Publishing type：Research paper (scientific journal)  

<jats:title>Abstract</jats:title>
<jats:p>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.</jats:p>

DOI： 10.1088/1361-6587/ad074a

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0159175

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

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.

DOI： 10.1088/1361-6587/acfbf7

Other Link： https://iopscience.iop.org/article/10.1088/1361-6587/acfbf7/pdf

Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

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.

DOI： 10.1063/5.0150748

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.92.033501

Publishing type：Research paper (scientific journal)  

Recently, a new method that applies vector tomography to laser-induced fluorescence has been introduced, enabling the measurement of quasi-periodic two-dimensional velocity field in plasma [H. Arakawa et al., Plasma Fusion Res. 18, 1401032 (2023)]. In this study, we experimentally demonstrated this method in a linear magnetized plasma and presented the initial measurement results. The observed two-dimensional velocity field allowed the evaluation of Reynolds force and its energy transfer.

DOI： 10.1585/PFR.18.1201086

Scopus

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0122295

Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PORTFOLIO  

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.

DOI： 10.1038/s41598-022-23559-1

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC PLASMA SCIENCE & NUCLEAR FUSION RESEARCH  

Prediction of time evolution of multi-scale turbulence is performed by using Long-short term memory networks. The time series data is obtained by Langmuir probes in a linear magnetized plasma device, PANTA. The simultaneous prediction of high and low frequency components of turbulence is shown to be possible within several tens percent accuracy. The prediction accuracy depends on the initial network, which can be controlled by reducing the learning rate. (C) 2022 The Japan Society of Plasma Science and Nuclear Fusion Research

DOI： 10.1585/pfr.17.1201048

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>The velocity distribution function is a statistical description that connects particle kinetics and macroscopic parameters in many-body systems. Laser-induced fluorescence (LIF) spectroscopy is utilized to measure the local velocity distribution function in spatially inhomogeneous plasmas. However, the analytic form of such a function for the system of interest is not always clear under the intricate factors in non-equilibrium states. Here, we propose a novel approach to select the valid form of the velocity distribution function based on Bayesian statistics. We formulate the Bayesian inference of ion velocity distribution function and apply it to LIF spectra locally observed at several positions in a linear magnetized plasma. We demonstrate evaluating the spatial inhomogeneity by verifying each analytic form of the local velocity distribution function. Our approach is widely applicable to experimentally establish the velocity distribution function in plasmas and fluids, including gases and liquids.

DOI： 10.1038/s41598-021-00138-4

Other Link： https://www.nature.com/articles/s41598-021-00138-4

Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

DOI： 10.1063/5.0065887

Publishing type：Research paper (scientific journal)  

Interaction between drift wave turbulence and streamer flow in magnetized plasmas is investigated in the framework of wave-kinetic theory. By performing a wave-kinetic simulation of turbulence coupling with the streamer flow, the role of the phase-space dynamics of turbulence is clarified. Here, the phase-space consists of the real space and wavenumber space. As a consequence of the phase-space dynamics, the turbulence is trapped in the flow and propagates with the streamer. Because of the trapping, the asymmetry of the turbulence intensity with respect to the sign of the flow curvature appears. The trajectories of the turbulence wave-packet, which are the characteristic curves of the wave-kinetic solution, can be categorized by the trapping, passing, and scattering. Therefore, the obtained phase-space structure of the turbulence can be understood by their trajectories. The propagation direction and speed are found to be governed by the turbulence spectrum. The obtained characteristics can be explained by the theoretical analysis.

DOI： 10.1063/5.0059839

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC PLASMA SCIENCE & NUCLEAR FUSION RESEARCH  

Effects of electron temperature fluctuation on measurement of other fluctuations by using of Langmuir probe is investigated in PANTA plasma with finite temperature gradient, where significant electron temperature fluctuations are observed. The temperature fluctuation and its effects are evaluated from spatiotemporal structures of ion saturation current and floating potential, derived by conditional sampling/averaging technique. It is found that electron density fluctuation is in phase with ion saturation current fluctuation, but plasma potential fluctuation is anti-phase with the floating potential fluctuation. (C) 2021 The Japan Society of Plasma Science and Nuclear Fusion Research

DOI： 10.1585/pfr.16.1202081

Web of Science

Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.1088/1367-2630/abeffc

Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/5.0037352

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1088/1361-6587/abd980

Other Link： https://iopscience.iop.org/article/10.1088/1361-6587/abd980/pdf

Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.1088/1361-6587/abcb46

Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE RESEARCH  

A tomography system is installed as one of the diagnostics of new age to examine the three-dimensional characteristics of structure and dynamics including fluctuations of a linear magnetized helicon plasma. The system is composed of three sets of tomography components located at different axial positions. Each tomography component can measure the two-dimensional emission profile over the entire cross-section of plasma at different axial positions in a sufficient temporal scale to detect the fluctuations. The four-dimensional measurement including time and space successfully obtains the following three results that have never been found without three-dimensional measurement: (1) in the production phase, the plasma front propagates from the antenna toward the end plate with an ion acoustic velocity. (2) In the steady state, the plasma emission profile is inhomogeneous, and decreases along the axial direction in the presence of the azimuthal asymmetry. Furthermore, (3) in the steady state, the fluctuations should originate from a particular axial position located downward from the helicon antenna.

DOI： 10.1038/s41598-021-83191-3

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Emission intensity epsilon from plasmas is a complicated function of electron temperature T-e and electron density n(e). To understand the dependence of epsilon on T-e and n(e), an experimental comparison between tomography data (local epsilon of ArII) and Langmuir probe data (T-e and n(e)) in a linear plasma device PANTA is presented in this paper. In the comparison, the local emission intensity is modeled as epsilon(model) alpha T(e)(alpha)n(e)(2), and the model is validated by calculating both cross-correlation function and least squares of residuals between the fluctuations of epsilon and epsilon(model). The comparison reveals that the both methods provide alpha = 2.3-2.7, higher than 2 that of the exponent of n(e). This result confirms that dependence of T-e on epsilon is larger than that of n(e) when T-e is low, as in PANTA.

DOI： 10.7566/JPSJ.89.093501

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0007899

Language：English   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

In this paper, we evaluated electron temperature fluctuations by using a conditional sampling and a statistical phase detection method. These methods allow us to reconstruct probe I - V characteristics at each time delay; that is, we can extract electron temperature and ion saturation current fluctuations in high temporal resolution without complex electronic circuits. Our method was applied to two quasi-coherent fluctuations observed in a linear magnetized plasma device, PANTA. We observed spatiotemporal structures of electron temperature and ion saturation current fluctuations. The results revealed the amplitudes and phase relation between electron temperature and density fluctuations.

DOI： 10.1088/1361-6587/ab8132

Web of Science

Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.1088/1361-6587/ab471b

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC PLASMA SCIENCE & NUCLEAR FUSION RESEARCH  

A frequency comb sweep microwave reflectometer is developed in PANTA to realize multi-point electron density measurements with high temporal resolution. A radial profile of the electron density is successfully reconstructed from simultaneous 17-point measurements with 2-mu s temporal resolution. This result demonstrates the reliability and feasibility of the system for electron density profile diagnostics. (C) 2019 The Japan Society of Plasma Science and Nuclear Fusion Research

DOI： 10.1585/pfr.14.1201131

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC PLASMA SCIENCE & NUCLEAR FUSION RESEARCH  

We proposed a new method to evaluate particle flux in magnetized turbulent plasma. To extract fundamental fluctuation pattern, we used template method which is a kind of statistical conditional averaging technique. The method allows as not only to extract coherent patterns of density and potential fluctuations but also to evaluate time delay between them. We succeeded to calculate particle flux driven by fluctuation pattern and observed this pattern drives inward particle flux. Result obtained by the method is in good agreement with that obtained by a conventional method. (C) 2019 The Japan Society of Plasma Science and Nuclear Fusion Research

DOI： 10.1585/pfr.14.1402090

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5093218

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC PLASMA SCIENCE & NUCLEAR FUSION RESEARCH  

Spatiotemporal structure of fluctuation with 11 kHz excited in a linear magnetized plasma was observed in details by applying pattern recognition method based on statistical averaging. Statistical behaviors of instantaneous period of the fluctuation and the temporal behavior of the radial profile of the fluctuation are clarified. Two-dimensional structure of the fluctuation is reconstructed and distortion of the wave-front of the fluctuation was inferred. (C) 2018 The Japan Society of Plasma Science and Nuclear Fusion Research

DOI： 10.1585/pfr.13.3401105

Web of Science

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Language：Japanese