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.

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.

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.

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.

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.

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.

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 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.

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.

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.