記述言語：英語   出版者・発行元：一般社団法人 プラズマ・核融合学会  

A high-speed floating potential measurement system has been developed for investigating turbulence at frequencies above the ion cyclotron frequency in the PANTA linear plasma device. The system is designed to have a phase-flat response up to 1MHz, enabling accurate measurement of the phase relationship between floating potential and density fluctuations extending into the high-frequency regime. Initial results show that the attenuation of gain in the high-frequency range is significantly reduced compared to conventional measurements. A cross-phase analysis between the estimated electric field and density fluctuations shows a clear difference.

DOI： 10.1585/pfr.21.1302021

Web of Science

Scopus

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 プラズマ・核融合学会  

<p>To properly study plasma fluctuations and to develop new plasma processing technologies, precise calibration of probes is required. We exploit plasma fluctuations properties to develop a novel simple method for calibrating the radial position of a probe in a linear magnetized plasma device. We determine the probe radial position with respect to the plasma column axis by using a previously calibrated reference probe. This study provides the experimental evidence that plasma fluctuations can be used not only to investigate plasma physics but also to determine the radial position of probes.</p>

DOI： 10.1585/pfr.21.1201026

Web of Science

CiNii Research

researchmap

掲載種別：研究論文（学術雑誌）   出版者・発行元：Plasma Physics and Controlled Fusion  

An atomic-orbital close-coupling method is applied to Ne¹⁰⁺–H(1 s) and Ne¹⁰⁺–H(2 s) collisions to calculate the total electron capture cross-section, state-selective electron capture cross-section, and total ionization cross-section. Results are shown from 1 to 50keV u⁻¹ for the H(1s) target and from 1 to 25keV u⁻¹ for the H(2s) target. The present results for the H(1s) target are overall in good agreement with other theoretical results. The results for the H(2s) target are less reliable, but may be useful for future studies.

DOI： 10.1088/1361-6587/ae0cfd

Web of Science

Scopus

researchmap

その他リンク： https://iopscience.iop.org/article/10.1088/1361-6587/ae0cfd/pdf

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：Fusion Engineering and Design  

In order to save flux consumption and lessen engineering requirements, the achievable toroidal loop voltage tends to be lowered in modern tokamaks. In such devices, careful optimization of the plasma startup scenario is often required to achieve a successful tokamak discharge. The PLATO tokamak, which is recently built at Kyushu University, also faces challenges in the plasma startup. PLATO employs an air-cored central solenoid that limits the loop voltage and creates stray magnetic fields. In addition, only simple capacitor banks with modest maximum charging voltages are available for the generation of the poloidal magnetic and toroidal electric fields in the initial operation. To realize the breakdown and plasma current ramp-up under those constraints, the configuration of coils and capacitor bank settings has been optimized to produce sufficient loop voltage while minimizing the stray field inside the vacuum chamber at the onset of the discharge. Through this optimization, a tokamak discharge with a plasma current of 30 kA and a pulse duration of 20 ms has been achieved.

DOI： 10.1016/j.fusengdes.2025.115222

Web of Science

Scopus

researchmap

掲載種別：研究論文（学術雑誌）   出版者・発行元：Plasma Physics and Controlled Fusion  

In this study, we investigate the spatiotemporal dynamics of fluctuations in the presence of a toroidally directed zonal flow using two pairs of radial capacitive arrays with finer spatial resolution than the observed toroidal zonal flow. Conditional sampling is employed to evaluate frequency- and time-resolved behavior with respect to the toroidal zonal flow phase. We identify three distinct types of fluctuations, each exhibiting different spatiotemporal dynamics synchronized with the toroidal zonal flow. These modes are modulated in space and time in accordance with the phase of the toroidal zonal flow. The mechanism underlying the formation of spatiotemporal structures is discussed in terms of long-range correlations and wave-kinetic characteristics

DOI： 10.1088/1361-6587/adf009

Web of Science

Scopus

researchmap

その他リンク： https://iopscience.iop.org/article/10.1088/1361-6587/adf009/pdf

掲載種別：研究論文（学術雑誌）   出版者・発行元：Physics of Plasmas  

Reliable estimation of equilibrium flow shear from reciprocating probe measurements is challenging since the quantity of interest corresponds to the second derivative of the observable plasma potential. In addition, a time series of the plasma potential obtained by plunging a probe is affected by both the probe head position and plasma fluctuations, complicating the estimation of equilibrium components and their errors. We tackle this problem by employing Gaussian process regression that is able to infer even the derivatives of a spatial or temporal profile in the form of a probability distribution function. The proposed inference framework is validated by using synthetic data generated by gyrofluid simulations. While the inference result based on a single plunge is unstable in certain spatial locations, we have obtained reasonable agreement between the inference result and the true flow shear profile by combining data sets taken from several plunges.

DOI： 10.1063/5.0254473

Web of Science

Scopus

researchmap

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：Plasma Physics and Controlled Fusion  

Reliable equilibrium reconstruction is indispensable for understanding and controlling hot magnetized plasmas to achieve fusion reactors. In axisymmetric systems, current and pressure profiles that satisfy the force balance conditions are given by the Grad-Shafranov (GS) equation. While many novel approaches have been developed to swiftly and robustly find an optimum solution of the GS equation, approaches based on a single solution search may not be adaptable if diagnostics fail to provide sufficient constraints. Here, we investigate the solution space of the GS equation when only basic edge magnetic measurements are available. By combining Gaussian process regression and Markov chain Monte Carlo sampling within the Bayesian framework, we treat each current element as an independent variable and evaluate the probability distribution that describes all possible solutions. We have applied this inference frame to the geometry of the PLATO tokamak and shown that the flux surface locations can be determined relatively well only from 16 pick-up coils, 4 flux loops and a diamagnetic loop. On the other hand, the toroidal current density is inferred with limited success, and the inferences of the safety factor and pressure profiles are difficult. The characterization of possible choices of equilibria realized by this inference framework will help optimize diagnostic setups for equilibrium reconstruction.

DOI： 10.1088/1361-6587/ad9521

Web of Science

Scopus

researchmap

その他リンク： https://iopscience.iop.org/article/10.1088/1361-6587/ad9521/pdf

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 プラズマ・核融合学会  

The Cormack inversion is a well-known procedure for tomography. However, the Fourier-Bessel function expansion could be preferably used for plasma tomography, since plasma images obtained with the Cormack inversion tend to produce a false plasma image in the boundary without regards to its advantage: the direct structural analysis of plasma is possible using a set of line-integrated data. Here we propose an application of function modification technique to revive the advantage of the Cormack inversion and present successful experimental result in a tomography system of a cylindrical plasma.

DOI： 10.1585/pfr.20.1201052

Web of Science

Scopus

CiNii Research

researchmap

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 プラズマ・核融合学会  

We report on the nonlinear coupling properties of high-wavenumber fluctuation, which exhibits discrete frequency spectra with a frequency span comparable to the ion cyclotron frequency observed in PANTA. Bicoherence analysis is used to investigate this nonlinear coupling. The results indicate a finite coupling between individual discrete peaks but no coupling between the ion cyclotron frequency components of these discrete peaks. This suggests that the observed turbulence with a discrete spectrum is destabilized linearly rather than through nonlinear coupling.

DOI： 10.1585/pfr.1201031

Web of Science

Scopus

CiNii Research

researchmap

出版者・発行元：Nuclear Fusion  

Recently, experiments on basic plasma physics issues for solving future problems in fusion energy have been performed on a Large Helical Device. There are several problems to be solved in future devices for fusion energy. Emerging issues in burning plasma are: alpha-channeling (ion heating by alpha particles), turbulence and transport in electron dominant heating helium ash exhaust, reduction of the divertor heat load. To solve these problems, understanding the basic plasma physics of (1) wave-particle interaction through (inverse) Landau damping, (2) characteristics of electron-scale (high-k) turbulence, (3) ion mixing and the isotope effect, and (4) turbulence spreading and detachment, is necessary. This overview discusses the experimental studies on these issues and turbulent transport in multi-ion plasma and other issues in the appendix.

DOI： 10.1088/1741-4326/ad3a7a

Web of Science

Scopus

出版者・発行元：Nuclear Fusion  

Experiments on ASDEX Upgrade (AUG) in 2021 and 2022 have addressed a number of critical issues for ITER and EU DEMO. A major objective of the AUG programme is to shed light on the underlying physics of confinement, stability, and plasma exhaust in order to allow reliable extrapolation of results obtained on present day machines to these reactor-grade devices. Concerning pedestal physics, the mitigation of edge localised modes (ELMs) using resonant magnetic perturbations (RMPs) was found to be consistent with a reduction of the linear peeling-ballooning stability threshold due to the helical deformation of the plasma. Conversely, ELM suppression by RMPs is ascribed to an increased pedestal transport that keeps the plasma away from this boundary. Candidates for this increased transport are locally enhanced turbulence and a locked magnetic island in the pedestal. The enhanced D-alpha (EDA) and quasi-continuous exhaust (QCE) regimes have been established as promising ELM-free scenarios. Here, the pressure gradient at the foot of the H-mode pedestal is reduced by a quasi-coherent mode, consistent with violation of the high-n ballooning mode stability limit there. This is suggestive that the EDA and QCE regimes have a common underlying physics origin. In the area of transport physics, full radius models for both L- and H-modes have been developed. These models predict energy confinement in AUG better than the commonly used global scaling laws, representing a large step towards the goal of predictive capability. A new momentum transport analysis framework has been developed that provides access to the intrinsic torque in the plasma core. In the field of exhaust, the X-Point Radiator (XPR), a cold and dense plasma region on closed flux surfaces close to the X-point, was described by an analytical model that provides an understanding of its formation as well as its stability, i.e., the conditions under which it transitions into a deleterious MARFE with the potential to result in a disruptive termination. With the XPR close to the divertor target, a new detached divertor concept, the compact radiative divertor, was developed. Here, the exhaust power is radiated before reaching the target, allowing close proximity of the X-point to the target. No limitations by the shallow field line angle due to the large flux expansion were observed, and sufficient compression of neutral density was demonstrated. With respect to the pumping of non-recycling impurities, the divertor enrichment was found to mainly depend on the ionisation energy of the impurity under consideration. In the area of MHD physics, analysis of the hot plasma core motion in sawtooth crashes showed good agreement with nonlinear 2-fluid simulations. This indicates that the fast reconnection observed in these events is adequately described including the pressure gradient and the electron inertia in the parallel Ohm’s law. Concerning disruption physics, a shattered pellet injection system was installed in collaboration with the ITER International Organisation. Thanks to the ability to vary the shard size distribution independently of the injection velocity, as well as its impurity admixture, it was possible to tailor the current quench rate, which is an important requirement for future large devices such as ITER. Progress was also made modelling the force reduction of VDEs induced by massive gas injection on AUG. The H-mode density limit was characterised in terms of safe operational space with a newly developed active feedback control method that allowed the stability boundary to be probed several times within a single discharge without inducing a disruptive termination. Regarding integrated operation scenarios, the role of density peaking in the confinement of the ITER baseline scenario (high plasma current) was clarified. The usual energy confinement scaling ITER98(p,y) does not capture this effect, but the more recent H20 scaling does, highlighting again the importance of developing adequate physics based models. Advanced tokamak scenarios, aiming at large non-inductive current fraction due to non-standard profiles of the safety factor in combination with high normalised plasma pressure were studied with a focus on their access conditions. A method to guide the approach of the targeted safety factor profiles was developed, and the conditions for achieving good confinement were clarified. Based on this, two types of advanced scenarios (‘hybrid’ and ‘elevated’ q-profile) were established on AUG and characterised concerning their plasma performance.

DOI： 10.1088/1741-4326/ad249d

Scopus

掲載種別：研究論文（学術雑誌）   出版者・発行元：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

researchmap

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 プラズマ・核融合学会  

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.

DOI： 10.1585/pfr.19.1201014

Web of Science

Scopus

CiNii Research

researchmap

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 プラズマ・核融合学会  

This article presents a new method, called extended Rotational Movement Analysis (e-RoMA), to estimate the velocity field of a cylindrical plasma from its two-dimensional images based on Fourier-Bessel function expansion. The proposed method is applied to tomography images of a plasma produced in a linear cylindrical device PANTA and succeeded in obtaining the two- dimensional velocity field. The first results suggest an issue to be investigated, that is, the presence of azimuthal asymmetry in the velocity field.

DOI： 10.1585/pfr.19.1201005

Web of Science

Scopus

CiNii Research

researchmap

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Plasma Physics and Controlled Fusion  

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

Web of Science

Scopus

researchmap

その他リンク： https://iopscience.iop.org/article/10.1088/1361-6587/ad074a/pdf

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Plasma Physics and Controlled Fusion  

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

Web of Science

Scopus

researchmap

その他リンク： https://iopscience.iop.org/article/10.1088/1361-6587/acfbf7/pdf

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

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

Web of Science

Scopus

researchmap

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physics of Plasmas  

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

Web of Science

Scopus

researchmap

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Contributions to Plasma Physics  

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

Web of Science

Scopus

researchmap

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Reports  

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

Web of Science

Scopus

PubMed

researchmap

その他リンク： https://www.nature.com/articles/s41598-022-23559-1

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Nuclear Materials and Energy  

The L-mode density limit in ASDEX Upgrade is investigated with focus on the scrape-off layer. We observe a chain of events that involves the collapse of the E×B outer shear layer (just outside the separatrix), widening of the electron pressure fall-off length, outer divertor detachment, density shoulder formation, reduction and flattening of the parallel Mach number, and enhanced turbulent cross-field transport. The latter effect was associated with higher amplitude density fluctuations at low frequencies, where the turbulent cross-phase between density and velocity fluctuations pointing radially outward was favourable.

DOI： 10.1016/j.nme.2022.101277

Web of Science

Scopus

researchmap

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physics of Plasmas  

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

Web of Science

Scopus

researchmap

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Nuclear Fusion  

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

Web of Science

Scopus

researchmap

その他リンク： https://iopscience.iop.org/article/10.1088/1741-4326/ac60e8/pdf

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Nuclear Fusion  

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

Scopus

researchmap

その他リンク： https://iopscience.iop.org/article/10.1088/1741-4326/ac6071/pdf

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physics of Plasmas  

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

Scopus

researchmap

出版者・発行元：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

出版者・発行元：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

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Review of Scientific Instruments  

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.

DOI： 10.1063/5.0075846

Scopus

researchmap

出版者・発行元：48th EPS Conference on Plasma Physics, EPS 2022  

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/5.0062436

記述言語：その他   掲載種別：研究論文（学術雑誌）  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/5.0039011

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/5.0025696

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1741-4326/ab9be7

記述言語：その他   掲載種別：研究論文（学術雑誌）  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1361-6587/aaf9e0

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevlett.122.105001

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/1.5035093

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevlett.121.165002

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevlett.120.225002

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/1.4999450

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/1.4955491

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1063/1.4960073

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1109/tns.2014.2321657