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It is demonstrated that the zonal flow is controlled by the parallel flow in the cylindrical plasmas. A three-dimensional turbulence simulation is performed, based on the reduced fluid model. The background parallel flow is applied by introducing the time independent parallel momentum source. Changing the magnitude of the momentum source, the behavior of the zonal flow is investigated. The drift wave turbulence is affected by the parallel flow, and it shows the spatial competition between turbulence modes. The spatial competition appears/disappears abruptly, depending on the intensity of the parallel momentum source. As a consequence of the transition between the turbulence state, the radial profile of the turbulence drastically changes, which leads to the change of the turbulence force to drive the zonal flow. In this way, the parallel flow indirectly affects the zonal flow through the deformation of the turbulence., source:https://doi.org/10.1585/pfr.14.1401161

In order to observe spatial profiles of energetic particle driven modes, measurement signal of Heavy Ion Beam Probe (HIBP) is formulated theoretically, focusing on effects of the fast ion density fluctuation and attenuation of the injected beam (line-integral effect). Obtained formula is applied to the density fluctuation measurement of energetic-particle driven geodesic acoustic modes (EGAMs). Intensity of the fluctuation obtained by the HIBP can be up-down asymmetric in the poloidal cross-section due to the effect of the fast ion density fluctuation, when the electron loss cross-section of HIBP due to fast ions is comparable to that due to electrons. A possibility to measure the fast ion density by HIBP is discussed, and the experimental guideline for the specification of the resonance which drives the EGAM is presented., source:https://doi.org/10.1585/pfr.13.3403040

Changing the experimental conditions such as the neutral pressure and the magnetic field strength provides a drastic change in the turbulence states in linear magnetized helicon plasmas. In order to define the turbulence states and their occurrence region a throughout parameter scan in the two-dimensional parameter space was performed. The classification of the turbulence states is carried out phenomenologically based on the turbulence spectrum and the waveform., source:https://doi.org/10.1585/pfr.12.1401019, identifier:0000-0001-5669-1937

Multiple free energy sources for instabilities coexist in magnetized plasmas with density gradient and velocity shear. Linear stabilities are investigated, and the mutual relation between resistive drift wave, D'Angelo mode and flute mode is systematically clarified. By evaluating the linear growth rates, dominant instability is categorized in a parameter space. The parallel wavenumber spectrum could be used as a guideline for the identification of the instabilities., source:https://doi.org/10.1585/pfr.12.1401042

A ka-band multi-channel Doppler reflectometer system was constructed for the Large Helical Device (LHD) using a comb frequency generator as a source. A filter bank system is utilized for precise quadrature phase detection, and preliminary back-scattered waves were obtained in LHD plasma experiments. In addition, a direct digital signal acquisition system was successfully demonstrated for providing a greater number of multi-channel measurements., source:https://doi.org/10.1585/pfr.9.1402149, identifier:0000-0001-5473-2109

By employing a new method for studying dynamical transport, hysteresis in the flux-gradient relation was recently discovered for modulated heating. In this paper, the new method is compared to the conventional heat-pulse propagation method. We also demonstrate integration of results from the new method with the power balance thermal conductivity., source:https://doi.org/10.1585/pfr.8.1202172

A new method is presented for identifying waveforms of fluctuations in turbulent plasmas. The method is based on heartbeat analysis in which the convolution of a waveform is obtained by employing the phase tracking method. Phase tracking is performed by correlating raw time-series data with a template waveform; the template is evaluated through iteration procedure. The method is applied to fluctuations in a PANTA plasma, and the nonlinear waveform and its distribution of periods are obtained., source:https://doi.org/10.1585/pfr.9.1201016

We have used the convolution method to obtain higher harmonics of the temperature perturbation in a heating modulation experiment on LHD plasma. In comparison with predictions based on the diffusive model, amplitudes of higher harmonics (such as the seventh) decayed much more slowly as they propagated radially. Changes in the time derivative of temperature at the onset and turning-off of heating power remained much sharper after radial propagation than those predicted by the diffusion model., source:https://doi.org/10.1585/pfr.8.1202173

We propose the use of a microwave comb for measuring the density profile as a continuous function of radius with high temporal resolution. The analysis focuses on phase-sensitive convolution of the reflected waves: convolution of pulses can eliminate statistical noise. The time resolution for constructing the density profile is also discussed., source:https://doi.org/10.1585/pfr.8.1201171

To study the nonlinear saturation mechanism in turbulent plasmas, we plan to obseve ion temperature gradient (ITG) modes in the Plasma Assembly for Nonlinear Turbulence Analysis (PANTA). Linear growth rates of ITG modes in the linear device are analyzed to determine their excitation conditions using a three-field fluid model to describe the ion motion. Parameter scans of the linear growth rate show the threshold for ITG mode excitation, and the typical plasma parameters of the PANTA are found in the unstable domain., source:https://doi.org/10.1585/pfr.8.2403133

Turbulence Diagnostic Simulator is an assembly of simulation codes to clarify the formation mechanism of turbulent structures by numerical diagnostics in magnetically confined plasmas. Global simulations are carried out using a reduced MHD model of drift-interchange mode in helical plasmas, and time series data of 3-D fluctuation fields are produced. It includes localized modes in their rational surfaces, and broad modes spread in the radial direction. Magnitudes of nonlinear couplings from the convective derivative are evaluated in the nonlinear saturated states. The radial profile shows that there exist strong mode excitation near the center, various modes and nonlinear couplings with higher m modes in the middle radius, small number of propagating modes near the edge, which contribute to the pressure profile modification. For the detection of the different features, combination of several diagnostics is necessary., source:https://doi.org/10.1585/pfr.8.2403070, identifier:0000-0001-8210-5700

A nonlinear simulation of resistive drift wave turbulence in a cylindrical plasma is carried out. Long time evolution of turbulence with formation of a zonal flow is obtained for more than 1000 times of the typical drift wave period, which is sufficient for statistical analyses. Dynamical particle and momentum balance for the formation of the mean reveals that the radial turbulent fluxes are dominant contributors for the evolution of fluctuations. The particle flux is found to precede the momentum flux for 0.4 times of the typical drift wave period with large temporal variance. The analyses of the time series data of 3-D fields give the understanding of the dynamical structural formation mechanism., source:https://doi.org/10.1585/pfr.8.2401113, identifier:0000-0001-6835-1569

The nonlinear dispersion relation of the geodesic acoustic modes (GAMs) is investigated for tokamaks with a high safety factor and low magnetic shear. We focus on the Reynolds stress as a nonlinearity, which is truncated at the third order of the GAM amplitude. The real frequency of the GAM is modified according to the phase of the nonlinear force acting on the GAM, which depends on the turbulence decorrelation rate. The nonlinear frequency shift is much larger than that from the finite gyro-radius effects in the linear theory, when the poloidal turbulent E × B velocities are comparable to the diamagnetic drift velocity. Under such circumstances, the group velocity is strongly enhanced and becomes comparable with the radial phase velocity. In addition, the magnitude of the nonlinear effects is also evaluated using experimental parameters., source:https://doi.org/10.1585/pfr.8.1403010, identifier:0000-0002-7696-6405

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., source:Physics of Plasmas 23, 102311 (2016); https://doi.org/10.1063/1.4965915, source:https://doi.org/10.1063/1.4965915, identifier:0000-0001-5669-1937

The spatio-temporal dynamics of turbulence with the interaction of geodesic acoustic modes (GAMs) are investigated, focusing on the phase-space structure of turbulence, where the phase-space consists of real-space and wavenumber-space. Based on the wave-kinetic framework, the coupling equation between the GAM and the turbulence is numerically solved. The turbulence trapped by the GAM velocity field is obtained. Due to the trapping effect, the turbulence intensity increases where the second derivative of the GAM velocity (curvature of the GAM) is negative. While, in the positive-curvature region, the turbulence is suppressed. Since the trapped turbulence propagates with the GAMs, this relationship is sustained spatially and temporally. The dynamics of the turbulence in the wavenumber spectrum are converted in the evolution of the frequency spectrum, and the simulation result is compared with the experimental observation in JFT-2M tokamak, where the similar patterns are obtained. The turbulence trapping effect is a key to understand the spatial structure of the turbulence in the presence of sheared flows., source:Physics of Plasmas 25, 012316 (2018); https://doi.org/10.1063/1.5008541, source:https://doi.org/10.1063/1.5008541

The neutral drag effect on the parallel velocity gradient driven instability (PVG) in the presence of density inhomogeneity is theoretically investigated. The dispersion relation of PVG mode with the effect of the density gradient and neutral particle drag is derived, and its solution is analytically obtained. The neutral particle drag gives rise to the phase shift between parallel flow and electrostatic potential fluctuations and modifies the parallel compression. As a result, the stability of the PVG mode changes. It is found that the neutral particle drag does not only reduce but also enhances the instability. Specifically, near the marginal condition, the neutral particle effect suppresses the density gradient effect, and the parameter region where the PVG mode is linearly unstable significantly expands., source:Physics of Plasmas 27, 082105 (2020); https://doi.org/10.1063/5.0011025, source:https://doi.org/10.1063/5.0011025, identifier:0000-0001-6835-1569

We have upgraded the multi-channel microwave reflectometer system which uses a frequency comb as a source and measure the distribution of the density fluctuation caused by magneto-hydro dynamics instability. The previous multi-channel system was composed of the Ka-band, and the U-band system has been developed. Currently, the U-band system has eight frequency channels, which are 43.0, 45.0, 47.0, 49.0, 51.0, 53.0, 55.0, and 57.0 GHz, in U-band. Before the installation to the Large Helical Device (LHD), several tests for understanding the system characteristics, which are the phase responsibility, the linearity of output signal, and others, have been carried out. The in situ calibration in LHD has been done for the cross reference. In the neutral beam injected plasma experiments, we can observe the density fluctuation of the interchange mode and obtain the radial distribution of fluctuation amplitude., source:Citation R. Soga et al 2016 JINST 11 C02009, source:https://doi.org/10.1088/1748-0221/11/02/C02009