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Helium plasma-induced surface modifications of unirradiated and pre-ion irradiated tungsten at LHD divertor leg position
Full text linkORCID 0000-0001-5089-3642Understanding helium (He) plasma-induced tungsten (W) surface modifications and the effect of irradiation defects on He plasma-induced W surface modifications under a real divertor environment are important for the operation of fusion reactors. In this study, two iron (Fe) ions pre-irradiated in W samples with dislocation loops and voids, and two unirradiated W samples were exposed to He plasma at the divertor leg position of the Large Helical Device. The gross erosion rate is 1.0 × 1020 atoms m−2 s−1, 1.0 × 1020 atoms m−2 s−1, 9.3 × 1019 atoms m−2 s−1 and 7.4 × 1019 atoms m−2 s−1 for W9, W10, W11 and W12, respectively. The surface of each sample after the exposure was different at the strike point and the two regions on either side of the strike point, the scrape-off layer (SOL) region and the private region. The typical He plasma-induced structures in the SOL region are stripe structure, sawtooth structure and non-undulating structure, which are collectively called He-structures in the present study. At the strike point, the typical He plasma-induced structures are dense W protrusions. In the private flux region, the typical He plasma-induced structures are semi-formed He-structures. The formation of these structures is dependent on the grain orientation. Pinholes were observed in these structures. No significant difference in He plasma-induced structures is found between the pre-irradiated W and the unirradiated W. The formation of non-undulating structure and stripe structure is discussed based on the observed semi-formed He-structures.journal articl
Measurement of Ion Temperature during the Intermittent Negative Spike in Floating Potential
Full text linkThe ion temperature in an ECR plasma is determined from the ratio of wavelength-integrated spectra obtained using a spectrometer with sufficient time resolution. The ion temperature during the intermittent negative spikes in floating potential is extracted using the conditional averaging method. For the first time, we have successfully observed a decrease in ion temperature during intermittent events.journal articl
Accident-tolerant hybrid ceramics for fusion breeding blanket
Full text linkORCID 0000-0001-8067-8732In a water-cooled ceramic breeding blanket for fusion reactors, hydrogen gas generation by steam oxidation of metallic Be compounds (i.e. neutron multiplier) in a loss-of-coolant accident (LOCA) raises major safety concerns. Li–Be hybrid ceramics has a potential to reduce hydrogen generation significantly, however, the stable compositions and structures for quaternary compositions have not been comprehensively understood. Herein, we report machine-learning based prediction, synthesis, structure, and properties of chemically stabile two-phase Li–Be–X–O hybrid ceramics. The steam exposure tests demonstrated a negligibly small H2 generation from the two-phase powder of Li2BeSiO4 and 5 at.% BeO below 1200 °C. The stability is explained by the intrinsic ionic/covalent bonding characters and little capacity for further oxidation by steam. Neutronic calculations with simplified one-dimensional model show that the two-phase hybrid cermaics has a sufficient tritium breeding capability without having metallic Be-based multiplier in the blanket. The hybrid ceramics is the first example of multi-functional oxide to breed sufficient fuel tritium with no metallic neutron multiplier, which allows a novel design of ceramic breeding blanket with enhanced safety margins during in-box LOCA.journal articl
Benchmark tests of the efficient electrostatic particle-in-cell simulation code on various high-performance computing systems
Full text linkORCID 0000-0001-7641-6908The “up3bd” code, which is an efficient three-dimensional (3D) electrostatic particle-in-cell (PIC) simulation code for study of transport dynamics in fusion boundary layer plasmas or other plasmas in nature, has been tested on various high-performance computing (HPC) systems which consist of different types of processor. The results of benchmark tests indicate that the up3bd code works faster on processors in which cache memory is larger or memory bandwidth is broader. Also, the types of computations that each architecture excels at and struggles with has been revealed.journal articl
Effect of vacancy, self-interstitial atoms and transmutation rhenium on lattice thermal conductivity in tungsten
No full textTungsten (W) is a promising candidate for plasma-facing components (PFCs) because of its excellent properties, such as a high melting point and high thermal conductivity (TC). However, neutron irradiation introduces lattice defects and transmutation, leading to the degradation of material properties. In this study, the reduction of lattice thermal conductivity (LTC) in W caused by point defects, such as vacancies, self-interstitial atoms, substitutional rhenium (Re) and interstitial Re were calculated using molecular dynamics at 300 K and 800 K using equilibrium molecular dynamics (EMD). Also, we calculate phonon density of state (DOS) and discuss how the point defects change the phonon DOS of crystalline W. It was found that interstitial Re decreased LTC most significantly, followed by random self-interstitial atoms, vacancies, and substitutional Re. Additionally, it was found that the calculation results of the phonon-phonon scattering rate by EMD deviate significantly from those by the ab initio method.journal articl
Experimental discoveries of a variety of turbulent states of magnetic fusion plasma
Full text linkORCID 0000-0002-0585-4561This paper presents significant experimental discoveries of various turbulent states of magnetically confined plasmas in far non-equilibrium states, providing a wealth of new knowledge to the field. The intricate interplay between a non-uniform radial electric field (mean E × B flow shear and zonal flow) and turbulence in plasma, which gives rise to various turbulent states, is a complex yet fascinating area of study. The symmetry breaking of turbulence produces intrinsic torque and toroidal flow shear, while the pressure gradient produces poloidal flow shear. The turbulence produces the zonal flow (fluctuating E × B flow with zero toroidal and poloidal mode numbers). Both the mean E × B flow shear and zonal flow suppress the turbu- lence, which is the origin of this flow. The turbulence-dominant state and E × B flow dominant state co-exist in the plasma in space and finally cause the various turbulent states and radial profiles of temperature and density of magnetically confined plas- mas. The E × B shear also significantly impacts the blocking of turbulence spread- ing, which is one of the origins of non-local transport. Interaction between micro- scale, mezo-scale, and macro-scale fluctuation is another mechanism that causes non-local transport. The magnetic topology (nested flux surface, magnetic island, and stochastic magnetic field) is also important in determining the characteristics of turbulence transport. The magnetic island provides an excellent platform for the study of turbulence spreading. Isotope mixing, a new concept in particle transport, is also discussed. The experimental discovery and comprehensive understanding of these processes are described.journal articl
Linear Landau damping, Schrödinger equation, and fluctuation theorem
Full text linkORCID 0000-0001-5444-1758A linearized Vlasov–Poisson system of equations is transformed into a Schrödinger equation, which is used to demonstrate that the fluctuation theorem holds for the relative stochastic entropy, defined in terms of the probability density functional of the particle velocity distribution function in the Landau damping process. The difference between the energy perturbation, normalized by the equilibrium temperature, and the entropy perturbation constitutes a time-independent invariant of the system. This invariant takes the quadratic form of the perturbed velocity distribution function and corresponds to the squared amplitude of the state vector that satisfies the Schrödinger equation. Exact solutions, constructed from a discrete set of Hamiltonian eigenvectors, are employed to formulate and numerically validate the fluctuation theorem for the Landau damping process. The results offer new insight into the formulations of collisionless plasma processes within the framework of nonequilibrium statistical mechanics.journal articl
Prediction of mode structure using a novel physics-embedded neural ODE method
No full textWe designed a new artificial neural network called Exposed latent state neural ordinary differential equation with physics (ExpNODE-p) by modifying the neural ordinary differential equation (NODE) framework to successfully predict the time evolution of the two-dimensional mode profile in nonlinear saturated stage. Starting from the magnetohydrodynamic equations, simplifying assumptions were applied based on physical properties and symmetry considerations of the energetic-particle-driven geodesic acoustic mode (EGAM) to reduce complexity. Our approach embeds known physical characteristics directly into the neural network architecture by exposing latent differential states, enabling the model to capture complex features in the nonlinear saturated stage that are difficult to describe analytically. ExpNODE-p was evaluated using a dataset generated from first-principles simulations of the EGAM instability, focusing on the nonlinear saturated stage where the mode properties (e.g. frequency) are quite difficult to capture. Compared to state-of-the-art models such as ConvLSTM, ExpNODE-p achieved superior performance in both accuracy and training efficiency for multi-step predictions. Additionally, the model exhibited strong generalization capabilities, accurately predicting mode profiles outside the training dataset and capturing detailed features and asymmetries inherent in the EGAM dynamics. Our results establish ExpNODE-p as a powerful tool for creating fast, accurate surrogate models of complex plasma phenomena, opening the door to applications that are computationally intractable with first-principles simulations.journal articl
Statistical analysis on level energies and effective line strengths of singly ionized lanthanides for opacity of kilonovae
No full textORCID 0000-0002-5302-073XLanthanides play essential roles in opacities for the kilonova, the ultraviolet-optical-infrared emission from the neutron star merger detected by the gravitational wave (GW170817). In this paper, we examined suitable statistical representations for quasi-continuous spectra of singly ionized lanthanides ( = 59 − 70) based on our previous ab-initio calculations using the multi-configuration Dirac-Hartree-Fock and parametric potential methods. Using higher order Gram-Charlier expansions, an improved representation was obtained for the statistical distribution of the level energies. Systematic features that appear in mean, variance, and skewness of the distribution are explained in terms of atomic structure theories. We found that the effective line strength, that is defined as a product of the line strength and the Boltzmann factor at a give temperature, follows the log-logistic distribution. The scaling parameter of the log-logistic distribution gives an indicator down to which transitions should be included in opacity calculations for neutron star mergers.journal articl
Impurity Powder Injection Experiments in the Large Helical Device
No full textThe Impurity Powder Dropper (IPD) is a device capable of injecting controlled amounts of sub-millimetre powder into the plasma under the action of gravity. In 2019 the IPD was first installed on the Large Helical Device (LHD) in Japan, with the aim of improving the plasma performances through real time boronization and assessing the compatibility of this technique with steady state operation. Extensive series of experiments have been performed using the IPD, focused on the improvement of the plasma performance via low-Z powder injection and the understanding of the underlying physical phenomena. In this article, we review the experiments that took place in the period 2019-2024. The main results include the demonstration of the improvement of the wall conditions (reduction of intrinsic impurity content, wall recycling) both on a shot-to-shot basis and in real time. Furthermore, a reduced-turbulence improved confinement regime has been observed coincident with powder injection, resulting in an increase of the plasma temperature of the order of 25%, with enhancements that can reach up to 50% for ion temperature.journal articl