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.

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.

This work reports a novel approach employing a retarding field analyzer (RFA) to simultaneously measure parallel and perpendicular ion temperatures. I-V curves obtained by the RFA were analyzed considering the influence of selective ion transmission, and the ion temperatures were evaluated. The RFA analysis yielded parallel and perpendicular ion temperatures of 1.2 eV and 2.1 eV, respectively. The perpendicular ion temperature obtained by an ion sensitive probe was 2.0 eV, demonstrating good agreement with that evaluated by the RFA.

The response of energetic particle (EP)-driven magnetohydrodynamic instability to electron cyclotron heating (ECH) was experimentally studied in Heliotron J. The neutral beam injection power remained constant throughout the experiment, and the ECH power was systematically modulated. When the ECH power was gradually decreased to a certain threshold, the excitation of an EP mode with a frequency of approximately 100 kHz was observed. The response to ECH showed a close linkage with variations in electron density and temperature. In addition, a distinct delay response effect concerning plasma pressure suggests a complex delay response of mode excitation and suppression.

The safety factor and magnetic shear profiles play an important role in plasma confinement, particularly in cases of nearly zero or reversed magnetic shear. This may be closely related to the formation of internal transport barriers. We have upgraded the low-aspect ratio reversed field pinch (RFP) machine RELAX to produce tokamak plasmas, which will be used to investigate the physics of magnetic shear within a single machine. The preliminary measurements of the radial profile of the magnetic field have been conducted under different conditions in two separate experiments. The initial results indicate that standard tokamak and RFP plasmas are produced within a single machine due to an upgraded power supply that increased the toroidal field. This unique feature enables systematic studies of the physics of magnetic shear across the negative-to-positive region.

Data assimilation technique implemented in fusion research has enhanced the modeling capability. The quantitative “gap” between the original model (typically based on physics considerations and/or empirical approach) and the optimized model (obtained through data assimilation) can be utilized to improve the original model to align with the measured data. Such a procedure is proposed here by taking the model of the heat diffusivity of plasmas as an example. It successfully elucidates relevant parameters recognized in the experiment but were missing in the original model, demonstrating the efficiency of the proposed procedure.

The water-cooled ceramic breeder system is the leading choice for research and development in Japan’s ITER and DEMO blankets. This type of blanket uses reduced activation ferritic/martensitic (RAFM) steel as structural material in the flow of the high-temperature and high-pressure water as coolant. There is however insufficient information regarding corrosion under fusion reactor operating conditions despite previous water chemistry and environmental strength evaluations. In particular, there are no specific studies of the effects of strong magnetic fields on corrosion behavior. Therefore, it is of great engineering importance to clarify the influence of magnetic fields on the corrosion of RAFM since the RAFM itself shows inherent ferromagnetic characteristics. In this study, we investigated whether a magnetic field affects the corrosion of Japanese RAFM steel, i.e., F82H, in high-temperature and high-pressure water. The results showed that 1.3 T of the magnetic field made no difference in the thickness of the inner oxide, but a difference in the size of the oxide particles on the surface.

We developed a new silica (SiO₂) removal technique that can maximize biomass-based activated carbon adsorption performance. SiO₂ removal is one of the key processes in making activated carbon suitable for cryosorption pumps in fusion machines. In this study, we employed an evaporation process to remove SiO₂ by high-temperature vacuum heating. The charcoal made from rice straw was heated at 1800 ℃ for 1 h at approximately 10 Pa in a vacuum furnace. We found that SiO₂ amount was significantly reduced from 15.8 wt% to 4.20 wt% due to vacuum heating. In addition, the result of surface element mapping analysis using energy-dispersive X-ray spectroscopy (EDX) indicated a considerable decrease in the oxygen content of SiO₂. We demonstrated the great potential of activated carbon derived from rice straw as an adsorbent for high-performance cryosorption pumps.

Photoinduced processes such as photoionization and photoexcitation in the vacuum ultra violet (VUV) energy range are considered important for the divertor region in nuclear fusion reactors and in interstellar space because the cross sections (photoionization, photoexcitation) of these processes in relevant species (hydrogen, helium, neon, argon, and biomolecules) become large in that energy range. Herein, a photoionization experiment was conducted for the first time in the synchrotron light source UVSOR-III with VUV photon energies. The synchrotron light source has the advantage of capability to change photon energy continuously over a wide range and high beam repetition rates. These features allow the simulation of the divertor region and interstellar radiation field to systematically investigate photoinduced processes. Using argon as the sample gas, plasma production was evidenced by the detection of electron current in Langmuir probe measurements. Although an accurate evaluation of the plasma parameters was challenging because of the large scatter of probe data, the possible ranges of plasma parameters are discussed based on a 0D model of photoionization plasmas [R.M. van der Horst et al., J. Phys. D: Appl. Phys. 48, 285203 (2015)] and a newly proposed 1D model in the steady state. Analysis result indicates that plasma density is in the range of 10¹⁰ - 10¹¹ m^-3. Additionally, the further development of experiments is discussed for realizing higher plasma densities and for studying photoinduced processes in the divertor region in nuclear fusion reactors and interstellar plasma in terms of the chemical evolution of biomolecules.

Recent technological advances have significantly improved the quality of virtual reality (VR) provided by head-mounted display (HMD) systems. The immersive experiences provided by contemporary HMDs provide an effective environment for scientific research, especially for three-dimensional data visualization. Plasma physics has decades of history of using VR technologies for visualizations with particular emphasis on room-sized, large-scale display systems called CAVE. Modern HMDs are a cost-effective alternative to CAVEs. This study focuses on replicating the CAVE’s VR software, VFIVE, developed at the National Institute for Fusion Science. The newly developed software for HMDs, VOIR, is built on Unity with the OpenXR plugin. It allows for interactive analysis of 3D scalar and vector fields in VR. One of the key features of VOIR is the ability to enlarge and analyze specific regions of large-scale simulation data in real time.

We experimentally investigate the effect of external resonant magnetic perturbation (RMP) on“Edge MHD instability”, which is observed in a discharge with relatively low magnetic Reynolds number and middle beta in the Large Helical Device (LHD) and leads to minor collapse (rapid degradation of volum-averaged beta value by 10%). When the external RMP is small, the instability is continuously observed in a discharge, and the fluctuation amplitude decreases a little with the increment of the RMP. When the RMP is large enough, the appearance of the instability becomes intermittent, and the fluctuation amplitude rapidly decreases with the increment of the RMP. Moreover, the confinement performance degraded by the instability also recovers when the intermittent instability appears.

We have measured the emission line spectra of neutral helium (2¹P - 3¹D, 667.8 nm) in a poloidal cross-section of the plasma in the Large Helical Device (LHD). The measurement has been made from two different observation ports with 40 lines-of-sight each. A tomographic analysis was done to derive the line intensity distribution, using the Hopfield Neural Network method with the Phillips-Tikhonov regularization term. The algorithm was expanded to incorporate Zeeman splittings, a feature prominently evident in the measured spectra, providing information on the position of the emission. The results revealed a concentrated radiation distribution near the X-points and on the divertor legs, specifically those connected to the inboard side divertor plates. We have confirmed that reconstructed images are available even when the analysis is performed solely with data from one of the two observation ports. This suggests the potential applicability of our approach under highly limited options for observation port arrangements. By reconstructing the line emission distribution only using the π peak signals, we show that our method can estimate the vertical emission positions from the Zeeman effect. The robustness of our new method was also checked.

Impurity generation mechanisms including RF sheath sputtering and heating from fast electrons were explored in LHW sustained plasmas on TST-2 spherical tokamak. Molybdenum impurity was measured with a high-resolution spectrometer and the heating effect on a molybdenum target plate was estimated with a fast camera system. The LHW power modulation experiment indicates that the RF sheath sputtering dominated impurity generation from the antenna limiters (molybdenum) under the current plasma parameters. In addition, the target plate insertion experiment shows that molybdenum atoms were released from the target when heated by fast electrons accelerated by LHW. Although the heating effect was negligible for the antenna limiters, it could become significant under higher plasma parameters or during longer pulses.

We simultaneously measured the electron temperature (T_e) and electron density (n_e) using a low-dispersion near-infrared spectrometer in a small-size pellet ablation cloud in Heliotron J, a medium-sized helical-axis heliotron device. We applied the intensity ratio of the Paschen-α, β, γ and to determine T_e based on the collisional-radiative model, which was fairly consistent with the partial local thermodynamic equilibrium (LTE) in the upper principal quantum numbers of 4, 5, and 6. For a typical pellet injection discharge, T_e and n_e were determined to be 0.9 eV and 4 × 10²¹ m^－3, respectively. Our derived generalized empirical calibration curve demonstrates a weak influence of T_e on n_e evaluation, particularly in the range of 0.4 - 2.0 eV. Subsequently, we determined the region where the LTE is achieved for the Paschen series.

Tungsten (W) is one of the major impurities in ITER and future DEMO reactors. However, diagnosing ion density, temperature, and spatial distribution for tungsten ions in low charge states such as W¹⁷⁺-W²⁷⁺ is difficult due to a lack of spectral line data. In this study, we observed tungsten Unresolved Transition Array (UTA) spectra around W²⁰⁺ in Large Helical Device. Furthermore, the emission spectra of tungsten ions ranging from W¹⁹⁺-W²³⁺ were also measured using Compact electron Beam Ion Trap (CoBIT). Two spectral peaks were detected in the CoBIT experimental setup. Subsequently, these peaks were theoretically identified as 5s-5p and 5_p3/2-5d transitions using Flexible Atomic Code (FAC). The identified peaks are useful for impurity diagnostics of ITER edge plasma.

The successful operation of a tokamak requires effective and appropriate methods of plasma fueling. In the development plan for Thailand Tokamak-1 (TT-1), the use of supersonic molecular beam injection (SMBI) has been proposed as a method that can more effectively and deeply deliver fueling gas compared to the gas puffing method. In this study, we used 2D fluid simulation to investigate the impact of SMBI on plasma transport in TT-1. Our model incorporated the continuity equations, energy balance equations, momentum equation, continuity of fuel equations, and momentum equation of fuel. BOUT++ is then used to solve these equations by a finite difference method with the field-aligned coordinates in the edge region of the tokamak. Our simulation results showed that when hydrogen fuel gas is injected into the plasma via SMBI from the low-field side at the speed in the range of 600 - 1200 m/s, the electron density in the edge region locally increases due to dissociation and ionization in the region where the fuel gas meets the plasma. This subsequently leads to a decrease in the ion and electron temperatures. The increased density then spreads throughout the plasma volume within approximately 10 ms. Increasing the injection speed leads to a deeper penetration length for the fuel deposition.

Divertor heat loads are one of the most significant issues affecting fusion reactors. Atomic processes play a crucial role in reduction of the divertor heat load. Notably, elastic scattering between ions and neutral particles can be characterized as large-angle scattering. A large fraction of ion energy is transferred to neutral particles, and the ion direction can be significantly changed by a single large-angle scattering event. In abundant neutral particle regions such as divertor plasmas, the large-angle elastic scattering results in additional ion transport perpendicular to magnetic field lines. Effect of the additional ion transport is expected to be significant at low magnetic field strength and long Larmor radii, such as in a case of advanced divertors (e.g., Super-X and Snowflake divertors). In this study, we investigated the effect of the large-angle elastic scattering at low magnetic field strength and long divertor legs with reference to advanced divertor configurations using an orbital calculation. The large-angle elastic scattering transport is seen to cause a spread in density profiles and a reduction of heat flux. The results of this study show that for the short (long) leg divertor configuration like JT-60U (advanced divertor), the peak heat flux is reduced by around 15% (21%) when the magnetic field strength is 0.5 T in comparison to the model that assumes no guiding center movement due to the elastic scattering. It is also shown that the assumption of isotropic elastic scattering with neutral particles leads to excessive suppression of ion flows.

Turbulent transport by the ballooning mode in tokamak plasmas with edge pedestals is simulated using a reduced set of two-fluid equations. In the absence of the equilibrium poloidal flow, global heat transport by the secondary nonlinear evolution of the resistive ballooning mode turbulence is observed. By examining the effect of the edge shear flow, the global heat transport is suppressed to be almost half, if the edge shear flow is strong enough. A detailed analysis on the radial profile and the poloidal spectrum of the heat flux is newly performed. It is revealed that such confinement improvement is caused by the suppression of the formation of streamer structures that lead to the strong convective heat transport.

Herein, a new aspect of the parallel velocity gradient (PVG)- driven instability is explored. We present its linear stability analysis and investigate the transport properties of the instability, focusing on a specific electron motion called hydrodynamic. In the realm of hydrodynamic electrons, electron motions across the magnetic field are much faster than those along the magnetic field. This electron motion plays an important role in fluctuation transport. This analysis reveals that the PVG convective cell is newly excited, and its feature of particle transport is favorable, since the particle pinch by PVG with adiabatic electrons disappears.

Time evolution of plasma vertical position is estimated by using long-short term memory networks (LSTM) with Time2Vec technique which incorporates temporal information into a neural network. Since many tokamak devices have elongated cross-section in achieving high performance whereas accurate vertical position feedback control is required in order to avoid vertical displacement events (VDEs). Our data-driven model, using experimental data obtained from a small tokamak device PHiX in Tokyo Institute of Technology, can estimate the plasma vertical displacement by incorporating operational scenario coils current data. The model achieved high performance by combining Time2Vec with LSTM. We can also interpret the weights extracted from a trained, data-driven model by comparing the model’s predictions.

Simulating waves in hot plasmas in configuration space is a difficult problem because of the non-local property of the plasma response, which makes the wave equation an integro-differential one. In this research, we conducted an axisymmetric hot plasma full wave simulation at the lower hybrid frequency range with the finite element method. Kinetic effects were introduced in the direction parallel to the magnetic field. We implemented the code so that it can handle an arbitrary velocity distribution function to introduce electron kinetics. An iterative method was utilized to introduce the non-local hot plasma contribution, which is more memory efficient than direct solving. The hot plasma perturbed current density was iteratively calculated in our scheme. For the present simulation, we used the plasma equilibrium obtained by the experiment with the TST-2 spherical tokamak, which is located at the University of Tokyo. The simulation with the realistic profiles successfully converged. The electron heating power deposition profile was estimated for the obtained electric field solutions.

Tungsten is candidate material for plasma facing armor. In our initial study, to improve the plasma facing tungsten on fusion divertor, a new oxide dispersion strengthened tungsten (ODS-W) has been developed using by REDOX (oxidation-reduction reaction) process between W and TiC, including titanium oxide as strengthening nano-particles in matrix, fabricated by mechanical alloying (MA)-hot isostatic pressing (HIP), which can inhibit the decrease of mechanical property even after recrystallization occurs. Our past studies showed that the condition of MA process affects the mechanical and the thermal properties of the products. In the present study, the optimal ball size to be used in the MA process of preparing ODS-W has been proposed by investigation of the relationship between different ball sizes and the MA effect, focusing the optimization of manufacturing process for DS-W. The evolutions of the lattice constant and microstructure were shown to indicate the progress of mechanical alloying. The effect of the ball size was interpreted as that of collision energy delivered by the weight of MA balls.

ITER toroidal field coils are electrically connected to 68-kA main busbars (terminal joints). We propose the measurement of the electric potential distribution in a terminal joint using electrical probes (e-probe method) to inspect the contact resistance in the joint. In this study, we experimented with a mockup of a terminal joint. The test current was 20 A, and the electric potential was measured using the e-probe method at room temperature and 77 K. Nine different degradation patterns were prepared by distributing polyimide films in the joint interface. Next, we performed finite element analysis to investigate the detailed relationship between the electric potential distribution and contact resistance at 300 K and 77 K. In the numerical analysis, the same degradation patterns and test current as in the experiment were assumed. The analysis results agree with the experimental results. Different degradation patterns exhibit different electric potential profiles with 10-µV-scale differences. The analysis results also indicate that the e-probe method works when the contact resistance in the degraded area is larger than 1.0 × 10^－5 Ωmm² at 77 K and 300 K.

Signals generated by spontaneous alpha decay of uranium in fission chambers are very small and are buried in background noise. These signals are often simply discriminated by amplitude as unwanted background noise. However, if these signals can be distinguished from white noise, they could function as a checking source that is electric-noise-free and is embedded in the detector itself. To confirm our hypothesis, we estimated and measured the pulse height of alpha decay signals in the fission chambers of the Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS). When background noise was sufficiently low, the plateau curves of alpha decay signals could be measured by using the maintenance function of the neutron monitoring system. When background noise was high, instruments such as an oscilloscope and a multi-channel analyzer (MCA) successfully caught alpha decay signals by using their high-performance trigger functions. Measuring alpha decay pulses is also an indicator of the health of the detector and its preamplifier because if something were wrong with either component, the alpha decay signals would prove difficult to measure. This practical application of alpha decay signals to verify neutron instrumentation systems that use the noise-sensitive Campbelling method will be beneficial for fusion facilities and fission reactors and help meet ALARA policy because this method does not require an external checking source.

The ITER blanket remote handling system (BRHS) must operate with high precision within the confines of the ITER vacuum vessel, which will become a gamma-ray environment filled with radioactive dust. The main structural component of the BRHS, a telescopic arm, will be covered with a sleeve that must prevent not only the radioactive dust from infiltrating into the system but also lubricant from leaking out into the vacuum vessel. Verifying the radiation resistance of this sleeve is crucial and will ultimately affect the operation scenarios and timing of preventive maintenance for the BRHS. Thermoplastic polyurethane was selected as a candidate material for the sleeve. This paper presents the results of gamma-ray irradiation tests up to either 1 MGy or 2 MGy for polyurethane sheets and the results of subsequent bending, tensile, and hardness tests. Based on the results of these tests, mock-ups of the sleeve were manufactured and were also irradiated up to either 1 MGy or 2 MGy, after which the mock-ups underwent expansion and contraction tests to simulate actual operations in ITER. We have concluded from conservative estimates that, although these polyurethane sleeve mock-ups have over 1 MGy of radiation resistance, the sleeves of the BRHS should be replaced at 1 MGy.

Start-up of a tokamak DEMO reactor using only deuterium (D) fuel without the initial loading of tritium (T) has been assessed from the aspect of fuel cycle and electric power balance. The temporal evolution of plasma parameters and electric power demands for current drive and BOP have been analyzed by means of system dynamics. One of the critical issues for a fusion DEMO reactor is securing the initial loading of T. While this “DD start-up” scenario would be a solution of this critical issue, this operation requires external energy input to maintain the burning plasma condition for several months until sufficient DT burning is established. The plasma temperature increases with the increase of α heating by the growth of T fraction. Consequently, the current drive power, hence, its electric power demand is reduced while the electric power generation increases. The positive net electric power output is anticipated well before the full DT operation is built up. Operational options to improve the performance and sensitivity of the performance to assumed parameters have been also discussed.

A broad, low-energy electron beam extracted from a multi-cusp plasma source through a pair of mesh electrodes produced metallic silver (Ag) from the Ag compound prepared on the surface of titanium dioxide (TiO₂) thin films. The TiO₂ films were grown by thermal oxidation of magnetron-sputtered titanium (Ti) films. The reduced metallic Ag formed the Ag-TiO₂ heterostructure. The reduction of the Ag compound was investigated by varying the operating pressure and the electron beam exposure time. The electron energy distribution function was measured using a retarding potential analyzer, while the electron current density measurements were obtained by collecting the electrons passing through the aperture of a shielded Faraday cup. Characterization of the synthesized heterostructure was performed using X-ray diffraction and Raman spectroscopy to check the chemical structure and composition, UV-Vis spectroscopy and 4-point probe to investigate the optoelectronic properties, and electron microscopy to observe the surface morphology of the films. The electron beam characteristics were correlated with the structure and properties of the synthesized Ag-TiO₂ heterostructure.