In a reactor grade device, the role of core fueling is to replace the D and T consumed in the fusion reactions (almost negligible) and to compensate the plasma losses through the separatrix - including the material expelled out by the ELMs. For this purpose, deep material deposition is an advantage and pellet injection the best candidate for fueling the future machines. Fueling by pellet injection consists in two phases: First, the pellet ablation itself, then the ablated material homogenization and drift in the discharge. The former is a self-regulated process, which depends only of the local plasma characteristics. The second is a global phenomenon, which depends on the whole magnetic configuration. In this paper, we discuss first the basics of the ablation physics, emphasizing the role of the fast particles - ions and electrons - resulting from NBI or wave heating; then we describe the homogenization process and associated ∇B-induced drift. The drift acceleration and damping processes are described as well as the influence of the magnetic configuration (tokamak, stellarator and reversed field pinch) on the predominance of a given damping process and its consequence on the resulting deposition profile. We finally review the last results relative to pellet fueling in these different kind of devices and present the ongoing projects for future large-scale machines.

A chaos control system based on the time delayed-feedback method was constructed using the LabVIEW system in a laboratory plasma. When the appropriate delay-time and proportionality constant are set as control parameters, the chaotic oscillations are stabilized to be periodic. However, if they are not, the system makes the transition to a more chaotic state. The constructed chaos control system can be widely applied to laboratory plasma by feeding the system back to the dominant location in an electric circuit.

For the development of a high nitrogen atom density source, we produced nitrogen plasmas and measured the nitrogen atom density using vacuum ultraviolet absorption spectroscopy (VUVAS) at high neutral gas pressure (>1.5 mTorr) and discharge power (>500 W) in the NAGDIS-T, which could generate spiral-shaped dissociative recombining plasmas. It was seen that the nitrogen atom density increases with increasing neutral gas pressure and discharge power, and it reached 6.2 × 10¹⁷ m⁻³. In the low gas pressure case, the estimated atom density was confirmed to be consistent with that measured by the actinometry method.

Prediction of time evolution of multi-scale turbulence is performed by using Long-short term memory networks. The time series data is obtained by Langmuir probes in a linear magnetized plasma device, PANTA. The simultaneous prediction of high and low frequency components of turbulence is shown to be possible within several tens percent accuracy. The prediction accuracy depends on the initial network, which can be controlled by reducing the learning rate.

Ion mass dependence of the resistive drift wave instability is investigated to understand wave number spectra in magnetized cylindrical plasmas. Modes with larger axial mode numbers are linearly unstable in the case of smaller mass ions as helium. Analytical expression is obtained, which shows that not only the azimuthal mode number but also the axial mode number has the preferable one depending on the mass number. Therefore, 3-D observation of the spatial structure is important to capture the variation including this parameter.

The rollover of electron density, a common indicator for the onset of volumetric recombination, was observed in hydrogen plasma, for the first time in the DT-ALPHA. Electron density and electron temperature were measured near the plasma production region and secondary gas feeding position with various conditions regarding the amount of hydrogen secondary gas. A rollover of electron density was observed at the secondary gas feeding position, whereas electron density near the plasma production region remained nearly constant. This behavior indicates an enhancement of hydrogen molecular-activated recombination.

The study of pellet ablation mechanisms is an important subject for plasma fueling in fusion plasmas. In GAMMA 10/PDX, sub-millimeter hydrogen pellet injection experiments are conducted in the higher electron density plasma for detached plasma experiments. We observed the pellet ablation cloud by two-directional simultaneous photography during the GAMMA 10/PDX pellet injection experiments. The three-dimensional pellet ablation cloud and its trajectory were clearly obtained for the first time.

The impact of magnetic geometry on zonal-flow generation in ion temperature gradient driven turbulence is investigated by means of linear and nonlinear gyrokinetic simulations. The modulation of geodesic curvature on various configurations has revealed amplification of the zonal-flow intensity in relatively smaller geodesic curvature. Based on these findings, a nonlinear proxy model for explorations of novel magnetic geometry to activate the zonal-flow dynamics is proposed.

Ion- and electron-scale microinstabilities and the linear zonal-flow response in mixture plasmas with the medium-Z impurity and He-ash are investigated by means of a multi-species gyrokinetic model. In addition to the charge and dilution effects, stabilization/destabilization by nonthermal He-ash ions is clarified.

We developed a prototype low reflection dummy load for calorimetric power measurements in high-power millimeter wave systems. The dummy load is featured by low reflection of millimeter waves by an internal cylinder designed for increasing the length of the ray trajectory in the dummy load. This structure prevents the millimeter wave reflection from the dummy load by absorbing the waves by the inner surface of the cylinder. Initial tests at high powers of up to 0.5 MW for 0.25 s were successfully able to measure output power with lower reflection back to the source than that of conventional short pulse dummy loads used in the past.

Liquid phase H₂O and D₂O were measured with mid-IR lasers with wavelengths of 2.9 µm and 3.9 µm. The laser power change over time was observed when the water isotope specie was continuously replaced. Additionally, the concentration ratio of H₂O and D₂O as a function of time was obtained. These results indicate that quantitative measurement of water isotopes using mid-IR lasers in real-time has been successful.

Size and temperature dependence of the point defect binding free energy has numerically evaluated for self-interstitial atom (SIA) clusters and vacancy clusters in bcc Fe by using continuum models based on thermodynamics and linear elasticity. The estimated binding free energy of SIAs to SIA-clusters is much higher than that of vacancies to vacancy clusters, indicating that SIA-clusters are more thermally stable than vacancy clusters. For relatively small clusters, the estimated binding free energy at 0 K is comparably consistent with atomistic calculation data; and then, the SIA binding free energy at 850 K is averagely about 35% lower than that at 0 K, while the vacancy binding free energy is about 6% lower; which may remarkably affect the formation kinetics of those defect clusters under irradiation. These kinds of information will be one of the basic parameters for a theoretical model of the microstructural evolution of Fe-based materials in the nuclear fusion DEMO environment.

A diamond-like carbon film with a film hardness of 12 GPa was prepared at a low substrate temperature of 60^◦C using a repetitive nanosecond pulsed glow hydrogen/methane discharge plasma operated in burst mode under a gas pressure of 1.2 kPa. As the peak electrical power of the pulsed discharge increased, the hydrogen content in the films estimated by Raman spectroscopy decreased and the film hardness increased. The ion irradiation to the substrate is considered to be the main factor determining the film properties, since the hydrogen abstraction reaction and the etching of graphite components by hydrogen radical irradiation are weakened in the low-temperature deposition.

A solitary vortex organization process in drift-wave type fluctuations interacting with the zonal flow was identified experimentally in a linear magnetized plasma. An azimuthal probe array was used to evaluate temporal changes in the amplitude and phase in the density fluctuations. Excitation/damping of the solitary vortex is synchronized with zonal perturbation, and the waveform of drift-wave type fluctuation and its harmonics also changes synchronously. The solitary vortex is formed primarily through the phase modulation of the fundamental drift-wave type fluctuation and its harmonics.

Helium (He)-Tungsten (W) co-deposition experiments were conducted in the linear plasma device Co-NAGDIS at a temperature of 1223 K to observe the characteristics of fuzz growth on the W surface with auxiliary W deposition. The dependence on the deposition rate of W was investigated and a clear difference in fuzz thickness was found between He-only and co-deposition experiments. In addition, the fuzz structures on a single sample exhibited a spatially nonuniform distribution, which was probably caused by the nonuniformity in the deposition rate.

In order to detect the wave pattern of electron Bernstein waves (EBWs), an excitation and detection system has been developed in Low Aspect ratio Torus Experiment (LATE). The system consists of a waveguide launcher with arbitrary polarization, a specially designed five-pin probe antenna, a two-dimensional (2-D) mechanical probe driving system, and a homodyne-type mixer circuit. The excitation and detection system has been tested in the air and shows reliable results.

A surface-localized mode governing resistive drift-Alfvén mode is experimentally observed in a collisional, current-carrying finite-β plasma, accompanied by an electron diamagnetic-drift frequency spectrum. The half-wavelength travelling along the plasma column with the Alfvén velocity determines the Alfvén mode. The spatial distribution and the phase relation of magnetic components are presented.

The particle flux onto a material is an important parameter in the study of the plasma-material interaction. With conventional Doppler spectroscopy, it is difficult to measure the flow velocity perpendicular to the material because only the velocity component projected on the wave number vector can be measured. To overcome the limitation, we are developing a transverse flow measurement method using the azimuthal Doppler shift of a Laguerre-Gaussian (LG) beam absorption spectroscopy. In this paper, the feasibility of this spectroscopy method has been examined by numerical analysis. The LG beam is anisotropically absorbed in the transverse flow due to the azimuthal Doppler shift. Since the anisotropic LG beam rotates with propagation, the spatial structure of resonance absorption in plasma and the intensity structure of the LG beam that has propagated through the plasma are inevitably different in the LG beam absorption spectroscopy. It was shown that the deviation from the original azimuthal Doppler shift is reduced to several percent at the position where the intensity distribution of the LG beam reaches its maximum value. Therefore, the transverse flow can be measured with sufficient accuracy by properly selecting the position on the beam cross-section used to evaluate the azimuthal Doppler shift.

In the TST-2 spherical tokamak (ST), non-inductive start-up by lower-hybrid waves (200 MHz) has been studied and a plasma current of 27 kA was achieved. For a comprehensive understanding of the wave sustained plasmas, a fast electron transport model combined with an X-ray emission model is constructed. The electrons in the model show a velocity random walk induced by the wave and collisional slowing down. Simultaneously, they show diffusion in real space. Electron generation and loss at the limiters are also considered. Using the model we can calculate the powers, such as the power from the wave to electrons (i.e., deposition power), collisional bulk electron heating power, power to the limiters. In addition, plasma current, electron density, neutral density, X-ray spectrum expected by a certain measurement system are obtained. Comparison with experimental data shows that a major part of the LHW deposition power is lost by fast electrons hitting the outboard limiter, while a minor part is used to heat cold bulk electrons. The diffusion in real space is well described by the RF induced radial transport, which is often used to interpret fast ion diffusion in ICRF heating. The present work suggests that the RF induced transport of fast electrons is the dominant loss mechanism.

With the help of the linear divertor simulation device Magnum-PSI, a fluctuation investigation of the impact of the target plate geometry was conducted. We simultaneously quantify coherent low-frequency fluctuations with a newly built 70-GHz microwave reflectometry system, a reciprocating probe, a light emission detector system, and a fast-framing camera system. The strong low-frequency fluctuations were observed at both the electron density and the plasma radiations by moving the target plate along the magnetic field line. Furthermore, a strong peak in fluctuation intensity and the influence of the target plate tilt angle on the fluctuation intensity were noted.

CO₂ laser interferometers are a promising option for high-density plasma measurements. However, in low- and middle-density measurements, noise due to mechanical vibrations is a serious problem. To remove this noise, we developed a two-color laser imaging interferometer using a CO₂ laser and quantum cascade (QC) laser, called the CO₂/QC laser imaging interferometer, through benchtop experiments and installed it in the Large Helical Device (LHD). Benchtop experiments provided optical design guidelines for the CO₂/QC laser imaging interferometer to minimize the influence of the unstable output wavelength of the QC laser. The optical system in LHD was designed according to this guideline, and the vibration noise was successfully reduced to 2.80 × 10¹⁸ m^-3. We also demonstrate measurement examples of hollowed and peaked electron density profiles evaluated using Abel inversion and macro-scale instability. This is the first study to present the measurement results of high-temperature plasma using a CO₂/QC two-color laser interferometer. The study outcomes provide important insights for the development of two-color laser interferometers in future fusion devices.

Edge transport barrier (ETB) models are developed and introduced into an integrated transport code TOTAL. The transitions between L-mode and H-mode are triggered by comparing the net heating power with the threshold powers. At the L to H transition, quick reduction in transport in the pedestal region causes back transition due to reduced net heating power and then gradual change in the transport in the pedestal region is needed. The pedestal pressure is adjusted to the value predicted by an empirical scaling. Three models, the PID control model, the ELM model, and the empirical continuous ELM model, are compared for the pedestal pressure control. The control is possible in all of three models but it is observed that larger pellet injection is needed to increase the density and a lower pedestal density is obtained in the ELM model. During tungsten injection, the pedestal pressure is well controlled in the empirical continuous ELM model and in the PID control model.

We develop a new simulation code for solving the Poisson equation, based on Monte Carlo methods. When static resonant magnetic perturbations (RMPs) are used in tokamak plasma to mitigate or suppress edge-localized modes, the RMPs generate an electric field in the ergodized edge region. The electrostatic potential should be calculated only in the edge region to reduce the computational cost of solving the Poisson equation in the complicated three-dimensional magnetic structure, which is assumed to be fixed in time. In this study, we propose a basic idea for evaluating an electrostatic potential given by the Poisson equation in only a part of the domain in curvilinear coordinates. This Poisson solver allows for the boundary condition to be set not only inside the selected region in which the potential is evaluated, but also outside the selected region. Several benchmarks for the developed code are also presented.

In this study, in order to evaluate the SOL width of both density and temperature, we analyze Hasegawa-Wakatani model which includes the temperature fluctuations. Different instability regimes, i.e. drift waves and linearly unstable convective cells are studied. It is shown that convective cells are favorable in terms of the heat load reduction on the divertor. Implications on future devices, such as JT-60 SA, ITER, etc, are discussed.

Neoclassical transport is caused by the non-equilibrium distribution function produced by the driving forces due to quasi-steady but non-uniform plasma state parameters and electromagnetic fields as well as by the Coulomb interactions. In this article, we present a method to evaluate the impact of each driving force on neoclassical transport by a single global drift-kinetic simulation. This method can be used to evaluate the impacts of each driving force not only in one-dimensional forms as transport coefficients, but also in multidimensional forms as how the impacts of each driving force are distributed over the phase space. As an application of the method, we investigate the impacts of each driving force on particle density variations in an impurity hole plasma and demonstrate that the impact of the outward driving force of the temperature gradient on the radial impurity flux becomes as large as the impact of the inward driving force of the negative ambipolar radial electric field. Further, we show that the variation of electrostatic potential on each flux surface, Φ₁, which is involved in several factors in a drift-kinetic equation, affects the density variations specifically through the radial E × B drift.

Ion temperature gradient (ITG) and trapped electron modes (TEM) driven turbulent transport in an ITER-like plasma is investigated by means of multi-species gyrokinetic Vlasov simulations with D, T, He, and real-mass kinetic electrons including their inter-species collisions. Beyond the conventional zero-dimensional power balance analysis presuming the global energy and particle confinement times, gyrokinetic-simulation-based evaluation of a steady burning condition with He-ash exhaust and D-T fuel inward pinch is demonstrated. It is clarified that a significant imbalance appears in the turbulent particle Žflux for the fuel ions of D and T, depending on the D-T density ratio and the He-ash accumulation. Then several profile regimes to satisfy Reiter’s steady burning condition are, for the first time, identified by the gyrokinetic simulation. Also, the impacts of zonal Žflows and nonthermal He-ash on the optimal profile regimes are examined.

An electric current is induced on the vacuum vessel when the toroidal current of a tokamak plasma varies in time, and it is called the eddy current. During disruption, the eddy current becomes large and influences the process of disruption by interacting with the confined plasma. We have developed a new non-axisymmetric eddy current simulation code, the Keddy3D that solves the eddy current equation based on the thin-wall approximation. The Keddy3D is suitable for simulating long-term non-axisymmetric disruption processes with low computational costs.

A method is presented for controlling the trajectory of a vertically injected charged spherical laser fusion target. The position and time of the injected target in flŽight are measured in a position measurement unit using the Arago spot. After the target passes between the first pair of deŽflection plates, where there exists a constant electric field, the mass-to-charge ratio of the target in flŽight is obtained from the shift in its trajectory. The amplitude of the electric field applied between the second and third pairs of deŽflection plates is calculated using this ratio. After passing through the second and third pairs of deflŽection plates, the target deflŽects its trajectory to pass through the reactor center. The design parameters of the trajectory control system for a tabletop plasma device and a laser fusion reactor are presented.

A deuterium-tritium (DT) neutron generator in Osaka University with a continuous intense neutron source emitting 3 × 10¹² fusion neutrons per second has been in operation since 1981. However, radioactivation for the parts of the accelerator body is a serious issue. Hence, in this study, we investigated the radioactivation of the intense irradiation room containing the continuous intense neutron source. Core samples of the concrete wall were collected at various positions in the irradiation room and the radionuclides in them were determined by performing gamma-ray spectrometry. Major long-lived radionuclides found were ⁵⁴Mn, ⁶⁰Co, and ¹⁵²Eu. The radioactivity of ¹⁵²Eu may possibly be consistent with the result obtained using the simulation code. The radioactivities of ⁵⁴Mn and ⁶⁰Co were minimal compared with that of ¹⁵²Eu. The tritium amount in the core sample was measured employing a tritium sampling system and a liquid scintillation detector and was found to be considerably larger than the amount estimated using the simulation code. Tritium diffused from the titanium-tritium target was attached to the wall surface. However, most of it did not penetrate the concrete wall. These results reveal the radioactivity issue of fusion neutron generator facilities and are expected to aid in the maintenance of their operation.

We carried out a fundamental investigation of a uniaxial direct W-to-Cu bonding at relatively low temperatures in ambient air, which would potentially allow for simple preparation and maintenance of divertor wall components. W/Cu bonds formed at 500^◦C with a bonding pressure of 0.1 MPa, but the mechanical interfacial strength was about 1 MPa, significantly lower than the state-of-the-art values for bonding around at 1000^◦C in vacuum. Higher degree of interfacial oxidation and atomic interdiffusion were observed for higher bonding temperature, through x-ray photoelectron spectroscopy. The electrical conductivity across the bonded W/Cu interface, an indicator of thermal conductance, was measured to be lower for higher bonding temperature, presumably due to the interfacial oxidation.

Lithium titanate (Li₂TiO₃) pebble is a candidate tritium breeder of solid breeder blanket systems of fusion reactors. The oscillation of coolant tubes can be induced by the coolant flow. Fretting corrosion is caused between the Li₂TiO₃ pebbles and the coolant tubes which are made of reduced activation ferritic martensitic steel F82H. The purpose of the present study is to clarify the fretting behaviors at the temperatures of blanket conditions. The Li₂TiO₃ pebble produced by sol-gel method was pushed onto the surface of the oscillating F82H plate in the fretting tests which were performed for 10 min in an air atmosphere up to 573 K. The fretting scars of the Li₂TiO₃ pebble and the F82H plate were observed and analyzed by SEM/EDX and 3D laser scanning microscope. The fretting wear was mitigated at the temperatures of 373 K and 473 K due to the formation of the oxide layer, which might reduce the friction. The pebble was partially destructed by the fretting motion in the test performed at 573 K. The fretting wear of the pebble and the F82H plate was mitigated when the pebble was not fixed on the holder since the pebble could vibrate together with the oscillating plate.

In order to control or suppress edge localized modes in nuclear fusion reactors, an accurate pedestal pressure profile measurement is necessary. A line integrated backward Thomson scattering measurement is an attractive method because of its long scattering length. Assuming that the first mirror is located far from the plasma to avoid degradation of the mirror due to erosion and impurity deposition, the measurement accuracies of density and temperature and pressure are estimated. For the target plasma, we adopt the pedestal profile with the shoulder density of 10¹⁹ - 10²⁰ m^-3 and the dimensions of the JA DEMO reactor. The calculation results show that, the Poisson noise due to finite detected scattered photon number is much larger than that due to bremsstrahlung emission. In addition, noise is enhanced by reconstruction process. The resultant total noise levels of reconstructed density, temperature and pressure profiles are at most 1.5%, 3%, 3%, respectively in the steep gradient region, and this method is feasible in the reactor.

In the FFHR power reactor equipped with a supercritical CO₂ gas turbine power generation system, an divertor cooling system is connected to this power generation system [S. Ishiyama et al., Prog. Nucl. Energy 50, No.12-6, 325 (2008)[1]]. In this paper, for the purpose of developing a diverter by supercritical CO₂ gas cooling that can cope with a neutron heavy irradiation environment with a heat load of 15 MW/m² or more, CFD heat transfer flow analysis was carried out for performance evaluation and its design optimization by a structural analysis models of a supercritical CO₂ gas cooled divertors. As a result, in the supercritical CO₂ gas cooled tungsten mono-block divertors (50 × 50 mm × 5 channel × 5,000 mL) with a flow path length of 5 m or less, the engineering designable range of these advanced diverters having the same cooling performance as the water cooling divertor was clarified, and its practicality is extremely high from the feature that the structural model has an extremely low risk during operation as compared with the water cooled divertor.

A detailed experimental study has been conducted to demonstrate the efficient confinement of ions in the popular four-rod configuration of a linear Paul trap without exciting the transverse radio-frequency (rf) quadrupole field. The three-dimensional (3D) ion confinement is achieved with an identical rf voltage applied to the end electrodes. The optimum operating region is visualized in the stability tune diagram, which indicates that a large number of ions can be stored by adjusting a few fundamental parameters. The lifetime of an ion cloud in the present linear trap is over a second (corresponding to a million rf cycles), long enough for various practical applications. It is also shown through 3D numerical simulations that one can easily extract ions from the trap at a low loss rate below 10%.

Mo/Si multilayer mirrors are used for extreme ultraviolet (EUV) lithography. The formation of hydrogen-induced blisters in the Mo/Si multilayer is a problem that reduces the reflectance of the mirror. To evaluate the blister-resistance of EUV mirrors, the blister formation processes of Mo/Si multilayers with a capping layer were investigated using a high-frequency hydrogen plasma system as a hydrogen ion source under varying hydrogen ion exposure conditions. As a result, it was observed that blister formation by low-energy hydrogen ion irradiation of about 10 eV increases the blister-occupied area, depending on the amount of the ion dose. Furthermore, the sample was heated to promote the diffusion of hydrogen atoms, and the activation energy of blister formation was examined using the Arrhenius plot of the ion dose required for blister formation with respect to the heating temperature. The analysis showed that when the ion flux is known, the blister formation time can be predicted.

This paper demonstrates the estimation of vibrational temperatures of N₂ and CO₂ in low-pressure plasmas by threshold ionization mass spectrometry. The principle for the estimation is the decrease in the ionization potential by the vibrational excitation. We observed that the threshold ionization curves of N₂ and CO₂, which were measured using a quadrupole mass spectrometer with an energy-variable electron beam, shifted toward the low-energy side, when they were sampled from the plasmas. We constructed a model which assumed a Boltzmann distribution for the population densities of vibrational excited states and the same cross sections of electron impact ionization for vibrational excited states except the shifts of the threshold energies. The vibrational temperatures were estimated by fitting the experimental threshold ionization curves with the model.

In this study, we investigated the enhancement of cellular lipid that forms in Lipomyces starkeyi under the electro chemical stimulation by discharge in the dielectric barrier electrode at a gas-liquid boundary. The plasma injection sequence and composition of recovery media were optimized for the minimum deterioration of cellular density and maximum increase in the lipid volumes. Under optimum conditions, the volume of lipid globules increased to 1.35 and the count of cells per milliliter to 1.16.

In this paper, we measured the one-dimensional phase and polarization structure of a millimeter-wave with a helical wavefront (vortex beam) using a simultaneous heterodyne detection system at two spatial points. This is a novel approach, and it provides us with the relative phase of the vortex beam. We measured the phase discontinuity of the vortex beam by scanning the one-dimensional path where an antenna passes through the optical axis. It was found that relative phases gradually increase or decrease when the antenna does not pass through the optical axis. Additionally, we measured polarization parameters, which were indefinite near the optical axis. In other words, the optical axis of the vortex beam is a singular point about the phase.

Conditions for the emergence of chaotic orbit of low energy positrons are numerically calculated in a compact dipole trap for use in positron and electron-positron plasma experiments. Due to its relatively weak field strength and existence of magnetic null line near the confinement region, coupling between gyro and bounce motions is pronounced even for low energy particles in this trapping geometry. Breakdown of first and second adiabatic invariants and the resultant non-integrable chaotic motion are realized for positrons with kinetic energy below the order of 10 eV. This kinetic energy value is two orders of magnitude smaller than the threshold value for a chaotic orbit in the Ring Trap-1 (RT-1) experiment. The stochastic long orbit is potentially applicable for efficient injection and compression schemes of positrons in a compact levitated dipole experiment.

We perform the principal verification of reconstructing object surface images by using deep learning. Using the deep learning neural network based on convolutional neural networks, simple object surface images with 128×128 pixels are reasonably reconstructed with up-converting from rough microwave signal images with 16×16 pixels. The model captures large structural features of the object surface images even with small number of training data. As the number of training data increases, it captures small structures of objects. It is also found that noises of input signal images affect reconstructions of small structures of objects.

The conjugate gradient (CG) method with the acceleration technique of using both H-matrix arithmetic and H-matrix-based preconditioning is applied to the linear system that appeared in the shielding current analysis of the uncracked high-temperature superconducting film and its performance is investigated numerically. The computational results show that the proposed acceleration technique is extremely effective for improving the speed of the CG method.

A capacitive coupled radio frequency (RF) plasma system has been developed for producing tungsten coated graphite tiles using plasma assisted chemical vapor deposition (PACVD) technique. To characterize the deposition chamber for optimal plasma parameters, small amount of air is released into the hydrogen plasma purposefully to measure its gas temperature using spectral bands of nitrogen molecule. Optical emission spectra in the wavelength range 350 to 900 nm have been recorded with a miniature spectrometer. Molecular spectral bands of N₂ (B³Π_g-A³ Σ_u⁺ ) have been observed and identified as three bands from the nitrogen 1PS (Δν = 2, 3 & 4). These bands are simulated using MATLAB code developed in-house by considering Boltzmann distribution of particles in the vibrational states. The experimental spectra have been modelled with the simulated spectrum through the best-fit technique by iterating the latter one with different temperature values. Boltzmann plot method is also utilized to evaluate plasma gas temperature using identified vibrational bands. The estimated temperature using spectral modelling method matches fairly well with Boltzmann plot method. The estimated vibrational temperatures are in the range of ∼7000 - 8000 K, an order higher than the room temperature ∼300 K.

The commissioning of three different types of D-D neutron energy spectrometer has been performed in the Large Helical Device (LHD) to accelerate energetic-ion physics studies in a non-axisymmetric system. Because the LHD is equipped with negative-ion-source-based tangential neutral beam injectors (N-NBs) characterized by high energy up to 180∼190 keV, a significant Doppler shift of D-D neutron energy from 2.45 MeV is expected. Two different compact neutron energy spectrometers, i.e., a conventional liquid organic scintillator, designated as EJ-301, and a newly developed Cs₂LiYCl₆:Ce with ⁷Li-enrichment called CLYC7, having tangential sightlines,have shown up- and/or down-shifted D-D neutron energy, as expected according to the direction of N-NB injection. In addition, with the aim of study on a perpendicular energetic ion tail, created by wave heating with ion cyclotron resonance frequency, a neutron energy spectrometer named the Time of Flight Enhanced Diagnostic (TOFED) is being developed. The TOFED is based on a time-of-flight technique and is characterized by high-energy-resolution and a high-counting-rate capability. Commissioning of the TOFED is now ongoing. Recent advances of neutron energy spectrometer development for LHD deuterium plasmas are described.

Impurity behavior has been studied for the high performance Ohmically heated ADITYA tokamak plasmas operated with higher toroidal magnetic field and multiple gas puffs. The neutral hydrogen and impurity emissions in the visible range were monitored by photo multiplier tube (PMT) based system in which interference filter used for wavelength selection. The VUV spectral line emissions from impurities, such as C⁴⁺, O⁵⁺ and Fe¹⁴⁺, were also recorded by a VUV survey spectrometer operated in the 10 - 180 nm. It has been found that H_α, O¹⁺, and C²⁺ emissions normalized with electron density (n_e), and visible continuum normalized with n_e² show a gradual decrease with increase in density indicating lower impurity concentration in these discharges. The VUV emission also shows the similar trend with increasing n_e. Indeed, the impurity transport study using iron emissions confirms that the iron concentration reduces with increasing n_e. This is also corroborated by the observed reduction in plasma effective charge, Z_eff and radiation power loss with the increase in n_e. These results clearly indicate that the improved confinement for ADITYA plasma are correlated with reduction of impurities concentration in those discharges.

A feasibility study for measuring a deuterium-deuterium (D-D) fusion reaction radial profile by promptly lost D-D fusion born 3 MeV protons, whose energy Larmor radius is the same as the minor radius of CFQS, was performed. The Lorentz orbit code was utilized to estimate the predicted signals of collimated proton detectors using the D-D fusion radial profile calculated by the analytical Fokker-Planck code for steady-state plasma FIT3D-DD code. The inversion of the D-D fusion profile using the estimated signals was performed using a linear matrix solution library. The coarse agreement between input and inverted profiles shows the possibility of D-D fusion profile diagnostics by a 3 MeV proton in CFQS.

Cryogenic neon pellets of 3 mm in diameter were injected into neutral beam injection (NBI) heated discharges on the Large Helical Device (LHD). The time response of far infrared (FIR) interferometer has pointed out a relatively slow assimilation of the ablated materials compared to the cases of hydrogen injection. This is consistent with the neutral gas and plasma shielding (NGPS) model prediction, showing that strong line emission inside the ablation cloud limits the cloud temperature and the expansion velocity along the magnetic field line. Measured penetration depths were also compared, being well reproduced by the code prediction when the contribution from 180 keV fast ions produced by tangential NBI is taken into account.

In the Large Helical Device, the divertor detachment has been attempted by application of resonant magnetic perturbation (RMP) field and Ne gas puffing in electron cyclotron resonance- heated discharges for compatibility of high central electron temperature and low divertor heat load. Two kinds of divertor detachment phases were observed. The first one appeared transiently just after the Ne gas puffing (1st detachment), and the second one appeared steadily in the latter half of the discharge (2nd detachment). Space-resolved extreme ultraviolet spectroscopy revealed that NeVI-NeVIII emissions increased slightly outside the last closed flux surface (LCFS), while NeIX and NeX emissions increased inside the LCFS in the 1st detachment phase. Although in the 1st detachment the divertor heat load was significantly reduced, the central electron temperature also decreased because the Ne ions were penetrated inside the LCFS as a radiation source. In the 2nd detachment phase, NeVINeVIII emissions increased outside the LCFS while NeIX and NeX emissions kept low intensity inside the LCFS. In this phase, low divertor heat load and high central electron temperature were obtained simultaneously because the Ne ions were localized outside the LCFS as a radiation source. The profile measurements of Ne emission show that the edge island structure created by the RMP application impacts on the impurity emission distribution, where the peak of the emission shifts radially stepwise as the detachment proceeds.

Tritium yields due to the deuterium-deuterium fusion reaction during the 22nd LHD experiment campaign are numerically estimated. As usual, the total tritium yields are assumed to be the same total neutron yields. In the Large Helical Device (LHD), however, it is considered that fusion reactivity of the D(d,p)T branch is lower than that of the D(d,n)³He one because the fusion reaction between a fast-deuteron and a thermal deuteron is dominant. By integrated simulation, considering the velocity distribution function of fast-deuteron, the ratio of the tritium yields to the neutron yields is estimated to be Y_t/Y_n ∼ 0.936. From the assumptions applied in the simulation, it is expected that this value should be still an over-estimation rather than the actual value.

To investigate the isotope effect on the plasma detachment in helium (He) and hydrogen (H)/deuterium (D) mixture plasmas, we performed H₂, D₂ or He gas puffing into He plasma in the linear plasma device NAGDIS-II. Axial distributions of electron density (n_e) and electron temperature (T_e) were obtained using a movable Langmuir probe. Additionally, optical emission spectroscopy (OES) was applied to measure axial distributions of Balmer lines and He I lines. When the neutral gas pressure was high (ΔP_n = 7 ∼10 mTorr), n_e distribution in He-D₂ mixture plasma was similar to that in pure He plasma, showing a sharp decrease at the downstream region where T_e < 1 eV. In contrast, in He-H₂ mixture plasma, a decrease in n_e was confirmed from upstream region where T_e > 1 eV. The upstream H_α/H_γ in He-H₂ plasma was significantly larger than the D_α/D_γ in He-D₂ plasma at the same T_e. This result indicated that molecular activated recombination (MAR) processes significantly occurred in He-H₂ plasma, while electron-ion recombination (EIR) processes were dominant in He-D₂ and pure He plasmas.

An advanced tomography method based on Bayesian probability theory is presented in this article. In the method, Gaussian Process (GP) prior is adopted as an effective approach to smoothness regularization which can be optimized based on the balance between model complexity and data constraint. In particular, to address the problem of varying smoothness in space, a non-stationary version of the GP has been developed and resolved via Bayesian hierarchical algorithm to implement locally adaptive smoothness regularization such that the accuracy of the reconstruction can be improve significantly. The Bayesian formulism allows the reliability of the reconstruction result to be examined by the confidence interval of a posterior probability. Through a wide range of applications, this tomography method is proved to be a robust tool for the study of magnetohydrodynamics (MHD) activity and impurity transport during HL-2A experimental campaigns.