Inorganic solid-state materials comprise several types of polymorphs with different structures and properties. Among these, only one of the polymorphs is stable, whereas the others are metastable at ambient temperature and pressure. Sometimes, metastable phases exhibit more curious structures and properties and excellent functionalities than those of the stable phase. Such a phenomenon has been observed in magnetic properties of oxides as well. This paper presents some metastable oxides that manifest magnetic properties and functionalities never observed in their stable counterparts; ferrimagnetism of spinel-type ZnFe₂O₄ with high magnetization and a high magnetic-phase transition temperature; ferromagnetism observed in metastable Eu²⁺-based perovskite-type oxides, such as EuTiO₃, EuZrO₃, and EuHfO₃; and ferromagnetism of the amorphous oxides containing a large amount of Eu²⁺ ions. Amorphous oxides clearly exhibiting ferromagnetic transition are very rare.

This paper reviews the control and the application of ultrahigh hydrogen flux in materials for fusion reactors and future hydrogen societies. Ultrahigh hydrogen flux can be efficiently formed by the combination of hydrogen plasma exposure and the surface modification method, similar to super-permeation. The paper discusses the possibility of the fabrication of oversaturated hydrogen storage materials and the effective tritium removal from the components using the ultrahigh hydrogen flux.

In this study, we demonstrated the principle underlying the three-dimensional (3D) reconstruction of plasma structure by using a soft X-ray camera with multiple pinholes. Time evolutions of seven independent soft X-ray images were captured by the camera. The obtained data were used to reconstruct a 3D soft X-ray emission profile in the reversed-field pinch produced in the RELAX machine, which appeared to be consistent with the forecast in our feasibility study.

Multi-field singular value decompositions (SVDs) is applied to turbulence obtained in a cylindrical magnetized plasma, PANTA. This method enables us to obtain the spatial mode structures with common temporal evolution of different physical quantities, such as the fluctuations of density and flows. Turbulence driven particle transport is evaluated by the method. It is shown that only the coupling of the same mode drives the transport, which stems from the orthogonality of the SVD. Thanks to this characteristics, the number of degrees of freedom which plays roles for the transport dynamics could be significantly reduced.

Anisotropic-Ion-Pressure (AIP) fluid model describes more accurately the characteristics of plasmas in open-magnetic-field systems, e.g., magnetic mirror, scrape-off layer, and divertor. AIP model combined with the virtual divertor model has a merit to be free from the boundary condition of flow speed at the end plate [S. Togo et al., J. Comput. Phys. 310, 109 (2016); Contrib. Plasma Phys. 58, 556 (2018)]. In order to compare the AIP modeling with the conventional plasma fluid modeling by using only a single AIP code, we introduce a kind of “viscous-flux approximation (VFA)” into the AIP energy equations, where the anisotropic pressure relaxation term is modified artificially to equalize the pressure anisotropy with the parallel viscous stress. One-dimensional simulations are carried out with the original AIP modeling and with the VFA-AIP modeling, respectively. Expected numerical results are obtained for a simple mirror configuration.

Some local turbulence analyses in tokamaks have shown that inward particle fluxes are induced in inversed density gradient regions. To evaluate the particle transport effect, we carry out global simulations using a 5-field reduced MHD model with an inversed density gradient near the plasma edge. Outward particle fluxes in the steeper gradient region are dominant, which mainly determine the evolution of the density profile. On the other hand, inward particle fluxes are also induced in the inversed gradient region. The electron drift modes are possible to become unstable with negative density gradients. The contribution of the inward particle fluxes is found to be present and its fraction is increased with nonlinear excitation of low n modes in the nonlinear phase, where n is the toroidal mode number.

The runaway electron (RE) beam formation in JA DEMO is simulated using the integrated disruption code INDEX. It is shown that the gamma-ray flux is comparable with that of the published results for ITER and the Compton scattering of gamma rays governs an irreducible minimum of the RE seed (~ 0.01 - 0.1 A), which can lead to multi-mega-ampere RE beams in the absence of significant radial transport of REs.

This paper is the first to define a conceptual design of a fusion power complex with hydrogen storage capability in a superconducting magnet system using cryogenic hydrogen. The fusion power complex realizes a carbon-neutral society by combining a large-capacity power source of renewable energy with the flexibility of energy output. Therefore, the proposed design is positioned as one of the most important next-generation power sources.

The manufacturing of tungsten (W) - graphite bonded divertor components for the Large Helical Device (LHD) has been investigated. The spark plasma sintering method was used to bond W and graphite with titanium (Ti) interlayer. Small specimens were fabricated to investigate the bonding strength and to diagnose the bonding interface. The granular structure was formed in the grooved area on the graphite surface. It was suggested that this granular structure had affected the bonding strength.

Tungsten-rhenium alloys are more attractive due to their excellent characteristics, such as high ductility and improved mechanical properties. They can be expected to become some of the candidate materials for the next-generation of nuclear fusion reactors. We have successfully achieved crack-free laser processing in high-doped tungsten-rhenium alloys using a nanosecond Q-switched Nd:YAG laser system. In addition, the connection between the number of cracks and grain size on different rhenium doping concentrations is also characterized.

This article presents the concept of unlimited fusion-energy station (U-FESTA), comprising a fusion power plant and a lithium (Li) recovery facility to recover Li from seawater, for example the Li-separation method by ionic conductor (LiSMIC). To realize the U-FESTA, we discuss the relationship among the seawater-based Li-recovery efficiency, blanket technology on ⁶Li enrichment, intake cooling seawater in a power plant, and the blanket-replacement period.

We constructed the cost-effective polychromator system for multi-channel Thomson scattering measurement systems, in which scattered light is reflected between two angled interference filters. The number of interference filters was reduced to less than half of the number of wavelength channels, by using the reflections between two filters whose transmitted wavelength decreases with its angle of incidence. The developed polychromator successfully separated five different wavelength bands (1055.5, 1044.9, 1050.0, 1035.0, and 1024.3 nm), reducing its cost by saving the number of interference filters.

A database has been developed to provide a highly efficient data analysis environment by registering various types of data obtained in the QUEST (Q-shu University Experiment with Steady-State Spherical Tokamak) experiments and providing the data to researchers through standard SQL query output. In conventional experiments, measurements are usually obtained using several instruments, and the results are often stored separately in proprietary formats. An analysis to collect distributed files and convert them into appropriate formats individually was necessary, which led to a decrease in analysis efficiency. In order to build a system to centrally store and provide data, we constructed new servers, each for registering data in a database and for extracting data from the database and visualizing the data. As a result, various real-time monitoring data can be registered in the database, and data retrieval for analysis can be performed easily with a unified user interface. The construction of this database has made it possible to easily retrieve real-time monitoring data and a wide variety of table data, enabling analysis from a new perspective, and improving analysis efficiency.

To extract intense ion beams with good beam optics from ion sources, controlling the distance d_eff between the plasma meniscus (i.e., beam emission surface) and the beam extraction grid is important. This study conducts a novel investigation into the dependence of the effective distance d_eff on the amount of surface H^- production S_H-. For this purpose, a 3D PIC (three dimensional Particle-in-Cell) simulation is conducted to obtain a model geometry of the extraction region for a H^- ion source with S_H- as a parameter. Based on results, d_eff significantly depends on S_H- and the H^--electron density ratio (α = n_H-/n_e) in front of the extraction aperture for the same plasma density; as S_H- increases, d_eff decreases. The results suggest that S_H- is critical for controlling d_eff and the resultant beam optics extracted from the negative ion source.

A new experimental method using a Langmuir probe to measure negative-ion density is introduced in an area that cannot use lasers, such as the extraction hole. An electron reduction Langmuir probe model is established to distinguish negative ions from electron density. A comparison between density measured by the Langmuir probe with the electron reduction model and a photo-detachment Langmuir probe is performed to verify the use of the newly developed model.

A new method of vector tomography by laser-induced fluorescence (LIF) spectroscopy is proposed for the evaluation of the velocity field of quasi-coherent structure in linear magnetized plasma. Here, the two-dimensional velocity field was reconstructed with measuring synchronization of azimuthal reference probes and line-integrated LIF. The laser installation condition is designed to move in a radial direction perpendicular to the laser path. We applied the proposed method to a quasi-coherent flow obtained from a direct numerical turbulence simulation and demonstrated that the Reynolds stress and its force can be evaluated if its flow structure is maintained.

High precision measurements and accurate modeling of atomic polarization under three-dimensional radiation transfer are crucial to understand the structures of magnetized solar plasmas. To develop and validate spectropolarimetric measurements and analyses, we set up an inductively coupled plasma (ICP) generator designed especially for ∼1-eV plasmas interacting with radiation and weak magnetic fields. The device was put in front of the focal plane of the Horizontal Spectrograph of the Domeless Solar Telescope at Hida Observatory of Kyoto University. In helium discharges, the typical electron temperature, electron density, and helium column density of the ICP are comparable values to those of solar prominences, and the direct comparison of spectra shows almost the same opacity at He I 1083 nm. Magnetic and radiation fields were introduced to the ICP, and the system successfully reproduced reasonable spectropolarimetric signals as compared with those from the solar prominences.

We attempt to interpret the different trends of collected ion currents of the Langmuir probe and the ion sensitive probe (ISP) observed during N₂ and H₂ seeding in hydrogen plasmas in the divertor simulation experimental module (D-module) in GAMMA10/PDX. The current measured at the ion collector electrode of the ISP shows two distinct bumps. Results can be interpreted with the aid of spectroscopic measurements related to the dominant reaction processes, where nitrogen-induced molecule-activated-recombination (N-MAR) mechanisms produce various ion species, resulting in different measured currents. These experimental findings provide a further understanding of the most relevant plasma-neutral processes occurring in a detached-like plasma scenario in the presence of nitrogen, a candidate for impurity seeding in ITER.

Electron cyclotron heating (ECH) assisted start-up is considered to be necessary for reliable start-up of tokamaks with superconducting central solenoid (CS) because of the low loop voltage. Pure Ohmic start-up using the CS requires the field-null configuration to minimize electron loss. For ECH assisted Ohmic start-up, the trapped particle configuration (TPC) was found to have wider operational parameter space than the field-null configuration experimentally. In this work, we have analyzed electron transport under the TPC using the orbit-averaged kinetic equation. The global electron distribution function was simulated by solving for the steady-state distribution function. The parameter boundary for successful pre-ionization was estimated by evaluating the net particle number growth rate from the total ionization rate and the particle flux out of the limiter boundary. Upper limit of the ECH power was predicted as that for the net particle number to grow. In the absence of the inductive electric field, the simulated high ECH power limit increased with neutral pressure and vertical field strength, consistently with the experimental results. Application of loop voltage did not change this behavior qualitatively up to the inductive electric field of 0.48 V/m, which is the typical range of low voltage start-up experiments.

In the magnetic plasma fusion community and the solar physics research community, different velocimetry algorithms have been used. Those are: orthogonal dynamic programing based particle image velocimetry (ODP-PIV) and local correlation tracking (LCT), respectively. In this paper, a systematic comparison of these velocimetry codes is performed using synthetically produced turbulence data. The spatial scale of a typical turbulence pattern is scanned to examine the sensitivity of these codes on the tracer pattern property. Use of an LCT code is recommended when the ratio of the turbulence pattern size to the spatial resolution is small.

A digital phase analysis technique was applied to a 119-m wavelength far-infrared (FIR) laser interferometer on the Large Helical Device (LHD). High-density plasma measurement without phase jumping was achieved, representing an improvement over measurements made using a conventional analog phase counter. Digital phase analysis became operational at a quarter of the threshold of the analog phase counter. The phase sensitivity was also improved using digital analysis because the noise level was reduced to approximately half that of the conventional analog phase counter. The improved phase sensitivity enabled measurement of small-amplitude fluctuations.

Fast ion diagnostic is one of the most crucial plasma diagnostics for nuclear fusion investigation. A new diagnostic method for fast ions has been proposed using visible spectra of ³He produced by a deuteron-deuteron reaction. This diagnostic method has a better energy resolution than methods using neutron/γ-ray and is superior to conventional spectroscopy in measuring high energy (MeV order) ions. This diagnostic method has been predicted using numerical analysis for ITER, but no verification experiments have been performed yet. In this study, we examined the measurability of this diagnostic method in the large helical device (LHD) deuterium plasma. Although very dependent on the measurement geometry and the spectrometer performance, it may be possible to measure the fast ³He visible spectrum.

To aim at the realization of a helical fusion reactor, we study multi-objective optimization of coil shapes, which satisfy various requirements. In the magnetic field configuration created by these coils, several unfavorable examples are found: some of them have magnetic islands or doublet configurations. In order to automatically and quickly exclude such cases that hinder the optimization, we have developed a new method to detect unfavorable magnetic surfaces by using image recognition. Binarization and erosion are performed as preprocessing, and then blanks of magnetic islands and doublets are extracted as recognition targets. Consequently, we have developed a classifier with high performance. Using this trained classifier, we have shown that almost all cases with unfavorable magnetic surfaces in various magnetic configurations can be excluded in a short time and with high precision.

A laser fusion rocket has been proposed for interplanetary flight. This rocket generates a high-energy plasma via laser-fusion and expels it via a magnetic nozzle. The magnetic nozzle is a key component for the rocket performance, and we have investigated the impulse bit generated from the magnetic nozzle by using simulations and experiments in relatively low energy regime of a few joules. In addition, the energy dependence on the thrust performance is an essential factor to evaluate and design the laser fusion rocket. Here, we conducted numerical simulations in energy regimes from a few joules to mega joules to understand the thrust performance both for small-scale experiments and for full-scale fusion rockets. We find that the momentum efficiency does not depend on the propellant mass, material, and plasma energies, and the impulse bit is expressed as the power-law of the plasma energy and mass. These relations are important for designing missions and for estimating the thrust of fusion rockets.

The Thomson scattering control system has been modified to measure the time evolution of the electron density and temperature profiles during long duration discharges in the QUEST spherical tokamak. The system consists of a signal generator and a control circuit. The former accepts a QUEST main trigger and provides multiple triggers, each of which starts a short-term (e.g. 15 s) measurement. The latter provides triggers to synchronize the oscilloscope and laser oscillation during a short-term measurement. The system was used in 1000 s long duration discharges in QUEST, and the temporal evolutions of density and temperature profiles were obtained successfully. It was found the profiles are stationary after about 300 s.

The Remote Handling Electrical Connector (RHEC), crucial for in-vessel diagnostics in ITER, enables connection and disconnection through Remote Handling operations. Partitioning electrical lines between the Lower Port sidewall and the Diagnostic Rack, the RHEC connects the Mineral Insulated cable bundle. The design concept omits components like core wire extensions with ceramic sleeves, avoiding overheating and power loss due to impedance mismatching during the mirror cleaning system's plasma generation. The prototype was tested for torque measurements and positional misalignment, proving the RHEC feasibility and remote handling compatibility.

Insulated inter-coil structure components are installed to ITER TF coils to fix the aliment of the coils. In the cases of Intermediate Outer Intercoil Structures (IOIS) and Outer Intercoil Structures (OIS), custom machined sleeves (CMSs) with Al₂O₃ coating are installed in the holes of TF coil structure. The IOIS/ OIS CMSs are possibly stacked in the IOIS/ OIS holes during their cold installation, and surface of their Al₂O₃ coating would thermally contact with the IOIS/ OIS holes tightly. Thus we propose a method to remove the CMS without damaging its insulation. In the method, the IOIS/ OIS CMSs are removed by cooling with liquid nitrogen (cold removal). The gaps of the IOIS/ OIS CMSs and holes (∆) are influenced by contact thermal resistance between them (R_ct). Thus, in this study, we first numerically estimated the gap between the IOIS/ OIS CMSs and holes. Then, the cold removal was experimentally demonstrated with mock-ups of IOIS/ OIS holes and CMSs. In addition, an insulation test of the Al₂O₃ coating to confirm their soundness after the cold removal. It was numerically shown that ∆ became > 57 μm, which was sufficiently large considering the manufacturing tolerance of IOIS/OIS holes, even if R_ct is 0. The experimental result showed that IOIS/ OIS CMSs were successfully removed from the IOIS/ OIS holes. Moreover, the insulation test demonstrated that the cold removal did not influence on the insulation resistance of the IOIS/ OIS CMSs.

Metal surfaces with sub-wavelength structures form a plasmon polariton-like surface mode, i.e., spoof-plasmon. The spoof-plasmon on a corrugated disk propagates radially and is reflected at the edge, resulting in formation of plasmonic cavity. With a concentric circular corrugation, the excited spoof-plasmons form an axisymmetric plasmonic cavity. With a spiral corrugation, the spoof-plasmons have non-zero orbital angular momenta and form a non-axisymmetric plasmonic cavity. Spoof-plasmons consisting of the plasmonic cavity transfer their angular momenta to radiation waves via the corrugated hollow waveguide by conserving their topological charges.

A buncher and a magnetic lens were introduced to improve the beam flux density of a pulsed positron beam extracted from the low energy positron accumulator at National Institute of Advanced Industrial Science and Technology (AIST). The buncher made the pulse width ~ 1/4 and the magnetic lens reduced the beam cross section ~ 1/9, which resulted in about 36 times increase in the beam flux density. Possible applications for electron-positron plasma experiments are also discussed.

A low voltage compact ion mobility spectrometer type charged particle analyzer is being investigated to test the performance for diagnostics of ion/electron transport in an atmospheric pressure environment. The 44 mm diameter, 53 mm long device consists of a mesh gated shutter, a 35 mm long drift region, and an end-plate detector. By applying a cyclic positive bias followed by a negative bias to the shutter region, subsequent attraction and expulsion of charged species is observed with the applied voltage less than 100 V. Mesh size dependence of the shutter gated current at the detector shows that across 0, 30, 100, and 200 mesh sizes, the 30 mesh size realized the transport of the most number of ions towards the detector. This mesh size also presents a well-defined currenttime derivative which allows the measurement of ion mobilities in electric fields with the intensity less than 33.3 V/cm. Analysis of peaks observed after the onset of the positive phase shows the presence of O⁺ ions in the swarm with a reported experimental mobility of K₀ = 2.97 cm²/Vs.

Direct numerical simulations (DNS) and large eddy simulations (LES) of homogeneous Hall magnetohydrodynamic (MHD) simulations are carried out to verify the properties of a sub-grid-scale (SGS) model which has been developed for LES recently. LES with the new SGS model reproduces one-dimensional spectra of DNS. It is also shown that the probability density functions (PDFs) of the current density components of DNS and LES in the grid-scale coincide with each other by an appropriate normalization. We verify by this numerical study that our improved SGS model is applicable to homogeneous Hall MHD turbulence. We also find that the difference in the deviation of the current density components is smaller in the MHD-scale of LES than in that of DNS. These results provide a new insight to study the spectral anisotropy of turbulence, especially in relation to the sub-ion-scale.

Collisional merging experiments of field-reversed configurations (FRCs) have been conducted in the FRC Amplification via Translation-Collisional Merging device at Nihon University. Self-organized FRCs have been observed after the dynamic process with destructive disturbance of the collision/merging of plasmoids at super-Alfvénic speeds. A compact and component-based tomography camera (T-cam) has been developed to obtain the internal structure of plasmoids during and after the collisional merging process using a computed tomography technique. The plasmoids radially expand by a factor of 2 within a few microseconds in the collision and merging phases. Because the viewing angle of the T-cam was limited by the inner diameter of the observation port, the field of view was insufficient for the accurate reconstruction of the internal structure. In this study, the incident optical component of the T-cam is improved to provide a sufficient viewing angle to observe the whole process of collision and merging of plasmoids. The newly-built component, comprising a doublet lens with two planoconvex cylindrical lenses at different focal lengths, expands the viewing angle from 60^◦ - 100^◦. Simulation results with dummy data indicate that the internal structure of plasmoids during the dynamic process can be reconstructed with the developed system.

Measuring abundance of iron-group elements (IGEs; Cr, Mn, Fe, and Ni) is essential for understanding astrophysical phenomena such as supernovae. The L-shell emissions from IGEs will soon be resolved by next-generation X-ray satellites, enabling more accurate measurements of abundance. However, theoretical calculations for these L-shell transitions have not been sufficiently validated using experimental work. Herein, large helical device (LHD) experiments that measured L-shell emission from Ni and Mn are reported. The temporal evolution of LHD plasma is characterized by three phases, each of which resolves different L-shell emissions. We modeled EUV-Short spectra considering LHD plasma structure with AtomDB, an atomic database widely used in the X-ray astronomy community. We discovered that the observed intensity ratios of the 3C (2p⁵3d^{1 1}P₁ → 2p^{6 1}S₀) to 3D (2p⁵3d^{1 3}D₁ → 2p^{6 1}S₀) transition of Ne-like ions are 0.4 - 0.6 times lower than the theoretically predicted ratios.

We numerically investigated the ion behavior in radio frequency (RF) plasma with a pulsed magnetic field applied to a high-current ion source. A one-dimensional hybrid particle-in-cell (PIC) simulation was performed. The numerical results demonstrate the generation process of an ion beam by applying a pulsed magnetic field to the RF plasma. The pulsed magnetic field induces an electric field owing to the interaction between the plasma and the magnetic field. The induced electric field forms a density gradient, and an electrostatic field is generated. The electrostatic field propagates as an ion acoustic wave; some ions in the plasma are accelerated, and an ion beam is generated.

A challenge in the in-situ visualization is the proper setting of the visualization’s viewpoint. While visualizations should focus on a small part of the simulation region or region of interest (ROI), the appearance/disappearance and the motion of ROI are usually unknown beforehand in simulations of complex phenomena. In our previous paper, we proposed to make the visualization camera a kind of autonomous agent or a “smart camera”. In this approach, the camera agent locates itself near an ROI and keeps an appropriate distance to track the motion of the ROI. The next step in our approach is to increase the number of camera agents to visualize an ROI from multiple perspectives. A problem in introducing multiple camera agents resides in the rules for the agents’ motion. In this paper, we propose to use virtual materials, or pheromones, that trigger social responses in camera agents.

To understand the mechanism of particle acceleration in collisionless shocks, generating a collisionless plasma flowing through dilute gas is required. We have proposed a compact plasma focus (CPF) device to generate collisionless plasma by using pulsed-power discharge. To discuss a particle acceleration process in collisionless shocks, it is necessary to evaluate an ion energy distribution function (IEDF) of the plasma. In this study, we measured the IEDF of the plasma flow using a retarding field energy analyzer (RFA). The experimental results measured by the RFA showed that ions with a few eV are dominant in the plasma flow generated by the CPF device. The IEDF could be fitted with the shifted Maxwellian distribution. The plasma parameters were estimated to be the ion number density n_i = 4.6⁺₋⁰₀^._.²₈ × 10¹⁹ m⁻³, the ion temperature T_i = 0.8⁺₋⁰₀^._.⁶₄ eV, and the drift velocity v_d = 11⁺₋⁰₃^._.⁵₅ km/s. The estimated velocity approximately agreed with the result of a time-of-flight method calculated by the ion current waveform.

To study DEMO divertors, some linear devices have attempted to create DEMO-grade high-density steady-state divertor simulation plasma in a stronger magnetic field. Helicon plasma sources using a flat-type antenna, which are one of the methods of radio frequency (RF) plasma generation in magnetic field, are expected to achieve this. In this study, we developed a new RF plasma source with a two-turn flat-loop antenna with RF sources having a maximum output power of 30 kW in continuous waves. A two-turn flat-loop antenna is expected to have a large-diameter discharge, and its radial density profile can be controlled. In addition, a water-cooled, double-disc quartz window unit was installed for a high-density, high-power discharge. In the initial experiment, argon and hydrogen plasmas were generated, and the integrity of the new device up to 5 kW, was confirmed in a discharge experiment. The magnetic field conditions were changed in this experiment, and a mode change from capacitively coupled plasma (CCP) to inductively coupled plasma (ICP) during argon discharge was observed. Under the experimental conditions of this study, neither argon nor hydrogen produced helicon plasma. This study hereby, discusses the plasma characteristics and provides guidelines for future development.

We perform experiments where lithium ion and electron plasmas are simultaneously confined in a nested trap to detect two-fluid plasma states. However, the behavior of charged particles in the nested trap is unexplained. Results of experiments demonstrate that the number of charged particles in the outer wells of the nested trap rapidly decreases and that peculiar deformations in ion and electron plasmas cause in some conditions.

The plasmas whose current is driven by the lower-hybrid wave (LHW) alone have been investigated on the TST-2 spherical tokamak. We are developing a probe to obtain the information about energetic electrons generated by the LHW. We investigated a detector, motivated by the lost fast-ion probe (LIP) or fast ion loss detector (FILD), which can identify the energy and pitch angle of captured ions. However, the pitch angle of electrons of interest is expected to be so small that it was found that a conventional configuration with the normal vector of an entrance-orifice surface almost perpendicular to the magnetic field line was not applicable. Thus, we accessed the feasibility of the configuration with two tiny orifices by which orbits, or the energy and pitch angle,of detected electrons can be well specified. We carried out numerical calculations by using calculation conditions and geometries which are manufacturable. The results suggest that it is difficult to identify the energy of detected electrons, but that it is possible to identify their pitch angle well. Based on the results, we designed the diagnostic system with 3 channels which will detect energetic electrons with the pitch angle of 3 - 5, 5 - 9, and 9.5 - 12.5 degrees, respectively.

A large volume and fast response gamma ray diagnostic based on the LaBr₃(Ce) scintillator was installed to obtain the gamma ray spectrum in the Large Helical Device (LHD) for understanding energetic ion confinement. The advantages of the LaBr₃(Ce) scintillator are relatively sensitive to gamma rays due to its relatively heavy weight density of 5.3 g/cc, high counting operation because of a relatively short pulse width of ∼100 ns, and relatively better energy resolution of ∼3%. The gamma ray diagnostic was installed at the outboard side of LHD. The radiation shielding for the LaBr₃(Ce) detector was designed to avoid unwanted signals due to stray neutrons and gamma rays using the three-dimensional radiation transport calculation MCNP6. In-situ energy calibration of the LaBr₃(Ce) detector was performed using ⁶⁰Co and ¹³⁷Cs gamma ray sources. We surveyed a neutron effect on the LaBr₃(Ce) detector in an electron-cyclotron-heated deuterium plasma discharge. The pulse counting rate of LaBr₃(Ce) detector under the total neutron emission rate of 2×10¹¹ n/s was 110 kcps. Therefore, the LaBr₃(Ce) detector is expected to be utilized in most of ion cyclotron resonance frequency (ICRF) discharges, where the total neutron emission rate of ∼10¹¹ n/s. We plan to measure the gamma ray spectrum in deuterium ICRF discharges.

We have applied the multivariable analysis technique called the proper orthogonal decomposition (POD) to both the divertor particle flux distribution and the electron pressure distribution in the core region of LHD. The cross-correlation analysis indicates that 3rd, 4th, and 5th POD modes of the electron pressure distribution are highly correlated with the divertor footprint index which is a measure of where the peak position of the particle flux distribution is located on the inner divertor plate. Both the 3rd and 4th modes seem to correspond to the shift of the electron pressure peak position from the magnetic-axis radius. In contrast, 5th mode has a strong influence on the peripheral gradient of the electron pressure distribution. Their relationships with the divertor footprint could be explained by the finite βand the Pfirsch-Schlüter current effects.

The relationship between the edge toroidal field B_t and the rotation of the spatial structure of visible light emitted from reversed field pinch (RFP) plasmas is reported. The rotation of the spatial structure of visible light is captured by a high-speed camera attached to a horizontal viewing port of the RELAX, which was designed for RFP experiments. It is found that the rotation velocity and the direction of the spatial structure of visible light depend on B_t. In the non-reversal case, the structure rotates in the co-current direction, while in the counter-current direction in the reversal case. The frequency and direction of the spatial structure of visible light are consistent with magnetic fluctuations measured by magnetic probes.

A design study of a line integrated Thomson scattering (TS) system with a complete backscattering configuration has been performed to clarify practical problems. Installation on the TST-2 spherical tokamak was assumed, and an optical system design with ready-made components, are adopted. Some practical aspects, such as aberration, masking effect of the laser combining mirror, misalignment are investigated by ray tracing calculations. The performance of density profile reconstruction was also investigated. It was found that a tangential-multi-chord measurement configuration on the midplane shows a good effective localization, and error enhancement in the reconstruction is small. In addition, the efficiency of the system is about an order of magnitude larger than the present conventional TS system in TST-2. The attractiveness of the line integrated TS measurements was demonstrated.

Plasma radiation intensity distribution plays a crucial role in the energy balance and impurity transport during plasma detachment. An InfraRed imaging Video Bolometer (IRVB) is a radiation distribution measurement system, that uses an infrared camera to measure the temperature distribution on a thin foil caused by plasma radiation. Herein, an IRVB measurement system is installed in the west plug/barrier cell of the GAMMA 10/PDX, which is upstream of the divertor-simulated plasma. In the case of Ar injection, a clear difference is observed in the plasma radiation intensity depending on the incident gas pressure. However, during Ne injection, the difference in the plasma radiation intensity is not clear. Compared with the case of Ne, the radiation intensity is higher when Ar gas was injected.

In the magnetic reconnection in the existence of a high guide field, the inductive reconnection electric field and the static in-plane electric field are generated in the reconnection upstream and downstream regions. This self-generated static electric field determines the plasma motion and has large impacts on the energy conversion process in the guide field reconnection. In this paper, to elucidate the effect of the in-plane electric field, a novel experimental technique is proposed to actively control the electrical boundary condition of the magnetic field lines in the downstream region. Moreover, the capability of the developed technique was demonstrated by showing the controllability of the in-plane static electric field using separate electrodes with fast semiconductor switching devices.

© 2023 The Japan Society of Plasma Science and Nuclear Fusion Research

This report compares results ion-cyclotron range of frequencies (ICRF) plasma production at hydrogen minority regime in Uragan-2M (U-2M) and Large Helical Device (LHD). The condition of the presence of the fundamental harmonic ion cyclotron resonance zone for the hydrogen inside the plasma column should be fulfilled for this method. The scenario is successful at both machines and weakly sensitive to the variation of the hydrogen concentration in the H₂+He gas mixture. It should be noted that at LHD the start up is slower than at U-2M. The comparison of plasma production in ICRF with hydrogen minority at U-2M and LHD indicate that this scenario can be scaled to larger stellarator devices. The experiments made are the base for the proposal for usage this scenario for plasma production in ICRF at Wendelstein 7-X at magnetic field reduced to 1.7 T.

The collisional merging formation process of field-reversed configurations (FRCs) was observed by using a recently developed spectroscopy system. The shape and boundary of the two plasmoids during the merging process were visualized by the “contrast medium” mixing. One of the two initial plasmoids captured a small amount of contrast medium (e.g., helium) diffused in a part of the vacuum vessel during the translation process. The contrast medium-mixed plasmoid merged with the other plasmoid which is without a contrast medium. The contrast-enhanced shape and internal structure of two plasmoids were observed using the spectroscopic systems with the line spectrum of the contrast medium ion. The emission distribution of the contrast medium ions inside the plasmoid was verified by visible-light computed tomography, which revealed that the contrast medium ions were captured only inside one plasmoid. In addition, the emission distribution resembled the density profile in the rigid rotor model, which is a pressure equilibrium model of FRCs. After the collision, the contrast medium ions diffused into the other side. This suggests the beginning of the merging with the plasmoid on the other side. The optically evaluated timescale of the merging was comparable with that observed by the internal magnetic measurements.

In this study, a previously proposed three-dimensional (3D) tomography method using a multiple-pinhole camera is applied experimentally to detect 3D structures in reversed-field pinch plasmas. The 3D structure of the toroidal plasma was successfully reconstructed and found to be consistent with magnetic field measurements at the plasma’s edge. The results also indicate that toroidal mode number estimation may be possible using the images from the multiple-pinhole camera.

In fusion plasmas, energetic ions play a crucial role in plasma heating. Nuclear elastic scattering (NES) is a non-Coulombic scattering process that affects the energy transport between energetic and bulk ions. In the Large Helical Device (LHD), energetic protons produced by neutral beam injection (180 keV) formed a knock-on tail (KT) in deuterons via NES, and the DD neutron emission rate increased by one order of magnitude in relatively high-electron-temperature plasmas. Furthermore, the effect of NES among ion cyclotron range of frequency (ICRF) tail protons and bulk deuterons was investigated at high-electron-temperature plasmas in the LHD. It was found that DD neutron emission rate was increased by a factor of 2 to 4. Changes in ion temperature and plasma density cannot be the only reasons for the increase in DD neutron emission rate. The increment in DD neutron emission rate was reproduced by the Fokker-Planck simulation using the Boltzmann collision integral for NES by assuming the ICRF-tail protons having a high-temperature Maxwellian.

In this paper the prospect of experimental investigation of phase-space turbulence in magnetically confined fusion plasmas is assessed, motivated by a theoretical suggestion of its potential role in turbulent anomalous transport conundrums. Three experimental approaches regarding (i) fusion plasma experiment, (ii) basic plasma experiment, and (iii) simulation data analysis are considered, accounting for challenges and expected outcomes. Platforms for experiments ranging from high-temperature fusion plasmas to low-temperature linear plasmas, are overviewed. Interdisciplinary interests regarding phase-space turbulence in magnetosphere plasmas and laser wake-field acceleration are also discussed.