To achieve a steady-state fusion burning plasma, energetic particle confinement studies have been performed in fusion devices. Deuterium plasma experiments in the Large Helical Device starting from March 2017 have expanded the energetic ion confinement study toward a helical-type fusion reactor. To conduct this study, integrated neutron diagnostics, such as a wide dynamic range neutron flux monitor composed of three sets of detectors, a neutron activation system with two irradiation ends, three vertical neutron cameras, four types of scintillating fiber detectors, and a fast time response neutron fluctuation detector, were installed based on the plan and were working stably, as designed. Moreover, an energetic particle confinement study was advanced by utilizing integrated neutron diagnostics. Furthermore, the results of the energetic particle confinement study using neutron diagnostics obtained from FY2017 to FY2019 are reviewed in this paper.

The tomography measurement, with a help of a newly developed analysis called Fourier-rectangular function (FRF) transform, reveals the properties of the bursting phenomenon occurring at the lower operational boundary of the filling pressure in a cylindrical plasma produced with a helicon source. The analysis provided a clear difference in the spatiotemporal structure between the bursting and quiescent states in the phenomenon.

Time evolutions of the emission from helium (He) ion at 468.6 nm were observed in the magnetized co-axial plasma gun device NAGDIS-PG, and the broadening of the spectrum was analyzed in terms of Doppler and Stark broadenings to deduce the ion temperature and the electron density. A significant broadening of the He II spectrum was identified around a dip in the discharge current, suggesting that increases in the temperature and density occurred.

Divertor pumping was applied to plasma discharges for superior fuel particle control in the Large Helical Device (LHD). The LHD is equipped with two different pumping systems. One is the main pumping system, in which the pumping speed is 260 m³/s in hydrogen. The other pumping system is the divertor pumping system in which the pumping speed is 70 m³/s in hydrogen. Divertor pumping was applied to 40-second long pulse Electron Cyclotron Heating (ECH) discharges to assess the improvement in particle control provided by divertor pumping. The results show that without divertor pumping, the electron density was not controlled by gas puffing using the feedback signal of line-averaged electron density. Then, the plasma confinement deteriorated, finally leading to radiation collapse. On the other hand, with divertor pumping, the density was well-controlled by gas puffing using the feedback signal. The results indicate that divertor pumping is one of the key tools for controlling the particles in fusion plasmas.

We report the observation of arcing damage on the diagnostic shutter during the glow discharge wall conditioning in LHD. The diagnostic system has no experience of plasma discharge produced by electron or ion cyclotron resonance heating or neutral beam injection. The arc trails were observed on the aluminum surface but not on the stainless steel although both materials were exposed to the glow discharge with the same duration. The difference in work functions between two materials may be a cause to divide the conditions of arcing ignition.

We developed a new glass-tube-pair type Doppler probe array for 1D ion velocity vector and temperature measurement. It needs just two parallel glass-tube insertion, realizing low plasma perturbation and 1D ion flow vector measurement on a single discharge. Using four mirrors and optical fibers for one measurement point, this system can measure ion local light emissions of each measurement volume from four different directions, enabling us to measure local ion flow vector and temperature. All set of mirrors and optical fibers are aligned in the two parallel glass tubes for 1D measurement by a single discharge. This system measured successfully ion outflow speed of two merging tokamak plasmas, about 80% of poloidal Alfvén speed in agreement with recent reconnection experiments and theory.

The spatial radiation distribution has been measured with an infrared imaging video bolometer (IRVB) in a neutral beam injected (NBI) plasma from the Heliotron J device. The temperature distribution on the IRVB foil is consistent with radiation simulated by the three-dimensional transport code, EMC3-EIRENE. The foil temperature increase is proportional to the radiation intensity measured with an Absolute eXtreme UltraViolet (AXUV) diode diagnostic. These results suggest that the IRVB can be used for plasma radiation measurements in small and medium size devices.

Effects of electron temperature fluctuation on measurement of other fluctuations by using of Langmuir probe is investigated in PANTA plasma with finite temperature gradient, where significant electron temperature fluctuations are observed. The temperature fluctuation and its effects are evaluated from spatiotemporal structures of ion saturation current and floating potential, derived by conditional sampling/averaging technique. It is found that electron density fluctuation is in phase with ion saturation current fluctuation, but plasma potential fluctuation is anti-phase with the floating potential fluctuation.

Liquid metals are excellent coolants of fission and fusion reactors. However, the chemical compatibility of structural materials is important issue. The mass losses of the high-temperature materials such as FeCrAl-ODS, SiC, and refractory metals by corrosion in liquid metals are essential information to obtain their corrosion rates. The specimens must be cleaned to remove liquid metals solidified and adhered on the specimens after the corrosion tests, though the damage of the specimens in the cleaning procedure must be minimized. Cleaning methodologies appropriate for the specimens tested in liquid metals are urgently required for further compatibility study. The cleaning methodology with 0.1 M sodium hydroxide (NaOH) solution was developed, in which Sn was selectively dissolved without any damage on the specimens of the high-temperature materials. The cleaning procedure to remove Pb, Bi, and these alloys (i.e., Pb-16Li and Pb-Bi) adhered on the specimens in the solution mixture of acetic acid, ethanol, and H₂O₂ were also studied.

Large-scale fiberform nanostructures (LFN) are formed on the tungsten (W) surface with He-W co-deposition environments. In this study, we conducted annealing experiments at 1473 - 1673 K for 30 min using an infrared heating furnace. It was found that the LFN retained their structures after annealing at 1673 K. Through detailed observations using an optical microscope, a scanning electron microscope (SEM), and a transmission electron microscope (TEM), the morphological changes are discussed in relation to the high-temperature stability.

A repetitive nanosecond pulsed glow hydrogen/methane discharge plasma generated at 1.2 kPa gas pressure led to diamond-like carbon films with high hardness and a high-speed deposition rate of 0.13 µm/min. Film hardness showed strong substrate temperature dependence, reaching up to 15 GPa. Raman spectroscopy revealed that hydrogen content in the films decreased with increasing substrate temperature. The mechanisms of the changes in film hardness and hydrogen content are considered to be the substrate temperature dependence of the hydrogen abstraction reaction and etching by irradiation with hydrogen radicals.

Double-pass Thomson scattering is a simple and reliable scheme to measure two-directional (perpendicular and parallel) electron temperatures in plasmas. In this study, we configured a double-pass Thomson scattering configuration so that the laser beam passing through plasma is reflected by a mirror and passes through the plasma again to generate the second scattering light with a different scattering angle. To avoid direct re-entering of the beam to the laser, the reflected beam was tilted slightly. This study investigated the configuration in terms of the measurement performance and laser damage risk by the backward beam. Furthermore, this study clarified several requirements on the optical configuration and quantified the parameters' effects on the performance of the configuration. Through optimization procedures, three optimal configurations were figured out: (i) a simple configuration with two lenses and one mirror, but with a long distance from the laser to the plasma, (ii) another simple configuration that slightly breaks the requirement of sufficient deviation of the backward beam from the laser output, and (iii) a modified configuration with three lenses and one mirror.

We have investigated the excitation mechanism of n = 1 helical cores (HCs) with m/n = 2/1 neoclassical tearing modes (NTMs) in JT-60U. It is found that the n = 1 HC is observed with the mode frequency from several Hz to 6 kHz. This indicates that the resistive wall and the plasma control system do not induce n = 1 HCs because the both time scales are different from the mode frequency. In addition, n = 1 HCs appear to be the non-resonant mode from the two observations: n = 1 HCs do not rotate with the plasma around the q = 1 surface in the core and an n = 1 HC is observed even when q_min. > 1. It is also observed that the fluctuation due to an n = 1 HC in the core region disappears with the stabilization of an m/n = 2/1 NTM, implying that n = 1 HCs are driven by m/n = 2/1 NTMs. We revisit a quasi-linear MHD model where the n = 1 HC is induced directly by the sideband of the current for the m/n = 2/1 NTM, which potentially excites the non-resonant m/n = 1/1 mode.

Characteristics of the new vertical neutron camera (VNC3) installed for the study of energetic-particle transport in the relatively low neutron emission rate (S_n) in Large Helical Device (LHD) deuterium plasma is investigated. Dependence of signal of VNC3 operating with the current mode on S_n shows that accurate neutron signal is obtained using VNC3 in low S_n range with 10ms time bin where the error of neutron counts of first vertical neutron camera (VNC1) operating with the pulse counting mode is significantly large. Time-resolved measurements of neutron emission profiles in deuterium beam heated low S_n plasmas are performed. Although the line-integrated neutron obtained by VNC3 is wider due to its larger inner diameter of the collimator compared to VNC1, the neutron profile measured by VNC3 is almost matched with the neutron profile measured by VNC1. The time-resolved neutron profile measurement in low S_n discharge with relatively short time period becomes possible using VNC3.

We investigated electron cyclotron (EC) wave assisted low voltage Ohmic start-up in the conventional field null configuration (FNC) and the trapped-particle configuration (TPC) in the TST-2 spherical tokamak device. The upper pressure limit for successful burn-through increased when EC power was applied for both the FNC and TPC. On the other hand, at low prefill pressure, breakdown was delayed in the FNC start-up. The achievable plasma current also decreased especially at high EC power. By applying the TPC, fast breakdown was recovered even at high EC power. The plasma current ramp-up rate was also greater with TPC compared with FNC at the same loop voltage waveform. The lower prefill pressure limit for successful breakdown expanded in the TPC compared to that in the FNC. The higher vertical field decay index resulted in faster EC breakdown. The reduction of the upper pressure limit due to impurities was the same in the FNC and TPC indicating that the poloidal field configuration did not significantly affect the upper pressure limit for successful burn-through.

The lens-less technique of microwave holography is expected to provide information of three-dimensional structures of plasma with a wide field of view. From the complex amplitudes of waves, which are observed on a single planar array of antennas, we will be able to obtain an imaging of the three-dimensional object. With a geometry of back-scattering observation, the feasibility is examined with a numerical tool of electromagnetic analysis on dielectric objects. With respect to the variety of the dielectric constant and shape of object, it is shown that useful information can be acquired in regarding the complex amplitude distribution at planar detector.

Prediction and likelihood identification of high-beta disruption in JT-60U has been discussed by means of feature extraction based on sparse modeling. In disruption prediction studies using machine learning, the selection of input parameters is an essential issue. A disruption predictor has been developed by using a linear support vector machine with input parameters selected through an exhaustive search, which is one idea of sparse modeling. The investigated dataset includes not only global plasma parameters but also local parameters such as ion temperature and plasma rotation. As a result of the exhaustive search, five physical parameters, i.e., normalized beta β_N, plasma elongation κ, ion temperature T_i and magnetic shear s at the q = 2 rational surface, have been extracted as key parameters of high-beta disruption. The boundary between the disruptive and the non-disruptive zones in multidimensional space has been defined as the power law expression with these key parameters. Consequently, the disruption likelihood has been quantified in terms of probability based on this boundary expression. Careful deliberation of the expression of the disruption likelihood, which is derived with machine learning, could lead to the elucidation of the underlying physics behind disruptions.

A wide-range neutron flux measurement instrument is developed herein for monitoring the total neutron emission rate and yield of the Large Helical Device (LHD) during deuterium experiments implemented from March 2017 in the National Institute for Fusion Science (NIFS), Japan. The instrument is designed for and installed on the Neutron Flux Monitoring (NFM) system, which measures the counting rate using a ²³⁵U Fission Chamber. By combining the pulse counting and Campbell methods, the Digital Signal Processing Unit (DSPU) realized a wide dynamic range of over six orders of magnitude from 1 × 10³ counts/s (cps) to 5 × 10⁹ cps. This study explains and discusses how the instrument is developed, including topics from the predevelopment activities to the verification test at the Kyoto University Critical Assembly (KUCA). Experimental results in the LHD using the finished products suggest that the NFM system works well during deuterium experiments.

The objective of this article is to provide a database for the transmutation of LLFP (long-lived fission products) using neutrons of muon-catalyzed nuclear fusion (MCF). As examples of LLFP with a natural half-life of more than 10⁵ years, four nuclides, ¹⁰⁷Pd, ¹³⁵Cs, ⁷⁹Se and ⁹³Zr, are chosen. Taking simplified geometrical models of the neutron source and blanket, which appear in the conceptual design of in-flight MCF, the nuclide production yield was calculated by a three-dimensional Monte Carlo calculation based on nuclear data. The number of neutrons and flux, which are necessary to convert half of the initial LLFP amount into a stable nucleus, are obtained. We also investigated the method of controlling two competing reactions of the nuclear fractions by fast neutrons,called the (n, 2n) reaction, and the neutron captures of the thermal neutrons. Theoretical simulation studies have revealed the quantity of LLFP that is detoxified by transmutation under the condition that the fusion neutrons are continuously irradiated to LLFP for approximately 10 years with a flux of 10¹⁹ m⁻² s⁻¹.

One of the problems in extreme ultraviolet (EUV) lithography is the deterioration in the reflectivity of the EUV mirror owing to the deposition of tin (Sn) debris. Such Sn adhesion films can be etched by hydrogen ions and atoms through a chemical reaction, forming a volatile SnH₄ gas. In this study, the dependence of the hydrogen ion energy on the Sn etching was investigated. Samples covered by Sn thin films and with various applied bias voltages were exposed to hydrogen plasmas. The etched thicknesses of the Sn films were quantitatively analyzed using X-ray fluorescence. As a result, it was found that the threshold ion energy is approximately 7 eV, and that the peak of the Sn atom yield per hydrogen ion, which is the value indicating the efficiency of the reactive ion etching, is obtained at a hydrogen ion energy of approximately 14 eV.

Spatial uniformity of an atmospheric-pressure microwave line plasma is evaluated from surface hydrophilicity treatment of polyethylene terephthalate film as well as observation of optical emission from the plasma. Prior to the experiments, the structure of the waveguide-based plasma source is optimized using a three-dimensional electromagnetic simulation to suppress standing-wave generation for the uniformity of plasma production. The spatial distribution in the longitudinal direction of the Argon (Ar) plasma is investigated by operating the microscope parallel to the slot and by irradiating film with the plasma to improve surface wettability of the film. Uniform profile of water contact angle is obtained in 40cm with very high-speed processing.

The advanced oxidation of aromatic compounds in aqueous solution has been investigated using a multi-gas, dielectric barrier discharge, and the degradation rate was measured by high performance liquid chromatography (HPLC). In the degradation experiment of 2,5 - DNP, an accelerated degradation pathway was suggested in the transient state, using the molecular orbital calculation of the enhancement of the degradation of oxidation depending on the para-position of nitro-groups. From the nature-friendly technological point of view, a growth of the radish sprout in the hypo-culture was tested after the pH-neutralization of the air-plasma treated water.

The polarization-dependent Hall effect of light was investigated in full-wave simulations for propagation of electron cyclotron waves in magnetized plasma as an anisotropic medium. The transverse shift of the wave packet, which is comparable to the wavelength in the vacuum, was observed in propagation of extraordinary (X) waves under a static magnetic field. This transverse shift is produced by the Berry curvature for the X wave strongly enhanced at the right-hand cutoff. The direction of the transverse shift is perpendicular not only to the gradient of the refractive index but also to the static magnetic field.

In high-gas pressure helium arc plasmas, a forbidden line (1s ¹S-2p ³P : 59.1 nm) as well as the resonance lines 1s ¹S-np ¹P lines of He I have been observed. The intensity ratio of the 1s ¹S-2p ¹P line of He I to forbidden line calculated from the Einstein A coefficients (NIST database) is ∼10⁻⁷, whereas the value obtained experimentally was as small as ∼20. The reason for the discrepancy between the experiment and database can be interpreted from that the photoabsorption (self-absorption) of the resonance lines 1s ¹S-np ¹P lines can cause the drastic change in radiative processes in high-pressure plasmas. In order to validate our interpretation on this mismatch, we investigated the influence of self-absorption of the resonance lines 1s ¹S-np ¹P lines by numerical simulations. The simulation code calculated the photoabsorption process by He atom along a line-of-sight by using coupled rate equations incorporated with the radiation trapping effect. As a result, the simulation yielded the line intensity ratio of 25 because of the strong self-absorption.

Compact toroid injection has been proposed as a particle fueling technique for the core region of fusion plasmas. An accelerated plasmoid penetrates through confinement magnetic fields and reaches the core region of target plasmas. To inject plasmoids into the magnetically confined plasmas featuring strong confinement fields, the injection velocity should be increased. The injection velocity depends on the operating conditions of the compact toroid injector such as charging voltage and gas pressure. Changing these conditions is not preferable as it affected not only the injection velocity but also other plasmoid parameters. Pulsed magnetic coil has been introduced for the additional acceleration of the ejected plasmoid. The pulsed field was produced by the current flowing through a one-turn coil installed at the muzzle of the magnetized coaxial plasma gun. The acceleration of ejected plasmoid by pulsed magnetic coil was experimentally verified. Application of pulsed magnetic field resulted in velocity increase up to approximately 50% compared to the average velocity without additional acceleration.

The enhancement of the acceleration performance of a superconducting linear acceleration (SLA) system to inject the pellet container has been investigated numerically. To this end, a numerical code used in the finite element method has been developed for analyzing the shielding current density in a high-temperature superconducting film. In addition, the on/off method and the normalized Gaussian network (NGnet) method have been implemented in the code for the shape optimization of an acceleration coil, and the non-dominated sorting genetic algorithms-II have been used as the optimization method. The results of the computations show that the speed of the pellet container for the current profile of the optimized coil is significantly faster than that for the homogeneous current profile of the coil. However, for the on/off method, the current profile is scattered, whereas the coil shape becomes hollow for the NGnet method. Consequently, the NGnet method is an effective tool for improving the acceleration performance of the SLA system and for obtaining a coil shape that is easy to design.

A surface wave oscillator (SWO) is driven by an electron beam to generate intense microwaves. The electron beam possesses slow space-charge (SSC) and slow cyclotron (SC) modes that interact with the surface wave leading to microwave generation. The beam current and external magnetic field affect the relationship between SSC and SC modes. The SSC mode gradually approaches the SC mode when the beam current increases. Meanwhile the SC mode gradually approaches the SSC mode when the magnetic field decreases. The two modes merge in a low magnetic field and high beam current. In this work, we experimentally examine the operation of an F-band SWO in the low magnetic field region. The output power decreases with low beam current when magnetic field decreases. Meanwhile, the SWO maintains its power level with high beam current even though the magnetic field decreases to around 0.4 T. The merged instability enables a sustained power level in the low magnetic field region.

In this paper, a method for estimating the radial profile of electron density n_e using a single line-of-sight signal by the He I line intensity ratio method is proposed. By applying this method to cylindrical helium plasma,in which electron temperature was almost uniform and density was uniform in the center, we tried to estimate the parameters representing spatial distribution. It was confirmed that a good distribution estimation result could be obtained by considering the sensitivity factor, the rate at which the line intensity ratio changes as the parameters change, during optimization. Two methods of considering a sensitivity factor are proposed: using the best combination of intensity ratios for analysis in terms of the sensitivity factor, and weighting the objective function using the sensitivity factor. The former method can be analyzed in short computational time, although its applicability is limited. The latter method can be used when it is not obvious which set of intensity ratios is best to use, although it takes more computational time compared with the former method. Both methods reproduce the parameter of a radial density profile.

This study investigated the chemical distribution of an atmospheric-pressure plasma jet (APPJ) along its propagation direction using numerical simulation. Low-resolution spectral data were used to estimate the gas temperature and the excitation temperature. These estimations were used with a collisional-radiative model to elucidate population densities and the electron temperature. A global model was applied to investigate the chemical species distribution in the plasma jet. The thermodynamic properties of the APPJ corresponded well to the relation T_g < T_exc < T_e for all the positions along the jet propagation. Chemical species generation and propagation along the plasma jet were numerically simulated using the GM with input parameters derived from the CR model and the ideal gas law.

Extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) wavelength spectra including line emissions released from neon (Ne) ions ranging from low to high charge states observed simultaneously in a single discharge are summarized for contribution to compile a fundamental spectral dataset for the Ne-seeded divertor heat load reduction experiments in Large Helical Device (LHD). NeIX and NeX lines were observed in the EUV wavelength range of 10∼50 Å and NeIII-NeVIII lines were observed in the VUV wavelength range of 400∼1000 Å. The temporal evolutions of the line intensities exhibited different behaviors between the edge emissions of NeIII-NeVIII with the ionization potential, E_i, of 63∼239 eV and the core emission of NeX with E_i of 1362 eV. NeIX with E_i of 1196 eV exhibited a marginal behavior between the edge emission and the core emission.

The total neutron emission rate (S_n) characteristics of electron cyclotron heated plasma were surveyed in the Large Helical Device in order to exhibit the thermonuclear performance of helical plasma. The dependence of S_n on electron density showed that S_n increased with an electron density of power of 3.1. To understand S_n, characteristics in the electron cyclotron heated plasma, a numerical simulation considering thermal deuterium-deuterium fusion reactions was performed. Although the numerical simulation overestimated S_n in a relatively low S_n region, calculated S_n matched the experimental result for a relatively high S_n region. A possible reason for the disagreement in the low S_n region is that effective charge due to the impurities such as carbon is changed because of the low density.

A radiative collapse predictor has been developed using a machine-learning model based on high-density plasma experiments in the Large Helical Device (LHD). Concurrently, the physical background of radiative collapse was discussed based on the distinct features extracted by a sparse modeling, which is one of the frameworks of data-driven science. Electron density, CIV and OV line emissions, and electron temperature at the plasma edge have been extracted as the key parameters of radiative collapse. Those parameters are relevant to the physical knowledge that the major cause of radiative collapse is the enhancement of radiative loss by light impurities in the plasma-edge region. Using these four parameters, the likelihood of occurrence of radiative collapse has been estimated. The behavior of plasma at the edge—in particular, the carbon impurities outside the last closed flux surface—has been evaluated using EMC3-EIRENE code for the phase with increasing likelihood, that is, the plasma is getting close to the collapse. It is shown that the radiation caused by the C³⁺ ion, which corresponds to the CIV emission, is enhanced in the region where electron temperature is around 10 eV.

A novel divertor biasing using four biasing plates that are arranged toroidally every 90^◦ on the upper divertor plate is applied to low-density plasmas of the QUEST spherical tokamak. When some of these plates are biased in-phase by applying a sawtooth waveform voltage of 85-V amplitude and 50-Hz repetition, up to approximately 35% reduction of the particle flux to the divertor is observed during positive biasing. The input power for the flux reduction is approximately 0.2 kW for low-density tokamak plasmas produced by ∼130-kW electron cyclotron wave injection. Additionally, the signal of a plate probe placed in the low-field side of the mid-plane of the vacuum vessel indicates enhanced losses of fast electrons during positive biasing. The enhanced loss is attributed to small resonant magnetic perturbations produced by the bias-driven currents in the scrape-off layer. This novel divertor biasing is expected to provide a new experimental tool for studying divertor heat load control and fast electron confinement in a tokamak device.

Polarization of the hydrogen Lyman-α line is detected in the Large Helical Device. It is the first observation of a polarized atomic emission line in magnetically confined fusion plasma devices. With the help of an atomic model simulation, the anisotropy in the electron velocity distribution function (EVDF) in terms of T_∥/T_⊥ is evaluated, where T_∥ and T_⊥ represent the electron temperature in the parallel and perpendicular directions regarding the magnetic field, respectively. The results show that T_∥/T_⊥ has a tendency to decrease and deviate from unity with decreasing electron-electron collision frequency, which qualitatively agrees with an intuitive understanding of the anisotropic EVDF in the plasma boundary.

A plasma density fluctuation signal was measured by the recently-installed Beam Emission Spectroscopy (BES). A time-dependent analysis was performed for a discharge in the Large Helical Device (LHD) and compared with the magnetic fluctuation. While the fundamental frequency peak shows a high correlation between the density fluctuation and the magnetic fluctuation, the higher harmonic components have smaller or even negligible correlation. As a possible mechanism that makes the density fluctuation and the magnetic fluctuation different, the relation between the MHD mode and the transport is discussed.

In a small tokamak, the visible light emission is observed and used to investigate plasmas' behavior with a fast visible camera. However, the reflected light causes a systemic error in measuring visible light emitted from the plasma. In this paper, we managed to overcome the reflection effect with the ray-tracing technique which is utilized in a synthetic diagnostic platform of the small tokamak device PHiX at Tokyo Institute of Technology using Raysect and CHERAB python libraries. We successfully evaluated the amount of reflected light and obtained tomographic reconstruction images from simulated and experimental data with the Tikhonov-Phillips regularization and the L-curve method to choose an optimal regularization parameter. We also proposed to project the contour of a reconstruction image onto a camera image to validate tomography results.

The controllability of particle supply during long-term discharge in a high-temperature environment was investigated at the Q-shu University Experiment with steady state spherical tokamak (QUEST). QUEST has a high-temperature wall capable of active heating and cooling as a plasma-facing wall. With this hot wall, a temperature rise test was conducted with 673 K as the target temperature. It was confirmed that the hot wall could maintain the temperature above 600 K. Feedback control of particle fueling was conducted to control Hα emission, which is closely related to influx to the wall. Using this particle fueling control and setting the hot wall temperature to 473 K, it was possible to obtain a discharge of more than 6 h. In this discharge, the fueling rate of particles decreased with time, and finally became zero, losing the particle fueling controllability. However, as soon as the cooling water started to flow through the hot wall, particles could be supplied again, and controllability was restored. Thus, indicating that temperature control of the plasma first wall is important even in the high-temperature environment of 473 K to control particle retention of the wall.

Influences of nitrogen ratio on plasma detachment and molecular activated recombination (MAR) processes during combined seeding with hydrogen have been investigated utilizing end-loss plasma in the GAMMA 10/PDX tandem mirror. Additional gases were injected under the condition that hydrogen partial pressure was fixed and nitrogen partial pressure was changed from 0% - 10% compared to that of hydrogen. Electron density and ion flux further decrease with increasing nitrogen ratio. In addition, it is suggested that the hydrogen-MAR process that begins with dissociative attachment is suppressed during combined seeding of nitrogen and hydrogen. Observed emission spectrum of NH radicals suggests that the density of NH increases as nitrogen ratio increases and nitrogen-induced MAR efficiently contributes to the reduction of particle flux.

The behavior of energetic particles (EPs) associated with toroidal Alfvén eigenmode (TAE) activities during the combined injection of hydrogen and deuterium beams was investigated in the Large Helical Device (LHD). The enhanced transports of both proton and deuteron with TAE activities were simultaneously observed by a tangentially viewing and mass and energy resolved neutral particle analyzer (E||B-NPA). At the timing of the TAE bursts with the mode number n = 1, both proton and deuteron were transported to the outboard and observed with the similar energies of 137 - 138 keV. At the peak amplitude of the magnetic fluctuations measured by the Mirnov coils, the mixed frequencies of 64 kHz and 29 kHz were identified, and the observed frequencies did not chirp down. The observed timings of the transported hydrogen and deuterium were just after the magnetic fluctuations of 64 kHz and 29 kHz, respectively. By adapting the cross-correlation analysis, the delay times from the magnetic fluctuation to the detection of EPs by E||B-NPA are estimated to 95 µs and 145 µs for hydrogen and deuterium, respectively. These delays are considered to be the time of the radial transport, and the time delays depended on the velocities of the transported EPs.

GAMMA 10/PDX is a linear plasma confinement device that mainly uses slow waves in ion cyclotron range of frequencies (ICRF) for heating, and it can achieve a high ion temperature that is in the range of several keV. Although slow waves are effective for ion heating, it is difficult to excite them with antennas at high densities, such as those over 10¹⁹ m⁻³. In this study, we considered a way to excite a slow waves using the frequency difference of input waves. Two fast waves having different frequencies, which are both adequate for high-density plasma, were applied with antennas to excite a slow wave as a difference-frequency wave between the fast waves. As a result, the excitation of difference-frequency waves inside the plasma was confirmed from magnetic probe and reflectometry measurements, and an increase in diamagnetism was also observed. These results firstly demonstrate the possibility of slow-wave heating using a DF wave in a high-density linear plasma, where direct slow-wave heating is not feasible.

Transient coaxial helicity injection (t-CHI) current start-up using a new design simple electrode configuration has been implemented on the QUEST. Discharges injected from the low field side (LFS) and from the high field side (HFS) were examined. Compared to the LFS injection case, the HFS injection has the advantages of providing access to a higher toroidal field and better controlling the location of the injector flux footprint location. Although the present PF coils on QUEST are not well positioned to form the injector flux on the HFS injector region and there has been a frequent occurrence of the spurious arcs, known as absorber arcs, HFS injection has shown flux evolution in a shape that is suitable for the formation of closed flux surfaces. The discharges were improved by installing an in-vessel-coil and adding a new cylindrical electrode to the existing CHI electrode. The results show that the new cylindrical electrode allowed the flux to evolve stably while allowing both the inner and the outer injector flux footprint to remain in the vicinity of the cylindrical electrode. This configuration which inherently generates a narrow injector flux footprint width resulted in discharges that strongly suggested the persistence of the CHI generated plasma after the injector current was reduced to zero. These studies have informed us of the need to improve the CHI gas injection system so that the absorber arcs could be better controlled in the HFS injection configuration.

Energy flow in the collisional merging process of a field-reversed configuration (FRC) was experimentally evaluated. Collisional merging formation of an FRC was carried out in the FAT-CM (FRC amplification via translation-collisional merging) device. In this experiment, the field-reversed theta-pinch formed FRC-like plasmoids are accelerated due to a magnetic pressure gradient. Then, two plasmoids collide at a relative speed of ∼300 km/s, which is faster than typical Alfvén and ion sound speeds (∼50 km/s) on the separatrix. The kinetic and internal energy of plasmoids before and after collision are estimated by simultaneous multi-point measurements combining magnetic probes and interferometers. The energy flow in the collisional merging process is compared to an experimental case with single plasmoid translation. This comparison indicates that the kinetic energy of two accelerated plasmoids regenerates back into the internal thermal energy of the FRC after merging. Moreover, density and neutron measurements suggest excitation of shockwaves. These results indicate that shock heating may become a channel for energy regeneration.

Neutral beam (NB) fast-ion loss mechanisms in the large helical device (LHD) are investigated by the combination of the neutron measurement, the classical slowing-down simulation, and the neo-classical guiding center orbit following simulation. It is found that the neo-classical transport provides little contribution to the loss of tangentially injected NB fast-ions. For perpendicularly injected NB fast-ions, the neo-classical transport has more than 40% contribution to the NB fast ion loss. These results indicate that there are other loss mechanisms dominant in LHD plasmas. The charge exchange loss is one of the plausible candidates for the loss mechanism.

Fuel particle and impurity influxes have been investigated for ADITYA-U tokamak plasma operated with toroidal belt limiter using PMT based spectroscopic diagnostic system installed on machine. The influxes of hydrogen and impurity ions are estimated using various lines of sight (LoS) terminating on the graphite limiter and stainless steel wall to understand their contributions in recycled particle and impurities into the main plasma. It is found that the influxes of neutral hydrogen and oxygen are around 4 times higher in case of LoS terminating on the limiter than the wall while carbon influxes from the both LoSs are comparable. The comparable integrated particle influxes from both LoSs indicate the important role of the wall in the recycling and presence of the impurities in the plasma. The particle confinement time (τ_p) and recycling coefficient (R) are also estimated to quantify those from the estimated particle influxes. The τ_p values vary between 8 to 25 ms when plasma electron density is in the range of 2.0 - 3.2 × 10¹⁹ m⁻³. Analysis of recycling coefficient, R suggests that the Plasma Facing Component (PFC) acts as the particle sink at the beginning of the plasma operational campaign. The R values tend to become more than one as the campaign progresses suggesting that the PFC acting as the particle source.

The Steady state operation can be defined for various characteristic physical time scales. Usually one speaks of steady state plasmas when the discharges are stationary for several energy confinement times. The next longer time constant is the L/R time for the development of the toroidal plasma current. Ultimately, the gas equilibrium must also be achieved. W7-X is ideal for creating stationary conditions for the different plasma parameters. In the operation phase OP1.2, the experimental operation was neither limited by the confining magnetic field nor by the pulse length of the ECRH. Only the maximum tolerable temperature of the uncooled in-vessel components and the test divertor unit (TDU) limited the total input energy. Therefore this paper presents steady state operation different plasma parameters with different time scales.

A stationary direct current of a central solenoid (DC_CS) can expand the operation region of tokamak plasmas. For tokamak plasma formation experiments using electron cyclotron heating (ECH) alone, the minimum ECH power and plasma current necessary for tokamak plasma formation were reduced using the DC_CS, which was applied in the counter rotational direction to the plasma current. In TST-2, the minimum ECH power necessary for the formation of a tokamak plasma was reduced from 3.3 to 1.6 kW when the DC_CS was changed from 0 to 184 A. Simultaneously, the plasma current that was needed to sustain the tokamak plasma configuration was reduced to 0.6 kA.

The physical processes of ion heating during high guide field (B_g/B_rec ∼ 6) reconnection is explored utilizing Doppler tomography measurement in the TS-6 ST (Spherical Tokamak) merging experiment. The newly developed 288CH extensive/high-resolution (Δr ∼ 1.5 cm; ΔZ = 1.0 cm) ion Doppler spectroscopy system has revealed the detailed characteristic of ion heating in the downstream region. In the high guide field regime,ion heating occurs not only around the diffusion region but also more globally in the downstream region. This was beyond the scope of the previous ion temperature measurement and hence we have confirmed a different heating mechanism which is attributed to strong in-plane electric field produced extensively in high guide field reconnection regime.

Full vertical profiles of visible bremsstrahlung continuum have been measured for diagnostics of effective ion charge, Z_eff , with parallel optical fiber arrays at horizontally elongated plasma cross section in the Large Helical Device (LHD). Measured bremsstrahlung profiles were asymmetric due to non-uniform bremsstrahlung emissions located in the stochastic magnetic field layer. Then, the Z_eff profile analysis has been done only for a magnetic configuration with relatively thin stochastic magnetic field layer, i.e. for plasmas at magnetic axis position of R_ax = 3.60 m. The effect of the non-uniform bremsstrahlung emission disappears in the lower half profile at R_ax = 3.60 m, while the Z_eff profile analysis is extremely difficult in other magnetic configurations due to the presence of the non-uniform bremsstrahlung emission over the entire vertical profile from top to bottom. To improve the difficult situation in the Z_eff diagnostic the fan array optical fiber system was newly installed at vertically elongated plasma cross section for full horizontal profile measurement. It is found that the non-uniform bremsstrahlung emissions disappear from the signal and the observed bremsstrahlung profile is almost symmetric between inboard and outboard radial profiles. The analysis on the Z_eff profile has been carried out based on the plasma equilibrium database in LHD, TSMAP. A preliminary result is obtained in high-density discharges, which shows a flat Z_eff profile and values of Z_eff ∼ 1.

For accelerating a transport simulation with an advanced physics turbulent transport model like TGLF, we have been developing a surrogate model that mimics the behavior of the model based on a neural network model. With a steady-state transport solver GOTRESS used, the surrogate model has shown its ability to successfully predict temperature profiles almost equivalent to those by TGLF. The performance of the surrogate model is improved by optimizing hyperparameters and eliminating outliers from training data. Extrapolability of the optimized model is examined by changing the normalized temperature gradient. The objective is to better investigate the nature of the model in addition to measuring its utility in transport simulations. The versatile model, which has been trained with data of multiple cases, is developed applicable to many situations. It shows the same reproducibility as the model specific to each individual case, a fact which unveils great potential of the surrogate model in transport simulations.

Long pulse discharges in the Large Helical Device have often been interrupted by large amounts of dust particle emission from the divertor region caused by the exfoliation of carbon-rich mixed material deposition layers. The plasma wall interaction code ERO2.0 has provided the simulation results of the three-dimensional distribution of the carbon flux density in the divertor region which is quite reasonable with the observed distribution of the carbon-rich deposition layers. The code has also succeeded in reproducing the reduction of the carbon deposition layers on dome plates by changing the target plate configuration in the divertor region. The ERO2.0 simulations have also successfully explained dust particle emission from the inboard side near the equatorial plane for the new target plate configuration at the termination of a long pulse discharge. These simulation results prove that the ERO2.0 code is applicable to predicting the possible position from where the dust particles are released, and to designing an optimized divertor configuration for performing stable long pulse discharges with controlled dust particle emission.