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Tritium retention characteristics of the dust in LHD after the deuterium plasma experiment
Full text linkTritium (T) retention of the dust particles collected at ten toroidal sections of the Large Helical Device (LHD) after the 21st experimental campaign, in which deuterium (DD) plasma experiments were conducted, were evaluated by an enhanced full combustion method (EFCM), and their individual T retention was characterized by a tritium imaging plate technique (TIPT) in combination with a scanning electron microscopy (SEM-EDS). A new method of fixation and storage of the dust particles containing T, called the electroconductive Resin surface Embedding Method (eREM), was successfully demonstrated for T retention characterization of the individual dust particles.
From the EFCM results, total T retention in dust particles collected from all toroidal sections of LHD after the 21st campaign was found to be several hundred MBq. Much higher T retention was found in the flaky C dust particles at the toroidal section with the open helical divertor than in the other closed helical divertor sections. T retention in dust particles at the section of W-coated C divertor tiles was lower than at the other sections of C divertor tiles. Zr-Ti-V dust particles originating from in-vessel getter pump materials were found, and T retention in these particles was much lower than in the C dust particles.
The results from TIPT in combination with SEM-EDS indicated not all the dust particles but a half of the dust particles retained T. Such the T retention characteristics of the dust particles could be attributed to T retention characteristics of the dust source resulting in the dust particles produced and transported, and the other is depth distribution of T in the individual dust particle thicker than T β-electron escaping range.journal articl
Comprehensive gyrokinetic study of eigenstate transitions in fast ion-driven electrostatic drift instabilities
No full textORCID 0000-0001-5899-1917This study comprehensively investigates fast ion-driven drift instability, extending the theory in Kang and Hahm (2019) [18]. The eigenmode equation, including the resonant contribution of passing fast ions, is derived and solved using the shooting method. Passing fast ions significantly affect the instability in weak negative shear or moderate positive shear plasmas. Eigenstate transitions to non-ground states occur more readily in weak magnetic shear, high safety factor, and long wavelength perturbations. Linear gyrokinetic simulations using the GKV code verify the theory, showing good agreement with shooting method results. The estimated quasilinear transport indicates that the net energy flux can be inward, without contradicting the second law of thermodynamics. These findings have important implications for heating efficiency and plasma confinement in the heating process, such as Ion Cyclotron Resonance Heating (ICRH) in future fusion devices.journal articl
Three-Dimensional Analysis of Eddy Current in TOKASTAR-2
Full text linkThe accuracy of magnetic field analysis including eddy current is important in MHD equilibrium reconstruction of tokamak plasmas. In a small toroidal plasma experimental device TOKASTAR-2, the eddy current calculations were done with an axisymmetric model of the vacuum vessel though its vacuum vessel has periodic three-dimensionality every 90 degrees of toroidal angle due to large horizontal ports. The three-dimensional (3D) eddy current magnetic field is evaluated by 3D magnetic field calculations using ANSYS for the first time in TOKASTAR-2. The results are compared with the conventional axisymmetric magnetic field calculation and measurements using magnetic probes located inside and outside of the vacuum vessel. The resistivity of the vacuum vessel model in ANSYS is modified to reduce the error from the experimental values. Using the developed model, the effect of the presence of the port on the eddy current magnetic field is evaluated. The results show that the toroidal-average of the eddy current magnetic field becomes smaller by presence of the ports but the non-uniformity in the toroidal direction is relatively small. This implies that the effects of the port would be introduced in an axisymmetric model by using poloidally nonuniform resistivity to suppress the eddy current on the midplane.journal articl
"Plasma and Fusion Cloud" Data analysis environment
Full text linkORCID 0000-0002-0441-6340The Fusion Cloud concept is currently underway at the National Institute for Fusion Science (NIFS). This concept is to deploy data collection and data analysis systems, which are assets developed through LHD experiments, to other laboratories in Japan through a high-speed network.
The Open Data Server discussed in this paper is part of the Fusion Cloud concept, and data collected at LHD is currently being made publicly available. In the future, the data collected by various devices will be made widely available for use in fusion research, and will also be used for research beyond fusion and across different fields. In addition, in the future, not only the data but also a portable environment for analyzing these data will be provided on the cloud.journal articl
Experimental Study of Triton Burnup in EAST Using Neutron Activation Method
Full text linkA neutron activation system with two sets of activation terminals was developed in the experimental advanced superconducting tokamak (EAST), and it was implemented in the measurements of the time-integrated triton burnup for the deuterium plasma experiment in EAST. Indium and silicon samples with an energy threshold of 0.3 and 4.0 MeV were used to measure the DD and DT neutron, respectively. The shot-averaged triton burnup ratio was determined by simultaneously measuring the DD and DT neutron yields during plasma discharges. The triton burnup ratio of EAST was found to range from 0.035% to 0.65%, depending on plasma parameters. Due to a better confinement of fast triton, the triton burnup ratio increases with the plasma current and the magnetic field in the range of 400–600 kA and 1.74–2.57 T, respectively. Additionally, the triton burnup ratio initially increased and then decreased with electron density in the range of 3.8×1019 – 6.0×1019 m−3, with a turning point observed at approximately 4.4×1019 m−3.journal articl
Dynamic response of atomic processes in detached helium plasma induced by high-density transient pulse in Magnum-PSI
Full text linkPulsed plasma experiments simulating the transient divertor heat loads caused by edge-localized modes (ELMs) were conducted in Magnum-PSI, a linear plasma device capable of generating ITER-relevant conditions. The dynamic responses of detached recombining helium (He) plasmas to the pulsed plasma were observed through time-resolved laser Thomson scattering measurements and optical emission spectroscopy. Experimental results indicate that He atom depletion occurs in high-density pulsed plasmas due to a reduced mean-free-path and increased plasma pressure. This limits plasma-neutral interactions, suppressing energy dissipation via direct excitation and electron–ion recombination processes. The He II and He I emissions were found to peak sequentially, suggesting that doubly-ionized He ions recombine first, followed by the recombination of singly-ionized He ions. These findings suggest that the neutral penetration and recombination efficiency are significantly reduced in pulsed plasmas, challenging heat flux mitigation in fusion divertors. The results provide critical insights into the behavior of He ash transported by ELMs and plasma detachment under high-density pulsed conditions, highlighting the need for further studies on neutral fueling strategies in ITER-relevant environments.journal articl
EUV and Soft X-ray Spectroscopy of Highly Charged Heavy Ions Using LHD: Research Ranging from Industrial Light Sources to Basic Atomic Physics
No full textORCID 0000-0001-6536-9034This article reviews various achievements in spectroscopy of highly charged ions of a variety of heavy elements injected into the Large Helical Device (LHD) plasmas. We focus on discrete and quasi-continuum spectra observed in extreme ultraviolet (EUV) and soft X-ray wavelength ranges using multiple grazing incidence spectrometers. In particular, the atomic number dependence and temperature dependence of the spectral features have been investigated more comprehensively than ever before over extremely wide ranges based on comparisons with theoretical models and other experimental data. Consequently, the series of studies could provide an experimental database valuable for investigations of basic atomic physics issues specific to highly charged heavy ions, as well as the applications to industrial light source developments.journal articl
Kinetic estimation of tritium and neutron yields in all LHD deuterium experiment campaigns
Full text linkORCID 0000-0003-3293-488XTritium yields in all six Large Helical Device deuterium experiment campaigns, or 41 064 discharges, have been numerically estimated. Usually, tritium yields are estimated from neutron yields by assuming that they are the same. In present fusion devices, this assumption is inaccurate because a fusion reaction between thermal-deuterons and fast-deuterons is dominant.
In this paper, the energy distribution of fast-deuterons injected by neutral beam injectors is taken into account for the estimation of the ratio of the tritium yields to the neutron yields, Yt/Yn. By integrated simulation, the Yt/Yn is approximately 0.94 in each campaign. Because assumptions applied in the simulation aim to avoid under-estimation of the tritium yields compared to the actual value, the Yt/Yn should be 0.85 < Yt/Yn < 0.94.journal articl
Cross-scale nonlinear interaction and bifurcation in multi-scale turbulence of high-temperature plasmas
No full textORCID 0000-0001-5473-2109The structural formation of magnetized high-temperature plasmas, which are ubiquitous in nature, is often controlled by turbulence. Such plasma turbulence is characterized by multiple scales and thus cross-scale nonlinear interactions are key to understanding its dynamics and structural formation. This is also the case for laboratory plasmas, for which multi-scale turbulence data are most abundant. The interactions of components on the meso- and ion-gyroradius (micro)-scales have been extensively studied. Here we report, the experimental discovery of the cross-scale nonlinear interactions between fluctuations at micro-scale and hyper-fine (HF)-scale (whose scale is about an electron gyroradius). Cross-scale bifurcation is found, in which micro-scale turbulence is suppressed, and the amplitude of HF-scale turbulence simultaneously increases. There is also an abrupt change in the isotropy of HF-scale components. We discuss a possible mechanisms in which nonlinear interactions, related to the weakening of the electric field generated by micro-scale turbulence, enhance HF-scale turbulence.journal articl
Enhanced beam transport via space charge mitigation in a multistage accelerator for fusion plasma diagnostics
No full textORCID 0000-0002-2752-3333Efficient transport of high-current negative ion beams is critical for accurate plasma potential diagnostics using heavy-ion beam probe (HIBP) systems in magnetically confined fusion plasmas. However, strong space-charge effects often degrade transport efficiency, particularly for heavy ions such as Au−. In this study, we demonstrate a substantial improvement in beam transport by introducing an electrostatic lens effect through optimized voltage allocation in a multistage acceleration system. Numerical simulations using IGUN, supported by experiments with the Large Helical Device-HIBP system, show that this approach effectively suppresses space-charge-induced beam divergence and loss. Without requiring mechanical modifications to the beamline, the optimized configuration enables a 2–3 fold increase in Au− beam current injected into the tandem accelerator. Consequently, plasma potential measurements were extended to higher-density plasmas, reaching line-averaged electron densities up to 1.75×1019 m−3 with improved signal-to-noise ratio. This technique offers a compact, practical, and highly effective solution for transporting high-current heavy-ion beams under space-charge-dominated conditions. Beyond its impact on plasma diagnostics, the method is broadly applicable to a wide range of accelerator systems, including those used in scientific and industrial applications where high-intensity beam transport is required.journal articl