As a signal processing method for fast digitizers of the switched-capacitor-type in Thomson scattering diagnostics, a “model fitting” method is proposed. An ideal shape of the signal is estimated by this method by averaging many Thomson scattering signals. After applying this method to a relatively low density LHD plasma,the scattering of electron temperature profiles becomes small. The magnitude of error is also reduced by about 60% at some spatial channels in the core plasma. Simulations of signals with some noises based on the JT-60SA Thomson scattering system enables a showing of the expected error in electron temperature. The error can be suppressed by the “model fitting” method.

The results of the plasma start-up with ICRH of U-2M RF discharges in H₂+He mixture with newly implemented controlled gas H₂ concentration are presented. The W7-X like ICRH antenna operated in monopole phasing with applied RF power of ∼ 100 kW. We investigated plasma start-up in the pressure range p = 6×10⁻⁴ - 9 × 10⁻² Pa. Plasma production with an average density of up to N_e ∼ 10¹³ cm⁻³ was observed at frequencies the fundamental harmonic of the hydrogen cyclotron frequency.

The capability of plasma current control by the second harmonic electron cyclotron current drive in the quasiaxisymmetric stellarator CFQS is investigated. We used the ray-tracing code TRAVIS to evaluate the electron cyclotron (EC) wave power deposition and driven current. In the standard magnetic field configuration of CFQS, the poloidal distribution of the magnetic field is nearly axisymmetric, i.e., equivalent at all toroidal positions as tokamaks. In the calculation, a flat electron density profile at the core region with n_e0 = 1 × 10¹⁹ m⁻³ and a center-peaked electron temperature profile with T_e0 = 3.5 keV are assumed. The EC wave beam direction is scanned mainly in the toroidal direction, aiming at the plasma axis. The vertical injection angle of the beam and magnetic field strength are varied and optimized to keep on-axis power deposition to maximize driven current at each toroidal direction of the EC wave beam. According to the calculation, the maximum driven current at optimum beam direction, with an expected maximum EC wave power of 400 kW, is approximately 80 kA. Meanwhile, approximately 26 kA of bootstrap current in CFQS with the volume-averaged β value of 1.2% is estimated using the BOOTSJ code. Hence, sufficient on-axis EC-driven current can be expected for compensation of the possible bootstrap current, although the current profiles are different. Moreover, a driven current of over 30 kA can be expected even in extreme cases where the magnetic field on-axis has ripples by modified modular coil currents by 20%. The possibility of compensation of bootstrap current in total amount and current profile is also discussed.

A radiative collapse predictor has been developed using a machine-learning model with high-density plasma experiments in the Large Helical Device (LHD). The model is based on the collapse likelihood, which is quantified by the parameters selected by the sparse modeling, including n_e, CIV, OV, and T_e,edge. The control system implementing this model has been constructed with a single-board computer to apply this predictor model to the LHD experiment. The controller calculates the collapse likelihood and regulates gas-puff fueling and boosts electron cyclotron resonance heating in real-time. In density ramp-up experiments with hydrogen plasma, high-density plasma has been maintained by the control system while avoiding radiative collapse. This result has shown that the predictor based on the collapse likelihood has the capability to predict a radiative collapse in real-time.

A super-Alfvenic/sonic collisional merging formation of field-reversed configurations (FRCs) with low-density and high-temperature initial-FRCs was attempted on the FAT-CM device at Nihon University. To vary the density and temperature of initial-FRCs, the low-density/high-temperature (LD/HT) FRC formation technique was applied to the initial-FRC formation. The electron density of initial-FRCs formed using the LD/HT FRC formation technique was reduced to about 50% of that in the standard cases. The ion temperature was increased as the electron density decreased because the plasma pressure completely balances with the external magnetic pressure in an ideal FRC. The ion mean-free-path also increased to the equivalent value of the diameter of the initial-FRCs. Therefore, the initial-FRCs will be collision-less. These collision-less initial-FRCs were successfully translated. The observation results of the collisional merging formation process of FRC from the internal magnetic probe array and two axially arranged interferometers indicate that the performance of the FRC formed after the collision and merging declined in cases with collision-less FRCs and it depends on the kinetic energy in the collision process.

Plasma equilibrium in ADITYA/ADITYA-U is provided by two pairs of vertical field coils (BV1 & BV2) placed outside the vessel. A peak loop voltage of ∼20 V is required for successful breakdown and start-up in ADITYA, which leads to a higher I_P ramp-rate ∼6 - 8 MA/Sec during the first ∼7 ms of discharge. To hold the plasma column in equilibrium, the vertical field should also be ramped-up at the same rate. Series connections of vertical field (BV) coils do not provide the required ramp rate due to the high L/R time-constant of the coils and 12 pulse converter firing. Therefore, additional arrangements are made to achieve it. The addition of a pre-charged capacitor of 500 μF/3 kV with VF converter based power supply allows successful start-up but causes concern about a slight dip that is observed in the plasma current. To obtain proper stabilization, two techniques are used. One is the paralleling of BV coils, and second is using the combination of another capacitor bank of 19.5 mF/1.2 kV and IGBT based power supply have improved the plasma performance and raised the I_P ∼ 150 kA with dI_P/dt ∼ 3.0 - 3.5 MA/s in ADITYA. In this paper, the effect of the equilibrium field in accordance with plasma performance is discussed in detail.

The effects of collision velocity and mirror ratio on the collision/merging processes of field-reversed configurations (FRCs) were experimentally evaluated. In the collisional merging experiment, the reversal field structure is reformed despite experiencing destructive perturbations resulting from collisions at supersonic/Alfvénic velocity. However, reformation of the reversal field structure was not observed under some experimental conditions, such as slow collision velocity or low mirror ratio. The radial profile of the reversal field structure and its time evolution were observed using an internal magnetic probe array. Collisional merging of FRCs was attempted by adjusting the external field profile. Experimental results suggest the dependence of the trapped poloidal flux of the formed FRC after merging on collision velocity and external magnetic boundary.

In Thomson scattering diagnostics systems, a combination of the lookup table and the minimum χ² methods has been widely used to determine electron temperature. The concept of the minimum χ² method is based on clearly defined mathematical statistics. However, the minimum χ² method calculation requires a large amount of time because all χ² values have to be calculated at all temperatures included in the lookup table. Thus, this method is unsuitable for the real-time data analysis required for the next generation of fusion devices, e.g., the International Thermonuclear Experimental Reactor in France. To establish real-time data analysis for Thomson scattering diagnostics, we have developed a neural network program for the large helical device (LHD) Thomson scattering (TS) system. First, we systematically studied the number of nodes and training cycles required to obtain satisfactory results, and then applied them to the LHD TS system. The calculation time was successfully reduced by approximately 1/50 - 1/100 of the χ² method calculation time. In addition, experimental error estimation has been performed according to the concept of the neural network method used in this study.

In this study, the effect of the collision axes offset in the collisional merging process of field-reversed configuration (FRC) in the FAT-CM (FRC amplification via translation-collisional merging) device was experimentally investigated for the first time. The offset of incident axes during collision does not exhibit any considerable effect on particle inventory and trapped magnetic flux of the merged FRC, which is inconsistent with the results predicted via the three-dimensional magnetohydrodynamic (MHD) simulation using the MHD infrastructure for plasma simulation (MIPS) code. Based on the obtained results, the FRC exhibits robust stability and it does not collapse even when subjected to destructive perturbations during the dynamic translation and collision processes.

By using newly installed local helical coils (ULT coils), it is expected that helical magnetic field will be reinforced and then the rotational transform and the cross-sectional area of the last closed flux surface (LCFS) will get larger in TOKASTAR-2. Electron beam mapping and plasma measurement with an electrostatic probe were made to confirm improvement in the helical field. As the result, large closed flux surfaces were measured by using the ULT coils. In plasma measurement, it was observed that the plasma pressure changed according to the movement of the calculated LCFS, though change in the plasma pressure at the position of the calculated LCFS was not clear. Furthermore, it was confirmed that helical magnetic field confined plasma from plasma decay after turning off the plasma heating power.

The field-reversed configuration (FRC) collisional merging experiment in the FAT-CM device at Nihon University has been conducted for the generation of collisionless shock waves that are considered to cause the generation of nonthermal particles. Two FRC-like plasmoids with extremely high-beta formed by field-reversed theta-pinch are translated directly toward each other and collide at super-sonic/Alfvénic velocity. The acceleration performance is improved to generate high-Mach number shocks by generating the high-magnetic pressure gradient at the boundary between the formation and confinement sections. In this study, an increase in the translation velocity in the shortened coil geometry is presented. Changing of coil geometry may affect not only the velocity but also other parameters.

The carbon deposition profile on a Directional Material Probe (DMP) installed in the inboard side of the torus in the Large Helical Device (LHD) is investigated using the ERO2.0 code. The experimental result of the carbon deposition profile with short and wide shadows (lower deposition density areas) on the DMP is reasonably explained by the carbons sputtered from the carbon divertor plates installed in the inboard side. The simulation proves that the short and wide shadows are produced by carbons sputtered from the right and left divertor plate arrays, respectively. The experimental carbon deposition profile accumulated in the previous experimental campaign (FY2010) was successfully reproduced by the simulation, which provides detailed understanding of material (carbon) migration in the divertor region in the LHD.

A novel method is proposed for solving an EFG-type Saddle-Point (EFG-SP) problem. Although the null-space method and the variable-reduction method (VRM) were developed as a solver of a saddle-point problem, both methods are extremely time-consuming in solving an EFG-SP problem. This is attributable to the QR decomposition that is indispensable for both methods. For the purpose of resolving this problem, the improved Variable-Reduction Method (iVRM) is formulated without using the QR decomposition. A numerical code has been developed for solving an EFG-SP problem with the iVRM, the VRM and the ICCG method. By means of the code, the performance of the three methods is investigated numerically. The results of computations show that, from the standpoint of convergence property and computational speed, the iVRM is even superior to either of the VRM and the ICCG method.

The relations among turbulent amplitude, zonal-flow amplitude, and transport level are discussed for the time series data of nonlinear gyrokinetic simulations for magnetized toroidal plasmas. Since it was shown that the transport coefficient can be expressed as a function of the time-averaged turbulent fluctuation level and the zonal flow amplitude [M. Nunami et al., Phys. Plasmas 20, 092307 (2013)], we apply the results to a model function for the turbulent plasma transport coefficient to extend to a functional relation which includes not the time-averaged data but the time-series data of gyrokinetic simulations. We obtain a new functional relation to the turbulent fluctuations, the zonal-flow amplitudes, and the transport coefficients as a function of the input parameters of the gyrokinetic simulations such as plasma temperature gradients. It is also confirmed that the obtained functional relation can reduce relative error which is compared with the original function with time-averages.

Ideal magnetohydrodynamics (MHD) stability of tokamak plasmas is compared between positive and negative triangularity cross sections. The bootstrap current is included that is determined from the density and temperature profiles. This is crucial for a DEMO reactor design. The density and temperature profiles are chosen to have internal transport barriers (ITBs), which are necessary if an H-mode edge cannot be expected. The ideal MHD stability is examined in a wide range of the ITB position and the central temperature. We confirmed that ballooning modes are prone to be unstable when the ITB is located near the plasma edge for negative triangularity. Internal kink modes become dominant instability when the ITB is located inner side of the plasma for both positive and negative triangularities. We have succeeded to stabilize both ballooning and internal kink modes by introducing additional currents to control the safety factor profiles in a favorable manner.

Electron and proton energy-loss rate coefficients in a fusion detached plasma, formed through collisions with molecular hydrogen, are investigated using a rovibrationally resolved collisional-radiative model of molecular hydrogen [K. Sawada and M. Goto, Atoms 4, 29 (2016)]. The rovibrational population distribution of X¹Σ_g⁺ in electrons and protons, both with a temperature of 1 eV and density of 10¹⁶ cm⁻³, is solved time-dependently with an initial 300 K Boltzmann distribution. Energy loss of electrons by rovibrational excitation in X¹Σ_g⁺ is approximately one or two orders of magnitude larger than that by excitation to the triplet b³Σ_u⁺ state.

A tokamak is a toroidal device in which the donut shaped plasma is confined by means of external magnetic field. The externally measured magnetic field due to the plasma column carries several information about the plasma and these include total plasma current, the position of plasma column centroid, the shape of plasma etc. To diagnose these mentioned parameters, several magnetic diagnostics are used and this requires adequate data processing, which is not always very straight forward. In contrary, present theoretical study focus on a rather simple approach of estimating magnetic field at a given location with respect to circular plasma to figure out the change in size of the plasma column. Moreover, this study reveals that the required estimation is very much sensitive to the choice of location for measurement of magnetic field and completely depends on the geometry of the tokamak, and hence tokamak machine-specific. Finally, a subtle approach is explored to make these observations more generalized and hence the usefulness of this approach for any other tokamak-like machine is established.

As the first step to analyze the orbit of the triton produced by the fusion reaction between the beam-injected deuteron and the deuteron in the bulk plasma, we developed the code to calculate the velocity and spatial profiles of the fusion products in the Large Helical Device (LHD) that properly treats the particle loss boundary and the velocity anisotropy of the triton at birth. The neutron emission and neutron counting rates calculated by the code were compared with experimental results. The experimental results are reproduced by the developed code when the bulk plasma density, including the impurity ion, was used.

FIT3D-DD code, which is a neutron emission and heating efficiency evaluation code based on a simple analytical solution of the Fokker-Planck equation, was applied to deuterium Chinese first quasi-axisymmetric stellarator (CFQS) plasmas heated by a deuterium neutral beam injector. A database of the neutron emission rate due to the beam-bulk D-D fusion reaction in the CFQS was created with the electron temperature and density, which were evaluated from the energy confinement time using ISS04 scaling. The neutron emission rate increases monotonically with the increase in the electron temperature and density. The neutron emission rate is expected to be up to ∼2.7×10¹² [n/s] for an electron density of less than 4×10¹⁹ m⁻³.

The stabilizing effect of tokamak plasma vertical position with saddle coils, which had been experimentally confirmed on the small tokamak PHiX, is evaluated numerically. The growth rate of the vertical position instability was evaluated by the three-dimensional linearized ideal MHD stability analysis code TERPSICHORE. The results of the evaluation showed that the growth rate decreased (increased) when the saddle coils were excited in the normal (inverted) direction, where the plasma vertical position had been stabilized (destabilized) in the experiments. It was also found that the growth rate was inversely proportional to the square of the saddle coils current, which was consistent with the tendency obtained experimentally. These results show that the TERPSICHORE code qualitatively reproduces the stabilizing effect of saddle coils.

Gyrokinetic simulations are important for analyzing magnetically confined plasmas. However, the data obtained from the gyrokinetic simulations are time-series of a five-dimensional phase space distribution function, making analyzing the transport phenomena extremely difficult because of its high dimensionality and large data size. We propose a novel method for analyzing such phase space distribution functions. First, the two-dimensional velocity space distribution function is mapped into the wavenumber space and visualized as an image. This enables us to easily capture the global features and the features of the individual velocity space distribution functions. Second, we apply similarity analysis based on the local features of images and cluster analysis based on distances between images and the velocity space distribution function. The proposed method enables us to automatically extract similar structures in the velocity space distribution function and quantify the duration of these structures.

Gas target injection system is newly fabricated to observe behaviors of injected fast ignition targets. It can eject a mimic fast ignition target by pressured nitrogen gas and magnetic separator. The flight attitude of injected target is observed by high-speed cameras. Analysis of high-speed camera images indicates that the target velocity is increased with the injection gas pressure up to around 100 m/s which meets the demanded specification of a reactor and the target flight angle is varied in wide range shot by shot. The technical problem how to control flight angle is recognized for a fast ignition reactor.

We herein propose a fuel pellet with a flower-shaped tamper and an ablator layer filled with a heavier material as a heavy-ion inertial fusion target. The proposed fuel pellet structure successfully mitigates the non-uniformity of energy deposition owing to the different penetration depths at different parts of the incident heavy-ion beams. The heavier material in the ablator interferes with the movement of the ablator material. The low-density region in the ablator layer at the edge of the irradiated area is mitigated, and direct heating of the fuel by heavy-ion beams is prevented.

Applying a solenoidal magnetic field to a laser ion source is a method to produce a high current beam. In this study, we measured the effect of the solenoidal magnetic field on the time evolution of the ion beam emittance using the double-slit method for the laser ion source. Ablation plasma was produced by irradiating an Al target with an Nd:YAG laser. From the emittance measurements, the phase difference shown in the time evolution of the emittance ellipses increased as the ion current increased by applying a magnetic field. In addition, the emittance increased with increasing time range for the averaged current in the waveform. However, the emittance using the averaged current in a pulse was almost constant with the magnetic field. These results indicate that the brightness of the beam increased as the ion beam current increased using the solenoidal magnetic field.

In the counter-irradiation, which is one of the fast ignition schemes, higher core energy coupling can be expected when there are two hot electron flows in counter directions. Two plasma mirrors were installed for the counter irradiation at about 180 degrees. The hot electron effective temperatures (T_eff) were measured by using electron energy spectrometers. T_eff vs the laser intensity on a foil target followed Wilkes' scaling law. The energy incident on the target could be calculated by estimating the laser intensity on the target from T_eff and estimating the focusing radius from the X-ray pinhole camera image. As a result, the reflectivity could be estimated to be 17±3%.

We investigated the effects of radial thermal conduction and radiation transport from a fuel pre-heating for during the implosion process. We proposed a target structure using a Pb pusher to prevent the pre-heating phenomenon. We compared the electron heat and radiation fluxes, optical thickness, radiation temperature of the fuel, and fuel compression ratio for the Al and Pb pushers. For the Pb pusher, the compression ratio of the fuel increased when pre-heating was prevented. The results indicated that a pusher with a high-Z and dense material could minimize pre-heating and achieve a high fuel compression ratio. This is because such a material can maintain higher opacity during the implosion process.

High-temperature superconducting (HTS) conductor is a feasible candidate to make magnets for the next generation fusion devices because of its higher temperature margins and higher critical current in a high magnetic field in comparison to low-temperature superconducting (LTS) conductors. The recently proposed concept of the HTS-WISE (Wound and Impregnated Stacked Elastic tapes) conductor was studied to clarify its characteristics under certain magnetic fields. The WISE conductor, including 30-stacked REBCO (Rare-Earth Barium Copper Oxide) tapes, was fabricated and energized in a 9-T test facility which produced the condition of magnetic field B = 5 - 8 T and a temperature T = 30 - 50 K. Obtained critical currents (5.4 - 10.8 kA) increased with a decreasing magnetic field and/or temperature under the condition of T > 40 K. The maximum current of 16.9 kA was obtained at T = 30 K, which corresponded to the engineering current density j_E = 60 A/mm². Experimental results showed qualitative agreement with numerical calculations of the critical current. We confirmed the operation of the WISE conductor under a high magnetic field and low temperature.

For the high-power and long-pulse ion cyclotron range of frequencies (ICRF) heating of plasma in the Large Helical Device (LHD), two types of ICRF antennas are used. One is the Field-Aligned-Impedance-Transforming (FAIT) antenna. It has an In-Vessel Impedance Transformer (IVIT) in the vacuum region of the antenna and shows the possibility of high-power injection despite the short antenna head. To enhance the performance more, an Ex-Vessel Impedance Transformer (EVIT) was attached outside the LHD vacuum vessel. As a result, the injectable power increased. The other is the Handshake form (HAS) antenna. Plasma can be efficiently heated by adjusting the phase difference between currents in straps. However, the injectable power from the HAS antenna was originally small. Therefore, later an EVIT was attached to it. Moreover, the transmission line in the vacuum region was remodeled to form an IVIT. By utilizing these impedance transformers, the performance of the HAS antenna was drastically improved.

This study evaluated the applicability of high-frequency ultrasonic tests to the non-destructive inspection of the bonded interface between a cooling pipe and a divertor monoblock. Samples prepared in this study were an ITER-grade tungsten block bonded with a 2.5 mm-thick oxygen-free copper using diffusion bonding. The high-frequency ultrasonic test was performed using an acoustic microscope. A probe, operated in pulse-echo mode, scanned the copper surface of the sample two-dimensionally. Five probes with operating frequencies ranging from 15 to 50 MHz were used. The measured ultrasonic signals were converted into ultrasonic images on the assumption that the samples had a uniform and isotropic speed of sound to evaluate reflections from the interface. Whereas an interface without any artificial flaw partly reflected ultrasonics, setting the decision threshold properly, based on the distribution of the surface echo amplitudes, enabled the smallest flaw to be detected clearly. Ultrasonic signals measured around 30MHz showed the best signal-to-noise ratio in detecting an artificial flaw introduced at the bonding layer. The results of the ultrasonic tests were consistent with those of subsequent lock-in thermography and destructive test. However the thermography test could not detect small flaws that the high-frequency ultrasonic test confirmed.

As a qualification test of ITER-TF coils, each joint sample was tested prior to manufacture of each TF coil. Eleven joint samples were tested by a conductor test facility with a 9 T split coil. The joint sample comprised two short TF conductors that had twin-box joint terminals at both ends. The lower joint is a testing part that is a full-size joint of the TF coil. Hall probes were attached on the lower joint box to measure the field induced by shielding currents. In order to simulate the shielding currents, an equivalent current circuit has been considered. The main loop of the shielding currents is the current flow in two conductors with crossing the jointed plane twice. The other two loops are the current loops inside superconducting cables in the two conductors. The resistance of the latter loops is given by twice the value of an average contact resistance between the superconducting strands. The calculated results are in good accordance with the experimental data, with an assumption that the overall joint resistance is decreased at low current. This phenomenon can be explained by the existence of a few strands contacting the joint box with extremely low joint resistances.

The space vector of an output voltage can be rotated smoothly in a matrix converter. However, a zero-sequence component appears causing problems in the load, such as a motor. A quaternion is a four-dimensional hypercomplex number with an imaginary part that can simultaneously represent and deal with three-phase voltages. In addition, the quaternion is expressed in the exponential form; thus, it can easily represent the space vector rotation. Two zero configurations were used to optimize the ripple characteristics in fictitious pulse-width-modulated voltage-source inverter. The two zero configurations are used to remove the zero-sequence component from the matrix converter. The quaternion can be differentiated in time as well as rotate in space. Therefore, it is used to analyze transient phenomena in the matrix converter's rise-up and rise-down phases, and the switching's transition phase.

Thermal conductivity of 9.8 at% Ytterbium-doped Yttrium Aluminum Garnet (Yb:YAG) ceramics has been measured by three systems at Commissariat à l'Énergie Atomique et aux énergies alternatives (CEA Paris-Saclay) and at the National Institute for Fusion Science (NIFS). For accurate measurements, thermal impedances in thermal systems should be negligibly small. However, each system includes some thermal impedance in a heat flow path. This paper describes that thermal conductivity of the 9.8% doped Yb:YAG ceramics and discusses the compensation method for thermal impedance in each system.

A numerical code has been developed in order to investigate the applicability of the permanent magnet method to vertically-stacked multiple high-temperature superconducting (HTS) tapes. The permanent magnet method was proposed to simply and contactlessly measure the critical current density in the HTS tape. By this method, the critical current density is estimated from the proportional relation between the critical current density and the Lorentz force working on the permanent magnet. The Maxwell equation, coupled with the superconductivity characteristics, is solved by the edge-based finite element method to investigate the effect of the number of HTS tapes on the Lorentz force. As a result, it is clear that the permanent magnet method can be applied to measurement of the critical current density in the multiple HTS tapes although there is an upper limit of the number of HTS tapes in which the critical current density can be measured. In addition, by using stronger magnet, the permanent magnet method can measure the critical current density when more number of HTS tapes are stacked.

For ITER spectroscopic measurements, it is important to understand the optical reflection characteristics of the divertor surface for an accurate measurement because the stray light in the divertor may be large. We set up a goniophotometer that measures the optical reflection characteristics and investigated the bidirectional reflectance distribution function of tungsten samples sputtered in an argon plasma. The specimens sputtered at temperatures lower than the recrystallization temperature of tungsten exhibited smooth surfaces and strong specular reflections in their optical reflectance characteristics. Recrystallized crystals likely grew for the specimens sputtered at temperatures approximately equal to the recrystallization temperature, resulting in a weak specular reflection.

Thermal neutron flux evaluation using a single crystal diamond detector (SDD) was carried out in the core region of the UTR-KINKI reactor where a mixed radiation field by thermal and fast neutrons and gamma-ray exists. The pulse shape discrimination method to extract pulses with a rectangular shape as well as a wide pulse-width was established to exclude pulses induced by gamma-rays. The SDD, using a ⁶LiF thermal neutron converter, is able to detect pulse events caused not only by fast neutrons but also by thermal neutrons through energy depositions into the diamond by energetic alpha and triton particles induced by thermal neutrons. Additionally, the SDD without the thermal neutron converter was used for the measurement of the energy deposition events only by fast neutrons. A comparison of the pulse counts of the SDD with or without the thermal neutron convertor deduced the energy deposition spectra by thermal neutrons. The thermal neutron flux in the core region of the UTR-KINKI reactor was evaluated to be 7.6×10⁶ n cm⁻² s⁻¹ W⁻¹ up to a reactor power of 1 W.

For the purpose of further improving the power generation performance by the supercritical CO₂ gas turbine power generation system, aerodynamic optimum and heat transfer flow analysis were carried out for vertical single-axial bypass control type supercritical CO₂ gas turbine power generation system model in the 0.6 GW class FFHR-b1 nuclear fusion power reactor model. As a result, the following conclusions were obtained.

(1) Since the outlet temperature of the 5-stage final stage of the improved main compressor as an alternative to the low/high pressure compressor can be lowered to 318 K compared to the conventional design (outlet temperature 334 K), there are design cases that do not require an intercooler in the conventional design.

(2) As a result of reviewing the structural design and operating conditions of the turbine, the output increased by about 1.1%.

(3) Since a compact design with a total length of about 2.2 m is possible in the design of the above CO₂ gas turbine power generation system (excluding the generator), the feasibility of designing a vertical single-axial bypass control type supercritical CO₂ gas turbine power generation system is clarified.

From these results, the redesigned vertical uniaxial bypass control type supercritical CO₂ gas turbine power generation system is expected to be a compact and economical power generation system that exceeds the power generation efficiency of the conventional design model up to about 0.6%.

A fusion DEMO will be designed as a long-term D-T reaction machine, and it will provide electricity to the grid. The core components of the fusion DEMO such as superconducting TF coils and vacuum vessel sectors become larger and heavier than those of ITER. The difficulties of manufacturing and handling increase significantly. Therefore, the conceptual design of the plasma vacuum vessel and its leg are carried out based on the consideration of assembly and disassembly processes of the core components. When the global density of the device is assumed to be same as the ITER, the total weight of the fusion DEMO would be about 77,500 tons and one TF coil weigh wound be about 1,200 tons. To carry out the easy works for assembling and disassembling, demountable superconducting TF coils are proposed which will be made of high temperature superconducting tapes so that the enough workspace can be secured. The weld joint of the vacuum vessel with a double wall structure is investigated, and the support leg is designed considering the three-dimensional displacement and the integration processes.

The dynamic behaviors of oxide layer formation, growth and self-healing in liquid metal lead (Pb) were studied by on-line monitoring with electrochemical impedance spectroscopy (EIS). The metal rod made of zirconium (Zr) was immersed to liquid Pb at 773 K as the working electrode of EIS. The capacitance semicircle due to the oxide layer formation on the rod surface was detected by EIS after the immersion for 45 hr. The increase of impedance due to the growth of the oxide layer in liquid Pb was continuously detected. The rod was taken out from liquid Pb after the immersion for 150 hr, and the oxide layer formed on the rod was partially exfoliated using lathe to simulate the damage of the oxide layer. The rod was immersed again to liquid Pb for 200 hr. The capacitance semicircle due to the self-healing of the oxide layer was recognized after the immersion for 126 hr. The self-healing of the artificially damaged area on the oxide layer was recognized by the microscopic analysis.

The formation of helium (He) induced “fuzz layer” significantly changes the deuterium (D) retention in tungsten (W). In this study, the D retention in W with an identical fuzz layer is investigated using thermal desorption spectroscopy, with various D ion fluences. It is found that the D retention substantially decreases by ∼80% at relatively low fluence of ∼10²⁴ D/m², and ∼40% at the higher fluence of 6×10²⁵ D/m². A new broad desorption peak at ∼650 K appearing at the higher D ion fluences of >10²⁵ D/m² originates from the interior of the fuzz layer, not the W bulk space. This suggests that the fuzz layer can force D ions to stay near the surface of W regardless of the D ion fluence, working as a diffusion barrier of hydrogen isotope fuels.

Low-Temperature Superconductors (LTS) are sensitive to non-uniform current distribution, which produces quenching. So, transposition of strands is indispensable in LTS cables to help current redistribution. In contrast, High-Temperature Superconductors (HTS) have higher thermal stability, which is expected to help current redistribution among strands (tapes) without quenching. Generally, HTS cable designs consider transposition to reduce quench likelihood and better handling AC operation. However, transposition causes mechanical strain in the tapes, reducing their performance. Recently, a 20-kA-class Stacked Tapes Assembled in Rigid Structure (STARS) conductor is being developed at NIFS, for the next-generation helical devices. To weigh the simple stacking feasibility of HTS tapes, a previous experiment confirmed, that 5 non-transposed HTS tapes can stably conduct a worst-case non-uniform current distribution without quenching. This further suggests that when using HTS tapes for DC HTS cables, transposition may be optional, but not strictly required. A numerical simulation was developed, dealing with the current distribution among the HTS tapes in a worst-case scenario, reproducing the previous experimental observation, and a second experiment was performed to give insights into the contact resistance between HTS tapes. The self-magnetic field effect and temperature fluctuations are to be explored for quench scenarios.

Understanding the temperature distribution in a liquid metal under plasma bombardment is required for characterizing and controlling their own impurity releases and heat transfer. To achieve this, a 2-D heat conduction inside a static liquid bombarded by a plasma is numerically solved in this study. Thin layers consist of Al, Li, In, Sn and Ga. Plasma constituents are D and Ar. The liquid temperature is initialized by melting temperature. The upper surface is heated by ions and electrons but cooled by evaporation and thermal radiation. The lower surface is in contact with the boundary conditions: the fixed melting temperature for characterizing the temperature spread influenced by different thermal diffusivities and ion masses; and the floating temperature governed by convective cooling and thermal radiation for characterizing the heat transfer across the liquid provided by the implementation of a coolant. It appears that Al conducts heat well so the temperature distribution is smoothed out, probably a choice for excessive heat flushing during abnormal events. Sn and In may be good undesirable impurity collectors because of low evaporation with less coolant concern, but not for Li and Ga.

Institute for Plasma Research (IPR) is developing Spherical Tokamak (ST) related technologies to realize a low aspect ratio compact device for plasma experiments. Center Stack is one of the critical components in ST which requires R&D to design, manufacture and assemble in a limited space within required tolerances. A Prototype Center Stack (PCS) is being developed based on reference ST parameters with major radius 0.28 m and minor radius 0.16 m for magnetic field 0.1 T. The main objective of PCS is to design and check its manufacturing feasibility to achieve required tolerances at component and assembly level and maintaining overall accuracy. The PCS assembly consists of two sets of coils, namely Toroidal Field (TF) coils and Ohmic (OH) coil made of Electrolytic Tough Pitch (ETP) Copper. It consists of 6 number of TF coils, with 3 turns per coil, all connected in series. The OH coil has two parallel layers, with 143 turns per layer. The SS 304 support structure consists of tension cylinder and Center Stack Casing (CSC) which are mounted on a pedestal support. Two different capacitor bank based power supplies are considered for TF and OH coils with a maximum operating current of 8.5 kA and 10 kA respectively. This paper discusses the conceptual design, engineering analysis including thermal, electromagnetic and structural analysis using ANSYS Mechanical, Multiphysics and Maxwell modules.

A High-Temperature Superconducting (HTS) magnet is being considered to for use in the next-generation helical experimental devices. Three types of large-current HTS conductors are being developed, and one of them is the STARS (Stacked Tapes Assembled in Rigid Structure) conductor which uses HTS tapes with a simple stacking technique. Following the proof-of-principle experimental results obtained in the former 100-kA-class prototype hand-made conductor sample, an actually applicable conductor is being developed with a rated current of 18 kA at a temperature of 20 K and a magnetic field of ∼10 T. One of the crucial requirements for this conductor is to have a high current density of 80 A/mm². In the first phase of the development, a 3-m short sample was fabricated by applying laser-beam welding to the stainless-steel jacket. It was tested in liquid nitrogen at 77 K with no external magnetic field. Then the sample was tested in gaseous helium at 20 - 40 K under a magnetic field of 6 - 8 T, and the results show that the basic requirements were satisfied.

The mass production of CuCrZr alloy - ITER Grade tubes for the ITER divertor outer vertical target (OVT) prototype and series production is now in progress as well as other material. It is required to manufacture the CuCrZr tube with the mechanical properties required by the design and with the grain growth suppressed in terms of prevention of water leakage and durability of electromagnetic load even after the completion of the OVT. Therefore, the mechanical properties are tested prior to a use for the OVT to confirm the mechanical properties and the quality during mass production of the CuCrZr tube. This paper reports the tensile properties, which are one of important mechanical properties required for the CuCrZr tubes used in the prototype and series production of the OVT. As the results of sampling inspection during the mass production, the CuCrZr tube was capable of meeting required values of the tensile properties even after the heat treatment simulated the solution annealing and aging in the PFU manufacturing.

On-line measurement of liquid metal level is an essential technique for the coolant systems of fusion reactors to keep their safety operation and to recognize accidents such as a coolant leakage and a coolant blockage. The purpose of the present study is to propose a static gas type level meter which is widely applicable in various coolant systems. This level meter consists of a differential pressure meter and a measurement compartment which is inserted into fluids. The change of the liquid level is measured on-line by the change of the differential pressure between the filler gas in the level meter and the cover gas. The experiments were performed with three fluids: water, viscous fluid (PVA starch) and liquid metal PbBi. The output of the level meter indicated good linearity and reproducibility. The meter output obtained in the experiments was slightly larger than that estimated by theory since the meter output was influenced by surplus gas which was unintentionally involved in the insertion procedure of the level meter. This technical issue was improved by the use of the measurement compartment with several slits, which can release the surplus gas through the slits.

In a thermonuclear fusion reactor, a fuel cycle system that recovers and reuses unburnt hydrogen isotopes is indispensable. Oxygen (O) and carbon (C) impurities are chemically combined with the unburnt hydrogen isotopes and exhausted from the plasma vessel of the fusion reactor. The impurities must be decomposed in the fuel cycle system to recover the hydrogen isotopes, especially tritium. In this study, a flow-type reactor using a radio-frequency (RF) plasma was applied to decompose water vapor (H₂O) molecules. The effect of C deposition on the vessel wall (stainless steel) was confirmed experimentally to promote reactions relating to the decomposition. At RF powers of 30 - 150 W, the decomposition ratio was around 30% with an argon gas mixed with 5% H₂O at the pressure of 100 Pa (5 Pa for H₂O) and the flow rate of 20 sccm (1 sccm for H₂O). The decomposition was enhanced by C depositions on the vessel wall; the decomposition ratio was largely increased to be 75% at the RF power of 150 W. Reasons for the increased ratio may be a reduction of O atoms and molecules and a production of carbon monoxide through the interaction with C depositions. The reduction of O products promoted the decomposition of H₂O in the plasma because the recombination of O and H can be suppressed.

The absorption efficiency of electron cyclotron heating is investigated theoretically and experimentally to understand the wave heating mechanisms under the overdense state and the density limit. The features of self-organizing mechanisms have been observed in the dipole confinement system [M. Nishiura et al., Nucl. Fusion 55, 053019 (2015)]. The modulated 2.45 GHz electromagnetic (EM) wave is applied to the RT-1 plasmas to evaluate the EM wave's absorption efficiency from the diamagnetic signals' response. The absorption efficiency maintains a constant 100% beyond the O-mode's cutoff density. However, it decreases rapidly near the 1.6 × 10¹⁷ m⁻³ line-averaged density, which is twice higher than the 2.45 GHz O-mode cutoff. At less than 0.6 × 10¹⁷ m⁻³, the absorption efficiency simulated by a ray-tracing code in a two-dimensional model explains the experimental absorption efficiency. However, it deviates from the experimental value near the cutoff density and is more significant at the density limit. We discuss the difference between the experimental and numerical results.

Fusion Information Science Centre is being planned as a research infrastructure integrating functions of ITER Remote Experimentation Centre, Computational Simulation Centre and replicated ITER DB. Conceptual design of storage system for the FISC which consists of short-term storage, long-term storage and data warehouse is proposed to satisfy the requirements for fast data transfer, intershot analysis, conventional offline analysis, and machine learning has been studied based on assessment of characteristics of each data access.

Neutrons are generated in a fusion plasma and induce various radionuclides via a nuclear reaction with fusion reactor materials. Evaluating the kinds of nuclide and the amount of induced radioactivity is important for decommissioning planning and regular maintenance. In this study, we verified a long-term prediction model of induced radioactivity in the large helical device (LHD) model by comparing induced radioactivity generated during deuterium plasma experiments in LHD with results calculated using a high-energy particle-induced radioactivity code. The metals employed for activation were SUS316L, Co, Mo, and Ni. During the deuterium plasma experiments, these materials were placed on an 8-O port of the LHD, and the induced radioactivity was measured weekly. To computed induced radioactivity using DCHAIN-SP, the neutron energy spectrum was computed using the LHD model with the Monte-Carlo simulation code PHITS. Although the calculated and measured radioactivity of ⁵⁸Co and ⁵⁴Mo agreed well, the calculated values of ⁶⁰Co and ⁹⁹Mo were underestimated. However,low-energy components could be improved by incorporating peripheral devices into the LHD model, resulting in more accurate radioactivity predictions.

This study investigated the applicability of eddy current testing (ECT) to the non-destructive inspection of cooling tubes in the blanket of a fusion DEMO reactor. Pipes made of F82H steel with inner and outer diameters of 9.0 and 11.0 mm, respectively, were prepared, and slits imitating cracks were fabricated on the pipe surfaces. ECT was performed using a differential type bobbin probe having one exciting and two detecting coils designed in this study. The results of the inspections and subsequent three-dimensional finite element simulations revealed that a bobbin probe is effective in detecting cracks appearing on the inner surface of a pipe. Moreover, the detectability does not deteriorate significantly when cracks oriented in the circumferential directions are targeted,unlike in the case of ECT of the heat exchanger tubes of the steam generators of the pressurized water reactors. This indicates that a probe with a more complicated structure, such as a plus-point probe, would be unnecessary to detect flaws on the inner surface of a pipe. In contrast, the ECT signals from a non-penetrating slit on the outer surface were buried in noise even though the slit was as deep as 0.9 mm.