In this study we developed an extrapolation code based on the MHD relaxation method to reconstruct a three-dimensional (3D) coronal magnetic field. A 3D magnetic field based on a nonlinear force-free field (NLFFF) was extrapolated from a photospheric vector-field map. A benchmark test using the semi-analytical solution introduced by Low & Lou (1990) found that the extrapolated solution were able to well reproduce an original Low and Lou solution. We then applied the NLFFF extrapolation to solar active region (AR) NOAA 10930. The energy accumulation region formed by the strong magnetic shear above the polarity inversion line was resultantly reproduced and this structure is greatly different from the potential field characterized by the minimum energy state. In this paper we discuss the reliability of our extrapolated field and a relationship between quasi-separatrix layers (QSLs) and a flare ribbon from the CaII image observed by Hinode/SOT.

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The variable preconditioned (VP) Krylov subspace method on multi Graphics Processing Unit (GPU) is numerically investigated. Besides, the linear system obtained by finite element method with an edge element is adopted for the problem. The results of computations show that VP conjugate gradient method on multi GPU demonstrated significant achievement than that of CPU. Especially, VP conjugate gradient method on multi GPU is 4.35 times faster than that of CPU. However, transmission rate between the PC using Gigabit Ethernet is the bottleneck of the performance.

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By directly discretizing the weak form used in the finite element method, meshless methods have been derived. Neither the Lagrange multiplier method nor the penalty method is employed in the derivation of the methods. The resulting methods are divided into two groups, depending on whether the discretization is based on the Galerkin or the Petrov-Galerkin approach. Each group is further subdivided into two groups, according to the method for imposing the essential boundary condition. Hence, four types of the meshless methods have been formulated. The accuracy of these methods is illustrated for two-dimensional Poisson problems.

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Particle-in-cell (PIC) is a simulation technique for plasma physics. The large number of particles in high-resolution plasma simulation increases the volume computation required, making it vital to increase computation speed. In this study, we attempt to accelerate computation speed on graphics processing units (GPUs) using KEMPO, a PIC simulation code package [H. Matsumoto and Y. Omura, Computer Space Plasma Physics, pp.21-65 (1985)]. We perform two tests for benchmarking, with small and large grid sizes. In these tests, we run KEMPO1 code using a CPU only, both a CPU and a GPU, and a GPU only. The results showed that performance using only a GPU was twice that of using a CPU alone. While, execution time for using both a CPU and GPU is comparable to the tests with a CPU alone, because of the significant bottleneck in communication between the CPU and GPU.

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A solitary wave like fluctuating structure is observed in the Large Mirror Device-Upgrade (LMD-U) under relatively high neutral pressure conditions. Ion saturation current fluctuations are measured and Fourier and wavelet power spectra are calculated. Temporal behavior of fluctuation power and strength of nonlinear coupling of low frequency coherent mode and background turbulence in a specific frequency region is analyzed. This is achieved using wavelet spectral and bispectral analyses with conditional averaging.

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The 3-dimensional gyrokinetic PIC (particle-in-cell) code for MHD simulation, Gpic-MHD, was installed on SR16000 (“Plasma Simulator”), which is a scalar cluster system consisting of 8,192 logical cores. The Gpic-MHD code advances particle and field quantities in time. In order to distribute calculations over large number of logical cores, the total simulation domain in cylindrical geometry was broken up into N_DD-r × N_DD-z (number of radial decomposition times number of axial decomposition) small domains including approximately the same number of particles. The axial direction was uniformly decomposed, while the radial direction was non-uniformly decomposed. N_RP replicas (copies) of each decomposed domain were used (“particle decomposition”). The hybrid parallelization model of multi-threads and multi-processes was employed: threads were parallelized by the auto-parallelization and N_DD-r ×N_DD-z ×N_RP processes were parallelized by MPI (message-passing interface). The parallelization performance of Gpic-MHD was investigated for the medium size system of N_r × N_θ × N_z = 1025 × 128 × 128 mesh with 4.196 or 8.192 billion particles. The highest speed for the fixed number of logical cores was obtained for two threads, the maximum number of N_DD-z, and optimum combination of N_DD-r and N_RP. The observed optimum speeds demonstrated good scaling up to 8,192 logical cores.

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A new method has been proposed for implementing essential boundary conditions to the Element-Free Galerkin Method (EFGM) without using the Lagrange multiplier. Furthermore, the performance of the proposed method has been investigated for a nonlinear Poisson problem. The results of computations show that, as interpolation functions become closer to delta functions, the accuracy of the solution is improved on the boundary. In addition, the accuracy of the proposed method is higher than that of the conventional EFGM. Therefore, it might be concluded that the proposed method is useful for solving the nonlinear Poisson problem.

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A simple parallelization approach using General Purpose computation on Graphics Processing Unit was applied for solving the MHD equations using the CIP-MOCCT method. We investigated the efficiency of this parallelization approach and found that the computational speed of the modified code is significantly improved despite the simple modification.

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A new scheme referred to as the multi-moment (MM) scheme is explored to develop a more reliable Vlasov code from the viewpoint of numerical properties. The MM scheme is based on the Eulerian approach, where spatial derivatives are evaluated by interpolation functions locally constructed by not only grid values but also 0th-, 1st-, and 2nd-order moment values between grids, which largely increases numerical accuracy and resolution. Through the Fourier analyses and benchmark tests of one-dimensional (1D) and 2D transport simulations, it is found that the MM scheme exhibits significantly smaller numerical dissipation and dispersion even near the Nyquist wave-number, and as a result, the MM scheme decreases the numerical cost. The MM scheme is also applied to a 1D Vlasov-Poisson simulation and we find that the scheme captures finer scale structure in velocity space compared to the conservative form of interpolated differential operator (IDO-CF) scheme, while also maintaining good energy conservation.

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Solar wind interaction with an artificial magnetosphere is investigated by means of a full particle-in-cell simulation. The resultant momentum transfer of solar wind plasmas may provide the propulsive force for a magnetic sail, which is a potential next-generation interplanetary flight system. These simulations are performed using two different simulation codes. One is a traditional code employing a uniform grid system, and the other is a newly developed code with an adaptive mesh refinement (AMR) technique. Even in a small magnetosphere having a scale smaller than the ion inertia length, ions are scattered at the front of the magnetosphere. In this region, an electron-scale current structure is observed, and the electromagnetic interaction with the coil current density, which creates the magnetosphere, causes a propulsive force. The current density structure observed in the AMR simulation is in good agreement with that resulting from the traditional code. The AMR code is expected to be a powerful tool to demonstrate this solar wind interaction under realistic conditions at a reasonable numerical cost.

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Charge-transfer cross sections in collisions of H⁺ ions with the ground state He, Ar, Kr, and Xe atoms have been measured in the energy range below 4.0 keV with the initial growth rate method. These observed cross sections are also compared with previously published experimental data and theoretical predictions. In the He and Ar targets, it is found that some previous experimental data deviate significantly from the present observed cross sections as the collision energy decreases. It has been found that in the Kr and Xe targets, the energy dependence of the present observed cross sections behaves as “near-resonant” charge transfer.

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A method proposed most recently for heat flux reduction in a divertor is to use a so-called V-shaped target plate. A numerical study of neutral behavior around the V-shaped target in the divertor simulator, Test Plasma produced by Directed current device for sheet plasma (TPD-SheetIV), using DEGAS 2 is reported. The neutral distribution and H-alpha emission profile are modeled and compared for three types of targets, mainly for the low-density attached condition. Under this condition, excited atoms are produced by electron impact with recycled atoms near the target, and hence, H-alpha emission has a peak there.

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For the purpose of speed-up of the three-dimensional eXtended Boundary-Node Method (X-BNM), an efficient algorithm for evaluating influence coefficients has been developed. The algorithm can be easily implemented into the X-BNM without using any integration cells. By applying the resulting X-BNM to the Laplace problem, the performance of the algorithm is numerically investigated. The numerical experiments show that, by using the algorithm, computational costs for evaluating influence coefficients in the X-BNM are reduced considerably. Especially for a large-sized problem, the algorithm is efficiently performed, and the computational costs of the X-BNM are close to those of the Boundary-Element Method (BEM). In addition, for the problem, the X-BNM shows almost the same accuracy as that of the BEM.

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We study the phase behavior of bolaamphiphilic solution performing the dissipative particle dynamics simulations of coarse-grained bolaamphiphilic molecules with explicit solvent molecules. Our simulations show that there are six kinds of phases: isotropic micellar, micellar, rod-shaped micellar, hexagonal, network-structure and lamellar. The network-structure and the lamellar phases disappear when the restoring potential against the bending of bolaamphiphilic molecules in our simulation model is excluded; and the isotropic micellar and the hexagonal phases disappear when the restoring potential is included. This suggests that the bending potential is important in the formation of the higher-ordered structures by the bolaamphiphilic molecules.

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In the internal coil device Mini-RT, overdense plasmas have been observed, and heating by electron Bernstein waves (EBWs) is expected. EBWs, which have no cutoff density, are electrostatic-mode waves converted from the electromagnetic mode. To examine mode conversion in Mini-RT experiments, we are attempting to investigate the propagation of waves in the electron cyclotron range of frequencies (ECRF) in overdense plasmas. We injected diagnostic microwaves (typically 1 GHz) into a plasma produced by 2.45 GHz microwaves and directly measured their characteristics by inserting various probing antennas into the plasma with an interference system. Electromagnetic field measurements revealed a long-wavelength mode in the area with lower density than the upper hybrid resonance (UHR) region. In the area with higher density than that of the UHR region, waves having the characteristics of EBWs were observed; namely, short wavelength (∼20 mm), electrostatic mode, longitudinal polarization, and backward wave mode. These results suggest that EBWs were mode converted from electromagnetic waves at the UHR.

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Several divergence-cleaning techniques for multi-dimensional Godunov-type magnetohydrodynamic schemes are comparatively investigated. We also propose a new divergence-cleaning technique that is improved from an earlier projection method to improve the robustness.

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We develop a collisional-radiative model for calculating the velocity distributions of excited hydrogen atoms. In the model, the velocity distributions are approximated using linear combinations of several basis functions which are treated as discrete velocity states, and only charge exchange collisions with proton are considered as velocity changing collisions. The populations in the velocity and excited states are calculated under an assumption of a quasi-stationary state. With the model we derive the velocity distribution of the ground state atoms from that of excited atoms in the n = 3 level observed for an LHD plasma, where n is the principal quantum number. The mean kinetic energy of the ground state atoms is estimated to be 5.6 eV, which is 0.05 eV less than that of the n = 3 atoms.

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A process of quiet auroral arc formation, in particular, the response of the magnetosphere to the effect of auroral energetic electrons, is studied using a holistic auroral simulation code. It consists of a three-dimensional magnetohydrodynamic code for a dipole magnetosphere-ionosphere coupling system and a one-dimensional electrostatic plasma particle code for auroral energetic electron production. Results of the holistic auroral simulation indicate that drastic variation in the ionospheric electric field, which is induced by auroral energetic electrons and propagates into the magnetosphere, influences the magnetospheric field-aligned current distribution. Furthermore, the characteristic time scale of this process is investigated.

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A detailed analysis of forward and inverse energy transfer processes due to the Hall term effect in freely decaying, homogeneous, isotropic Hall magnetohydrodynamics (HMHD) turbulence is performed through Fourier and wavelet analyses. We analyzed three snapshot datasets that were taken from such a period to allow the turbulence to develop sufficiently with a nearly constant magnetic Reynolds number. Because the Fourier energy spectra in these snapshots show remarkable agreement after the normalization in terms of the dissipation rates and the diffusion coefficients, they are considered as a universal equilibrium state. By analyzing the numerical solutions that are generated without any external forcing, it is confirmed that the inverse energy transfer due to the Hall term effect is intrinsic to HMHD dynamics. Orthonormal divergence-free wavelet analysis reveals that nonlinear mode interactions contributing to the inverse energy transfer exhibit a nonlocal feature, while those for the forward transfer are dominated by a local feature.

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Atomic processes of multiple-charged ions of high-Z elements from tin to dysprosium are investigated for their application to light sources for extreme-ultra-violet (EUV) lithography. Modeling of these ions in plasmas, including tungsten, which is being considered for use as a divertor and wall material in the fusion devices, is discussed. Atomic spectra become very complex in the case of low-charged ions below Pd-like ions, which calls for a new atomic code for calculating the energy levels and rate coefficients of collisional and radiative processes. The collisional radiative model is validated through code comparison workshop activities. An alternative method for investigating the formation of the non-uniform structure of the plasmas is also presented.

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At present, two stellarator type fusion devices, Uragan-2 M and Uragan-3 M, are in operation in Kharkov. At the Uragan-3 M machine, investigations of the low-density low-collisional plasma are continued. L-H-mode-like transition is studied with particular attention to the mechanisms of its formation and impact on divertor flow behaviour. For a higher density plasma, a scenario of the RF Alfvén resonance heating in high k_|| regime is realized both with continuous and pulsed gas injection. On Uragan-2 M the studies of RF wall conditioning are in progress. A new wall conditioning scheme which uses the high frequency slow wave is implemented.

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A new type of instability mode, related to suprathermal electrons, was found in extremely low-density plasmas, (n_e = (0.2-0.5) × 10¹⁹ m⁻³) with electron cyclotron resonance heating and current drive (ECRH/ECCD) in the TJ-II heliac. The quasi-monochromatic density and plasma potential oscillations in the frequency range 20-120 kHz tend to have several “branches” with constant frequency shift between them. The typical amplitude of the mode induced potential oscillations was estimated to be Δφ^EM ∼ 20 V. The mode branches have an individual and finite radial extent, odd low poloidal structure (m ≤ 5) and angular phase velocity of poloidal rotation of about 8 × 10⁴ rad/s in the ion diamagnetic drift direction. The contribution of the mode to the turbulent particle flux Γ_E×B for the observed wave vectors k_θ < 3 cm⁻¹ was found to be small in comparison with the contribution from broadband turbulence.

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This paper describes results from the first integrated analysis of the feasibility of a retro-reflector (RR) installed in the ITER vacuum vessel. We have carried out thermal and structural analyses of a passively-cooled RR and a terrace retro-reflector array (TERRA) using heat load conditions expected in ITER. In the case of the RR, the maximum temperature is approximately 620°C, and flatness of the deformed mirror surface of the RR is approximately 80 µm. Although maximal displacement of the mirror surface is large, 69% of the incident-laser power returns to the diagnostic room (89.9% returns when the RR is not deformed), phase disturbance is π/4 and the rotation angle of the polarization plane is less than 0.06°. On the other hand, the maximum temperature of TERRA is approximately 350°C, and flatness of the deformed mirror surface is less than 10 µm. The flatness of the mirror surface of a deformed TERRA is much smaller compared to the laser wavelength (118 µm), but the global shape curves in a manner which disrupts the coherent phase of laser light. The curvature radius of TERRA is 21 m, and 49% of the incident-laser power returns to the diagnostic room (88.6% returns when TERRA is flat), phase disturbance is π/5 and the rotation angle of the polarization plane is less than 0.06°. Since the optimized incident angles of a flat and curved TERRA are different, real-time feedback control of the incident angle is necessary for applying TERRA to the ITER poloidal polarimeter.

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The temporal evolution of ion temperature and flow was observed to investigate the heating effect of plasma merging in the UTST device by using a multipoint Doppler spectroscopy measurement system. The bulk plasma ion temperature measured by a carbon impurity line was about 15-20 eV after plasma merging. In addition, anomalous broadening of the carbon line width was observed only in the early phase of plasma merging. Since carbon impurity emission comes mainly from the X-point and the vicinity of the center stack, the broad spectrum is considered to consist of a radially bi-directional outflow component from the X-point and a stationary component near the center stack. The outflow velocity estimated from the spectroscopy was about 24 km/s, which agreed well with both an Alfvén velocity of 22 km/s and a velocity of the reconnected field line motion of 35 km/s.

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Edge electron temperature fluctuation is evaluated by the use of fast voltage sweeping technique on TST-2. The validity of obtained current-voltage characteristic curve was checked by comparing the time evolutions of floating potential between that obtained from the fast voltage sweeping technique and that measured with floating probe method. Good agreement between them was confirmed. We also found that fitting errors in the evaluation of the electron temperature itself are less than 10% of fluctuation levels of the electron temperature. Therefore the accuracy of the technique is applicable to study of plasma fluctuations.

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A new microwave imaging reflectometry (MIR) system for KSTAR is being developed based on the experience gained via the TEXTOR proof-of-principle system [H. Park et al., Rev. Sci. Instrum. 74, 4239 (2003)] which aimed to measure the poloidal image of the electron density fluctuations essential for transport studies. The KSTAR system will adopt a multi-frequency probe beam source in the range of 90 ∼ 100 GHz (X-mode case), which will enable the measurement of 2-D (radial and poloidal) fluctuations of the multiple cut-off layers, simultaneously. The optical system of the MIR system will be combined with the 2nd ECEI system (identical to the first ECEI system [G.S. Yun et al., Rev. Sci. Instrum. 81, 10D930 (2010)]) on KSTAR. The design of the launching and receiving optics of the MIR system will be constrained in order to maintain the performance of the ECEI system and thus it is necessary to consider sharing the zoom lens of the ECEI system. This stringent constraint is a challenge considering the tight wavefront matching requirement to obtain proper images for a wide range of cut-off layers within the focal depth. This paper discusses the details of the MIR system design that is compatible with the 2nd ECEI system on KSTAR.

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A new tangential 2D Soft X-Ray Imaging System (SXRIS) is being designed to examine the edge magnetic island structure in the lower X-point region of DIII-D. A synthetic diagnostic calculation coupled to 3D emissivity estimates is used to generate phantom images. Phillips-Tikhonov regularization is used to invert the phantom images for comparison to the original emissivity model. Noise level, island size, and equilibrium accuracy are scanned to assess the feasibility of detecting edge island structures. Models of typical DIII-D discharges indicate integration times > 1 ms with accurate equilibrium reconstruction are needed for small island (< 3 cm) detection.

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Spatiotemporal behavior of Alfvén-ion-cyclotron (AIC) waves excited in GAMMA10 is investigated using a microwave reflectometer and magnetic probes located at the edge. The frequency spectrum of the AIC waves has several discrete peaks. Simultaneous measurement of the internal density fluctuation and edge magnetic fluctuation of AIC waves shows that AIC waves have different radial structures in the initial excitation phase, but in the later steady state, each of them has the same structure in the radial direction. The results indicate that several AIC waves are excited as the same eigenmode in the radial direction.

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We have observed Fe¹⁴⁺ (3s^{2 1}S₀ - 3s3p ¹P₁) and Fe¹⁵⁺ (3s ²S_1/2 - 3p ²P_3/2) emissions from a LHD plasma with a space-resolved extreme-ultraviolet spectrometer. The observed intensity distributions against the viewing chord for the respective emissions are reconstructed to the emission flux distributions in the plasma against the normalized radius of the poloidal cross section with a maximum entropy method. Both of the emissions localize in the periphery region, and the Fe¹⁴⁺ emission is located outer side than that of Fe¹⁵⁺. We calculate the charge state distribution of Fe ions against the normalized radius assuming the ionization equilibrium at the electron temperature and density, which are measured by a Thomson scattering method. The calculated result is consistent with the experimental one.

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The effect of the magnetic configuration on fast-ion confinement is one of the most important topics for helical devices. Fast-ion velocity distributions have been investigated using ion cyclotron range of frequencies (ICRF) minority heating in Heliotron J with special emphasis on the effect of the toroidal ripple (bumpiness) of the magnetic field strength. In measurements of the fast-ion tail generated by ICRF minority heating, a high bumpiness configuration is found to be preferable for tail formation. However, the measurement area based on the line of sight of the fast-ion detector was restricted in this experiment. Due to the complexity of the magnetic field in Heliotron J, three-dimensional analysis is required to interpret the experimental results. Monte-Carlo simulations were performed. The calculation results agree well with the experimental results for high-energy tail formation. The effective temperature of minority protons was estimated.

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The collective Thomson scattering (CTS) technique has been utilized with the backscattering configuration to diagnose the bulk and fast ions in the Large Helical Device (LHD). The spectrogram of the scattered radiation presents the transient phenomena related to fast ion slowing down in auxiliary neutral beam heated plasmas. This is obtained by the broad band receiver resolving with 32 channels, while the probing beam from 77 GHz gyrotron is modulated with 50 Hz and is injected into plasmas. In the same shot, especially in low temperature plasma (T_e, T_i < 1 keV), a wave excitation is also observed in a narrow frequency band. The peaks in the CTS spectrum exist between 0.5 and 1.0 GHz and are observed in both frequency sides asymmetry. For the forthcoming experiments, the feasibility of CTS diagnostics at a new port location is assessed to extend the diagnostic capability for measuring the ion velocity components. Expected spectra for scattered radiation are the same signal level as the present location from a numerical calculation.

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Electron cyclotron current drive (ECCD) experiments were conducted in the Large Helical Device to investigate the characteristics of EC-driven current and its profile and the possibility of controlling current and rotational transform profiles by ECCD. Successful ECCD helps prevent magnetohydrodynamic instabilities in plasmas. Scanning the EC-wave beam direction with a long pulse width of 8 s revealed a systematic change in the plasma current. The current's direction was reversed by a reversal of the beam direction. The direction agrees with the prediction of Fisch-Boozer theory regarding EC-wave beam injection from low-field side. The maximum driven current is 9 kA with an EC-wave power of 100 kW. The optimum beam direction that maximizes the driven current is investigated with the help of ray-tracing code. This direction depends on the magnetic field, efficiency of power absorption, and fraction of the power absorbed by trapped electrons.

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Radial profiles of impurity spectral line emissions in extreme ultraviolet (EUV) range (60-400 Å) are surveyed for impurity seeded discharges in Large Helical Device (LHD). The line emissions from light impurities such as helium, carbon and neon are located in the edge ergodic layer (1.0 ≤ ρ ≤ 1.2) except for CV and CVI, which are located in region of 0.8 ≤ ρ ≤ 1. In contract, the line emissions from heavy impurities such as iron are widely located over the whole plasma region as a function of ionization energy. On the other hand, the radial profile of bremsstrahlung continuum radiation is also clearly recorded in the EUV spectrum observed from high-density discharges with hydrogen pellet injection. Wavelength intervals available for the absolute sensitivity calibration of the present space-resolved EUV spectrometer system are listed with dominant line emissions which are useful as a wavelength marker.

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We customized TASK3D in order to add a new function, TR-snap, for evaluating transport coefficients on the basis of a power balance analysis. To check the validity of the TR-snap's results, the thermal transport coefficient evaluated by TR-snap was compared with results of PROCTR, which is used as the transport analysis code in the Large Helical Device (LHD). The thermal transport coefficient evaluated by TR-snap was almost the same as that of PROCTR. The effect of re-entering fast ions on the transport coefficients in a typical LHD high-beta plasma was investigated using customized TASK3D. When the effect of the re-entering fast ions on the neutral-beam injection heating efficiency is considered, the thermal transport coefficient is slightly larger than that without considering the re-entering fast ions. However, the tendency of the thermal transport coefficient with considering the re-entering fast ions is rarely different from the previously identified tendency.

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TOKASTAR-2 device is characterized by a hybrid configuration between tokamak and helical confinement systems with outboard helical field coils. The improvement of plasma confinement by E × B poloidal flow was tried by applying radial electric field with spherical electrode biasing, and the effect of outboard helical magnetic field application was investigated on these electrode biased plasmas. It is measured that plasma radial density profile was improved in spherical electrode biased case, but the density fluctuation including density collapse was induced. This type of fluctuation was reduced by the application of outboard helical magnetic field.

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A broadband heterodyne radiometer system has been developed and installed on KSTAR to measure second harmonic electron cyclotron emission (ECE) at the magnetic field of 3 T. The system consisting of two radiometers (110-162 GHz and 164-196 GHz) can cover a frequency range of 110-196 GHz. The unique and key components to construct this ECE diagnostic instrument are specially-designed detector modules and a diplexer for splitting ECE radiation with high efficiency. The minimum detectable electron temperature with a time response of 1 µs is about 0.23 eV. The observed signal intensity is roughly consistent with the value estimated by using characteristics of various components (waveguide components, sub-harmonic mixers, amplifiers, and intermediate frequency detectors). In this article, design considerations and preliminary ECE measurements will be described.

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Short pulses of a deuterium neutral beam (NB) with a duration of less than ∼ 5 ms were co-injected into magnetohydrodynamic (MHD)-quiescent ohmic deuterium plasmas of the HL-2A tokamak to study the variation of the slowing-down time in two different electron temperature environments. Analyses were made for the decay rate of D-D neutrons produced by beam-plasma interaction following NB turn-off, i.e., experimentally observed neutron decay rates were compared with those predicted by a classical slowing-down model. The results suggest the beam ions decelerate without significant loss in the HL-2A tokamak in round terms. When the critical energy for beam ions is higher than the beam injection energy, it seems that a small fraction of the beam ions is lost.

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A new electron cyclotron emission (ECE) antenna was designed and installed outside of torus, when the heating and diagnostics systems were reconfigured in LHD. Its design is based on Gaussian beam optics and consists of two plane and two concave mirrors. The mirror surfaces are defined using the concept of constant phase to improve matching to the corrugated waveguide. The new ECE antenna was used in a 2010 experiment to measure the electron temperature profile and its fluctuations.

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Coupling the magnetic diagnostics and a 3D MHD equilibrium calculation code, the magnetic island is studied in the Large Helical Device (LHD) experiment. In an experiment, the collapse in the plasma core was observed in a configuration, which has large magnetic island produced by external perturbation coils. At the collapse, the temperatur profile was flattened. This suggests the magnetic island evolved. The magnetic island was observed by the magnetic diagnostics. The magnetic diagnostics also suggests evolving the magnetic island. A 3D MHD equilibrium is caluclated by the 3D MHD equilibrium code then signals of the magnetic diagnostics are simulated. Since the comparison of observed and calculated signals is comparable, the magnetic island in calculated equilibrium is similar to one of the experiment.

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The ion cyclotron wall conditioning (ICWC) is one of the conditioning methods to reduce impurities and to remove tritium from the plasma facing components. Among the advantages of ICWC are the possible operation under strong magnetic field for fully torus area based on the charge exchange damage observed in thin SS samples arranged on a hexahxedron block holder with three different facings, the areas influenced by ICWC is estimated. On the plasma facing area of the material holder, high density of helium bubbles is observed by transmission electron microscope (TEM). But the other areas show no observable damage. The fact that the bubble were observed only in a sample facing the plasma implies that the effective particles, most probably charge exchange neutrals come to the wall straightly Thus, cleaning of the surfaces un-exposed to plasma directly and those in shadow area is difficult by ICWC.

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The small device named TOKASTAR-2 has tokamak-helical hybrid coils to generate both configurations independently. We produced pre-ionized plasma with simple toroidal field in this TOKASTAR-2, and the spatial profiles of electron temperature and density were measured, which suggests both fundamental ECR heating process and other heating mechanisms. Within the limitation of static vertical field, around 0.1 kA plasma current was induced with strongly inward shifted configuration observed by the fast camera measurement, which is verified by the TOSCA equilibrium analysis. It was found that the eddy currents on the vacuum chamber prevent the plasma current build-up. The preliminary application of outer helical field was tried and the plasma current reduction was observed. For the optimization of tokamak operation to drive 1 kA plasma current, time-varying vertical field coil currents and eddy currents were evaluated by the TOSCA code, and the necessity of in-vessel vertical field coils was clarified. The pure current-less TOKASTAR configuration will be demonstrated by installing additional helical coils in the future.

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The results of experiments on plasma confinement at ECRH power density up to 1.5 MW/m³ are reported. It is shown that the results of measurements of the diamagnetism and poloidal rotation velocity of the plasma in the peripheral region agree with numerical calculations using a neoclassical model with allowance for anomalous losses. The correlation has been found between the low-frequency modulation of the gyrotron power radiation and the modulation of the radiation scattered by the turbulent plasma density fluctuations.

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The Tohoku University Heliac (TU-Heliac) surveyed the effects of island width on poloidal plasma flow. We determine that poloidal flow is driven externally by hot cathode biasing and an m = 3 magnetic island was produced by two pairs of external perturbation field coils. The electrode current required for poloidal flow at the plasma periphery jumping point increases with island width expansion and shows weak dependency at the core region of the plasma. These dependencies suggest that the magnetic island located at the plasma periphery affects poloidal flow as a drag term.

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For imaging of the emission profile of DD neutron produced by energetic deuterium ions, we propose a neutron pinhole camera with state-of-the-art nuclear emulsion technique. Recoiled proton due to elastic scattering of incident neutron passing though the pinhole makes a track in the nuclear emulsion. The incident neutron energy can be estimated by the energy of recoil proton and scattering angle that are derived from the track length and an angle between the track and the incident direction of neutron, respectively. DD neutron emission profile can be obtained with lower background events due to scattered neutrons and gamma-rays. We made preliminary designs on the pinhole collimator size and arrangement. The estimated spatial resolution would be better 100 mm with tungsten alloy collimator for the application to emission profile imaging of DD neutron in LHD plasma.

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The effects of the conducting wall on the FRC translation process have been investigated by using a confinement coil with a toroidal cut on the coil spool. Using the simple ring current model, the induced current on the conducting coil spool and chamber wall, and the separatrix shape, were estimated. A deceleration force produced by the induced currents equals to approximately 10% of the acceleration one generated by the gradient of the magnetic guide field. To investigate these effects experimentally, FRC translation was performed with a toroidal cut on the coil spool. In this case, typical translation velocity was increased from 100 to 150 km/s. However, a shift type (toroidal mode number n = 1) motion was triggered, likely by the asymmetry of the wall boundary.

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Suppression of the radial transport of the particles caused by low-frequency density fluctuations is one of the important subjects in the GAMMA10 tandem mirror. In GAMMA10, plasmas are mainly produced and heated with Ion Cyclotron Range of Frequency (ICRF) waves. The plasmas heated with ICRF waves have strong temperature anisotropy, because the ions are accelerated perpendicularly to the magnetic field line. In the GAMMA10 experiments, two types of low-frequency density fluctuations are observed; one is a drift-type fluctuation and the other is a flute-type fluctuation. In this paper, we study the radial transport of ICRF-produced high-energy ions which is caused by the low-frequency density fluctuations.

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In order to examine the effect of radiation power loss due to impurity gas puff in divertor plasma, we calculated time evolution of impurity ion densities and electron temperature for several cases with Ne gas puff rates 10¹² and 10¹³ cm⁻³ s⁻¹ during 0.01 s and initial electron temperature 10-300 eV for a plasma with electron density 10¹³ cm⁻³ with a one zone model. We found that the electron temperature decreases less than 1 eV for the cases with gas puff rate 10¹³ cm⁻³ s⁻¹, independent of initial electron temperature and in this case we expect plasma detachment. Detailed conditions for plasma detachment should also depend on time history of radiation power.

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Fluid flow can play a significant role in plasma equilibrium, possibly producing transport barriers and profile pedestals. To be consistent with hot ions, finite Larmor radius (FLR) effects should be included. Ito and Nakajima have developed a formulation for calculating magnetohydrodynamic (MHD) equilibria with poloidal-sonic flow and FLR effects using an inverse aspect-ratio expansion, known as the “Ito formulation” [A. Ito and N. Nakajima, AIP Conference Proceedings 1069, 121 (2008)]. The Ito formulation typically possess a singularity when the poloidal flow varies from sub- to super-poloidal sonic, known as the “poloidal-sonic (PS) singularity.” The presence of the PS singularity prevents the Ito formulation from being directly applied to equilibria with such flows. An investigation of the single-fluid MHD model shows that the PS singularity is due to an inverse aspect-ratio expansion of the Bernoulli equation. This suggests that, in order to use the Ito formulation for an accurate calculation of MHD equilibria with poloidal-sonic flows, the Bernoulli equation must be handled non-linearly near the singularity.

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In order to understand the confinement characteristics of different types of stellarators from the common point of view, characteristics of the geometric shape of the last closed magnetic surfaces are analyzed in terms of the Fourier modes. Magnetic configurations of LHD and CHS are compared for their different operational modes with shifted magnetic axis positions. Total number of Fourier modes is reduced for their outward shifted configurations to find out the essential modes for creating magnetic well. It is shown that the helical axis structure is important for the control of magnetic field configuration even for the planar-axis stellarators (heliotrons) of LHD and CHS.

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Possible fusion reactivity enhancement due to ³He minority ICRF heating in D-³He toroidal plasma is demonstrated in present numerical simulations. On this purpose the particle code based on test-particle approach is developed. This code solves guiding center equations for ³He ions in toroidal magnetic field including Coulomb collisions of these ions with the background deuterons and electrons. A simple Monte Carlo model for ICRF heating is implemented in this code as well. The transformation of ³He distribution function from Maxwellian to non-Maxwellian due to heating plays the key role for reactivity enhancement. The formation of significant energetic tail gives rise to the reactivity enhancement. This is an important issue for the performance of fusion reactors with minority heating of ICRF.

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