Helicon plasma is one of the radio frequency plasma source that can generate high-density and low-temperature plasmas by utilizing the helicon wave, i.e., the electromagnetic whistler wave in a bounded geometry. Helicon plasma is very useful for various applications due to its extremely efficient production of high-density plasma. Conversely, many unsolved physical issues remain regarding how an efficient production of the helicon plasma is realized in laboratories and what determines the density maximum. The past decades of the helicon studies have revealed that the “Trivelpiece-Gould” wave is responsible for the efficient power absorption. In recent years, the drift-wave type and the parametric-decay instabilities have been extensively studied, by using the linear magnetized helicon plasma sources. The present helicon study considers these non-linear effects. Consequently, it includes many interests for both industry and research fields. The mechanism of the helicon production is discussed, based on the several critical physical issues. In addition, some recent topics and the efforts to build a more refined physical model of the helicon plasma are highlighted.

As a result of the progresses in the study of plasma turbulence, the picture of ‘multiple-scale turbulence' is widely accepted, and the ‘nonlinear excitation, nonlocal interaction, probabilistic transition' views have been developed. Along the paths of these progresses, importance of new problem, i.e., the symmetry-breaking of turbulence structure (such as up-down asymmetry, excitation of streamer, etc.) is gradually recognized. In this topical review article, we revisit these issues, and illuminate the possibilities that the new progresses (which will be brought about by studying the symmetry-breaking of turbulence) are expected. Examining theoretical predictions and preceding experimental achievements, new advancements, which will be realized, are explained. The experimental study of symmetry-breaking of turbulence structure ultimately requires to measure fluctuations (at all scales) over the whole plasma cross-section simultaneously. In addition, new type of demands can be imposed in measuring of fluctuations over the whole plasma cross-section simultaneously. One of such demands is the accuracy of the measurement position. Problems in this aspect are also discussed. This concise review is used to identify what will be discovered and how it will be reached in future experiments, in the subject of the symmetry-breaking of turbulence structure.

We have performed laser Thomson scattering (LTS) measurements during the transition between attached and detached helium plasmas in the linear divertor simulator NAGDIS-II. In the detached plasma, the LTS spectrum shows a discrepancy with a single Gaussian function. The discrepancy is resolved by the spectrum fitting with a sum of two Gaussian functions, indicating that the electron energy distribution contains two different temperature components.

A high-density experiment using a novel fueling technique-high-intensity gas puffing (HIGP)-has been performed in Heliotron J. Using the recently developed charge exchange recombination spectroscopy (CXRS), a poloidal flow shear is observed at the peripheral region of high-density NBI plasmas. The evaluation of radial electric field based on the experimental results of toroidal and poloidal rotation velocities shows the formation of a large E_r shear in the peripheral region.

Plasma merging is a unique method of plasma start-up, and magnetic reconnection is a key process for plasma heating in this method. On the University of Tokyo Spherical Tokamak (UTST), we use a pickup coil array to obtain the macroscopic behavior of magnetic reconnection, but direct measurement of the localized current is still necessary to study the details of the magnetic reconnection process. A small Rogowski coil is a useful tool for measuring small scale currents, and we used five such coils to measure the radial structure near the X-points of merging plasmas on UTST. As a result, we have found current densities up to 1.5 MA/m², with a typical time scale of several micro seconds. The current density increases with the plasma current, and the average radial width is about 30 mm.

Spatial profile of deuterium/hydrogen (D/H) ratio is observed using the charge exchange emission of bulk deuterium/hydrogen ion and neutral beam perpendicularly injected into the torus plasma. Neutral beam injection (NBI) is turned on and off in order to subtract the deuterium/hydrogen emission from the plasma edge. The number of fitting parameters for the subtracted spectral signal is reduced from 12 to 3 by assuming that the bulk ion temperature and velocity are the same as those of the carbon ions (C⁶⁺). The initial result observed in Large Helical Device (LHD) plasma indicates that D/H ratio is constant in radial direction and is the same as that observed from passive spectroscopic measurement when the tangential NBI or the pellet injection are absent.

2D high-resolution magnetic probe arrays for local 2D magnetic field measurements of magnetic reconnection have been developed using advanced printed-circuit board (PCB) technology. Each of these arrays can be equipped with all three components of magnetic pickup coils (5 × 3 mm²). Each coil is composed of a two-sided coil circuit pattern with a line width of 0.05 mm. Based on this new probe technique, we conducted high-resolution and high-accuracy measurements of the current sheet to determine the formation and ejection of plasmoid in a current sheet of two merging tokamak plasmas for the first time.

Three-dimensional resistive MHD simulation was carried out in order to develop a method to fuel the plasma core by merging two Spherical Torus (ST) plasmas. A relatively small ST plasma was translated into the axial direction by external magnetic field control and made to collide with the larger ST. In this calculation, ballooning instability occurred in both plasmas during translation, while the magnetic surface was observed to collapse from the Poincare plot of the magnetic lines. It was observed that the magnetic lines in the core regions of the two STs were connected to each other and that the merging was completed.

An ingenious model for large-scale plasma simulations is proposed. An artificial permittivity ε_∗ much grater than ε₀ (permittivity of free space) is introduced to the Poisson equation, ∇· (ε_∗∇φ) = −ρ (φ is electrostatic potential and ρ is charge density), and the Debye length is artificially elongated. A large-scale system with a reasonable number of spatial grids can be treated stably even for including the self-consistent electron dynamics. A dielectric tensor is adopted for the three-dimensional global simulation of tokamak plasmas in the cylindrical coordinates (R, θ, Z). A diagonal element ε_θθ for toroidal axis θ is set much larger than those for the poloidal plane, ε_RR = ε_ZZ = ε_∗. Number of toroidal meshes N_θ can be reduced enough smaller than that of poloidal-plane meshes, N_R and N_Z, in comparison with N_θ > N_R, N_Z for the usual case of isotropic ε_∗.

We conduct the first simulation study of the neoclassical tearing mode (NTM) triggered by error field penetration. A series of processes including the error field penetration, formation of seed islands, and the triggering of the NTM occurs, when the plasma flow and the electron diamagnetic drift approximately cancel out each other. The excited NTM is the born-locked mode, which is locked from the beginning. In the case where the plasma flow and the electron diamagnetic drift completely cancel out each other, a vacuum island width necessary for triggering the born-locked NTM is much smaller than a seed island width necessary for triggering the original NTM. This tendency is consistent with a theoretical prediction. Thus, whether the born-locked NTM is excited depends on both the plasma flow velocity and the error field amplitude.

Fuzzy nanostructure growth occurs on tungsten (W) surfaces by the exposure to helium (He) plasmas. We investigated pulsation effect in the incident energy of He ions on W fuzz growth. It is shown that He irradiation contributes to the growth of fuzzy layer even if the incident ion energy was less than the threshold energy of 20 - 30 eV. When the duty cycle of the pulse was 1 - 10%, 7 - 8 eV He ion irradiation have contributed to the fuzz growth in addition to high (> 60 eV) energy ion irradiation, and the growth rate was enhanced.

Blistering was observed on surface of polycrystalline tungsten samples irradiated by helium ion beams with 4 MeV kinetic energy in a fluence of 10²² m⁻². The helium ion beam was used as the damage source to the tungsten plate, and the surface temperature of tungsten sample was increased up to 345 K by the ion beam irradiation. The surface was modified by the ion beam irradiation, and its maximum height was about 20 µm.

Evaluating the performance of the shielding walls and shielding doors at radiation facilities is important to ensure proper radiation safety management. In the first period of the deuterium experiment, the neutron-shielding ability of the shielding door of the experimental hall containing the large helical device (LHD) was evaluated using the gold-foil activation method. The gold foil was inserted into a small gap between the shielding door and the shielding wall. Meanwhile, the shielding effect of the wall against the neutrons and gamma rays was verified using the badge-type dosimeters that were placed both inside and outside the experimental hall near the wall. The radiation derived from Au-198, which was visualized using by imaging plate, reached approximately 35 cm inside the shielding door. The results provided visual evidence that the shielding door safely excluded the thermal neutrons that were generated in an LHD. Furthermore, the measurement results of the badge-type dosimeters confirmed that the neutrons and gamma rays were completely trapped by the concrete wall of the experimental hall.

Plasma grid bias has been utilized for reducing an electron density in the vicinity of a plasma grid in negative ion sources for fusion, resulting in achievement of low co-extracted electron current. In this study, the effectiveness and feasibility of the plasma grid bias on beam optics optimization are demonstrated. It is shown that the beam optics strongly depends on the bias voltage and is successfully optimized at different bias voltages depending on the discharge power. Responses of the beam properties such as the perveance and the current ratio to the plasma grid bias are also shown for both hydrogen and deuterium beams.

This study evaluates expiratory testing as a simple bioassay method for estimating the effective dose equivalent for individuals working inside radiation control areas in a large helical device (LHD) building at the National Institute for Fusion Science (NIFS) in Toki City, Japan. The expiratory testing apparatus is configured according to similar devices reported previously. The core components of the apparatus include the sample collection device, a water bubbler column, exhalation airflow controls, and a solution injection component. Exhaled water vapor from individual test subjects is collected and tritium levels are measured using a liquid scintillation counter. Tritium radioactivity measurements are then compared to the effective dose according to evaluation criteria estimated using the effective dose equation for tritium.

Laser fusion rocket is one of the candidate propulsion devices for Mars exploration. It obtains thrust from the interaction between plasma and magnetic field and this propulsion system is called magnetic thrust chamber. We constructed a spectrometer with high wavelength resolution of 35 pm to obtain plasma parameters by measuring ion feature of laser Thomson scattering from a laser-produced plasma in a magnetic thrust chamber. We obtain the plasma parameters such as electron temperature, electron density, and velocity as well as the plasma density structure showing the stagnation of the plasma by magnetic field.

In this study, we demonstrated the improvement in the biocompatibility of titanium alloy by atmosphericp-ressure-plasma nitriding, in which the pulsed-arc plasma jet is sprayed onto a titanium alloy to form TiN surface. The deposition properties of calcium phosphate on different samples were compared after immersion in simulated body fluid to investigate hard-tissue compatibility. It was determined that the growth of the calcium phosphate layer on the nitrided sample was the most rapid. This result suggests that atmospheric-pressure-plasma nitriding has the potential to easily improve the hard-tissue compatibility of titanium alloy.

In order to understand turbulent transport phenomena in magnetized plasmas, an excitation condition of the ion-temperature-gradient (ITG) instability is investigated in linear device PANTA. Numerical analyses using a global gyro-fluid code in linear devices are performed to obtain mode structures and parameter dependences of the ITG instability. Parameter scans of the linear growth rate show the destabilization condition of the ITG modes. The global analysis considers the boundary condition and determines the radial mode structure, which gives the values of the wavenumber in the direction perpendicular to the magnetic field. The local analysis confirms to reproduce the global analysis result by using the wavenumber obtained from the global analysis. The wavenumber is a parameter in the local model, and the global analysis of the radial mode structure is necessary for the selection of this important parameter.

In linear plasma devices, blob-like cross-field transport is enhanced especially in detached plasmas. Formation of blob-like plasmas in the linear plasma device NAGDIS-II was observed using a fast framing camera, and the mode analysis was conducted using the emission from the plasma column. From the Fourier analysis, it was seen that plasma instability was enhanced before the ejection of blob-like plasmas from the plasma column. It was found that blob-like plasmas are generated at the peripheral region of the plasma column associated with low mode number (m = 0 - 2) plasma instability inside the plasma column.

The electron temperature and density were measured with an electrostatic probe in a helical plasma in the TOKASTAR-2 device in order to determine the location and the shape of the last closed flux surface (LCFS). The electron density inside the calculated LCFS was found to be higher in a helical plasma than in a plasma without a helical field when the electron-cyclotron-resonance layer was located inside the LCFS. Although errors in the manufacturing and installation of coils have been a concern, this result indicates that the LCFS formed in this device does not differ greatly from the calculated LCFS.

In TOKASTAR-2, the tokamak plasma position was radially unstable in the outer region due to a large decay index (n-index) of the vertical field. To improve the radial position stability, the position of pulsed vertical field coils was moved toward the midplane. After the modification, improvement in the decay index profile and extension of the range of the stable radial plasma position toward the outer region were observed by the internal magnetic field measurement. The variation in the outermost position of the plasma center was about half of the radial shift of the decay index curve.

An indirect effect of the electron cyclotron current drive (ECCD) on the ion temperature in the plasma core region was observed in the Large Helical Device. The reference (no ECCD) discharge with a central ion temperature T_i0 of ∼3.0 keV is operated by a standard high ion temperature discharge procedure. To investigate the ECCD effect, a co- or counter-ECCD was applied to the reference discharge, and was turned off immediately before the T_i0 peaked in the reference discharge. In the co-ECCD and counter-ECCD applications, the T_i0 temporarily increased and decreased by ∼0.5 keV from T_i0 in the reference discharge, respectively. The mechanism of this phenomenon is presently unclear, but may be exploited as a practical knob for controlling the central ion temperature.

Tangential neutral beam injection deposits momentum in a fusion plasma, which produces macroscopic flow of bulk particles. Concurrently, equilibrium distribution of hot ions produced by the injection is determined by collisions with the bulk particles. Through these processes, macroscopic flow of hot ions can be produced. Effects of the flow of hot ions have not been focused on since their inertial force is thought to be much smaller than the bulk one due to smallness of density of hot ions. In this study, we derive an analytic representation for parallel flow of hot ions. From the analytic representation, we find that amplitude of the parallel flow is determined by three parameters, injection speed, injection pitch angle, and electron temperature. Also we find that the inertial force of hot ions is not negligible even if the density of hot ions is much smaller than that of bulk ions.

The stabilization effect of the Internal Flow Resistances (IFR) inserted into water jets has been investigated in order to apply the liquid metal divertor to the helical fusion reactor, FFHR. A high heat load exceeding 20 MW/m² is one of the important issues for the divertor in the nuclear fusion reactor. A new concept of ergodic limiter/divertor called the REVOLVER-D has been proposed for FFHR. This divertor is expected to tolerate a high heat load larger than a few tens of MW/m². Jets narrow after acceleration by gravity and transform to droplets due to the surface tension instability. Stabilization of jets is an important issue for realizing the REVOLVER-D. In this paper, stabilization of jet by various IFR has been examined. Here, the “sheath jet” is defined as a jet with an IFR inside. Experiments on sheath jet have been carried out and the stabilization effect of IFR has been confirmed. Numerical simulations on sheath jet using ANSYS have been carried out. It is shown that the velocity of the sheath jet reaches the terminal velocity, which is proportional to 0.36 ± 0.01 power of the flow rate and −0.26 ± 0.01 power of the outer circumference of the IFR.

Faraday rotation angle in Z-cut crystal quartz at a wavelength of 118.8 µm was measured for the first time. Z-cut crystal quartz is used in vacuum windows of polarimeters for plasma diagnostics that use far-infrared light. ITER poloidal polarimeter uses double vacuum windows of 10-mm Z-cut crystal quartz discs in compliance with nuclear safety. According to the Becquerel model, thick Z-cut crystal quartz may lead to non-negligible Faraday rotation that must be compensated for to measure the pure polarization change attributable to the plasma. For the first time, this paper measures Faraday rotation at the wavelength of 118.8 µm by means of the rotating linear polarizer method. The Becquerel model is not applicable at the wavelength of 118.8 µm and Faraday rotation resulting from the Z-cut quartz vacuum window is negligible even with a magnetic field of 0.3 T and a 10-mm thick vacuum window.

Intense beams of micro-particles, each attached with approximately 1.1 × 10⁷ electrons, were generated repeatedly. The particle was a spherical divinyl benzene polymer of 10 µm diameter, coated with 450 Å thick gold. More than 5000 charged particles were extracted outside in 0.5 s using an assembly composed of a reservoir, a helicoid conveyer, an oscillating sawtoothed plate and a charging section of a high electric field.

To deposit stable a-Si:H films at a high deposition rate (DR), we have studied time evolution of DR and amount of cluster incorporation (R) into films as a parameter of gas velocity, in the downstream region of a multi-hollow discharge plasma chemical vapor deposition reactor; because a-Si:H films containing less cluster incorporation show high stability. For a low gas velocity of 0.18 m/s, clusters are trapped between the multi-hollow electrode and the substrate and the trapped clusters absorb clusters and such absorption suppresses the cluster incorporation into films. By utilizing this phenomenon, we have realized a quite low R = 1.3 at a high DR = 0.06 nm/s.

Thermal motion of charged particles in the field of electrostatic trap under an influence of constant magnetic field is investigated analytically and numerically. For the first time the conditions for energy balance of these particles in the systems with the spatially non-uniform thermal sources are proposed. The numerical simulations were carried out for the ensembles consisting of one to thousands of particles in a wide range of parameters of the analyzed systems. Comparisons of their spectral characteristics are presented. We found that the shape of the spectral density distributions in these systems is practically independent of the number of particles in the analyzed ensembles, and their characteristic frequencies can be obtained by an analytical solution of motion equations for a single charged particle.

RIKEN has developed accelerator-driven compact pulse neutron systems for practical use in industrial applications and non-destructive infrastructure inspection that are called RIKEN accelerator-driven compact neutron source (RANS) and RANS2. The visualization method for corrosion, the corrosion's related water movement in painted steels, the analytical method for the quantitative estimation of the water movement in painted steels, a neutron engineering diffractometer for texture evaluation, and the austenite volume fraction estimation of iron and steel have been successfully observed through slow neutron applications. For fast neutron imaging applications, a pixel imaging detector for fast neutrons with energy levels above 1 MeV has been developed and used to produce images of a steel bar and an air gap through 30 cm of concrete. The salt concentrations of 4-cm and 5-cm thick mortar blocks have been measured, and a correlation diagram was obtained for a density of up to 1 kg/m³. RANS2 is now undergoing further development, particularly for outdoor use, as the first test model of an on-site compact neutron system. In this development, RANS2 is equipped with a 2.49-MeV three-fold proton accelerator with an RFQ of 200 MHz.

3 × 10⁸ deuterium-deuterium (D-D) neutron generation per pulse was achieved with one-sided laser irradiation through an inlet hole of a deuterated polystyrene shell and a laser intensity of (2-3) × 10¹⁶ W/cm². X-ray pinhole camera images displayed a surprisingly uniform hot core formation at the center of the shell. Neutron time-of-flight spectra were recorded at three different angles from the laser incident axis to investigate the directional dependence. The dependence of neutron yield on laser energy, shell diameter, and inlet hole diameter is also discussed. To explain the number of the observed neutrons, a simple model, based on a central expansion of two-electron-temperature (hot and cold) plasma, is presented under assumed hot and cold electron temperature. According to this model, more than 10¹⁰ neutrons per pulse, the average amount required for many industrial applications, could be generated by using a higher intensity laser.

A short and high-intensity neutron pulse can be produced efficiently by using photonuclear reactions caused by Bremsstrahlung hard X-rays in a lase-irradiated high-Z target. The efficient and repetitive neutron generation was demonstrated with the combination of 1 Hz, 0.5 J, 25 fs, 5 × 10¹⁹ W/cm² laser pulses and a rotating tungsten disc targe. Here we applied double laser pulse irradiation scheme to increase the neutron generation efficiency. The first low-intensity laser pulse produces a long-scale under-critical-density plasma on the tungsten target surface prior to the second pulse irradiatio. High energy electrons above the ponderomotive scaling value are accelerated by the second hig-intensity pulse in the preformed plasm, this results in the increment of hard X-ray photons and photonuclear neutron. 3.5 × 10⁴ neutron/pulse was obtained with optimized laser irradiation conditions.

We are studying the non-destructive detection of salt gradients in concrete using a neutron-captured prompt-gamma-ray analysis at the RIKEN Accelerator-driven compact Neutron Source (RANS). The neutrons at RANS are generated via a 7-MeV proton and 300-µm-thick beryllium target and are moderated via a polyethylene moderator. Samples, e.g., mortar samples with varying salt contents and part of an existing salt-damaged bridge, were irradiated by neutrons; then, the gamma rays following the neutron capture reaction were detected by germanium detectors placed near the samples. From the results, we obtained the detection sensitivity of the chloride content in a medium.

The laser Compton scattering gamma ray beamline at the synchrotron light facility NewSUBARU supplies a quasi-monochromatic, polarized, high-energy photon beam. A gamma ray flux of more than 10⁷ photons/s is generated at a photon energy range of 1 - 73 MeV. Emission distributions of the fast neutrons generated from different target materials were measured as a function of the polarization angle of the linear polarized gamma-ray beam using a time-of-flight method with fast plastic scintillators. The neutron distributions were consistent with a previous theoretical result. The relatively slow neutrons were measured via an activation method. The result demonstrated the small anisotropy of the slow neutron emission.

At 14 MeV, it is known that the absolute value of large angle scattering cross section is small. The contribution is thus thought to be neglected in the neutronics design of fusion reactor. However, in case that a neutron source can be regarded as a beam like a neutron streaming, large angle scattering cross sections might affect the nuclear design result largely. In fact, in fusion neutronics benchmark experiments using a neutron beam so far, there was a difference observed between experiment and simulation. Also it is known that there are differences in large angle scattering cross sections among nuclear data libraries. Then we have been carrying out preliminary benchmark experiments for verification of large angle scattering reaction cross sections of iron for a few years. The purpose of the present study is to optimize the experimental system design to realize an accurate benchmarking of large angle scattering reaction cross sections. Finally, we reached the optimized experimental system and developed the experimental procedure which was supposed to perform more accurate benchmark experiments for large angle scattering reaction cross sections.

This paper presents the study of prospective organic-based materials, trans-stilbene and p-terphenyl, as their potential application for new neutron scintillators. Growth of their single crystals by the self-seeding vertical Bridgman method is reported. And evaluation of grown crystals concerning the composition, optical and luminescence properties, and scintillation performance at room temperature is discussed. Their photo- and radio-luminesce spectra were peaking at 388 and 408 nm, their quantum yield was of 52.4% and 79.7%, and the light yield were of 0.89 and 1.80 times higher than that of GS-20 standard, respectively. The scintillation decay times of 8.2 ns (trans-stilbene) and 5.8 ns (p-terphenyl) were observed as well.

A flat-panel and resistor-type photomultiplier tube (FRP) system, which is a two-dimensional position-sensitive neutron detector, has been developed. The FRP detector consists of a scintillator, a multi-anode photomultiplier tube (MA-PMT), a resistor network board, and an FRP amplifier and a readout circuit which is similar to that for the old resistor type photomultiplier tube (RPMT) system. Therefore, the FRP system uses the same readout circuit of the RPMT system. Two types of FRP detector are available. One is the FRP4 detector, which consists of four MA-PMTs with a detection area of 10-cm square. The position resolution due to irradiation of neutron beam with approximately 0.5-mm diameter is 1.26 mm in the X axis and 1.61 mm in the Y axis. The other one is the FRP1, which consists of one MA-PMT with a detection area of 5-cm square. The position resolution due to irradiation of a neutron beam with approximately 0.5-mm diameter is 0.83mm in the X axis and 0.89 mm in the Y axis. The FRP detector can be used for high position-resolution and transmission experiments of a compact neutron source.

Currently, there is much demand for a non-destructive assay technique to quantify special nuclear materials of ²³⁵U and Pu isotopes in a field of nuclear nonproliferation. For this purpose, a compact NRTA system is under development. The performance of a proposed compact NRTA system was investigated by calculating a neutron transmission spectrum for a spent nuclear fuel. In this paper, we mainly evaluated how a transmission spectrum was affected by neutron pulse width and flight path length of the system and discuss the appropriate values from a viewpoint of measurement of special nuclear materials.

The authors have developed a nondestructive inspection method using backscattered neutrons to detect voids and water in concrete with a RIKEN Accelerator-driven Compact Neutron Source (RANS). In this study, neutrons are emitted to a slab sample that consists of a concrete layer and an asphalt layer, and position distribution of backscattered neutrons are measured at the timing from 0.1 to 0.5 ms. The two-dimensional position of water and voids in the concrete plate under asphalt (5 cm) of the slab sample are detected.

Clinical trials of the boron neutron capture therapy (BNCT) have been conducted using research reactors. Recent progresses in the accelerator and accelerator-driven-neutron-source technologies have rendered it possible to generate the substantial number of neutrons required for BNCT treatment, using a compact accelerator, which can be installed in a hospital. The University of Tsukuba launched a project for the development of a compact accelerator-based neutron source for BNCT. For this accelerator, we employed a linear particle accelerator (linac) and the energy of the proton beam was 8 MeV. Beryllium was selected as the neutron target material. To generate sufficient neutron intensity by the reaction between 8 MeV protons and beryllium, the linac accelerates a high current of 5 mA or more. As the target system is critical, we developed a beryllium target system with a threelayered structure to avoid target breakage, caused by massive heat load and blistering, within a short period. The linac-based neutron source for the BNCT is almost complete and we succeeded in generating neutrons, in 2015. Currently, several characteristic measurements are being carried out.

The Accelerator-driven System (ADS) is one of the candidates for transmuting long-lived nuclides, such as minor actinide (MA), produced by nuclear reactors. For the efficient transmutation of MA, a precise prediction of the neutronics of the ADS is required. To obtain neutronics data for the ADS, the Japan Proton Accelerator Research Complex (J-PARC) has a plan to build the Transmutation Physics Experimental Facility (TEF-P), in which a 400-MeV positive proton (H⁺) beam will be delivered from the J-PARC linac. Because the TEF-P requires a stable and low background proton beam with a power of less than 10 W, a stable and meticulous beam extraction method is required to extract the low power proton beam from the high power negative hydrogen (H⁻) beam of 250 kW. To fulfill this requirement, a new type of Laser Charge Exchange (LCE) device was developed. A feature of this LCE device is the elimination of the background protons that are not extracted by the LCE technique. To demonstrate the charge exchange of H⁻, an LCE experiment was conducted using a linac with an energy of 3 MeV in J-PARC. As a result of the experiment, a charge-exchanged H⁺ beam with a power of 7.99 ± 0.22 W equivalent was obtained under the J-PARC linac beam condition, and this value nearly satisfied the power requirement of the proton beam for the TEF-P.

A capillary plate (CP) gas detector is a type of a hole-type micropattern gaseous detector. The detector displays high spatial resolution characteristics in two-dimensional radiation detection by using a CP with a small channel pitch. In this study, a sealed-type CP gas detector for neutron imaging was developed. The detector converts the incident position of neutrons to an optical image. A novel imaging system was constructed, and it comprised the CP gas detector, an image intensifier unit, and a science-CMOS camera. The system was tested with a compact accelerator driven neutron source, “KUANS.” Each signal of charged particles generated by a nuclear reaction between a neutron and a ¹⁰B layer was obtained. The results indicated that neutron images were obtained clearly.

Feasibility studies on neutron radiography using an inertial electrostatic confinement (IEC) neutron source were carried out. A Monte Carlo analysis was carried out to evaluate the feasibility of neutron radiography experiment. Imaging tests using a medium-sized IEC neutron source were conducted with the indirect method using a dysprosium (Dy) foil and an imaging plate. Neutron images of objects consisting of six Cd pins and an array of B₄C powder contained in a stainless-steel blade were obtained. The numerical and experimental results confirmed that the IEC neutron source can be applied to neutron radiography even with relatively low neutron flux of ∼10² n/s/cm².

The plasma heating mechanism during magnetic reconnection in the presence of guide magnetic field is investigated by means of particle simulations and test particle simulations. Particle simulations demonstrate that ring-like structures of ion velocity distributions are formed in the downstream, where a large percentage of ions are effectively heated. Furthermore, a single ion motion is focused upon by using test particle simulations. The ion moves in the downstream direction owing to the E × B drift while in gyromotion. Finally, it is reported that the profile of the ion temperature obtained by the particle simulation fits well with the profile observed in the TS-3 experiment.

Tungsten (W) and tungsten based materials have been recommended as the plasma facing component in the current fusion devices. The electron induced processes on these materials are of prime importance for the application purposes. Electron impact total ionization cross sections (TICS) are reported for the W atoms and W⁺ ions. The TICSs have been calculated in the variants of distorted wave approximation (DWA) using Hartree-Fock wave functions and distorted potential with semi-classical exchange. Present TICS results have been compared with the available theoretical and experimental results. Reasonable agreement with the existing theoretical results have been obtained for the TICS of W atoms by present semi-relativistic distorted wave approach, however there are certain discrepancies for the TICS of W⁺ ions. Differential cross sections (DCSs) have also been calculated at projectile energy 100 eV for the ionization of W atoms and W⁺ ions and the DCSs have been found to be sensitive on the scattering angle.

Three-dimensional full particle simulation is performed on the electron beam plasma electrostatically confined in a linear chamber. Sausage instability and kink instability occurred within 1 µs from the initial state, and a twisted structure is also observed. The density distribution flattened with time and resultantly diffused in the radial direction. However, large scale collapse phenomenon until the beam plasma disappeared is not observed.

In order to study a new method of generating energetic ion, a linear plasma device NUMBER was designed and constructed. The device consists of a plasma production region and a test region connected axially. The radial profile of the electron density and the electron temperature were measured using a Langmuir probe. The radius of plasma is consistent with a field line trace calculation. The time evolution of the ion saturation current I_is was measured. A high I_is phase accompanied by rapid increase and rapid decrease of I_is was observed. From the scan of magnetic field in the test region, it is found that the high I_is phase may be caused by unexpected electron cyclotron resonance in the test region. The gas pressure is also related to the appearance of the high I_is phase.

A helium ion beam was injected into hydrogen ionizing plasma using a radio-frequency plasma source to study divertor plasma. Optical emissions from the hydrogen Balmer series and Fulcher-α band were collected. The emissions of the hydrogen Balmer series clearly increased following the onset of the ion beam injection. The behavior of excited hydrogen atoms slightly depend on its excitation energy. In addition to the Balmer series, the emission from hydrogen molecules increased under the existence of energetic ions. These results indicate that the energetic ion collision affects the population density of the excited hydrogen atoms and molecules.

Adaptive Particle Management (APM) method originally developped by Assous is improved for cylindrical 2-dimensional Particle-In-Cell code with 2nd-order shape function. Charge, momentum and energy for overall particles and charge and current density on each grid are rigorously conserved during particle refinements. In order to minimize the deformation of velocity distribution function, phase-space resampling of refinement particles was introduced. We tested our new code in Counter-Helicity Spheromak Merging simulation. Radial shift of X-point during magnetic reconnection is observed, which is consistent with the previously reported result.

We performed trajectory analysis of beam ions in such a magnetic field structure having a weak magnetic field region over a wide range by canceling the magnetic field created by the solenoid coil with a Helmholtz coil. The rate at which the beam ions injected in the axial direction are trapped in the magnetic field structure is statistically examined. In addition, by adjusting the Helmholtz coil current, the position of the field-null point is changed, and the influence on the ion trapping rate is also investigated. As a result, it is found that the larger the beam dispersion, the higher the trapping rate, and the higher the energy, the lower the trapping rate. Furthermore, it is also found that in the case where the magnetic field is not completely canceled at the center of the device, there is an energy which extremely decreases the trapping rate.

Dynamic behaviors of ionization waves influenced by feedback are experimentally studied. Delayed feedback is useful for stabilization of chaotic system; it has applicability in controlling chaos. However, delayed feedback can also result in a stable system becoming unstable, or even chaotic. The ionization wave system in our experiment has one spatial degree of freedom. Neon gas is introduced into a glass tube that has been evacuated to high vacuum, and a glow discharge Ne plasma is produced by an electric current between two electrodes. Fluctuations in the light intensity are sampled using two photodiodes placed a certain distance apart; the intensity sampled from one photodiode is fed back to the system through an external circuit. The largest Lyapunov exponents are calculated from the time series sampled from the photodiodes. The value of the largest Lyapunov exponent is positive for chaotic oscillations, with higher values for more chaotic systems. The value becomes close to zero for a system with periodic oscillations. In our studies, we found that a chaotic system can be made periodic by applying feedback while the distance between two photodiodes is a multiple of a particular value.