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.

Two-dimensional measurement becomes active to explore physics of plasma structure and dynamics. The paper proposes a set of new methods using the Fourier-Bessel function series expansion to characterize the two-dimensional images for plasma and illustrates an example of its application to a phenomenon of quasi-periodic oscillations observed with tomography in a linear plasma device PANTA. The analysis provides quantitative relations between the coherent structure and residual random fluctuations of the oscillations. The results suggest that similarity between their spatial patterns should be increased in the phase when both amplitudes become stronger.

Resistive drift wave instability is one of the driving sources of turbulence in linear devices, and its ion mass dependence on structural formation is investigated by turbulence simulation. Modes are less unstable in the case with smaller mass ions, because the normalized density gradient length becomes larger with larger Larmor radius, but can be unstable considering the change of ion-neutral collision frequency. Nonlinear calculations show that a large number of modes with larger axial mode numbers are linearly unstable and their nonlinear couplings induce turbulence with a zonal flow in the case of smaller mass ions as helium.

The formation and merging of the ST plasmas in the spherical tokamak have been studied by the means of the 2D toroidal MHD simulation for the two cases with a single and double PF coils. The following results were obtained: (i) during the ST plasmas formation, an internal toroidal flux is injected to the ST plasmas, (ii) this internal flux increases strongly during the plasma merging, and (iii) the internal toroidal flux damps after the merging.

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.

Line emissions from both silicon (Si) and boron (B) impurity ions introduced by a single tracer-encapsulated solid pellet (TESPEL) containing silicon hexaboride (SiB₆) powders were successfully observed using the extreme ultraviolet (EUV) spectrometer and charge-exchange spectroscopy (CXS) technique in the Large Helical Device. The CXS diagnostic shows clearly that a hollow radial profile of fully ionized B impurities was created immediately after the TESPEL injection, and such a hollow profile was relaxed with time. At the same time, Li-like emissions from the highly ionized Si impurities were also observed with the EUV spectrometer, SOXMOS. Therefore, the decay times of these impurities could be estimated under the same plasma conditions. The estimated decay time of the Si impurities, τ_Si = 0.12 ± 0.01s, was found to be slightly longer than that of the B impurities, τ_B = 0.09 ± 0.01s.

The compact neutron energy spectrometer (CNES), based on the Cs₂LiYCl₆ : Ce with a ⁷Li-enrichment (CLYC7) scintillator, has been developed for a neutron energy diagnostic in large helical device (LHD) plasma in the 2020 experimental campaign. The CNES, installed on a tangential sightline, is utilized to measure neutron energy spectra from tangentially-injected negative-ion-based neutral beam (N-NB) heated deuterium plasmas. In this paper, an initial result of the Doppler-shifted neutron spectrum is reported.

A modified leap-frog (LF) scheme is presented that keeps the correct Larmor radius even in case of a large time step Δt compared to the cyclotron period Ω⁻¹, ΩΔt » 1, for the particle simulation of a plasma in the strong magnetic field. The Larmor radius simulated by the conventional LF method becomes very large for ΩΔt » 1, and such a numerical condition has been avoided in general. If the LF method is applicable to such situations, new particle simulation codes can be more easily developed for a wide area of plasma physics. By repeating the LF steps doubly and adopting the averaged velocity to advance the particle position, the Larmor radius is kept real independently of the ΩΔt value. Proper nature on the energy conservation, magnetic moment conservation and drift-velocity realization is safely inherited from the LF method.

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.

Fiberform nanostructured tungsten (FN-W) samples were synthesized by helium plasma irradiation with different irradiation times. It is shown that the optical reflectivity decreases significantly with the increase of the irradiation time, even in a long-wavelength infrared range up to several dozen µm. These experimental results are of importance for the usage of retroflectors and will offer a promising direction for other practical applications.

Hollow cathode plasma discharge technology has several engineering and industrial applications. To further enhance these applications, information on plasma characteristics (such as its axial distribution inside the hollow cathode cavity) is essential. This work determines the axial distributions of electron temperature and density inside said cavity by inserting a triple probe. The temperature and density inside the hollow cathode cavity were approximately 4 eV and −10¹⁷ m⁻³, respectively. It was also confirmed that the plasma existed over almost the inside the cathode cavity and the electron temperature increased and the electron density decreased rapidly in the region near the anode.

We performed helium (He) plasma irradiation to molybdenum (Mo) substrate with auxiliary Mo deposition. Different from W cases, large-scale fiberform nanostructure (LFN) growth did not occur easily. The experiments suggested that the experimental condition of Mo LFN growth has a much narrower conditional window than that of fuzz growth or another hidden control factor exists for LFN growth.

A Hard X-ray monitor (HXRM) diagnostic system is being designed for ITER tokamak and will be utilized to detect runaway electrons for the safe operation of the tokamak. Runaway electrons produce X-ray photons by means of thick target and/or thin target bremsstrahlung emission process. In this diagnostic measurement system, the X-ray photons interact with scintillator detector volume and generate secondary UV-photons by luminescence. These UV-photons from the scintillator-crystal guided through the optics and detected by photomultiplier tubes. The light collection efficiency from the scintillator-crystals and light transmission efficiency of the optics determines the detectable energy range of X-rays and energy resolution. In this letter, we perform ray-tracing simulations of the luminescence to optical fiber bundle to assess light collection efficiency from the scintillator-crystal and show the effect on the total light coupling efficiency of the system.

A plasma window is an atmosphere-vacuum interface, formed by the interaction of the ideal gas pressure effect and dynamic viscosity effect of plasma. The application of a plasma window is the generation of a pressure difference between 1 and 7 × 10³ Pa without a large exhaust system. In this study, we designed an apparatus with a microhollow cathode discharge for plasma window generation. A resulting pressure difference between 0.889 and 8 × 10³ Pa and a pressure ratio of approximately 10⁴ were obtained.

An idea for shielding high energy ion and electron fluxes is proposed by applying external magnetic fields. In this work, we model a flowing plasma in a small region by utilizing one spatial dimension and three coordinates for velocities (1D3V) Particle-In-Cell (PIC) code. The plasma which consists of ion and electron is produced from the source region and absorbed at the conductor wall. The external magnetic field is modified by applying the change of the magnetic field in the direction perpendicular to the plasma flow. This magnetic field is localized and switched from strong negative values to strong positive values at several locations in the simulation region. We found that this localized reversed magnetic field traps the particles, and then reduces the particle and heat fluxes to the wall. Based on the modeling results, external localized-reversed magnetic fields can control the particle and heat fluxes to the wall. These results can be applied for shielding high energy ion and electron fluxes to the satellite or spacecraft in the space.

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.

The advanced tokamak (AT) scenario with the strong reversed magnetic shear is an attractive candidate of the steady state tokamak because the strong internal transport barrier leads to the high bootstrap current fraction, resulting in the reduction of the cost of the fusion reactor. In this paper, the causes of the collapses during or just after plasma current ramp-up of the experimental campaign of the AT scenario [Y. Sakamoto et al., Nucl. Fusion 49, 095017 (2009)] in 2007 and 2008 are investigated and the initial results are reported. As the observations are consistent with characteristics of the stability on the resistive wall mode (RWM) and the results of MARG2D code, the RWM is suggested as the candidate of the cause of the collapses in the analyzed AT scenario.

The internal mode structure of a precursor with the tearing-parity structure of the locked-mode-like instability is investigated. For the first time, the Thomson scattering system with high temporal and spatial resolution enables us to find the non-rotating temperature flattening region in the torus outboard side in addition to the rotating island during the slowing-down phase. Additionally, the width of the flattening region is 10% normalized by the plasma minor radius. Furthermore, the radial profiles of the pressure degradation during the slowing-down phase are evaluated. At the beginning of the slowing-down phase, the pressure degradation area is located slightly inside the resonant surface. After that, the peak location moves to the core region. Finally, the degradation area peaks at the plasma center.

A fast visible light measurement system was installed on GAMMA 10/PDX to measure plasma light fluctuations induced by ion cyclotron range of frequency (ICRF) (6360 kHz) waves. Time evolution and power spectrum were obtained and the relative amplitude of the fluctuations was the order of 0.1%. The difference of measured wavelength region was investigated by using three interference filters, but obtained time evolutions and the relative amplitudes were similar when we neglect a factor of difference. The RF pick-up noise is about two orders of magnitude smaller than the ICRF wave induced plasma signal, and the broad frequency band component is about an order of magnitude smaller than the signal. The latter arises from the shot-noise of the photon detection Poisson process, and an expression of the shot-noise, in which the effects of the frequency response of the system and the detector property are included, was obtained. The expression is confirmed experimentally using a stable light source.

Spectroscopic analyses of energetic particle (EP) driven bursts of MHD fluctuations in magnetically confined plasmas often exhibit chirps that occur simultaneously in groups of two or more. While the superposition of oscillations at multiple frequencies necessarily causes beating in the signal acquired by a localized external probe, self-consistent hybrid simulations of chirping EP modes in a JT-60U tokamak plasma have demonstrated the possibility of global beating, where the mode's electromagnetic field vanishes globally between beats and reappears with opposite phase [Bierwage et al., Nucl. Fusion 57, 016036 (2017)]. This implies that there can be a single coherent field mode that oscillates at multiple frequencies simultaneously when it is resonantly driven by multiple density waves in EP phase space. Conversely, this means that the EP density waves are mutually coupled and interfere with each other via the jointly driven field, a mechanism ignored in some theories of chirping. In this thesis-style treatise, we study the role of field pulsations in general and beating in particular using the Hamiltonian guiding center orbit-following code ORBIT with a reduced wave-particle interaction model in realistic geometry. Beating is found to drive the evolution of EP phase space structures. A key mechanism is the pulsation of effective phase space islands combined with the alternation of their effective O- and X-points due to phase jumps between each beat. Observations: (1) Beating causes density wave fronts to advance radially in a pulsed manner and the resulting chirps become staircase-like. (2) The pulsations facilitate convective transfer of material between neighboring layers of phase space density waves. On the one hand, this may inhibit the early detachment of solitary phase space vortices. On the other hand, it facilitates the accumulation of hole and clump fragments into larger structures. (3) Long-range chirping is observed when massive holes or clumps detach and drift away from the turbulent belt around the seed resonance. It is remarkable that the detached vortices remain robust and,on average, maintain their concentric nested layers while being visibly perturbed by the field's continued beating.

Transport simulations using the integrated code TASK are performed on the PLATO tokamak to forecast plasma performance. For transport simulations considering the experimental conditions, MHD equilibria are obtained by taking into account the external coil current condition. Dependences of the plasma parameters on externally controllable quantities in the experiment, such as the amount of particle fuel and values of the external coil currents, exhibit an increase on the order of 10% in the ion temperature without direct ion heating.

In this study, a new numerical method for constructing flux surfaces for three-dimensional (3D) toroidal magnetic fields is proposed. In the method, multiple field lines starting from all grid points in the computational domain are simultaneously followed to obtain the field-line average. The field-line average obtained for the entire domain is used to label flux surfaces as the radial coordinate based on a reasonable assumption that the field-line average approximates the flux surface average when continuous nested surfaces exist. It is demonstrated that a severe numerical discontinuity in the constructed surfaces, which is often observed near a low-order rational surface in a conventional method based on the Poincaré map, can be avoided using the proposed method, enabling the construction of smooth flux surfaces.

The infrared (IR) heating method for a central ignition target with spherical symmetry is modified for the axisymmetric Fast Ignition Realization EXperiment (FIREX) target. The challenge is that the FIREX target pretends to be a thermally spherical shell. Our previous simulation studies (A. Iwamoto et al., Fusion Sci. Technol. 56, 427 (2009), A. Iwamoto et al., J. Phys.: Conf. Ser. 244, 032039 (2010)) have shown that the combination of volumetric heating in a fuel and cone temperature control has the potential to finish a uniform fuel layer. We have developed the IR heating system, dedicated to the FIREX target, with exclusive cone temperature control. The ability of solid fuel layering was examined by using an 826 µm polystyrene (PS) shell with a gold cone of 1.2 mm in length instead of the 500 µm FIREX target for easy observation. The system could control the profile of a solid fuel layer in the PS shell target. Eventually, the solid layer with the best sphericity of 92% was formed, and the RMS roughness of the inner surface was 44 - 49 µm in modes 1 to 100 and 14 - 26 µm in modes 5 to 100.

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.

The effect of the pitch modulation of the helical coils on the core plasma performance of the LHD-type helical fusion reactor has been examined. The analysis of the MHD stability and neoclassical transport for the pitch modulation α = 0.0 and 0.1 has been conducted based on the finite-beta equilibrium calculated by the HINT code. It was found that the MHD stability is clearly improved without deteriorating the energy transport property by changing the pitch modulation α from 0.1 to 0.0. The reachable operation region expands to the higher density and the expected fusion gain can increase from ∼10 to ∼20. Because the change of the pitch modulation α from 0.1 to 0.0 requires only a slight change in the shape of the helical coils, the engineering design including the maintenance method that has been examined for the reactor with α = 0.1 can be applied without a major modification.

Erosion of first walls in tokamak fusion reactors due to transient heat loads during ELM and disruptions is a major concern and needs to be predicted. Studies have shown that the erosion amount is strongly dependent on the total energy density and duration of a transient heat pulse. Recently, it was pointed out that the erosion amount is also dependent on the pulse shape [J.H. Yu et al., Nucl. Fusion 55, 093027 (2015), and D. Motoi et al., Fusion Eng. Des. 165, 112209 (2021)]. Meanwhile, it is predicted that the erosion during the transient heat loads can be suppressed by vapor shieldings, and the efficiency of the vapor shielding may differ between the pulse shapes. Thus, in this study, we investigate the pulse shape dependence of the vapor shielding effect by a particle-in-cell based simulation code, PIXY. Two types of square shapes and three types of triangular shapes are examined. Among the triangular shapes, it is found that the vapor shielding is effective especially in “Negative Ramp” triangular shape, where the peak heat flux comes first.

An accelerator-based neutron source using d-Li reactions is one of the most promising neutron sources for fusion material irradiation facilities such as IFMIF, where 40 MeV deuterons bombard a liquid lithium target. The neutron yield estimation including angular neutron spectra is one of the most important issues in the design of such irradiation facilities. Recently, JAEA released deuteron nuclear data of JENDL/DEU-2020 in ACE format file for Monte Carlo codes such as MCNP, and in Frag-Data format for the PHITS code. We carry out the benchmark calculations of d-Li neutron yield by using PHITS with Frag-Data, MCNP with JENDL/DEU-2020, and MCNP/PHITS with built-in nuclear reaction models. Those calculation results are compared with experimental data. It is confirmed that PHITS with Frag Data and MCNP with JENDL/DEU-2020 reproduce well the experimental data. Those are useful for the neutron yield estimation and also the irradiation field characterization of IFMIF and similar facilities.

Tungsten fuzzy nanostructures commonly form on the plasma-facing walls of magnetic-confinement nuclear fusion reactors, induced by the helium plasma irradiation. We calculate the field enhancement factors at the fuzz tips of tungsten, molybdenum, and tantalum, quantitatively representing the degree of field focusing, based on the classical electromagnetic field theory under the quasistatic approximation, for a model system comprising a subwavelength-scale prolate metal hemispheroid protruding from a conducting plane. Field enhancement factors of 2.4 × 10³, 5.4 × 10⁶, and 2.3 × 10¹⁰ for the spheroidal aspect ratio of 10, 100, and 1000, respectively, are observed in the gigahertz regime for the incident electric field parallel to the fuzz, i.e., normal to the reactor wall. Such a potential large field focusing effect may be worth accounting for in the designing and operation of fusion reactors.

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.