16 results on '"Nishizawa, T"'
Search Results
2. Nonlinear simulation of resistive drift waves in cylindrical magnetized plasmas in the presence of symmetry breaking particle source.
- Author
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Sasaki, M., Kasuya, N., Kawachi, Y., Kobayashi, T., Nishizawa, T., Arakawa, H., Yamada, T., and Fujisawa, A.
- Subjects
SYMMETRY breaking ,CYLINDRICAL plasmas ,PARTICLE symmetries - Abstract
Nonlinear fluid simulation of drift wave turbulence in the presence of symmetry breaking particle source is performed for the cylindrical magnetized plasmas. It is demonstrated that the symmetry breaking of the system directly affects the selection rule of structure formations, the streamers disappear, and the zonal flows are enhanced in the case with the symmetry breaking. The symmetry breaking is introduced in the flux-driven simulation by inducing the particle source whose amplitude depends on the azimuthal angle. The symmetry breaking mode is driven stationarily, and the nonlinear process of the drift waves is significantly modified. By scanning the amplitude of the symmetry breaking source, the structure formation processes are systematically investigated. In addition, the nonlinear forces of the turbulence show the relaxation of the density gradient and the drive of the perpendicular/parallel flows in a two-dimensional manner. [ABSTRACT FROM AUTHOR]
- Published
- 2023
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3. Characterizing the flow and turbulence structure near the last closed flux surface in L-mode plasmas of ASDEX Upgrade.
- Author
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Nishizawa, T., Manz, P., Grenfell, G., Griener, M., Wendler, D., Brida, D., Kriete, D. M., Dux, R., Kobayashi, T., and Sasaki, M.
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PLASMA turbulence , *FUSION reactors , *TURBULENCE , *WAVENUMBER , *THERMAL plasmas , *SPECTRAL energy distribution - Abstract
Since high density operation is advantageous for building an efficient fusion reactor, understanding the density limit in tokamaks has been seen as one of the most important issues. This paper reports a series of measurements around the last-closed flux surface (LCFS) in L-mode plasmas by using a thermal helium beam diagnostic. Fluctuation analysis has been employed to characterize the poloidal flow and the turbulence structure. A reversal of the poloidal flow in the scrape-off layer and concomitant cooling of the outer divertor plasma are observed as the density is raised. While, in the confined region, the change in the density barely affects the poloidal flow, a higher density shifts the fluctuation power spectral densities toward lower frequencies and wave numbers. The eddy tilting of this region is consistent with what is expected from the magnetic shear effect. A radially coherent low frequency mode appears in the case of the highest density investigated in this study ( n ¯ e / n e , GW = 0.51), and higher frequencies near the LCFS are modulated by this mode. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
4. Edge turbulence measurements in L-mode and I-mode at ASDEX Upgrade.
- Author
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Bielajew, R., Conway, G. D., Griener, M., Happel, T., Höfler, K., Howard, N. T., Hubbard, A. E., McCarthy, W., Molina Cabrera, P. A., Nishizawa, T., Rodriguez-Fernandez, P., Silvagni, D., Vanovac, B., Wendler, D., Yoo, C., and White, A. E.
- Subjects
PLASMA turbulence ,TURBULENCE ,ELECTRON emission ,ELECTRON configuration ,EDGES (Geometry) - Abstract
The I-mode confinement regime is promising for future reactor operation due to high energy confinement without high particle confinement. However, the role of edge turbulence in creating I-mode's beneficial transport properties is still unknown. New measurements of edge turbulence ( ρ pol = 0.9 − 1.0) in L-modes and I-modes at low and high densities at ASDEX Upgrade are presented in this paper. A high radial resolution correlation electron cyclotron emission radiometer measures the broadband turbulence throughout the L-mode and I-mode edge and pedestal. The weakly coherent mode (WCM) is measured in both L-mode and I-mode near the last closed flux surface with T
e fluctuation levels of 2.3%–4.2%, with a frequency shift between the two phases related to a deeper Er well in I-mode. An n e T e phase diagnostic captures a change of the WCM n e T e phase between L-mode and I-mode from − 171 ° to − 143 °. The thermal He beam diagnostic measures a WCM wavenumber range of −0.5 to −1.0 cm−1 . A low-frequency edge oscillation (LFEO) appears in the I-mode phase of these discharges and displays coupling to the WCM, but the LFEO does not appear in the L-mode phase. Linear gyrokinetic simulations of the outer core and pedestal top turbulence indicate that while the dominant turbulent modes in the outer core are ion directed and electrostatic, the turbulence becomes increasingly electron directed and electromagnetic with increasing radius. Collisionality is not found to impact characteristics of the L-mode and I-mode edge turbulence with respect to the presence of the WCM; however, the quality of global confinement decreases with collisionality. [ABSTRACT FROM AUTHOR]- Published
- 2022
- Full Text
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5. High-heat flux ball-pen probe head in ASDEX-Upgrade.
- Author
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Grenfell, G., Adamek, J., Komm, M., Brida, D., Conway, G. D., Manz, P., Tolias, P., Eich, T., Sestak, D., Herrmann, A., Nishizawa, T., and Stroth, U.
- Subjects
PLASMA temperature ,PLASMA potentials ,HEAT flux ,PLASMA boundary layers ,ELECTRON plasma ,PLASMA sheaths ,ELECTRON emission ,ELECTRON field emission - Abstract
A new high heat flux ball-pen probe head installed on the midplane manipulator is currently being used in ASDEX-Upgrade (AUG). The probe was designed to withstand high heat fluxes making possible the investigation of the plasma edge under harsh conditions, such as low power H-mode. Composed of seven pins (four Langmuir probes, mounted in two Mach probe pairs, and three ball-pen probes), the new probe head allows us to measure several plasma parameters simultaneously and with high temporal resolution. A novel method to correct the sheath potential dynamically accounting for the total secondary electron emission is introduced together with applications to obtain the electron temperature and plasma potential profiles. The total secondary electron emission yield is obtained from particle in cell simulations in AUG condition and probe realistic impact angle with respect to the magnetic field. Finally, the probe capability to investigate turbulence around the separatrix of AUG is discussed. [ABSTRACT FROM AUTHOR]
- Published
- 2022
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6. Linearized spectrum correlation analysis for thermal helium beam diagnostics.
- Author
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Nishizawa, T., Griener, M., Dux, R., Grenfell, G., Wendler, D., Kado, S., Manz, P., and Cavedon, M.
- Subjects
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STATISTICAL correlation , *SPECTRUM analysis , *THERMAL analysis , *ELECTRON density , *ELECTRON temperature - Abstract
We introduce a new correlation analysis technique for thermal helium beam (THB) diagnostics. Instead of directly evaluating line ratios from fluctuating time series, we apply arithmetic operations to all available He I lines and construct time series with desired dependencies on the plasma parameters. By cross-correlating those quantities and by evaluating ensemble averages, uncorrelated noise contributions can be removed. Through the synthetic data analysis, we demonstrate that the proposed analysis technique is capable of providing the power spectral densities of meaningful plasma parameters, such as the electron density and the electron temperature, even under low-photon-count conditions. In addition, we have applied this analysis technique to the experimental THB data obtained at the ASDEX Upgrade tokamak and successfully resolved the electron density and temperature fluctuations up to 90 kHz in a reactor relevant high power scenario. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
7. Plasma parameter profile inference from limited data utilizing second-order derivative priors and physic-based constraints.
- Author
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Nishizawa, T., Cavedon, M., Dux, R., Reimold, F., and Toussaint, U. von
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MONTE Carlo method , *MARKOV chain Monte Carlo , *LAXATIVES , *KRIGING , *GAUSSIAN processes , *PLASMA diagnostics - Abstract
A Bayesian framework has been used to improve the quality of inferred plasma parameter profiles. An integrated data analysis allows for coherent combinations of different diagnostics, and Gaussian process regression provides a reliable regularization process and systematic uncertainty estimation. In this paper, we propose a new profile inference framework that utilizes our prior knowledge about plasma physics, along with integrated data analysis and a Gaussian process. In order to facilitate the use of the Markov chain Monte Carlo sampling, we use a Gaussian process to define quantities corresponding to the second derivatives of the profiles. We validate the analysis technique by using a synthetic one-dimensional plasma, in which the transport properties are known and demonstrate that the proposed analysis technique can infer plasma parameter profiles from line-integrated measurements only. Furthermore, we can even infer unknown parameters in our physics models when our physics knowledge on the system is incomplete. This analysis framework is applicable to laboratory plasmas and provides a means to investigate plasma parameters, to which standard diagnostics are not directly sensitive. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
8. Direct measurements of the 3D plasma velocity in single-helical-axis RFP plasmas.
- Author
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Boguski, J., Nornberg, M. D., Gupta, U., McCollam, K. J., Almagri, A. F., Chapman, B. E., Craig, D., Nishizawa, T., Sarff, J. S., Sovinec, C. R., Terry, P. W., and Xing, Z. A.
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AXIAL flow ,SHEAR flow ,CHARGE exchange ,FLOW measurement ,VELOCITY measurements ,PLASMA sheaths - Abstract
The first local velocity measurements of helical equilibrium plasmas in the Reversed Field Pinch (RFP) Single Helical Axis (SHAx) state using a Charge Exchange Recombination Spectroscopy (CHERS) diagnostic are presented. Measurements show strong axisymmetric and non-axisymmetric flow, with n = 5 components of flow related to the (m , n) = (1 , 5) dominant magnetic mode on the order of the axisymmetric flow in certain regions of the plasma, as well as significant n > 5 flow. Flow measurements are compared with NIMROD simulations of visco-resistive, single-fluid MHD in toroidal and cylindrical geometries with limited axial periodicity. Both measurements and the simulation with toroidal geometry show stronger inboard flows relative to the outboard flows, which is attributed to the toroidal geometry of the device. In the experiment, the n = 5 component of flow is phase shifted from the reconnection-like flow pattern observed in the single-fluid simulations, possibly due to decoupling of the ion and electron fluids over much of the plasma. Finally, the strength of the helical angular flow shear relative to the critical shear necessary to disrupt nonlinear coupling between tearing modes is calculated around the helical magnetic axis. The shear in the measured flow is on the order of the theoretical critical threshold needed to nonlinearly decouple modes, but the measurement uncertainty in the gradient of the flow is large. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
9. Intrinsic flow and tearing mode rotation in the RFP during improved confinement.
- Author
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Craig, D., Tan, E. H., Schott, B., Anderson, J. K., Boguski, J., Den Hartog, D. J., Nishizawa, T., Nornberg, M. D., and Xing, Z. A.
- Subjects
ROTATIONAL motion ,CHARGE exchange ,ION mobility ,PLASMA confinement ,PLASMA instabilities ,TOROIDAL plasma - Abstract
We use charge exchange recombination spectroscopy to make the first localized measurements of impurity ion flow velocity profiles in the reversed field pinch. Measurements in improved confinement plasmas reveal an intrinsic flow profile that is peaked on the axis and mostly parallel to the equilibrium magnetic field. The toroidal flow decreases in time at off-axis locations where tearing modes are resonant, giving rise to a highly sheared flow profile near the axis. The tearing mode phase velocity correlates strongly with toroidal flow near the resonant surface and weakly with flow in other locations, providing an opportunity to verify the commonly held assumption that the plasma and mode move together at the resonant surface. Mechanisms for the observed momentum loss during the improved confinement period are evaluated, and it is found that eddy currents in the conducting shell caused by the rotation of the dominant tearing mode dominate over other losses. [ABSTRACT FROM AUTHOR]
- Published
- 2019
- Full Text
- View/download PDF
10. Linearized spectrum correlation analysis for line emission measurements.
- Author
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Nishizawa, T., Nornberg, M. D., Den Hartog, D. J., and Sarff, J. S.
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PARTICLES , *PLASMA gases , *DOPPLER effect , *FLUCTUATIONS (Physics) , *PHOTONS - Abstract
A new spectral analysis method, Linearized Spectrum Correlation Analysis (LSCA), for charge exchange and passive ion Doppler spectroscopy is introduced to provide a means of measuring fast spectral line shape changes associated with ion-scale micro-instabilities. This analysis method is designed to resolve the fluctuations in the emission line shape from a stationary ion-scale wave. The method linearizes the fluctuations around a time-averaged line shape (e.g., Gaussian) and subdivides the spectral output channels into two sets to reduce contributions from uncorrelated fluctuations without averaging over the fast time dynamics. In principle, small fluctuations in the parameters used for a line shape model can be measured by evaluating the cross spectrum between different channel groupings to isolate a particular fluctuating quantity. High-frequency ion velocity measurements (100-200 kHz) were made by using this method. We also conducted simulations to compare LSCA with a moment analysis technique under a low photon count condition. Both experimental and synthetic measurements demonstrate the effectiveness of LSCA. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
- View/download PDF
11. Aerosol Retrieval from Dual‐wavelength Polarization Lidar Measurements over Tropical Pacific Ocean and Validation of a Global Aerosol Transport Model
- Author
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Nishizawa, T., primary, Sugimoto, N., additional, Matsui, I., additional, Shimizu, A., additional, Takemura, T., additional, and Okamoto, H., additional
- Published
- 2009
- Full Text
- View/download PDF
12. Development of aerosol and cloud retrieval algorithms using ATLID and MSI data of EarthCARE.
- Author
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Nishizawa, T., Higurashi, A., Sugimoto, N., Matsui, I., Shimizu, A., and Okamoto, H.
- Subjects
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ATMOSPHERIC radiation , *ALGORITHMS , *NATURAL satellite atmospheres , *CLOUD physics , *OPTICAL properties of atmospheric aerosols , *BACKSCATTERING , *PARTICLE size distribution , *LIGHT absorption - Abstract
EarthCARE (Earth Clouds, Aerosols and Radiation Explorer) is a joint Japanese (JAXA)-European (ESA) satellite observation mission for understanding the interaction between cloud, aerosol, and radiation processes in the earth climate. Four sensors of cloud profiling radar (CPR), multi-spectral imager (MSI), broadband radiometer (BBR), and high spectral resolution lidar (ATLID) are installed on the EarthCARE satellite. We develop an algorithm to derive aerosol and cloud optical properties and their vertical distributions using all the ATLID level 1 data of Mie copol (βmie,∥), Mie crosspol (βmie,⊥), and Rayleigh (βray) attenuated backscatter coefficients at 355nm (ATLID algorithm). The developed algorithm estimates extinction coefficients (α), backscatter coefficients (β) and depolarization ratio (δ) of particles (aerosols and clouds) without prescribing a particle lidar ratio (S=α/β), using a popular direct method. This algorithm identifies molecule-rich, aerosol-rich, or cloud-rich slab layers using the ratio of βmie and βray; it also classifies aerosol type (e.g., dust, maritime) and cloud type (e.g., water-droplet, ice-crystal) using the derived α, β, and δ by the threshold methods developed in this study. Planetary boundary layer (PBL) height is retrieved from the gradient of the ratio of βmie and βray. Furthermore, the algorithm retrieves extinction coefficients for dust, sea-salt, black carbon, and water-soluble particles using the difference in depolarization and light absorption properties of the aerosol components from the retrieved α, β, and δ. With this method, we assume an external mixture of aerosol components and prescribe the size distributions, refractive indexes, and particle shapes for the aerosol components. Water-soluble particles are defined as small particles with weak light absorption, consisting of sulfates, nitrates, and organic water-soluble substances. We also develop an aerosol retrieval algorithm using both the ATLID and MSI data (ATLID+MSI algorithm). The developed algorithm retrieves vertically mean mode-radii for water-soluble particles and dust as well as the extinction coefficients for the four aerosol components from the radiances at 670 and 865nm of MSI level 1 data and the derived α, β, and δ data. With this method, we use the spectral property of aerosols sensitive to particle size, as well as the depolarization and light absorption properties. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
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13. Upgrading a high-throughput spectrometer for high-frequency (<400 kHz) measurements.
- Author
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Nishizawa, T., Nornberg, M. D., Den Hartog, D. J., and Craig, D.
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SPECTROMETERS , *CHARGE exchange , *FLUCTUATIONS (Physics) , *PHOTONS , *SIGNAL processing - Abstract
The upgraded spectrometer used for charge exchange recombination spectroscopy on the Madison Symmetric Torus resolves emission fluctuations up to 400 kHz. The transimpedance amplifier's cutoff frequency was increased based upon simulations comparing the change in the measured photon counts for time-dynamic signals. We modeled each signal-processing stage of the diagnostic and scanned the filtering frequency to quantify the uncertainty in the photon counting rate. This modeling showed that uncertainties can be calculated based on assuming each amplification stage is a Poisson process and by calibrating the photon counting rate with a DC light source to address additional variation. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
14. Absolute wavelength calibration of a Doppler spectrometer with a custom Fabry-Perot optical system.
- Author
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Baltzer, M. M., Craig, D., Den Hartog, D. J., Nishizawa, T., and Nornberg, M. D.
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WAVELENGTHS ,CALIBRATION ,DOPPLER free spectroscopy ,HIGH resolution laser spectroscopy ,FABRY-Perot interferometers - Abstract
An Ion Doppler Spectrometer (IDS) is used for fast measurements of C VI line emission (343.4 nm) in the Madison Symmetric Torus. Absolutely calibrated flow measurements are difficult because the IDS records data within 0.25 nm of the line. Commercial calibration lamps do not produce lines in this narrow range. A light source using an ultraviolet LED and etalon was designed to provide a fiducial marker 0.08 nm wide. The light is coupled into the IDS at f/4, and a holographic diffuser increases homogeneity of the final image. Random and systematic errors in data analysis were assessed. The calibration is accurate to 0.003 nm, allowing for flow measurements accurate to 3 km/s. This calibration is superior to the previous method which used a time-averaged measurement along a chord believed to have zero net Doppler shift. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
15. Fabrication and characterization of n-type modulation-doped (Cd,Cr)Te quantum well.
- Author
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Takano, F., Nishizawa, T., Kinjo, H., Lee, J. W., Kuroda, S., Ofuchi, H., Imanaka, Y., Takamasu, T., Kido, G., Takita, K., and Akinaga, H.
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QUANTUM wells , *DILUTED magnetic semiconductors , *FERROMAGNETIC materials , *CADMIUM compounds , *CHROMIUM compounds , *TELLURIUM compounds , *ELECTRONS - Abstract
A novel ferromagnetic II-VI diluted magnetic semiconductor, (Cd,Cr)Te, has been combined with a two-dimensional electron system. The anomalous Hall effect accompanied by the hysteresis loop was successfully observed in the Hall measurement. This fact shows the ferromagnetic exchange interaction between carrier and magnetic moment was realized in the present system. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
- View/download PDF
16. Development of a multi-channel capacitive probe for electric field measurements with fine spatial and high time resolution.
- Author
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Nishizawa T, Almagri AF, Goodman W, Ohshima S, and Sarff JS
- Abstract
A capacitive probe [Tan et al. , Rev. Sci. Instrum. 88 , 023502 (2017)] is one of a few diagnostics that is directly sensitive to the plasma potential. Using this diagnostic technique, a Multi-channel Linear Capacitive Probe (MLCP) is developed for turbulence measurements. The MLCP has 10 spatial channels and provides 9 points of radial electric field measurements simultaneously with a spatial step of 7 mm. A new readout circuit and a correction technique for low frequency attenuation are also developed to achieve the required spatial and time resolution. A performance test of the MLCP using a reversed field pinch plasma confirms that the MLCP resolves sub-centimeter structures of the equilibrium radial electric field profile and fluctuations up to 680 kHz.
- Published
- 2018
- Full Text
- View/download PDF
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