S., Nakashima, H., Higashijima, A., and Watanabe, O.
- Subjects
HEAT flux ,PLASMA flow ,POLOIDAL magnetic fields ,TOKAMAKS ,SUPERSONIC flow ,ELECTRON plasma - Abstract
Heat flux and plasma flow in the scrape-off layer (SOL) are examined for the inboard poloidal field null (IPN) configuration of the spherical tokamak QUEST. In the plasma current (I
p ) ramp-up phase, high heat flux (>1MW/m²) and supersonic flow (Mach number M>1) are found to be present simultaneously in the far-SOL. The heat flux is generated by energetic electrons excursed from the last closed flux surface. Supersonic flows in the poloidal and toroidal directions are correlated with each other. In the quasi-steady state, sawtooth-like oscillation of Ip at 20 Hz is observed. Heat flux and subsonic plasma flow in the far-SOL are modified corresponding to the Ip -oscillation. The heat flow caused by motion of energetic electrons and the bulk-particle transport to the far-SOL is enhanced during the low-Ip phase. Modification of plasma flow in the far SOL occurs earlier than the Ip crash. The M-Ip curve has a limit-cycle characteristic with sawtooth-like oscillation. Such a core-SOL relationship indicates that the far-SOL flow plays an important role in sustaining the oscillation of Ip in the IPN configuration. [ABSTRACT FROM AUTHOR]- Published
- 2015
- Full Text
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25. Dynamical programming based turbulence velocimetry for fast visible imaging of tokamak plasma.
- Author
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Banerjee, Santanu, Zushi, H., Nishino, N., Mishra, K., Onchi, T., Kuzmin, A., Nagashima, Y., Hanada, K., Nakamura, K., Idei, H., Hasegawa, M., and Fujisawa, A.
- Subjects
TOKAMAKS ,PLASMA flow ,PARTICLE image velocimetry ,EMISSION spectroscopy ,TIME delay estimation - Abstract
An orthogonal dynamic programming (ODP) based particle image velocimetry (PIV) technique is developed to measure the time resolved flow field of the fluctuating structures at the plasma edge and scrape off layer (SOL) of tokamaks. This non-intrusive technique can provide two dimensional velocity fields at high spatial and temporal resolution from a fast framing image sequence and hence can provide better insights into plasma flow as compared to conventional probe measurements. Applicability of the technique is tested with simulated image pairs. Finally, it is applied to tangential fast visible images of QUEST plasma to estimate the SOL flow in inboard poloidal null-natural divertor configuration. This technique is also applied to investigate the intricate features of the core of the run-away dominated phase following the injection of a large amount of neutrals in the target Ohmic plasma. Development of the ODP-PIV code and its applicability on actual plasma images is reported. [ABSTRACT FROM AUTHOR]
- Published
- 2015
- Full Text
- View/download PDF
26. Local current density measurement using a Rogowski probe in Tokyo Spherical Tokamak-2.
- Author
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Furui, H., Nagashima, Y., Takase, Y., Ejiri, A., Kakuda, H., Sonehara, M., Oosako, T., Tsujii, N., Hiratsuka, J., Imamura, K., Inada, T., Nakamura, K., Nakanishi, A., Shinya, T., Togashi, H., Tsuda, S., Wakatsuki, T., and Yamaguchi, T.
- Subjects
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ELECTRONIC probes , *ELECTROMAGNETS , *LANGMUIR probes , *CURRENT density (Electromagnetism) , *TOKAMAKS , *SIGNAL-to-noise ratio - Abstract
A Rogowski probe consisting of a small multi-layer Rogowski coil, five magnetic pick-up coils, and a Langmuir probe was developed to measure the local current density and its direction. It can be moved along the major radius and can be turned around its axis. This probe was used to measure the current density profile near the last closed flux surface of Ohmic plasmas in Tokyo Spherical Tokamak-2. The current density profile was measured successfully with a signal to noise ratio of greater than 20. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
- View/download PDF
27. Thermal imaging of plasma with a phased array antenna in QUEST.
- Author
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Mishra, Kishore, Idei, H., Zushi, H., Nagata, K., Akimoto, R., Yamamoto, M. K., Hanada, K., Hasegawa, M., Nakamura, K., Fujisawa, A., Nagashima, Y., Banerjee, S., Onchi, T., and Kuzmin, A.
- Subjects
INFRARED imaging ,THERMOGRAPHY ,PLASMA gas research ,PHASED array antennas ,IMAGING systems - Abstract
A thermal imaging system to measure plasma Electron Bernstein Emission (EBE) emanating from the mode conversion region in overdense plasma is discussed. Unlike conventional ECE/EBE imaging, this diagnostics does not employ any active mechanical scanning mirrors or focusing optics to scan for the emission cones in plasma. Instead, a standard 3 x 3 waveguide array antenna is used as a passive receiver to collect emission from plasma and imaging reconstruction is done by accurate measurements of phase and intensity of these signals by heterodyne detection technique. A broadband noise source simulating the EBE, is installed near the expected mode conversion region and its position is successfully reconstructed using phase array technique which is done in post processing. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
- View/download PDF
28. Plans for Lower Hybrid Current Drive Experiment using a Dielectric Loaded Waveguide Array Antenna in TST-2.
- Author
-
Wakatsuki, T., Ejiri, A., Kakuda, H., Meneghini, O., Shiraiwa, S., Takase, Y., Wright, J. C., Ambo, T., Hanashima, K., Hiratsuka, J., Nagashima, Y., Sakamoto, T., Shino, R., Sonehara, M., Yamaguchi, T., and Watanabe, O.
- Subjects
ELECTRIC currents ,PHYSICS experiments ,DIELECTRICS ,WAVEGUIDES ,ANTENNA arrays ,FINITE element method ,ELECTRONIC excitation ,FOKKER-Planck equation - Abstract
Plasma current start-up experiments using the lower hybrid wave (LHW) excited by a new dielectric loaded waveguide array antenna are planned on TST-2. The wave excitation efficiency of this antenna is investigated using a versatile FEM solver package, COMSOL. Propagation and absorption of the LHW are calculated by iterating between the TORLH full wave code and the CQL3D Fokker-Planck code. Wave absorption is very weak for Maxwellian plasma, but becomes stronger as the quasilinear deformation of the distribution function develops. [ABSTRACT FROM AUTHOR]
- Published
- 2011
- Full Text
- View/download PDF
29. Plasma Curent Start-up Experiment using Waves in the Lower Hybrid Frequency Range in TST-2.
- Author
-
Takase, Y., Wakatsuki, T., Ejiri, A., Kakuda, H., Moeller, C. P., Ambo, T., Hanashima, K., Hiratsuka, J., Kasahara, H., Kumazawa, R., Nagashima, Y., Saito, K., Sakamoto, T., Seki, T., Shimpo, F., Shino, R., Sonehara, M., Watanabe, O., and Yamaguchi, T.
- Subjects
PLASMA gases ,ELECTRIC currents ,RADIO frequency ,ELECTRIC displacement ,TOKAMAKS ,PERMITTIVITY ,STATISTICAL bootstrapping ,ELECTRONIC excitation ,ELECTRON temperature - Abstract
Noninductive plasma current (I
p ) start-up experiments using RF power in the lower hybrid frequency range are being conducted on the TST-2 spherical tokamak. The lower hybrid wave (LHW) has demonstrated efficient current drive in conventional tokamaks. However, in spherical tokamak (ST) plasmas with very high dielectric constants (;[variant_greek_epsilon]∼ωpe 2 /Ωe 2 >1), accessibility of the LHW to the plasma core is severely limited. Our approach is to keep the plasma density low (such that [variant_greek_epsilon]∼1) during Ip ramp-up. Once Ip reaches a level sufficiently high for neutral beam current drive, plasma can be densified and transformed into an advanced tokamak plasma dominated by the self-driven bootstrap current. Initial plasma start-up experiments were performed on TST-2 using a combline antenna which excites a traveling fast wave. After formation of toroidal flux surfaces, RF power and vertical field were ramped up to increase Ip . Up to 12 kA of Ip has been obtained by this method. Soft X-ray measurements indicate that the electron temperature increases as Ip increases, and hard X-ray spectroscopy indicates that energetic electrons build up as Ip is ramped up. [ABSTRACT FROM AUTHOR]- Published
- 2011
- Full Text
- View/download PDF
30. Electron Cyclotron / Bernstein Wave Heating and Current Drive Experiments using Phased-array Antenna in QUEST.
- Author
-
Idei, H., Zushi, H., Hanada, K., Nakamura, K., Fujisawa, A., Hasegawa, M., Yoshida, N., Sakamoto, M., Watanebe, H., Tokunaga, K., Nagashima, Y., Ejiri, A., Sakaguchi, M., Kalinnikova, E., Ishiguro, M., Tashima, S., Fukuyama, A., Igami, H., Kubo, S., and Sharma, S. K.
- Subjects
ELECTRON cyclotron resonance sources ,ELECTRIC heating ,ELECTROMAGNETIC waves ,PHYSICS experiments ,PHASED array antennas ,PLASMA injection ,PERFORMANCE evaluation ,ELECTRIC discharges - Abstract
The phased-array antenna system for Electron Cyclotron/Bernstein Wave Heating and Current Drive experiments has been developed in the QUEST. The antenna was designed to excite a pure O-mode wave in the oblique injection for the O-X-B mode conversion experiments, and its good performances were confirmed at a low power level. The plasma current (<∼15 kA) with an aspect ratio of 1.5 was started up and sustained by only RF injection in the low-density operations. The long pulse discharge of 10 kA was also attained for 37 s. The new density window to sustain the plasma current was observed in the high-density plasmas. The single-null divertor configuration with the high plasma current (<∼25 kA) was attained in the 17 s plasma sustainment. [ABSTRACT FROM AUTHOR]
- Published
- 2011
- Full Text
- View/download PDF
31. High Harmonic Fast Wave experiments on TST-2 and UTST.
- Author
-
Oosako, T., Kobayashi, H., An, B. I., Kamio, S., Kurashina, H., Hayashi, H., Hihara, R., Matsuzawa, H., Yamada, K., Yamaguchi, T., Imazawa, R., Masuda, T., Nagashima, Y., Yamada, T., Watanabe, O., Inomoto, M., Ejiri, A., Ono, Y., and Takase, Y.
- Subjects
ELECTRICAL harmonics ,WAVE mechanics ,PHYSICS experiments ,ION cyclotron resonance spectrometry ,CYCLOTRONS - Abstract
HHFW experiments on TST-2 and UTST were performed. In the TST-2 HHFW experiment, the pickup probe mounted on the inboard wall detected the same PDI lower sideband peak as probes on the outboard side. Since the frequency difference from the pump wave corresponds to the ion cyclotron frequency on the low field side, the lower sideband wave is believed to be generated at the plasma edge on the low field side, and propagated to the high field side. This component may be the FW. In the UTST HHFW experiment, direct measurements of the RF fields in the plasma were made successfully. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
- View/download PDF
32. Equilibrium Analysis of EC-Sustained and RF-Sustained ST Plasmas.
- Author
-
Takase, Y., Ejiri, A., Nagashima, Y., Watanabe, O., An, B. I., Hayashi, H., Hanashima, K., Hiratsuka, J., Kakuda, H., Kobayashi, H., Kurashina, H., Matsuzawa, H., Oosako, T., Sakamoto, T., Wakatsuki, T., Yamada, K., and Yamaguchi, T.
- Subjects
HIGH-density plasmas ,SOLENOIDS ,RADIO frequency ,EQUILIBRIUM ,WAVE mechanics - Abstract
Plasma current start-up and formation of the ST configuration without the use of the central solenoid is a critical issue in ST research. In the TST-2 spherical tokamak (R = 0.38 m, a = 0.25 m), sustainment of an ECRF (2.45 GHz) produced ST plasma by low frequency (21 MHz) RF power alone was demonstrated. Since direct RF current drive can be ruled out, this result implies that the ST configuration is sustained entirely by pressure-driven currents. The Grad-Shafranov equilibrium was generalized to take into account the open field line region with finite plasma current and pressure (truncated equilibrium). In addition to the precessional current of trapped particles, Pfirsch-Schlüter current flowing along the open field line (and partially returning through the vacuum vessel) contributes to the toroidal plasma current. Three phases of plasma start-up are analyzed: (i) the current formation phase, (ii) the current jump phase, and (iii) the current sustainment phase. In the current formation phase, the plasma current is formed and increases slowly, roughly proportional to the stored energy. Closed flux surfaces do not exist and the current density profile is peaked on the outboard side. Once the plasma current increases beyond a critical value (approximately equal to the level necessary to satisfy the major radial force balance), the plasma current increases rapidly (current jump). Closed flux surfaces appear not at the beginning, but at the end of the current jump. In the sustainment phase, plasma has a high poloidal beta, β
p = O(1). The equilibrium is characterized by the hollowness of the current density profile, which also determines the fraction of the plasma current inside the last closed flux surface. The plasma equilibrium does not differ greatly between EC-sustained and RF-sustained plasmas. However, RF-sustained plasmas are more turbulent, with larger fluctuation levels over a wide frequency band (0–20 kHz), and in many cases MHD activity in the 5–10 kHz frequency band increases rapidly and terminates the discharge. [ABSTRACT FROM AUTHOR]- Published
- 2009
- Full Text
- View/download PDF
33. DEFECT SIZING METHOD USING ULTRASONIC GUIDED WAVES IN PIPES.
- Author
-
Nagashima, Y., Endou, M., Miki, M., Odakura, M., and Maniwa, K.
- Subjects
- *
SURFACE defects , *PIPE , *TORSION , *REFLECTANCE , *THREE-dimensional imaging , *MECHANICAL impedance - Abstract
This paper presents a defect-sizing method using swept-frequency guided waves. In this technique, reflection coefficients of multiple frequencies were measured, and the coefficients were compared with the 3-D analysis result that was calculated by integration of reflection waves from divided calculation-regions that had different characteristic acoustical impedances. The effectiveness of the method was evaluated experimentally using torsional mode guided waves that propagated in pipe specimens. The experimental results were in good agreement with analysis results. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
- View/download PDF
34. Physics of Zonal Flows.
- Author
-
Itoh, K., Itoh, S.-I., Diamond, P. H., Fujisawa, A., Nagashima, Y., Yagi, M., Fukuyama, A., Hahm, T. S., Hallatschek, K., and Tynan, G. R.
- Subjects
FLUID dynamics ,TURBULENCE ,DRIFT waves ,COMPUTATIONAL fluid dynamics ,PHYSICS - Abstract
Zonal flows, by which we mean azimuthally symmetric band-like shear flows, are ubiquitous phenomena in nature and the laboratory. It is now widely recognized that zonal flows are a key constituent in virtually all cases and regimes of drift wave turbulence, indeed, so much so that this classic problem is now frequently referred to as “drift wave-zonal flow turbulence.” In this lecture note, we present new viewpoints and unifying concepts which facilitate understanding of zonal flow physics, via theory, computation and their confrontation with the results of laboratory experiment. Special emphasis is placed on identifying avenues for further progress. We briefly survey issues such as (i) mechanism of zonal flows excitation, (ii) back interaction on turbulence, (iii) saturation mechanism of zonal flows, (iv) energy partition between fluctuations and flows, (v) turbulent transport coefficient dressed by zonal flows, and (vi) experimental efforts to verify these fundamental processes. [ABSTRACT FROM AUTHOR]
- Published
- 2008
- Full Text
- View/download PDF
35. Role of stochasticity in turbulence and convective intermittent transport at the scrape off layer of Ohmic plasma in QUEST.
- Author
-
Banerjee, Santanu, Zushi, H., Nishino, N., Hanada, K., Ishiguro, M., Tashima, S., Liu, H. Q., Mishra, K., Nakamura, K., Idei, H., Hasegawa, M., Fujisawa, A., Nagashima, Y., and Matsuoka, K.
- Subjects
STOCHASTIC processes ,PLASMA turbulence ,TRANSPORT theory ,PLASMA boundary layers ,PLASMA gases ,FLUCTUATIONS (Physics) - Abstract
Statistical features of fluctuations are investigated using the fast camera imaging technique in the scrape of layer (SOL) of electron cyclotron resonance heated Ohmic plasma. Fluctuations in the SOL towards low field side are dominated by coherent convective structures (blobs). Two dimensional structures of the higher order moments (skewness s and kurtosis k) representing the shape of probability density function (PDF) are studied. s and k are seen to be functions of the magnetic field lines. s and k are consistently higher towards the bottom half of the vessel in the SOL showing the blob trajectory along the field lines from the top towards bottom of the vessel. Parabolic relation (k = As² + C) is observed between s and k near the plasma boundary, featuring steep density gradient region and at the far SOL. The coefficient A, obtained experimentally, indicates a shift of prominence from pure drift-wave instabilities towards fully developed turbulence. Numerical coefficients characterizing the Pearson system are derived which demonstrates the progressive deviation of the PDF from Gaussian towards gamma from the density gradient region, towards the far SOL. Based on a simple stochastic differential equation, a direct correspondence between the multiplicative noise amplitude, increased intermittency, and hence change in PDF is discussed. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
- View/download PDF
36. A study of elastic and inelastic collisions of 58Ni+90,94Zr at two near-barrier energies.
- Author
-
Corradi, L., Napoli, D. R., Stefanini, A. M., Signorini, C., Sugiyama, Y., Ikezoe, H., Tomita, Y., Ideno, K., Kim, H. J., Nagashima, Y., Masuda, T., Fujiwara, H., Pu, Ye Hu, Tagishi, Y., Sugimitsu, T., and Qi, B.
- Published
- 1992
- Full Text
- View/download PDF
37. Two-dimensional bispectral analysis of drift wave turbulence in a cylindrical plasma.
- Author
-
Yamada, T., Itoh, S.-I., Inagaki, S., Nagashima, Y., Shinohara, S., Kasuya, N., Terasaka, K., Kamataki, K., Arakawa, H., Yagi, M., Fujisawa, A., and Itoh, K.
- Subjects
POWER (Mechanics) ,SPECTRUM analysis ,CLUTCHES (Machinery) ,POWER spectra - Abstract
Bispectral analysis and multichannel measurement are becoming attractive investigation tools in plasma fluctuation studies. In the Large Mirror Device-Upgrade, the measurement of fluctuations in the ion saturation-current with a 64-channel poloidal Langmuir probe array was performed. The two-dimensional (2D) (poloidal wave number and frequency) power spectrum showed a number of pronounced peaks and broadband fluctuations in the poloidal wave number-frequency space. We applied 2D bispectral analysis, which considers both the matching conditions of poloidal wave number and frequency, to the spatiotemporal waveform, and confirmed the nonlinear couplings between coherent-coherent, coherent-broadband, and broadband-broadband fluctuation components. More than ten peaks were revealed to have as their origins only three original parent modes generated in the plasma. Comparison between the theoretical estimate and experimental observation for the bicoherence showed good agreement. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
38. Nonlinear self-interaction of geodesic acoustic modes in toroidal plasmas.
- Author
-
Sasaki, M., Itoh, K., Nagashima, Y., Ejiri, A., and Takase, Y.
- Subjects
PLASMA gases ,PLASMA confinement ,PLASMA waves ,ACOUSTIC surface waves ,DRIFT waves - Abstract
A self-nonlinear process involving the geodesic acoustic mode (GAM) was analyzed. GAMs nonlinearly excite higher harmonics as quasimodes. In the fluid picture, the Reynolds stress is modeled by the drift wave action. Other nonlinear terms such as the coupling between parallel velocity and vorticity, and between density and vorticity, were also considered, and evaluated using linear eigenmodes. We considered nonlinearities up to the third order. Analytic expressions for the amplitude of the second harmonic GAM and the saturation level of the fundamental GAM were derived. The validity of the theory is discussed based on a comparison of experimental results with theoretical predictions. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
- View/download PDF
39. Fine positioning of a poloidal probe array.
- Author
-
Yamada, T., Nagashima, Y., Inagaki, S., Kawai, Y., Yagi, M., Itoh, S.-I., Maruta, T., Shinohara, S., Terasaka, K., Kawaguchi, M., Fukao, M., Fujisawa, A., and Itoh, K.
- Subjects
- *
PLASMA gases , *TURBULENCE , *VECTOR fields , *TOROIDAL harmonics , *LANGMUIR probes - Abstract
Multipoint detection is an essential requirement for investigating plasma turbulence which is a highly nonlinear phenomenon in space and time. We have fabricated an array of 64-channel poloidal probes surrounding the linear cylindrical plasma named LMD-U in order to study turbulence properties, particularly the nonlinear mode couplings, in the domain of poloidal wave number and frequency. However, misalignments of probe tips produce spurious modes, which do not exist in the real plasma, to distort the precise wave number measurements. The paper presents the description of the 64-channel poloidal probe array with means to adjust the probe positions, with discussion on the effects of the misalignments on the wave number measurements. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
- View/download PDF
40. Reduction effect of neutral density on the excitation of turbulent drift waves in a linear magnetized plasma with flow.
- Author
-
Saitou, Y., Yonesu, A., Shinohara, S., Ignatenko, M. V., Kasuya, N., Kawaguchi, M., Terasaka, K., Nishijima, T., Nagashima, Y., Kawai, Y., Yagi, M., Itoh, S.-I., Azumi, M., and Itoh, K.
- Subjects
DRIFT waves ,PLASMA gases ,TURBULENCE ,DENSITY ,ELECTRON distribution ,MONTE Carlo method - Abstract
The importance of reducing the neutral density to reach strong drift wave turbulence is clarified from the results of the extended magnetohydrodynamics and Monte Carlo simulations in a linear magnetized plasma. An upper bound of the neutral density relating to the ion-neutral collision frequency for the excitation of drift wave instability is shown, and the necessary flow velocity to excite this instability is also estimated from the neutral distributions. Measurements of the Mach number and the electron density distributions using Mach probe in the large mirror device (LMD) of Kyushu University [S. Shinohara et al., Plasma Phys. Control. Fusion 37, 1015 (1995)] are reported as well. The obtained results show a controllability of the neutral density and provide the basis for neutral density reduction and a possibility to excite strong drift wave turbulence in the LMD. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
- View/download PDF
41. Physics of zonal flows.
- Author
-
Itoh, K., Itoh, S.-I., Diamond, P. H., Hahm, T. S., Fujisawa, A., Tynan, G. R., Yagi, M., and Nagashima, Y.
- Subjects
SHEAR flow ,FLUID dynamics ,DRIFT waves ,PLASMA turbulence ,PLASMA confinement ,CONTROLLED fusion - Abstract
Zonal flows, which means azimuthally symmetric band-like shear flows, are ubiquitous phenomena in nature and the laboratory. It is now widely recognized that zonal flows are a key constituent in virtually all cases and regimes of drift wave turbulence, indeed, so much so that this classic problem is now frequently referred to as “drift wave-zonal flow turbulence.” In this review, new viewpoints and unifying concepts are presented, which facilitate understanding of zonal flow physics, via theory, computation and their confrontation with the results of laboratory experiment. Special emphasis is placed on identifying avenues for further progress. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
42. Convergence study of bispectral analysis in experiments of high temperature plasmas.
- Author
-
Nagashima, Y., Itoh, S.-I., Yagi, M., Itoh, K., Fujisawa, A., Hoshino, K., Shinohara, K., Uehara, K., Kusama, Y., Ejiri, A., and Takase, Y.
- Subjects
- *
RESEARCH , *PLASMA confinement , *TURBULENCE , *STOCHASTIC convergence , *DRIFT waves , *MAGNETICS - Abstract
A convergence study of bispectral analysis on toroidal plasma turbulence is presented with a theoretical estimate of bicoherence for the system of the drift wave and zonal flow. Examples are chosen from the analysis of floating potential fluctuation in the edge region of the JFT-2M tokamak. The convergence properties of the bispectral functions are clarified for an oscillatory branch of zonal flow (or geodesic acoustic mode) and drift waves. The total squared bicoherence or the biphase converges inversely proportional to the realization number or to the square root of the realization number, respectively. The numbers of realizations to qualitatively converge and obtain a quantitatively significant value of bicoherence are discussed for the drift wave fluctuations and for the zonal flow. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
43. On the bicoherence analysis of plasma turbulence.
- Author
-
Itoh, K., Nagashima, Y., Itoh, S.-I., Diamond, P. H., Fujisawa, A., Yagi, M., and Fukuyama, A.
- Subjects
- *
PLASMA turbulence , *TURBULENCE , *DRIFT waves , *FLUCTUATIONS (Physics) , *LANGEVIN equations , *PHYSICS - Abstract
The bicoherence of fluctuations in a system of drift waves and zonal flows is discussed. In strong drift-wave turbulence, where broadband fluctuations are excited, the bicoherence is examined. A Langevin equation formalism of turbulent interactions allows us to relate the bicoherence coefficient to the projection of nonlinear force onto the test mode. The dependence of the summed bicoherence on the amplitude of zonal flows is clarified. The importance of observing biphase is also stressed. The results provide a basis for measurement of nonlinear interaction in a system of drift waves and zonal flow. [ABSTRACT FROM AUTHOR]
- Published
- 2005
- Full Text
- View/download PDF
44. Electron Bernstein wave emission diagnostic assisted by reflectometry on TST-2 spherical tokamak.
- Author
-
Shiraiwa, S., Nagashima, Y., Ushigome, M., Yamada, T., Taniguchi, T., Ohara, S., Yamagishi, K., Kasahara, H., Iijima, D., Kobori, Y., Nishi, T., Aramasu, M., Ejiri, A., and Takase, Y.
- Subjects
- *
ELECTRON emission , *RADIOMETERS , *SCIENTIFIC apparatus & instruments - Abstract
A new electron Bemstein wave emission diagnostic consisting of a heterodyne radiometer and a reflectometer was installed on TST-2. The instrument has a frequency range from 5 to 12 GHz which covers partially the fundamental and fully covers the second and third harmonic emissions. X-mode emission perpendicular to the flux surface, generated by B-X-FX mode-conversion scenario, is measured. The mode-conversion efficiency is calculated using a one-dimensional full-wave code with the density profile obtained simultaneously from the reflectometer, and the electron temperature (T[sub e]) is estimated. The levels corresponding to the 100-150 eV range are measured from the plasma core region, and the mode-conversion efficiency is calculated to be 0.5-0.8, leading to T[sub e] of approximately 200 eV. In addition, it is found that the reflectivity measured by the reflectometer is consistent with the calculated mode-conversion efficiency. This property can be used for a direct measurement of the mode-conversion efficiency. [ABSTRACT FROM AUTHOR]
- Published
- 2003
- Full Text
- View/download PDF
45. Demonstration of improvement in the signal-to-noise ratio of Thomson scattering signal obtained by using a multi-pass optical cavity on the Tokyo Spherical Tokamak-2.
- Author
-
Togashi, H., Ejiri, A., Hiratsuka, J., Nakamura, K., Takase, Y., Yamaguchi, T., Furui, H., Imamura, K., Inada, T., Kakuda, H., Nakanishi, A., Oosako, T., Shinya, T., Sonehara, M., Tsuda, S., Tsujii, N., Wakatsuki, T., Hasegawa, M., Nagashima, Y., and Narihara, K.
- Subjects
THOMSON scattering ,SIGNAL-to-noise ratio ,PLASMA gas research ,ELECTROMAGNETIC wave scattering ,PHOTON scattering - Abstract
The multi-pass Thomson scattering (TS) scheme enables obtaining many photons by accumulating multiple TS signals. The signal-to-noise ratio (SNR) depends on the accumulation number. In this study, we performed multi-pass TS measurements for ohmically heated plasmas, and the relationship between SNR and the accumulation number was investigated. As a result, improvement of SNR in this experiment indicated similar tendency to that calculated for the background noise dominant situation. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
- View/download PDF
46. Note: Multi-pass Thomson scattering measurement on the TST-2 spherical tokamak.
- Author
-
Togashi, H., Ejiri, A., Hiratsuka, J., Nakamura, K., Takase, Y., Yamaguchi, T., Furui, H., Imamura, K., Inada, T., Kakuda, H., Nakanishi, A., Oosako, T., Shinya, T., Sonehara, M., Tsuda, S., Tsujii, N., Wakatsuki, T., Hasegawa, M., Nagashima, Y., and Narihara, K.
- Subjects
TOKAMAKS ,THOMSON scattering ,LASER pulses ,SIGNAL-to-noise ratio ,TEMPERATURE measurements - Abstract
In multi-pass Thomson scattering (TS) scheme, a laser pulse makes multiple round trips through the plasma, and the effective laser energy is enhanced, and we can increase the signal-to-noise ratio as a result. We have developed a coaxial optical cavity in which a laser pulse is confined, and we performed TS measurements using the coaxial cavity in tokamak plasmas for the first time. In the optical cavity, the laser energy attenuation was approximately 30% in each round trip, and we achieved a photon number gain of about 3 compared with that obtained in the first round trip. In addition, the temperature measurement accuracy was improved by accumulating the first three round trip waveforms. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
- View/download PDF
47. Vector and tensor analyzing powers for proton-deuteron elastic and inelastic scattering.
- Author
-
Sawada, M., Seki, S., Ishikawa, M., Furuno, K., Nagashima, Y., Schimizu, J., and Sanada, J.
- Published
- 1981
- Full Text
- View/download PDF
48. Wavelet analyses using parallel computing for plasma turbulence studies.
- Author
-
Fujisawa, A., Shimizu, A., Itoh, K., Nagashima, Y., Yamada, T., Inagaki, S., Matsuoka, K., and Itoh, S.-I.
- Subjects
PLASMA turbulence ,WAVELETS (Mathematics) ,PERSONAL computers ,ELECTRIC fields ,FLUCTUATIONS (Physics) ,ELECTRONIC probes ,STELLARATORS - Abstract
The wavelet analyses have been carried out, using a cluster of personal computer, on the signal of electric field fluctuations measured with heavy ion beam probes in the compact helical system stellarator. The results have revealed the intermittent characteristics of turbulence and of the nonlinear couplings between elemental waves of turbulence. The usage of parallel computing is found to successfully reduce the calculation time as inversely proportional to the CPU number used the cluster, which shows the nature of 'embarrassingly parallel computation.' The present example of the wavelet analyses clearly demonstrates the importance of the advanced analyzing methods and the parallel computation for the modern studies of plasma turbulence. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
49. Magnetic domain observation of Nd-Cu-diffused Nd-Fe-B magnets with submicron grains by Kerr effect microscopy.
- Author
-
Takezawa, M., Nagashima, Y., Kimura, Y., Morimoto, Y., Yamasaki, J., Nozawa, N., Nishiuchi, T., and Hirosawa, S.
- Subjects
- *
MAGNETIZATION , *FERROMAGNETISM , *MAGNETISM , *MAGNETS , *HYDROGENATION , *KERR electro-optical effect - Abstract
The domain structure and magnetization process of Nd-Cu-diffused magnets prepared from hydrogenation-disproportionation-desorption-recombination (HDDR)-processed powders and Nd-Cu powders were examined with a high-resolution Kerr microscope using an ultraviolet light source. We found that the coercivity of the HDDR magnet depends on the volume of magnetically coupled grains that simultaneously reverse their magnetization. In a magnet that has no Nd-Cu, and in which the Nd-rich phase along the grain boundaries is absent, magnetization reversal simultaneously occurs in several grains. In contrast, in a Nd-Cu-diffused magnet that contains adequate Nd-rich phases along the grain boundaries, magnetization reversal occurs independently in each grain. It follows that the change in microstructure caused by the presence of Nd-Cu leads to a decoupled magnetization reversal, and domain wall pinning is an important coercivity mechanism of an HDDR magnet compacted by hot-pressing. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
50. Magnetic domain observation of Nd-Fe-B magnets with submicron-sized grains by high-resolution Kerr microscopy.
- Author
-
Takezawa, M., Tani, N., Nagashima, Y., Morimoto, Y., Yamasaki, J., Nozawa, N., Nishiuchi, T., and Hirosawa, S.
- Subjects
MAGNETIZATION ,HYDROGENATION ,MAGNETIC moments ,MAGNETIC properties ,MAGNETS - Abstract
A Kerr microscope that uses UV light for high-resolution observation of magnetic domains was developed, and the domain structure and magnetization process of hot-pressed magnets prepared from hydrogenation-disproportionation-desorption-recombination (HDDR) processed powders were examined. This microscope is capable of distinguishing nanometer-sized domain patterns. The coercivity of the hot-pressed HDDR magnet depends on the area of magnetization reversal during a reversal process. Magnetization reversal occurs simultaneously in a few grains in a low-coercivity magnet because the Nd-rich phase along the grain boundaries is absent. In contrast, in a high-coercivity magnet, magnetization reversal in a grain independently occurs because of an adequate Nd-rich phase along the grain boundaries. It follows that the high coercivity of an HDDR powder compacted by hot pressing is due to domain wall pinning at the grain boundaries. [ABSTRACT FROM AUTHOR]
- Published
- 2011
- Full Text
- View/download PDF
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