Publisher：Physics of Plasmas  

We investigated the mass dependency of the high-wavenumber turbulence, which occurs at scales smaller than the ion effective Larmor radius, in a partially magnetized plasma column. In this system, two different types of fluctuations were observed: one exhibiting a coherent discrete spectrum, while the other displaying a broadband continuous spectrum. The phase velocities of both types showed a similar mass dependency, approximately matching the ion sound velocity or electron diamagnetic drift velocity. Additionally, we found that the discrete spectrum has a peak interval frequency comparable to the ion cyclotron frequency, which is consistent with ion cyclotron ranges of fluctuations, including ion Bernstein waves.

DOI： 10.1063/5.0187997

Web of Science

Scopus

Language：English   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

<p>A new method is proposed to analyze plasma image of tomography using modified Fourier-Bessel Functions (FBF) instead of the original FBF analysis. The application of the method to an assumed plasma image shows that the difference between the original and the fitting image is improved considerably to that of FBF, owing to giving a better fitting inside the plasma and eliminating the ghost values outside the plasma, without any disadvantages in analysis of plasma structures and patterns.</p>

DOI： 10.1585/pfr.19.1201014

Web of Science

Scopus

CiNii Research

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

DOI： 10.1585/pfr.19.1201005

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

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.

DOI： 10.1088/1361-6587/ad074a

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

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

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

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.

DOI： 10.1063/5.0165318

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

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：Others   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1002/ctpp.202200150

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

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.

DOI： 10.1038/s41598-022-23559-1

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

DOI： 10.1016/j.nme.2022.101277

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

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.

DOI： 10.1063/5.0098294

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

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 (&#36;< &#36;1 m s⁻²), a large diffusion coefficient and a strong outward convection are inferred right inside the pedestal top.

DOI： 10.1088/1741-4326/ac60e8

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

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 n_e and temperatures T_e 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 n_e and T_e 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.

DOI： 10.1088/1741-4326/ac6071

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

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 T_e fluctuation levels of 2.3&#37;–4.2&#37;, with a frequency shift between the two phases related to a deeper E_r 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.

DOI： 10.1063/5.0088062

Publisher：Nuclear Fusion  

We present recent highlights from the most recent operation phases of Wendelstein 7-X, the most advanced stellarator in the world. Stable detachment with good particle exhaust, low impurity content, and energy confinement times exceeding 100 ms, have been maintained for tens of seconds. Pellet fueling allows for plasma phases with reduced ion-temperature-gradient turbulence, and during such phases, the overall confinement is so good (energy confinement times often exceeding 200 ms) that the attained density and temperature profiles would not have been possible in less optimized devices, since they would have had neoclassical transport losses exceeding the heating applied in W7-X. This provides proof that the reduction of neoclassical transport through magnetic field optimization is successful. W7-X plasmas generally show good impurity screening and high plasma purity, but there is evidence of longer impurity confinement times during turbulence-suppressed phases.

DOI： 10.1088/1741-4326/ac2cf5

Web of Science

Scopus

Publisher：Nuclear Fusion  

An overview of recent results obtained at the tokamak ASDEX Upgrade (AUG) is given. A work flow for predictive profile modelling of AUG discharges was established which is able to reproduce experimental H-mode plasma profiles based on engineering parameters only. In the plasma center, theoretical predictions on plasma current redistribution by a dynamo effect were confirmed experimentally. For core transport, the stabilizing effect of fast ion distributions on turbulent transport is shown to be important to explain the core isotope effect and improves the description of hollow low-Z impurity profiles. The L-H power threshold of hydrogen plasmas is not affected by small helium admixtures and it increases continuously from the deuterium to the hydrogen level when the hydrogen concentration is raised from 0 to 100%. One focus of recent campaigns was the search for a fusion relevant integrated plasma scenario without large edge localised modes (ELMs). Results from six different ELM-free confinement regimes are compared with respect to reactor relevance: ELM suppression by magnetic perturbation coils could be attributed to toroidally asymmetric turbulent fluctuations in the vicinity of the separatrix. Stable improved confinement mode plasma phases with a detached inner divertor were obtained using a feedback control of the plasma β. The enhanced D α H-mode regime was extended to higher heating power by feedback controlled radiative cooling with argon. The quasi-coherent exhaust regime was developed into an integrated scenario at high heating power and energy confinement, with a detached divertor and without large ELMs. Small ELMs close to the separatrix lead to peeling-ballooning stability and quasi continuous power exhaust. Helium beam density fluctuation measurements confirm that transport close to the separatrix is important to achieve the different ELM-free regimes. Based on separatrix plasma parameters and interchange-drift-Alfvén turbulence, an analytic model was derived that reproduces the experimentally found important operational boundaries of the density limit and between L- and H-mode confinement. Feedback control for the X-point radiator (XPR) position was established as an important element for divertor detachment control. Stable and detached ELM-free phases with H-mode confinement quality were obtained when the XPR was moved 10 cm above the X-point. Investigations of the plasma in the future flexible snow-flake divertor of AUG by means of first SOLPS-ITER simulations with drifts activated predict beneficial detachment properties and the activation of an additional strike point by the drifts.

DOI： 10.1088/1741-4326/ac207f

Scopus

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

DOI： 10.1063/5.0075846

Publisher：48th EPS Conference on Plasma Physics, EPS 2022  

Scopus

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

DOI： 10.1063/5.0062436

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

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.

DOI： 10.1088/1361-6587/abe024

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

DOI： 10.1063/5.0039011

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

DOI： 10.1063/5.0025696

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

DOI： 10.1088/1741-4326/ab9be7

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

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.

DOI： 10.1088/1361-6587/ab9732

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

DOI： 10.1088/1361-6587/aaf9e0

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

DOI： 10.1103/physrevlett.122.105001

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

DOI： 10.1063/1.5035093

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

DOI： 10.1103/physrevlett.121.165002

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

DOI： 10.1103/physrevlett.120.225002

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

DOI： 10.1063/1.4999450

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

DOI： 10.1063/1.4955491

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

DOI： 10.1063/1.4960073

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

DOI： 10.1109/tns.2014.2321657