It can be advantageous to push particles with waves in tokamaks or other magnetic confinement devices, relying on wave-particle resonances to accomplish specific goals. Waves that damp on electrons or ions in toroidal fusion devises can drive currents if the waves are launched with toroidal asymmetry. Theses currents are important for tokamaks, since they operate in the absence of an electric field with curl, enabling steady state operation. The lower hybrid wave and the electron cyclotron wave have been demonstrated to drive significant currents. Non-inductive current also stabilizes deleterious tearing modes. Waves can also be used to broker the energy transfer between energetic alpha particles and the background plasma. Alpha particles born through fusion reactions in a tokamak reactor tend to slow down on electrons, but that could take up to hundreds of milliseconds. Before that happens, the energy in these alpha particles can destabilize on collisionless timescales toroidal Alfven modes and other waves, in a way deleterious to energy confinement. However, it has been speculated that this energy might be instead be channeled instead into useful energy, that heats fuel ions or drives current. An important question is the extent to which these effects can be accomplished together.

We improved the signal-to-noise ratio of a Reynolds stress (RS) probe in order to measure the Reynolds stress more accurately. By introducing a shield pipe in the probe, the power spectral density of the noise was suppressed to approximately one tenth in an electric biasing experiment. It was also confirmed that the leak current in the RS probe, which had been used in previous experiments, was small enough for the study of drift waves in our linear device.

The RF central solenoid operation is a method for generating an inductive RF electric field in a device by which plasma can be produced and ohmically heated. Furthermore, it may drive the DC current via heating. Experiments were carried out in TST-2 to clarify this potential under a limited amount of flux swing (±0.5 mVs) of the central solenoid. It was found that it can produce plasma and drive the DC current up to approximately 0.6 kA on average when an external equilibrium field is applied. In addition, the DC current can be ramped up when the inductive RF field is applied to ECW power-sustained plasmas.

Ion flux into a target plate in expanding and/or contracting magnetic field configuration has been measured under detached plasma condition in the linear plasma device NAGDIS-II. When the magnetic field was contracting towards the target plate, the strong reduction of the ion flux into the target was observed. This result indicates that the contraction in magnetic field should contribute to enhancing the degree of detachment. The magnetic-field-induced enhancement of electron density and the reduction of ion flow velocity due to the mirror effect in contracting magnetic field could lead to increasing volumetric recombination as well as the coupling between ions and electrons.

The flux of parallel momentum by parallel shear flow driven instability is calculated with the self-consistent mode dispersion. The result indicates that the diffusive component has two characteristic terms: ν_D1 ∼ ṽ_x² /γ₍₀₎ and ν_D2 ∼ ṽ_x² /(k_∥²D_∥) where ṽ_x is the fluctuation radial velocity, γ₍₀₎ is the growth rate of the mode, k_∥ is the parallel wave number, and D_∥ is the electron diffusivity along the magnetic field. ν_D1 results when the parallel flow shear is above the threshold, while ν_D2 is important around the marginal state. Since typically ν_D1 ≫ ν_D2 ∼ D_n, where D_n is the particle diffusivity, the Prandtl number (≡ ν/D_n) becomes large when parallel flow shear driven instability occurs. This feature may explain the experimental observation on the difference between profiles of density and toroidal flow in edge and SOL plasmas.

Impacts of electron inertia with an electron skin depth (ESD) longer than the realistic value used in early numerical studies on non-ideal ballooning modes (NIBMs) are numerically investigated by a linearized 3-field reduced MHD model. In this paper, 4 different ESDs d_e^∗ = 0, d_e, √10 d_e, 10d_e are used for a resistivity dependence study of the growth rate of NIBMs, where d_e = c₀ √ε₀m_e/n_ee² is the real ESD and d_e^∗ = 10d_e corresponds to an order of ESD used in a numerical study on collisionless ballooning mode (CBM) reported in [Kleva and Guzdar Phys. Plasmas 6, 116 (1999)]. In the case with the real ESD d_e ^∗= d_e, a transition from resistive ballooning mode (RBM) to CBM occurs in the edge relevant resistivity regime, while the electron inertia effect is overestimated and the growth rate is almost independent of resistivity in the cases with d_e ^∗= √10 d_e and 10d_e. These results indicate that the real ESD is one of key factors for the edge stability and turbulence analysis.

The in-vessel cryo-sorption pump for the Closed Helical Divertor (CHD) in the Large Helical Device (LHD) has been developed at the National Institute for Fusion Science (NIFS). An organic adhesive-free bonding technique for attaching activated carbon pellets to a copper cold panel was invented, which employs the indium solder with intermediate materials. The prototype of the CHD with the newly developed cryo-sorption pump was installed in the LHD. Performance of the cryo-sorption pump was estimated in the LHD vacuum vessel. A satisfactory result of the maximum pumping speed up to 9 m³/s was obtained with one divertor module in one toroidal section (10% of the torus), which is equivalent to the required pumping speed of the CHD.

A laser ablation plasma was characterized as a source for high-flux particle beams. The ablation plasma was biased systematically from positive to negative high voltages and the fluxes of charged particles through a pair of extraction electrodes were measured. The ratio of available flux of electrons to that of ions was around 10. The result means the ion extraction is basically provided by the flux of supersonically drifting plasma and the electron extraction is determined by the flux of co-moving electrons exceeding a reduced sheath potential at the extraction electrode.

A fiberform nanostructured layer is formed on a tungsten (W) surface using helium (He) plasma irradiation. The behavior of the nanostructures and He bubbles were observed during annealing via in situ transmission electron microscopy. Notable changes in the nanostructures occurred at 1223 K. A collapse of the nanostructures was observed along with a collapse of the He bubbles during annealing.

Large electromagnetic fluctuations inside the current sheet and large reconnection electric fields are observed during fast magnetic reconnection in the presence of a guide field. The fluctuations transport 2.5% of the dissipated magnetic energy from the reconnection region. Although the energy gains of the ions and electrons are approximately 60% and 12%, respectively, of the dissipated magnetic energy after the fast reconnection, the energy of fluctuations is not comparable to their energy gains. The fluctuations do not directly contribute to the energy conversion but might cause the fast reconnection leading to the rapid release of magnetic energy.

This paper describes recent advances in merging/reconnection experiments in MAST, namely tomographic ion Doppler spectroscopy capability from 2013 which solves the problem of the absence of ion temperature profile measurement during the solenoid-less startup. Providing 32 channel line-integrated spectra from 0.25 m < r_tangential < 1.1 m are connected to a grating spectrometer with the focal length of 1.0 m and grating frequency of 1800 L/mm, and tomographic reconstruction is applied to measure local ion temperature profile. This system successfully contributes to the study of ion heating during merging/reconnection startup and revealed that magnetic reconnection mostly heats ions in the downstream region of outflow jet and also found its contribution to bulk electron heating as well as the localized heating at X point with the time scale of energy relaxation between ions and electrons.

A high-accuracy numerical integration algorithm for a charged particle motion is developed. The algorithm is based on the Hamiltonian mechanics and the operator decomposition. The algorithm is made to be time-reversal symmetric, and its order of accuracy can be increased to any order by using a recurrence formula. One of the advantages is that it is an explicit method. An effective way to decompose the time evolution operator is examined; the Poisson tensor is decomposed and non-canonical variables are adopted. The algorithm is extended to a time dependent fields' case by introducing the extended phase space. Numerical tests showing the performance of the algorithm are presented. One is the pure cyclotron motion for a long time period, and the other is a charged particle motion in a rapidly oscillating field.

A deuterium plasma experiment is being planned at the Large Helical Device (LHD) at the National Institute for Fusion Sciences (NIFS). To delineate the regional background tritium concentration level before initiation of the experiment, we evaluated tritium concentrations in environmental water samples (river water, pond water, well water, tap water, and rainwater) collected at Tono area, Japan since 1982. Tritium concentrations in environmental water samples ranged widely from N.D. (below the instrumental detection limit of 0.27 Bq L⁻¹) to a maximum of 4.39 Bq L⁻¹. Tritium concentrations at 9 continuous monitoring locations over the 15 years ranged from N.D. to 1.36 Bq L⁻¹. This regional background concentration range will be used to evaluate environmental assessments after the initiation of the deuterium plasma experiment in LHD.

The purpose of the paper is to describe the decoupling processes of electron and ion dynamics in the reconnection layer (current sheet and separatrix regions) and how the in-plane electrostatic electric field and the parallel electric field are produced. During driven magnetic reconnection of oppositely directed magnetic field lines, both ions and electrons drift together with the merging field lines toward the neutral sheet where the magnetic field lines reconnect. Because the electron outflow velocity is much larger than the ion outflow velocity, a pair of currents flow inward toward the magnetic reconnection region and produce the quadrupole out-of-plane magnetic field concentrated around the separatrix regions. The parallel electric field is produced by the driving electric field and the quadrupole magnetic field and points toward the downstream direction. The parallel electric field accelerates electrons toward the reconnection region direction but ions move slowly across the separatrix field lines, which causes decoupling of the electron and ion flow dynamics around the separatrix regions to generate charge separation and produce electrostatic electric field pointing across the separatrix field lines toward the mid-plane direction. Around the magnetic reconnection region where the magnetic fields become weakened and reversed, the particle orbits perpendicular to the field lines become meandering. Because the ion meandering region width is much larger than the electron meandering region width, charge separation is produced inside the ion meandering region and produces a pair of strong bipolar in-plane electrostatic electric fields pointing toward the mid-plane direction. With the production of quadrupole magnetic field, the parallel electric field and the electrostatic electric field, particle dynamics and acceleration/heating can be understood.

Reflex discharge has a large number of applications. However, there is no analytical formula allowing for the specified geometry to calculate the current or spatial characteristics of the discharge a priori. To do this, we create two-component model of the discharge in a diffusion-drift approximation in a magnetic field. The conditions of breakdown have been investigated in the framework of the Townsend mechanism. Current-voltage (I-V) characteristics, temporal variation of the discharge current and average ion density, and spatial distributions of ions and electrons have been calculated. I-V discharge characteristics in the absence of magnetic field have been measured. Experiments have been carried out in the discharge with a cylindrical anode with a length of 180 cm, diameter of 90 cm and a flat circular cathode with a diameter of 4cm. The dependence of discharge current on the applied magnetic field has been determined. The dependence of steady-state discharge current on the order of electric and magnetic field switching has been studied. Comparison of theory with experiment provides qualitative agreement and it confirms the adequacy of the created model.

Effects due to kind of materials used for the collector electrode of a Gerdien condenser upon the current-voltage (I-V) characteristics were investigated. Aluminum, copper and nickel electrodes were initially polished and cleaned and the I-V characteristics were recorded with 15-minutes interval under exposure to ambient air. Results showed that the zero-crossing voltage in the I-V curve shifted toward negative electrical potential, while the saturation current decreased in accordance with the passage of time. The electrode surfaces showed clear change in the color after operation. Ion mobilities and ion species concentrations were determined from the measured I-V characteristics and they indicate sizable difference by the electrode aging effect. The aluminum electrode showed stable positive and negative saturation currents within 180 minutes, while the saturation current decreased down to 50% of the initial value.

Transient coaxial helicity injection (T-CHI) was successfully demonstrated in the Helicity Injected Spherical Torus (HIST) device for non-inductive plasma current start-up. The spherical torus (ST) requires no central solenoid coil. The characteristics of the T-CHI start-up discharges on the HIST were investigated in detail. The toroidal plasma current reached 60 - 80 kA, and the current density profile was significantly modified by variations in the bias poloidal coil flux. The Doppler ion temperature and electron temperatures reached 10 - 15 eV, and the electron density of 1 × 10²⁰ m⁻³, depending on the bias flux. Internal magnetic field measurements using a two-dimensional magnetic probe array verified that closed flux surfaces formed after fast magnetic reconnection during the current rising phase. The amount of closed flux was quantitatively related to the bias flux. We also present the first experimental measurements of helicity balance during the T-CHI process. These results validate the capability of T-CHI for higher current generation in the ST.

The influence of relative displacement of Mach probe (which is placed near the top of magnetic surface) on the interference of signals is discussed. An error can arise in measured value of poloidal electric field. The Mach number perturbation at the GAM frequency has an interference from the density perturbation. The interference from the density perturbation can propagate to all of Mach number measurement. By observing the signals associated with GAM oscillations, the error in setting the probe arrays can be detected. This result can be applied to correct the positioning of probes.

A beam emission spectroscopy (BES) system for density fluctuation measurements having the sight lines passing through the plasma in the toroidal direction was developed in the Large Helical Device (LHD). The coverage of the area sampled by 15 × 100 optical fibers is nearly from the core to the edge on the horizontally elongated poloidal cross section with the spatial pitch of around 1.0 cm, and 32 channels is detected simultaneously with the 4 × 8 pixel Avalanche Photodiode Detector camera. In order to improve the directional sensitivity for the propagation of the density fluctuation in the radial direction or the poloidal direction, we propose the use of slit-shaped sight lines. An initial result of the density fluctuation measurements for MHD activity in the edge region and its cross-correlation analysis is presented.

Neutron transport analysis is used to evaluate the effects of neutron calibration source position, support structure, and water coolant on the in situ calibration of the in-vessel neutron flux monitor using the micro-fission chamber (MFC) system by applying a Monte Carlo code for neutron and photon transport (MCNP). Results indicate that changing the position of a neutron calibration source leads to a longer calibration time of the MFC detectors. When positioned below the source, the supporting rail significantly affects the detection efficiency of the lower MFC detectors. On the other hand, though it has smaller impact when positioned adjacent to the neutron source, the analyses results suggest that the position and the size of the rail need to be optimized because the detection efficiency is sensitive to scattered neutrons by in-vessel components. Furthermore, water coolant can significantly affect the detection efficiency. This result indicates that when the in situ calibration is performed,the cooling water should be filled in the blanket module in the same manner as the ITER operations.

The effect of nuclear elastic scattering (NES) on energetic alpha-particles two-dimensional (2D) velocity distribution function in a magnetic fusion device is examined by particle orbit simulation. Energetic alpha-particles exhibit an anisotropic 2D velocity distribution function, because the alpha particle loss reduces the amount of particles moving in a particular direction, and an alpha particle orbit depends on the initial direction of particle. When NES changes the direction of fast alpha particle motion, its orbit can be changed. It is shown that NES causes particle orbit transitions and changes the energetic alpha particles 2D velocity distribution function. We discuss the manner in which the distribution function is changed and how it affects the confinement of fast alpha particles.

We present in this paper the code package SOLPS-ITER, initially introduced by S. Wiesen et al. [J. Nucl. Mater. 463, 480 (2015)], dedicated to simulations of plasmas in the edge region of fusion devices. This package brings together previously existing SOLPS implementations and aims to become the new standard SOLPS version. We summarize the benchmarking work done to ensure backward compatibility with previous work, with a strong requirement on maintaining the viability and usability of the already extensive database of SOLPS runs used for the ITER divertor design and edge plasma physics studies worldwide over the years. The SOLPS-ITER package includes not only the plasma (B2.5) and neutral (Eirene) transport solvers, but also a large set of software tools for input file build-up, conversion of old runs, inline run analysis, and post-processing, all within a standardized portable run environment and version control system. Ongoing and planned upgrades to the code, such as extending the computational domain to the full vacuum vessel wall and a new graphical user interface, are also discussed.

SOLPS-ITER is a new edge code package that will be developed and maintained at the ITER Organization [X. Bonnin et al., Plasma Fusion Res. 11, 1403102 (2016)], in close collaboration with the wider SOLPS community, and will be used to support the design of the ITER divertor [A.S. Kukushkin et al., Fusion Eng. Des. 86, 2865 (2011)]. In this paper, we report on the first application of the code to the modeling of the Alcator C-Mod divertor. With its high density, high magnetic field, and strong ITER-like target shaping, C-Mod is of particular interest to ITER in terms of plasma and neutral parameters in the divertor. We show that with a fluid neutral model, we can qualitatively reproduce the observed particle fluxes to inner and outer targets under partially detached conditions. However, simulated electron temperatures in the divertor are much too low. A number of physics and numerical reasons are proposed to resolve this issue and serve as a guideline for further development of the code.

Particle and energy fluxes to the plasma facing components (PFCs) during edge localized modes (ELMs) are expected to unacceptably shorten the PFCs lifetime in ITER. In order to understand the consequences of kinetic effects of ELMs to PFCs, PARASOL simulations have been carried. Initial 1-D simulations showed that both the in/out asymmetry of divertor parameters before ELMs as well as the magnitude of the ELM energy loss itself have an influence on the in/out asymmetry of the ELM divertor fluxes with the total energy deposited at the divertor being larger at the hotter/lower recycling divertor. The role of the thermoelectric current (I_SOL) has been studied with further 1-D simulations in which decreasing I_SOL leads to an increase of the ELM power deposition at the colder/higher recycling divertor but the degree of in/out asymmetry is smaller than in the experiment. PARASOL-2D simulations have been carried out to study the effects of plasma drifts on the asymmetries of ELM energy and particle transport. It shows that for the favourable ∇B direction the ELM energy flux is predominantly deposited at the inner divertor while for the unfavourable ∇B direction it is at the outer divertor, which is in agreement with experimental findings.

A knock-on tail formation in deuteron velocity distribution function due to nuclear elastic scattering (NES) by energetic protons and its observation method using γ-ray-generating ⁶Li(d,pγ)⁷Li reaction are examined for proton-beam-injected deuterium plasmas. The proton velocity distribution function is obtained by means of the ion trajectory analysis in a Tokamak magnetic configuration. The knock-on tail in two-dimensional (2D) deuteron velocity distribution function due to NES by energetic protons is evaluated via Boltzmann collision integral and 2D Fokker-Planck simulation. From the 2D deuteron velocity distribution function obtained, enhancement of the emission rate of 0.48-MeV γ-rays by ⁶Li(d,p)⁷Li*, ⁷Li*→⁷Li+γ reaction due to NES is evaluated. It is shown that the γ-ray emission rate is significantly influenced by the magnitude of the knock-on tail, and the γ-ray-generating reaction can be a useful tool for the knock-on tail observation.

An electrostatic kicker is used for heavy ion beam injection into the KEK digital accelerator (DA) ring. A voltage of 20 kV, which must be immediately turned off after injection, is applied across the electrostatic electrodes before injection so as to deflect the injected beam into the ring orbit. An SI-Thyristor Matrix Array (SI-Thy MA) has been developed to replace the conventional thyratron switching device. Long ringing in the turn-offvoltage affects the longitudinal motion of the injected beam bunch, resulting in the formation of microstructure. The physics behind the microstructure formation is discussed in detail.

An integrated two-dimensional ablation simulation code DECORE-2D (DEsign COde for REactor) has been developed to estimate the environment of the liquid wall chamber of KOYO-fast. Density profiles of ablated lead are estimated using DECORE-2D for the case of first ignition with 200 MJ fusion power output. To discuss the stagnation of ablated materials in the chamber, the divergence of ablated material with phase change was analyzed in two-dimensional planar geometry. The spread angle of a plume obtained by this simulation is roughly 1^◦. This result indicates that the stagnation of ablated materials can be discussed with straight flows from tiles tilted by 30^◦ to the tangential direction.

In this study, the free-surface characteristics of a liquid Li wall jet for the Li target of the International Fusion Materials Irradiation Facility (IFMIF) are comprehensively reviewed. In developing the IFMIF Li target, a scientific understanding of the free-surface wave characteristics and the development of diagnostic tools to measure these characteristics were critical issues. The same issues must be faced in other liquid metal applications in fusion engineering, such as liquid first walls or liquid diverters. Thus far, diagnostic tools and methods to measure all of the characteristics of waves (i.e., wavelength, wave period, wave speed (free-surface speed), wave height (amplitude)), and average jet thickness have been developed, and the probability distributions applicable to these wave parameters, as well as their statistical characteristic values, have been determined, validating the stability of the IFMIF Li target. Our findings, both the wave characteristics and the diagnostic tools, can be applied to not only the IFMIF Li target but also innovative liquid metal diverters or first walls in fusion engineering.

It is widely known that cavitation bubbles are not static bubbles but have the dynamics of the expansion, the shrinkage, and the collapse. In this work, we produced electrical discharges in acoustic cavitation bubbles for the first time with the intension of enhancing the reactivity of sonochemical processes. Glow-like discharges were observed in cavitation bubbles on the bottom surface of the cylindrical electrode which was connected to a pulsed high-voltage power supply. Bright optical emission was observed from the region corresponding to the cloud of expanded cavitation bubbles, while we also observed electrical discharges even in the shrinking phase of cavitation bubbles. If discharge-produced reactive species have lifetimes that are longer than the interval between the discharge and the collapse of the cavitation bubble, the species composition in the collapsed cavitation bubble becomes different from that in conventional cavitation bubble, which may result in the enhancement of reactivity in sonochemical processes.

The basis of the operating principle of Cockcroft-Walton (CW) circuit, which produces a high-voltage, is studied using a simulator of an electrical circuit. The CW circuit experimentally contributed to the breakthrough of nuclear physics for the development of a particle accelerator. High-voltage discharges play an important role not only in producing plasmas but are also applicable in a wide variety of fields. However, studies on the operating principle of CW circuits are not sufficient, though improvement and application of it have been studied. Through our studies, we have found that a part of electrical charge stored in the capacitors remains, the output value of the voltage can be expressed in recurrence formula, and more time is needed for boosting circuits that have higher number of steps. Furthermore, we studied combinations between frequencies of alternating current and capacitance in a capacitor in order to boost effectively.

Through the process of inward diffusion, a strongly localized clump of plasma is created in a magnetosphere. The creation of the density gradient, instead of the usual flattening by a diffusion process, can be explained by the topological constraints given by the adiabatic invariants of magnetized particles [ Z. Yoshida and S.M. Mahajan, Prog. Theor. Exp. Phys. 2014, 073J01 (2014). N. Sato and Z. Yoshida, J. Phys. A: Math. Theor. 48, 205501 (2015).]. After developing a canonical formalism for the standard guiding center dynamics in a dipole magnetic field, we complete our attempt to build a statistical mechanics on a constrained phase space by discussing the construction principles of the associated diffusion operator. We then investigate the heating mechanism associated with inward diffusion: as particles move toward regions of higher magnetic field, they experience preferential heating of the perpendicular (with respect to the magnetic field) temperature in order to preserve the magnetic moment. A relationship between conservation of bounce action and temperature isotropy emerged. We further show that this behavior is scaled by the diffusion parameter of the Fokker-Planck equation. These results are confirmed by numerical simulations.

Dark current analysis to estimate the current density of the field-emitted electrons have been developed for the prediction of the yoltage holding capability of multi-aperture multi-grid accelerators used in JT-60SA and ITER neutral beam injectors. From the experimental measurement of the field-emitted electrons in multi-aperture multi-grid accelerators, βE_BD, which represent the critical current density to trigger the breakdown, were found to be constant with around 6 × 10³ kV/mm regardless of the electric field profiles and the surface conditions in an accelerator with a surface area of 0.1 m². In addition, the dark current was measured in the region with the electric field above ∼60% of the electric field at the breakdown, which leads to the determination of the emission region in the analytical estimation of the dark current. Furthermore, from the measurement of βE_BD on electrodes with various surface area, βE_BD was found to decreases with an increase of the surface area S (βE_BD = 4 × 10³S^−0.3). From these results, the dark current profile at the critical current density can be estimated from the electric field analysis, which leads to the development of the breakdown model and the prediction of the voltage holding capability on the multi-aperture multi-grid accelerators.

A compact Gerdien condenser is being developed as a new characterization tool for laboratory-produced atmospheric pressure plasma. The Gerdien condenser is capable of determining the ion density and mobility, thus ion species composition of the plasma. The device has a 1.0 cm diameter, 6.0 cm long cylindrical current collector electrode mounted on a bias electrode of 1.7 cm inner diameter. The flow rate of the fan attached at the end of the condenser is set from 1.9 to 7.5 × 10² cm³ s⁻¹. Some of the values of ion mobility calculated from the I − V characteristics obtained for atmospheric plasmas produced by RF excited atmospheric pressure plasma pen are in the range of O⁺, O⁻, O₂⁻, O₂⁺, Ar⁺, N⁺ and N₂⁺. The total positive ion densities at 2.0 cm away from the plasma pen are determined to be in the range of 10⁶ cm⁻³.

Detectability of the multiple cracks in a high-temperature superconducting (HTS) film by a scanning permanent magnet method has been investigated numerically. To this end, a numerical code has been developed for performing a shielding current analysis in an HTS film with cracks. Furthermore, the GMRES(k) method and the H-matrix method are adopted in the code. By using the code, the scanning permanent magnet method can be simulated successfully. The results of the computations show that the x-coordinate of the crack endpoint cannot be estimated with increasing the magnet radius. On the other hand, the y-coordinate of the crack endpoint can be detected accurately.

The reduction rate of the magnetohydrodynamic (MHD) potential energy δW_M is defined as the energy difference between the vacuum magnetic field energy before a tokamak plasma formation and the MHD potential energy of an equilibrium tokamak plasma. Stationary direct current in a central solenoidal coil (DC_CS) can reduce δW_M, which means a reduction of the required heat energy for the tokamak plasma formation. The increase of the hole component ration in a toroidal plasma current distribution increases the toroidal plasma current and the stored thermal energy without increase of the plasma size. For tokamak plasmas with DC_CS = 100 A, δW_M has a local minimum with respect to the toroidal plasma current distribution. In this state, a perturbation of the toroidal plasma current is energetically inhibited.

Hall effect on counter-helicity spheromak merging is experimentally investigated in a low-S^∗ region, where S^∗ is the ratio of plasma minor radius to ion skin depth. Direct measurement of magnetic field, ion flow, electron density revealed that Hall effect affect not only merging reconnection phase but also relaxation phase through formation of ion flow and current density profiles. Hall effect breaks the symmetry of the “case-O” and “case-I” counter-helicity spheromak merging, which are defined by combinations of poloidal flux and toroidal flux of initial spheromaks. Formation of toroidal current profile and radial outflow pattern are different in case-O and case-I during merging process, and that results in great difference of profiles of electron density after merging.

The extended boundary-node method (X-BNM) with the hierarchical-matrix (H-matrix) method has been developed and its performance has been investigated numerically. The results of computations show that the solver speed of the X-BNM with the H-matrix method is much faster than that of the standard X-BNM for the case where the number of boundary nodes exceeds a certain limit. Furthermore, the accuracy of the X-BNM with the H-matrix method is almost equal to that of the standard X-BNM. From these results, it is found that the H-matrix method is useful as the acceleration technique of the X-BNM.

We performed detailed magnetic probe measurement to detect magnetic fluctuations during magnetic reconnection in counter-helicity spheromak merging experiment in TS-4. The fluctuation power in the frequency range of 0.5 - 5 MHz was concentrated in the outboard-side downstream region of the reconnection point while the fluctuation power in the frequency range of 7 - 20 MHz was localized inside the current sheet. It has been found that the detected fluctuations inside the current sheet had similar characteristics to obliquely propagating Whistler waves, but showed different power spectra inside the current sheet and in the downstream region.

A two-dimensional particle in cell code has been used to demonstrate the formation mechanism of a periodic nanograting structure in the hydrogen plasma. By using a linearly polarized, ultrafast laser beam with a wavelength of 800 nm, an incidence angle of 0^◦, and an intensity of 10¹⁶ W/cm²-µm², the periodic nanograting structure was clearly self-organized at the boundary between a preformed and dense plasma at t = 600 fs. The bidirectional surface plasma wave plays a significant role together with the oscillating two-stream instability in producing the periodic nanograting structure.

The self-assembly of flexible bolaamphiphilic molecules in a solution is studied by dissipative particle dynamics simulations. In particular, we investigate the effect of the interaction difference, Δa, between the two different hydrophilic end groups on the self-assembly in a bolaamphiphilic solution. Our simulations show that two types of self-assembled structures, spherical vesicles and worm-like micelles, are obtained from a random configuration of bolaamphiphilic molecules in a solution. We find that the worm-like micelles are formed when Δa > 0, whereas spherical vesicles are obtained when Δa ≤ 0. It is also ascertained that the size of the spherical vesicles decreases as Δa decreases.

Using the binary-collision approximation simulation with atomic collision in any structured target code AC∀T, we calculated sputtering yield, range, and retention rate for tungsten with a rough surface under argon atom irradiation. The simulation revealed the sputtering yield decreases and the retention rate increases as the surface becomes rougher. Because these quantities strongly depend on the surface, we suggest that it is necessary to consider the surface structure of the tungsten target when estimating the effects of walls.

We performed magnetic reconnection simulations with our simulation code. Our code is a multi-scale three-dimensional plasma simulation code in which Particle-in-Cell (PIC) method is used in a restricted region and magnetohydrodynamics (MHD) simulation is performed in the rest of a simulation box. A hierarchical mesh is used in the MHD region, which is controlled by an adaptive mesh refinement (AMR) technique. Restriction of the PIC region and the adaptively controlled mesh realize treatment both of micro-scale particle dynamics and macro-scale structure with low computational costs. In this paper, we describe methods of our simulation and results of calculations.

In order to determine the densities and temperatures in high-density helium plasmas by emission spectroscopy, a numerical study of He I spectral line shapes have been performed taking into account radiation trapping. A computational simulation code has been developed, consisting of two parts. The first part solves coupled rate equations to obtain population densities in each energy level of the He atom. The other describes the absorption rate, emission rate, and spectral line shapes by solving the equation of radiation transfer. In a homogeneous plasma, the calculated line profile has a central dip caused by photoabsorption by residual cold atoms. In addition, the results show that radiation trapping significantly alters the population dynamics. The population densities estimated from the intensity of the line profile considering the photoabsorption are lower by one order of magnitude than those without the photoabsorption. For application to a He arc plasma, the temporal evolution of this central dip is also examined as a preliminary calculation.

Real-time polarization scan experiments were performed on the Large Helical Device (LHD) to search an optimal incident wave polarization for electron cyclotron resonance heating. The obtained optimal polarization state to maximize the power absorption to the LHD plasma is compared with the ray-tracing code that includes mode content analyses, which indicates that the calculated results are generally in good agreement with the experimental results. The analyses show that optimal coupling to plasma waves requires a fine adjustment for an incident wave polarization even for perpendicular injection due to the finite density profile and the magnetic shear at the peripheral region.

Detached plasma has been successfully achieved with use of m/n = 1/1 resonant magnetic perturbation (RMP) coils in a large helical device (LHD) without any additional impurity gas puffing. Study of the impurity radiation is then important to clarify the physical mechanism triggering the RMP-assisted detachment. Resonance lines of CIII (977.02 Å, 2s2p-2s²), CIV (1548.2 Å, 2p-2s), CV (40.27 Å, 1s2p-1s²), and CVI (33.73 Å, 2p-1s) measured by vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) spectrometers are used to estimate the radiation power from C²⁺-C⁵⁺ ions because carbon is the most abundant impurity element in LHD. For the purpose the spectral intensity from the VUV spectrometer is absolutely calibrated using the EUV spectrometer,with which the spectral intensity has already been calibrated. The partial carbon radiation at each ionization stage of C²⁺ to C⁵⁺ ions, P_rad(C^q+), can be estimated for attached and detached plasmas by calculating the ratio of the partial carbon radiation to the resonance line based on ADAS atomic code. It is found that the radiation from C³⁺ ions existing near a radial location of ι/2π = 1 in the ergodic layer increases up to 40% of the total radiation loss and becomes a dominant origin to trigger the detached plasma, whereas carbon radiation is negligible in the attached plasma.

Doppler reflectometry is currently used as a powerful technique to measure the perpendicular (to the magnetic field) velocity of density fluctuation, the radial electric field, and the perpendicular wave number spectrum in many fusion plasma devices. In GAMMA 10 a Doppler reflectometer has recently been installed to measure perpendicular rotation velocity of density fluctuation in the cylindrical plasma. The Doppler reflectometer has an antenna system with a launching/receiving scalar feed antenna and focusing mirrors. By rotating one of the mirrors the tilt angle of the incident microwave can be controlled against the normal of cutoff layers. A frequency synthesizer is used in the range 11.5 - 18 GHz as the stable microwave source of X-mode probing beam for GAMMA 10 plasma having typical peak density ∼2 × 10¹⁸ m⁻³. The first preliminary results of Doppler shifted spectra and radial profiles of the perpendicular velocity of density fluctuations are presented for ICRF start-up plasma with additional ECH. The rotation of fluctuations during additional ECH period is found to become opposite direction comparing to the case of ICRF heating alone.

Temperature anisotropy in broad component of Hα line was found in the ring trap 1 (RT-1) device by Doppler spectroscopy. Since hot hydrogen neutrals emitting a broad component are mainly produced by charge exchange between neutrals and protons, the anisotropy in the broad component is the evidence of proton temperature anisotropy generated by betatron acceleration.

The power injection system for electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD) was modified and upgraded. An outside horizontal port 2-O on the Large Helical Device (LHD) was furnished with two antenna systems for the EC-waves of the frequencies of 77 and 154 GHz, respectively. In addition to them, two new antenna systems for 77 and 154 GHz waves were installed in the 2-O port. Each antenna in the 2-O port has wide range of EC-wave beam direction control so that these are suitable for ECCD which requires toroidally oblique EC-wave beam injection. In the LHD 18th experimental campaign in 2014-2015, an ECCD experiment with second harmonic resonance condition, on-axis magnetic field of 1.375 T for 77 GHz waves, was performed in which some combination patterns of two 77 GHz ECCDs were applied. The discharges of dual co- and dual counter-ECCDs showed remarkable plasma currents of ∼±26 kA in both of the co- and counter-directions, by 6 s pulse duration and injection powers of 366 and 365 kW. The new antenna has nearly the same capability for ECCD with that of the existing antenna. The improvement in the flexibility of the ways of applying plural ECCDs will offer a highly useful tool for investigations on the phenomena concerning with the plasma current such as magnetohydro-dynamics.

This research investigated the radiation cooling mechanism and formation of detached plasma in the case of gas injection in the D-module of GAMMA 10/PDX. In GAMMA 10/PDX, divertor simulation experiments have been started by using a divertor simulation experimental module (D-module). A V-shaped target made of tungsten has been installed in this module. In order to understand the effect of impurity injection into divertor simulation experimental module, we injected H₂ and Ar gases to the D-module and measured the heat flux and ion flux. According to the increase of gas injection, reduction of ion and heat fluxes have been observed. In the Ar injection experiments, H₂ gas has been injected simultaneously to examine the effect of molecular process on detached plasma formation. In this case, both the heat flux and ion flux are drastically reduced. These results indicate radiation cooling and formation of detached plasma due to gas injection. Simultaneous injection of noble gas and hydrogen gas showed the most effective results on detached plasma generation.

Amplitude modulated helium atomic magnetometers (AM He AOM) are proposed as a new method, which allows spatially resolved magnetic field measurement in the inner regions of burning plasmas without heating beams nor impurity atoms. In order to understand the fundamental properties of AM He AOM, we have developed a prototype, and Helium discharge lamp as well as its power supply has been also elaborated as a light source. 1083 nm AM emission was successfully obtained in the frequency range between 40 kHz and 500 kHz. We have performed an exploratory experiment of test magnetic field measurement with the developed AM He AOM. For the test magnetic fields: 2.6 µT and 5.0 µT, possible signs of magnetic field have been observed.