九州大学 研究者情報
論文一覧
西澤 敬之(にしざわたかし) データ更新日:2024.06.03

助教 /  応用力学研究所 核融合力学部門


原著論文
1. T Nishizawa, S Tokuda, T Kobayashi, K Tanaka, H Funaba, I Yamada, Y Takemura, T Tokuzawa, R Yasuhara, H Uehara, K Ida, H Takahashi, Y Kawachi, S Inagaki, S Kado, M Sasaki, A Fujisawa, Estimation of plasma parameter profiles and their derivatives from linear observations by using Gaussian processes, Plasma Physics and Controlled Fusion, 10.1088/1361-6587/ad074a, 65, 12, 125006-125006, 2023.11, Abstract

Gaussian process regression (GPR) has been utilized to provide fast and robust estimates of plasma parameter profiles and their derivatives. We present an alternative GPR technique that performs profile regression analyses based on arbitrary linear observations. This method takes into account finite spatial resolution of diagnostics by introducing a sensitivity matrix. In addition, the profiles of interest and their derivatives can be estimated in the form of a multivariate normal distribution even when only integrated quantities are observable. We show that this GPR provides meaningful measurements of the electron density profile and its derivative in a toroidal plasma by utilizing only ten line-integrated data points given that the locations of magnetic flux surfaces are known..
2. Daiki NISHIMURA, Akihide FUJISAWA, Yoshihiko NAGASHIMA, Chanho MOON, Kotaro YAMASAKI, Takashi NISHIZAWA, Taiki KOBAYASHI, Tatsuya KOBAYASHI, Akihiro SHIMIZU, Tokihiko TOKUZAWA, Velocity Field Estimation using Tomography in a Cylindrical Plasma, Plasma and Fusion Research, 10.1585/pfr.19.1201005, 19, 1201005-1201005, 2024.01.
3. Yuichi Kawachi, Makoto Sasaki, Yusuke Kosuga, Kenichiro Terasaka, Takashi Nishizawa, Takuma Yamada, Naohiro Kasuya, Chanho Moon, Shigeru Inagaki, Spatiotemporal dynamics of high-wavenumber turbulence in a basic laboratory plasma, Scientific Reports, 10.1038/s41598-022-23559-1, 12, 1, 2022.12, Abstract

High-spatial resolution observation of high-wavenumber broadband turbulence is achieved by controlling the magnetic field to be relatively low and measuring with a azimuthally arranged multi-channel Langmuir array in a basic laboratory plasma. The observed turbulence consists of narrowband low-frequency fluctuations and broadband high-frequency turbulent fluctuations. The low-frequency fluctuations have a frequency of about 0.7 times the ion cyclotron frequency and a spatial scale of 1/10 of the ion inertial scale. In comparison, high-frequency fluctuations have a higher frequency than the ion cyclotron frequency and spatial scales of 1/10–1/40 of the ion inertial scale. Two-dimensional correlation analysis evaluates the spatial and temporal correlation lengths and reveals that the high-wavenumber broadband fluctuations have turbulent characteristics. The measurements give us further understanding of small scale turbulence in space and fusion plasmas..
4. Yuichi Kawachi, Makoto Sasaki, Takashi Nishizawa, Yusuke Kosuga, Kenichiro Terasaka, Shigeru Inagaki, Takuma Yamada, Naohiro Kasuya, Chanho Moon, Yoshihiko Nagashima, Akihide Fujisawa, Transition between continuous and discrete spectra of high-wavenumber turbulence in neutral gas pressure scan experiments on a linear magnetized plasma, Plasma Physics and Controlled Fusion, 10.1088/1361-6587/acfbf7, 65, 11, 115001-115001, 2023.09, Abstract

Spectral transition between continuous and discrete spectra has been observed in high-wavenumber turbulence excitation experiments. The transition appeared in neutral gas scanning, which shows that continuous spectra dominate at low gas pressures and discrete spectra dominate at high gas pressures. Both spectra have well smaller spatial scale than the ion effective Larmor radius. The discrete spectra have 6–11 peaks and maximum peak values at 4th or 5th peak. Correlation length analysis reveals that the continuous component is turbulent and the discrete component is coherent. The bicoherence analysis shows that the turbulent spectra have finite nonlinear coupling, whereas the discrete spectra have no corresponding coupling..
5. Takashi Nishizawa, Simultaneous inference of multiple plasma parameter profiles by utilizing transport properties, Contributions to Plasma Physics, 10.1002/ctpp.202200150, 63, 5-6, 2023.03, Abstract

Numerical techniques for the computation of posterior distribution ease restrictions on inference models, and multiple plasma parameters can be simultaneously inferred with detailed modelling of uncertainties. In addition, data that depend on multiple parameters in a complex manner can also be utilized. This paper discusses frameworks that further expand the inference capability by including transport properties in plasmas. A prior belief based on a transport equation helps to localize the posterior distribution, and when ample diagnostic capability is available, even unknown parameters in the transport model can be estimated. Furthermore, these transport parameters can be directly inferred when the profiles of the measurable plasma parameters can be easily calculated for the given transport properties..
6. D. Nishimura, A. Fujisawa, K. Yamasaki, Y. Nagashima, C. Moon, T. Nishizawa, T.-K. Kobayashi, T. Kobayashi, A. Shimizu, T. Tokuzawa, T. Ido, Rotational movement analysis based on Fourier-rectangular function transform for cylindrical plasma, Journal of Applied Physics, 10.1063/5.0165318, 134, 11, 2023.09, This article presents a method to estimate the rotational velocity of a cylindrical plasma from its two-dimensional images by an extended use of the Fourier-rectangular function transform, which was proposed to analyze the structure and dynamics of a cylindrical plasma [K. Yamasaki etal., J. Appl. Phys. 126, 043304 (2019)]. The proposed method is applied to tomography images of plasmas produced in a linear cylindrical device and succeeds in obtaining the radial distribution of rotational velocity and its fluctuations, providing an interesting finding, that is, the existence of flow modulation associated with m=1 mode fluctuations..
7. M. Sasaki, N. Kasuya, Y. Kawachi, T. Kobayashi, T. Nishizawa, H. Arakawa, T. Yamada, A. Fujisawa, Nonlinear simulation of resistive drift waves in cylindrical magnetized plasmas in the presence of symmetry breaking particle source, Physics of Plasmas, 10.1063/5.0150748, 30, 8, 2023.08, 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..
8. T. Nishizawa, Y. Fujii, H. Kanda, M. Kaneta, Y. Kasai, J. Kusaka, K. Maeda, S. Nagao, S. N. Nakamura, K. Tsukada, F. Yamamoto, Development of a Fast Timing Counter with a Monolithic MPPC Array, IEEE Transactions on Nuclear Science, 10.1109/tns.2014.2321657, 61, 3, 1278-1283, 2014.06.
9. M. M. Baltzer, D. Craig, D. J. Den Hartog, T. Nishizawa, M. D. Nornberg, Absolute wavelength calibration of a Doppler spectrometer with a custom Fabry-Perot optical system, Review of Scientific Instruments, 10.1063/1.4955491, 87, 11, 11E509-11E509, 2016.11.
10. T. Nishizawa, M. D. Nornberg, D. J. Den Hartog, D. Craig, Upgrading a high-throughput spectrometer for high-frequency (Review of Scientific Instruments, 10.1063/1.4960073, 87, 11, 11E530-11E530, 2016.11.
11. T. Nishizawa, M. D. Nornberg, D. J. Den Hartog, J. S. Sarff, Linearized spectrum correlation analysis for line emission measurements, Review of Scientific Instruments, 10.1063/1.4999450, 88, 8, 083513-083513, 2017.08.
12. R. Fridström, B. E. Chapman, A. F. Almagri, L. Frassinetti, P. R. Brunsell, T. Nishizawa, J. S. Sarff, Dependence of Perpendicular Viscosity on Magnetic Fluctuations in a Stochastic Topology, Physical Review Letters, 10.1103/physrevlett.120.225002, 120, 22, 2018.05.
13. T. Nishizawa, A. F. Almagri, W. Goodman, S. Ohshima, J. S. Sarff, Development of a multi-channel capacitive probe for electric field measurements with fine spatial and high time resolution, Review of Scientific Instruments, 10.1063/1.5035093, 89, 10, 10J118-10J118, 2018.10.
14. T. Nishizawa, M. D. Nornberg, J. Boguski, D. J. Den Hartog, J. S. Sarff, Z. R. Williams, Z. A. Xing, D. Craig, Measurements of Impurity Transport Due to Drift-Wave Turbulence in a Toroidal Plasma, Physical Review Letters, 10.1103/physrevlett.121.165002, 121, 16, 2018.10.
15. T. Nishizawa, A. F. Almagri, J. K. Anderson, W. Goodman, M. J. Pueschel, M. D. Nornberg, S. Ohshima, J. S. Sarff, P. W. Terry, Z. R. Williams, Direct Measurement of a Toroidally Directed Zonal Flow in a Toroidal Plasma, Physical Review Letters, 10.1103/physrevlett.122.105001, 122, 10, 2019.03.
16. Zichao Li, K J McCollam, T Nishizawa, E Parke, J S Sarff, Z A Xing, Hong Li, Wandong Liu, Weixing Ding, Effects of oscillating poloidal current drive on magnetic relaxation in the Madison Symmetric Torus reversed-field pinch, Plasma Physics and Controlled Fusion, 10.1088/1361-6587/aaf9e0, 61, 4, 045004-045004, 2019.04.
17. T Nishizawa, M Cavedon, F Reimold, R Dux, D Brida, H Wu, Bayesian inference of particle source and sink in a closed-divertor using Balmer line spectroscopy, Plasma Physics and Controlled Fusion, 10.1088/1361-6587/ab9732, 62, 8, 085005-085005, 2020.08, Abstract

A new analysis technique for Balmer line spectroscopy that enables recombination rate (particle sink) and ionization rate (particle source) inference in a closed divertor configuration is reported. Bayesian inference is employed to systematically utilize all available information from multiple Balmer lines and constrain parameter ranges by using prior knowledge about plasmas. While a closed-divertor facilitates detachment, neutral plugging typically leads to large spatial variations in plasma parameters. A forward model is developed to take into account non-uniformity in the plasma parameters and applied to test data generated by divertor plasma simulations. It is shown that the forward model robustly provides particle source and sink inference over a wide parameter range. In addition, the precision improves as more Balmer lines are resolved simultaneously. The new analysis technique is also applied to an L-mode ASDEX Upgrade plasma in the high-recycling regime. The inferred quantities and their profiles are consistent with the expectations of a high-recycling divertor plasma. The further insight into the detachment physics will be provided by using this new analysis technique..
18. Z.R. Williams, M.J. Pueschel, P.W. Terry, T. Nishizawa, D.M. Kriete, M.D. Nornberg, J.S. Sarff, G.R. McKee, D.M. Orlov, S.H. Nogami, Impact of resonant magnetic perturbations on zonal flows and microturbulence, Nuclear Fusion, 10.1088/1741-4326/ab9be7, 60, 9, 096004-096004, 2020.09.
19. J. Boguski, M. D. Nornberg, U. Gupta, K. J. McCollam, A. F. Almagri, B. E. Chapman, D. Craig, T. Nishizawa, J. S. Sarff, C. R. Sovinec, P. W. Terry, Z. A. Xing, Direct measurements of the 3D plasma velocity in single-helical-axis RFP plasmas, Physics of Plasmas, 10.1063/5.0025696, 28, 1, 012510-012510, 2021.01.
20. T. Nishizawa, M. Cavedon, R. Dux, F. Reimold, U. von Toussaint, Plasma parameter profile inference from limited data utilizing second-order derivative priors and physic-based constraints, Physics of Plasmas, 10.1063/5.0039011, 28, 3, 032504-032504, 2021.03.
21. T Kobayashi, T Nishizawa, M Sasaki, M Yoshinuma, K Ida, Method for estimating the frequency-wavenumber resolved power spectrum density using the maximum entropy method for limited spatial points, Plasma Physics and Controlled Fusion, 10.1088/1361-6587/abe024, 63, 4, 045011-045011, 2021.04, Abstract

A combination of the Fourier transform and the maximum entropy method for estimating the frequency-wavenumber resolved power spectrum density is proposed. After illustrating the physical insight of the maximum entropy method by using synthetic test data, capability of the proposed method is tested using numerical simulation data. The method is also applied to experimental data obtained by the beam emission spectroscopy in the Large Helical Device. All of those examinations show that the proposed method provides more plausible results than conventional methods when the available spatial points are limited..
22. T. Nishizawa, M. Griener, R. Dux, G. Grenfell, D. Wendler, S. Kado, P. Manz, M. Cavedon, Linearized spectrum correlation analysis for thermal helium beam diagnostics, Review of Scientific Instruments, 10.1063/5.0062436, 92, 10, 103501-103501, 2021.10.
23. G. Grenfell, J. Adamek, M. Komm, D. Brida, G. D. Conway, P. Manz, P. Tolias, T. Eich, D. Sestak, A. Herrmann, T. Nishizawa, U. Stroth, High-heat flux ball-pen probe head in ASDEX-Upgrade, Review of Scientific Instruments, 10.1063/5.0075846, 93, 2, 023507-023507, 2022.02.
24. R. Bielajew, G. D. Conway, M. Griener, T. Happel, K. Höfler, N. T. Howard, A. E. Hubbard, W. McCarthy, P. A. Molina Cabrera, T. Nishizawa, P. Rodriguez-Fernandez, D. Silvagni, B. Vanovac, D. Wendler, C. Yoo, A. E. White, Edge turbulence measurements in L-mode and I-mode at ASDEX Upgrade, Physics of Plasmas, 10.1063/5.0088062, 29, 5, 052504-052504, 2022.05, 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 ([Formula: see text]) 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 Te 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 [Formula: see text] phase diagnostic captures a change of the WCM [Formula: see text] phase between L-mode and I-mode from [Formula: see text] to [Formula: see text]. 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..
25. M. Cavedon, B. Kurzan, M. Bernert, D. Brida, R. Dux, M. Griener, S. Henderson, E. Huett, T. Nishizawa, T. Lunt, O. Pan, U. Stroth, M. Wischmeier, E. Wolfrum, the ASDEX Upgrade Team, Experimental investigation of L- and H-mode detachment via the divertor Thomson scattering at ASDEX Upgrade, Nuclear Fusion, 10.1088/1741-4326/ac6071, 62, 6, 066027-066027, 2022.06, Abstract

The capabilities of the newly installed divertor Thomson Scattering (DTS) diagnostic (Kurzan et al 2021 J. Inst.16 C09012) at ASDEX Upgrade (AUG) have been demonstrated by measuring 2D electron densities ne and temperatures Te from attached to fully detached divertor conditions in L- and H-mode. The collected dataset is a breakthrough for divertor studies at AUG in which such measurements have been so far missing. Besides highlighting the strengths and limits of the DTS system, this work provides confirmations and new insights into detachment physics. The transition between partial and pronounced detachment correlates with a 2D redistribution of the electron density from the inner to the outer divertor in both L- and H-mode. In pronounced detachment, a strong parallel pressure gradient could be confirmed throughout the complete SOL. Finally, measurements of ne and Te of a stable X-point radiator revealed local temperatures as low as 1 eV within the confined plasma, whereas the pressure is shown to be constant on closed field lines..
26. T. Nishizawa, P. Manz, G. Grenfell, M. Griener, D. Wendler, D. Brida, D. M. Kriete, R. Dux, T. Kobayashi, M. Sasaki, Characterizing the flow and turbulence structure near the last closed flux surface in L-mode plasmas of ASDEX Upgrade, Physics of Plasmas, 10.1063/5.0098294, 29, 7, 072304-072304, 2022.07, 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 ([Formula: see text] = 0.51), and higher frequencies near the LCFS are modulated by this mode..
27. T. Nishizawa, R. Dux, R.M. McDermott, F. Sciortino, M. Cavedon, C. Schuster, E. Wolfrum, U. von Toussaint, A.Jansen Van Vuuren, D.J. Cruz-Zabala, P. Cano-Megias, C. Moon, the ASDEX Upgrade Team, Non-parametric inference of impurity transport coefficients in the ASDEX Upgrade tokamak, Nuclear Fusion, 10.1088/1741-4326/ac60e8, 62, 7, 076021-076021, 2022.07, Abstract

We present a non-parametric inference of impurity transport coefficients by using charge exchange recombination spectroscopy measurements of Ne X, Ne VIII, O VIII, and C VI lines. Due to their close atomic numbers, neon, oxygen and carbon impurity ions are assumed to have the same diffusion coefficient D and convection velocity v. Unlike conventional techniques that modulate or perturb the impurity contents, we employ a quasi-stationary plasma with static impurity profiles. Since the ratio of v to D only describes the equilibrated profile of the sum of all impurity charge states, steady-state measurements can still decouple D and v if different charge states are simultaneously observed. We have formulated a non-parametric analysis framework based on the Bayesian probability theory and conducted transport coefficient measurements for a Type III ELMy H-mode plasma at ASDEX Upgrade. The charge exchange reactions with the background neutrals, which are known to affect the impurity charge state balance, are taken into account by introducing additional free parameters. While D at the pedestal is close to the neoclassical level ($−2), a large diffusion coefficient and a strong outward convection are inferred right inside the pedestal top..
28. G. Grenfell, P. Manz, G.D. Conway, T. Eich, J. Adamek, D. Brida, M. Komm, T. Nishizawa, M. Griener, B. Tal, U. Stroth, Turbulence in the near scrape-off layer towards the L-mode density limit in ASDEX-Upgrade, Nuclear Materials and Energy, 10.1016/j.nme.2022.101277, 33, 101277-101277, 2022.10.

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