This paper reviews plasma-related atomic physics experiments performed with an electron beam ion trap (EBIT). In particular, activities with two types of EBITs at the University of Electro-Communications are reported after introducing the principle and the design of the devices. Spectroscopic and collisional data which are useful for plasma diagnostics, technology development based on plasmas are presented.

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This paper reviews various global instabilities destabilized by tangential neutral beam injection (NBI) in the Large Helical Device (LHD) plasmas. These global modes are toroidal Alfvén eigenmodes (TAEs), which are also observed in tokamak plasmas, and helicity-induced Alfvén eigenmodes (HAEs) which are observed only in three-dimensional plasmas such as LHD plasmas. Moreover, reversed magnetic shear Alfvén eigenmodes (RSAEs) are observed in a reversed magnetic shear (RS) plasma in the LHD, where the sign of the magnetic shear changes from positive in the plasma central region to negative in the plasma peripheral region. The RSAEs exhibit a characteristic frequency sweeping due to temporal evolution of the rotational transform profile. In the RS plasma, the energetic-ion-driven geodesic acoustic mode (GAM) is also excited. The GAM interacts nonlinearly with the RSAEs and generates a multitude of frequency sweeping modes through a three-wavecoupling process. The TAEs and GAM exhibit various types of nonlinear evolution, that is, pitchfork splitting and rapid frequency chirp-up and/or chirp-down. The linear and nonlinear characteristics of these energetic-iondriven global instabilities in the LHD are compared with those observed in tokamak plasmas. TAE bursts having rapid frequency chirp-down induce redistribution and/or loss of energetic ions. Future important issues are briefly described.

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In this article, experimental observations of limit cycle oscillations (LCO) that precede L-to-H transition are discussed. Issues are: (1) the existence of zonal flows, (2) spatio-temporal evolutions of turbulence intensity, and (3) periodic generations/decays of mean radial electric field and density. The role of Reynolds stress to accelerate the LCO flow is also addressed. The propagation of changes of the density gradient and turbulence amplitude into the core is commented. Varieties in experimental reports on these issues are explained, and possible origins of different interpretations are discussed. Problem definitions for the future research for resolution are presented.

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Recent research on turbulent transport of particle and toroidal momentum in the core of tokamak plasmas is reviewed. Similarities and differences between these two transport channels are briefly presented, highlighting the common feature that both include large off-diagonal components in the radial flux. The main goal of the review is to provide selected recent examples of validation studies in these topical areas, and, thereby, to outline an efficient route to validation in the complex field of transport studies dedicated to transport channels which include important off-diagonal components.

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On present-day devices much effort is devoted to develop state-of-the-art diagnostics with a continuous drive towards higher accuracy, better spatial and temporal resolution and more diagnostic channels. Diagnostic innovations often lead to better physics insight and they are often a driver for improving theoretical models. In future fusion devices the operation of diagnostics is strongly limited by the hostile environment. In ITER many of the presently used diagnostics are still marginally applicable, but in DEMO the amount of diagnostics that can be used is severely constrained – at the one hand because of the tough environmental effects and at the other hand because access to the machine will be limited. Theoretical modeling will be very important for DEMO diagnostics. Firstly, simulations based on synthetic diagnostics should lead to the optimum choice of diagnostics and secondly, theoretical modeling should complement the rather sparse diagnostic data set that can be obtained in DEMO.

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In order to test extended MHD models experimentally, we develop a new linear machine in which pure ion and electron plasmas are not only produced separately but also trapped simultaneously. At the first series of experiments, both lithium ion and electron plasmas are successfully confined at the same time. Values of density of those plasmas are in each range of critical values where ion and electron fluids could keep their own motions independently as a two-fluid plasma.

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Based on the noncanonical Lie perturbation theory, we derived a new formula for relativistic ponderomotive force in a transversely localized laser field, which is accessible to the regime where the conventional formula described in terms of the local field gradient can hardly be applied. The formula involves new terms represented by second and third spatial derivatives; therefore, the force depends not only on the local field gradient, but also on the curvature and its variation. A physical explanation for these terms is given.

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A high-speed rendering method for three-dimensional animated volume rendering in a CAVE visualization environment is developed. The proposed method accelerates the standard three-dimensional texture-slicing approach to volume rendering by making use of asynchronous data transfer with the pixel buffer object of graphics processors. The method enables stereoscopic animation of volume rendering at five frames per second for scalar data of 512³ grid points in a four-screen CAVE system.

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We have applied the theory of the nonlocal ponderomotive force which we derived recently using the noncanonical Lie perturbation approach to investigate a long time scale particle motion in a super Gaussian laser beam. In such a flat-top beam profile, the local field gradient is diminished near the axis, so that the conventional ponderomotive formula is hardly applied. Numerical analyses of the interaction time and its dependence on the initial position and momentum of particles show that the nonlocal effect of the ponderomotive force, which is associated with higher order spatial derivatives, regulates the dynamics predominantly and sensitively.

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We propose the use of a microwave comb for measuring the density profile as a continuous function of radius with high temporal resolution. The analysis focuses on phase-sensitive convolution of the reflected waves: convolution of pulses can eliminate statistical noise. The time resolution for constructing the density profile is also discussed.

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Anomalous ion heating has been observed for the first time during flux/current amplification by double-pulsing coaxial helicity injection (CHI) in the helicity injected spherical torus (HIST) device. Doppler ion temperature increases significantly near the separatrix on the inboard side during the second CHI pulse, whereas electron temperature remains constant. The experimental results indicate that selective ion heating may be associated with viscous damping of poloidal flows driven by the CHI pulse.

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A simple new tool for deposition layer studies, a directional material probe, is proposed. The probe, which examines the directionality of deposition layer formation, consists of a flat disk and pin. If deposits have directionality, a shadow of the pin is formed on the deposition layer on the disk. If no shadow appears on the deposition layer, this suggests that the deposition layer was formed isotropically. The probe can be applied to plasma-wall interaction studies in fusion devices and laboratory plasma devices such as linear divertor simulators to reveal the material migration mechanisms in such devices. The directional material probe method has been applied to plasma-wall interaction studies in the Large Helical Device (LHD), and a position-dependent variation in the directionality of deposition layer formation was found.

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By employing a new method for studying dynamical transport, hysteresis in the flux-gradient relation was recently discovered for modulated heating. In this paper, the new method is compared to the conventional heat-pulse propagation method. We also demonstrate integration of results from the new method with the power balance thermal conductivity.

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We have used the convolution method to obtain higher harmonics of the temperature perturbation in a heating modulation experiment on LHD plasma. In comparison with predictions based on the diffusive model, amplitudes of higher harmonics (such as the seventh) decayed much more slowly as they propagated radially. Changes in the time derivative of temperature at the onset and turning-off of heating power remained much sharper after radial propagation than those predicted by the diffusion model.

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In this paper, classical particle transport processes in field-reversed configuration plasma is investigated by particle-tracking calculations. The end-loss rate is found to increase with ion temperature, and the temperature dependence is much stronger than that of the Bohm scaling and the empirical scaling.

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Possibility of the poloidal current drive by low energy axial neutral beam injection is studied by a Monte-Carlo method for field-reversed configuration (FRC) plasma. In order to suppress the end-loss rate of beam ions, it is found that the separatrix beta of target plasma should be low (presumably less than 0.1). If a beam source of a few 100 eV can be developed, beam ions could be trapped inside the separatrix and poloidal current drive would be possible even for FRC plasma embedded in the open-field region.

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Relation among the ion temperature gradient (ITG) turbulence, zonal flows, and the transport in helical plasmas is investigated by nonlinear gyrokinetic simulations. Local gyrokinetic simulations for helical field configurations are carried out employing various parameters such as the density and temperature gradients and local shears. From the simulation results, we construct a simple model function to represent ion heat diffusivity in terms of the turbulent fluctuations and zonal flow amplitude in helical plasmas.

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A flux sustainment condition of field-reversed configurations (FRCs) is given in the present paper for the first time. Here, maintenance of the poloidal flux is considered to be accomplished by tangential neutral beam injection (NBI). The beam current of 700 A is estimated analytically to be required for a flux balance of an FRC with the resistive anomaly factor of 10.

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Use of an ‘inter-linked' (IL) central solenoid (CS) in a tokamak fusion reactor is proposed for achieving a sufficient amount of the CS magnetic flux swing for the plasma current I_p ramp-up with keeping the reactor size reasonable. It is shown that a large amount of the flux swing by the IL-CS, compared to the conventional (C) CS, is expected for a tokamak fusion reactor with the fusion power P_fus = 2 GW and the major radius R_p < 8.0 m, and that the IL-CS can generate a marginal amount of the magnetic flux swing to ramp up I_p only by the inductive way for a tokamak reactor with R_p = 6.5 m while the C-CS cannot a sufficient amount of the flux swing for the I_p ramp-up.

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Target injection is a key technology to realizing inertial fusion energy. Here we present the first demonstration of target injection and neutron generation. We injected more than 600 spherical deuterated polystyrene (C₈D₈) bead targets during 10 minutes at 1Hz. After the targets fell for a distance of 18cm, we applied the synchronized laser-diode-pumped ultra-intense laser HAMA and successfully generated neutrons repeatedly. The result is a step toward fusion power and also suggests possible industrial neutron sources.

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An experimental and design study has been commenced for a dual-frequency gyrotron (28 and 35 GHz). The target output power at 28 GHz is 2 MW. For a modified 28 GHz 1 MW gyrotron, an output power of 1.25 MW and operation of 0.6 MW for 2 s have been achieved at 28 GHz. For the 35.45 GHz oscillation test, a cavity oscillation power of 1.2 MW and an efficiency of 33.9% have been confirmed by considering the calculated transmission efficiency of 72%. These results support the development of dual-frequency gyrotrons for lower frequency ranges.

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A high-power sub-THz gyrotron is under development as a power source of collective Thomson scattering diagnostic of fusion plasmas. It operates at a fundamental harmonic frequency of 295 GHz. A cavity which realizes stable and efficient single mode oscillation, an electron gun with an intense laminar electron beam, and an internal mode convertor are designed. A maximum oscillation power of 234 kW is achieved with a Gaussian like radiation pattern. The duration of 130 kW pulse is extended up to 30 microseconds, which is limited by the configuration of power supply.

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An effective method to increase Thomson scattering (TS) signals is required in low-density plasmas. Multi-pass TS has the advantage of enhancing scattered signals. We constructed a double-pass TS system for a polarization-based system modelled on the GAMMA 10 TS system. In the second step, we have developed the multi-pass TS system based on the former double-pass TS system. In order to evaluate the effectiveness of the polarization-based configuration, the multi-pass system was installed in the GAMMA 10 TS system, which is capable of multi-pass scattering. We undertook Rayleigh scattering experiments. The integrated scattering signal was magnified to be approximately three times as large by using the multi-pass system with 6 passes.

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The feasibility of plasma transport in periodic magnetic fields was studied for the pulse length control and modulation of laser ablation ion sources. The field was created by permanent magnet rings and the plasma ion flux was investigated as a function of the field profile and the transport distance. The results showed that the field is effective for the plasma guiding and the transport efficiency depends on the field profiles. The results also indicated that collective effects play an important role in the plasma guiding.

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We developed a new imaging spectroscopy diagnostic tool for H_α emission and installed it on a negative hydrogen ion (H⁻) source to investigate the H⁻ dynamics in the extraction region. During beam extraction, the H_α emission dropped; the same drop also appeared in the H⁻ density (as measured by cavity ring-down spectroscopy). The reduction in the H_α emission results from the reduction in the excited hydrogen population caused by mutual neutralization processes between H⁺ and H⁻ ions, which in turn are due to a decrease in the H⁻ density. We find a reduction structure in H_α that is observed inside the plasma farther than 20 mm from the plasma grid (PG) surface. The result indicates that H⁻ ions produced at the PG surface accumulate in the extraction region, so we conclude that they flow toward the PG apertures.

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On QUEST, we are carrying out the steady state operation by RF current drive. A Thomson scattering (TS) system has been constructed to measure the plasma electron temperature. This system consists of a Nd:YAG laser of energy 1.65 J, a spherical mirror for collection of the scattering light, and a polychromator for spectroscopy. Because the Thomson scattering signal is weak for the low-density RF-sustained plasma, many scattering pulse signals were accumulated from steady-state plasmas. Electron temperature profiles were obtained for the steadystate spherical tokamak (ST) plasma for the first time. Six spatial points were measured by moving the fiber position, and the plasma electron temperature was in the range 10-500 eV.

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Kinetic damping in linear gyrokinetic (GK) Vlasov simulations is found to exhibit a bifurcation at the collisionality β_c = β^*_c , above which, i.e. β_c >β^*_c , the damping is represented by a Landau eigenmode in velocity space, while below which, i.e. β_c < β^*_c , by the phase mixing of a finite number of marginally stable, discretized Case-van Kampen eigenmodes. The latter causes a recurrence that restricts the damping and then the energy transfer from wave to particles within a finite recurrence time. In order to address whether the stabilization effect due to such stable damped modes on unstable modes via mode coupling can be evaluated in long timescale GK simulations, we introduced a triad model consisting of stable and unstable modes incorporated with a tertiary vortex flow. We identified β^*_c numerically and found that the stabilization effect works properly beyond the recurrence time even in the phase mixing regime across β_c = β^*_c .

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The laser Thomson scattering (LTS) method has been applied to measure the electron density n_e and electron temperature T_e of very-high-frequency (VHF) argon plasmas. When the probing laser wavelength was 532 nm and the laser power density was ∼10¹⁵ W/m², the Thomson scattering spectrum was obviously deformed by the effect of the photo-ionization of metastable argon atoms. The threshold laser power density at which the scattered light intensity from electrons in the plasma and that from electrons produced by photo-ionization are equivalent was found to be unexpectedly low (4×10¹³ W/m²). To avoid the photo-ionization of metastable argon atoms, the laser power density was decreased to around 1×10¹³ W/m² by using a cylindrical lens as the focusing lens. Then, the n_e and T_e values measured by LTS and the probe method were compared for a VHF plasma using argon gas at a pressure of 100 mTorr. This comparison confirmed that the LTS method gave reasonable n_e and T_e values.

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While expressing the ideal fluid/plasma equations in terms of Eulerian variables, we encounter a non-canonical Hamiltonian structure. In other words, Poisson operators determining symplectic geometry have nontrivial kernels that foliate phase spaces. There are several different recipes for “canonicalizing" such Hamiltonian formalisms by either reducing or extending phase spaces. Clebsch parametrization is a well-known method for reducing phase spaces. Here we introduce a new scheme that generalizes the Clebsch parametrization. Using the new set of variables, we delineate a fundamental difference between the reduced magnetohydrodynamic equations and the two-dimensional Euler equations.

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The onset and evolution of an m/n = 2/1 neoclassical tearing mode (NTM) in JT-60U under nearly constant discharge conditions are investigated (m and n are the poloidal and toroidal mode numbers, respectively.). In about 80% of the discharges, the 2/1 NTM appears from a small amplitude without a noticeable triggering event, and in the rest of the discharges, the 2/1 NTM is triggered by a collapse localized at the mode location. Although the former grows with regular oscillations of the magnetic perturbations, the latter grows with irregular oscillations from the beginning. In addition, although a collapse causing a temperature change of ∼20% triggers a 2/1 NTM, a smaller-amplitude collapse, e.g., a temperature change of ∼7%, does not trigger an NTM. This suggests the existence of a threshold amplitude that triggers an NTM. The characteristics of the localized collapse are similar to those of a previously observed barrier localized mode.

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Spherical tokamak (ST) merging start-up for high-beta ST formation was developed in the TS-4 merging/reconnection experiment under a high external toroidal magnetic field. We optimized the formation of a stable high-beta ST (β_t > 30%) by adjusting the initial parameters of two low-beta STs. We precisely analyzed the produced high-beta STs by MEUDAS using the measured and fitted profiles, and found that they exhibited low-plasma internal inductance because of a hollow current profile, a paramagnetic toroidal field, and weakly reversed shear. A pressure-driven instability analysis indicates that the merging STs become unstable when their q-values are less than 1. In the first high-power neutral beam injection experiment (#1 and #2, P_NBI ∼ 0.4 MW) in TS-4, the decay time and magnetic flux of the produced STs were improved, mainly because of pre-ionization and merging/reconnection effects.

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Sustaining a detached plasma is important for the reduction of the heat load on a divertor. The externally controlled methods such as gas puffing in the divertor region, etc. have been studied for maintaining detached plasmas. In the recent LHD experiments, a resonant magnetic perturbation (RMP) is utilized to establish the detached plasma, which is one of the candidates to reduce the divertor heat load. The finite plasma response field has been detected when the RMP is imposed, and its behavior is investigated from the viewpoint of the magnetic island dynamics. A parameter of the phase difference, Δθ (defined as the phase difference between the plasma response field and the RMP), changes from antiphase to in-phase while the plasma transits from the attached to the detached states. Finally, when the Δθ reaches at certain critical value, Δθ_crit, the plasma enters the detached state. Since this critical value does not have a dependency on plasma beta and collisionality, Δθ_crit seems to be a threshold to establish the detached plasma. From the viewpoint of magnetic island dynamics, the behavior of Δθ approaching in-phase implies the “trend” of island growth. The detached plasma might be established in the case of the tendency for the plasma response field to reinforce the island width.

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A perpendicular-view fast video camera has been installed in Heliotron J to observe the behavior of filamentary structures of edge plasma turbulence across the last closed flux surface (LCFS). Supersonic molecular-beam injection (SMBI) can greatly increase the edge Hα emission; hence, we used the high imaging rate and shutter speed of the camera to capture the behavior of the fast propagating filamentary structures. A high-pass fast Fourier transform filter on the time dimension was adopted to extract the fluctuation component from the raw data for each pixel. The motion of the filamentary structures was clearly visible when we applied an amplitude threshold to identify the intense structures. In addition, a time-resolved 2D cross-correlation technique was adopted to estimate the poloidal phase velocity of turbulence. The motion direction was found to be reversed dramatically just after an SMBI pulse.

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First mirrors in fusion devices have to retain the reflectivity and the degradation mechanism and dominant plasma operations that affect the reflectivity have to be understood. The reflectivity of visible laser light (635 nm) of a corner cube mirror is almost entirely determined by the initial hydrogen glow discharges for wall conditioning before conducting a main plasma experiment in the Large Helical Devices (LHD). The hydrogen glow discharge forms a carbon deposition layer on the mirror surface, which degrades its reflectivity. A neon glow discharge and subsequent main discharges had less effect.

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The rotation of resistive interchange instabilities excited in the periphery of plasmas compared to plasma flows under various plasma conditions in the Large Helical Device. The observed magnetohydrodynamic modes exhibit nominal rotation in the electron diamagnetic direction in the laboratory frame. We assumed that the ion (electron) flows consist of the E × B and ion (electron) diamagnetic drift flows. These drift flows were individually evaluated and geometrically compensated using a finite beta equilibrium. Experiments with different neutral beam injection conditions and different electron densities show that the frequencies of the observed modes were quantitatively consistent with those of the electron flow.

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In the previous 3D Cauchy-condition surface (CCS) method analysis to reconstruct the magnetic field profile in the Large Helical Device (LHD), one assumed an impractically large number of magnetic sensors, i.e., 440 field sensors and 126 flux loops. In the singular value decomposition (SVD) process employed in the CCS method, a gap is found in the magnitude of the singular values. The most accurate field results can be obtained if all the singular values smaller than the gap threshold are eliminated, independent of the number of boundary elements on the CCS and the number of sensors as well. With the reduction in the number of boundary elements, the required numbers of field sensors and flux loops are significantly reduced to 110 and 25, respectively, without losing the solution accuracy. They can be further reduced to 58 and 13 respectively if considering the symmetry of the field profile in the LHD. This result suggests the possibility of actual application to the LHD.

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A novel equilibrium field (EF) system has been developed for use in the merging start-up of spherical tokamak (ST) plasma in the University of Tokyo Spherical Tokamak (UTST) device. Since the UTST device utilizes external poloidal field (PF) coils to form two STs on the top and bottom of the device by huge induction voltages, it is necessary to decouple the slow EF coil system from the fast-swing PF coils. A new EF system using electric double-layer capacitors was constructed and evaluated through the merging start-up experiment in the UTST. Using a very long duration of the EF current waveform and a thick magnetic shield successfully reduced the induction voltages from the PF coils and prolonged the equilibrium time constant.

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A collimated neutron flux array system in JT-60U successfully upgraded performance of higher counting rate and the capability to detect 14 MeV neutrons as well by using a fast digitizer in 2006. Additionally, detailed analysis of the recorded waveform on JT-60U and FNS has provided us new findings. In this paper, firstly, characteristics of pulse shapes and neutron-gamma discrimination parameters investigated by the detailed analysis of digitized waveform data are described. Next, new data analysis procedure for neutron-gamma discrimination based on the characteristics is proposed. In the new procedure, an appropriate projection surface, on which we can define the discrimination boundary, has been introduced in three-dimensional discrimination parameter space. The problem of “pulse height variation”, which occurs when a counting rate is high in high performance plasma, and its countermeasures are also presented. The system using the countermeasure of a booster method successfully has avoided the problem in the high neutron flux condition of ∼2 × 10⁵ counts/s on FNS.

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The nonlinear dispersion relation of the geodesic acoustic modes (GAMs) is investigated for tokamaks with a high safety factor and low magnetic shear. We focus on the Reynolds stress as a nonlinearity, which is truncated at the third order of the GAM amplitude. The real frequency of the GAM is modified according to the phase of the nonlinear force acting on the GAM, which depends on the turbulence decorrelation rate. The nonlinear frequency shift is much larger than that from the finite gyro-radius effects in the linear theory, when the poloidal turbulent E × B velocities are comparable to the diamagnetic drift velocity. Under such circumstances, the group velocity is strongly enhanced and becomes comparable with the radial phase velocity. In addition, the magnitude of the nonlinear effects is also evaluated using experimental parameters.

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Neoclassical transport analyses have been performed for a high electron temperature LHD plasma with steep temperature gradient using a neoclassical transport simulation code, FORTEC-3D. It is shown that the large positive radial electric field is spontaneously formed at the core along with the increase in the electron temperature, while the neoclassical heat diffusivity remains almost unchanged. This indicates that the 1/ν-type increase expected in the neoclassical transport in helical plasmas can be avoided by the spontaneous formation of the radial electric field. At the same time, it is found that the experimentally estimated heat diffusivity is significantly reduced. This suggests that the formation process of the transport barrier in the high electron temperature plasma can be caused by the spontaneous formation of the radial electric field.

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In the International Global H-mode Confinement Database, there are data that have the H_H98(y,2) factor exceeding unity in the region where the electron temperature is higher than the ion one. This high value of H_H98(y,2) can be attributed mainly to the improved electron confinement. The conditions required for this improvement are investigated with the local flux-tube gyrokinetic code GS2. When the ion temperature gradient length is shorter than the electron one, ion temperature gradient (ITG) mode is dominant, whereas trapped electron mode (TEM) is stabilized. Under this situation, the electron heat diffusivity is suppressed. In addition, an effect of the magnetic shear s is also studied in the positive shear range, 0.6 ≤ s ≤ 1.4, and the reduction in the electron and ion heat transport with increasing s is found.

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One of the important phenomena in magnetically-confined fusion plasma is plasma turbulence, which causes particle and heat transport and degrades plasma confinement. To address multi-scale turbulence including temporal and spatial scales of electrons and ions, we extend our gyrokinetic Vlasov simulation code GKV to run efficiently on peta-scale supercomputers. A key numerical technique is the parallel Fast Fourier Transform (FFT) required for parallel spectral calculations, where masking of the cost of inter-node transpose communications is essential to improve strong scaling. To mask communication costs, computation-communication overlap techniques are applied for FFTs and transpose with the help of the hybrid parallelization of message passing interface and open multi-processing. Integrated overlaps including whole spectral calculation procedures show better scaling than simple overlaps of FFTs and transpose. The masking of communication costs significantly improves strong scaling of the GKV code, and makes substantial speed-up toward multi-scale turbulence simulations.

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We analyze linear instability in low beta tokamak plasmas with nonmonotonic q-profiles using resistive magnetohydrodynamic equations. We consider n = 2 (where n is the toroidal mode number) modes for configurations where q_min is close to 1.5 and sufficiently below 2 so that the separation of the rational surfaces with q = 2 is large. Similar instability characteristics arise when q_min is slightly and moderately above 1.5.

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Guiding-center equations for relativistic particles are presented in axisymmetric toroidal geometry using Boozer coordinates. Effects of slow equilibrium changes are included for describing electron acceleration due to the induction field, which is a fundamental process of runaway electron generation during disruptions. For a consistent treatment of the runaway orbit in finite-pressure plasmas, the equations are given in both canonical and noncanonical forms by retaining the radial covariant component of the equilibrium magnetic field. For this purpose, the Lagrangian formulation by White and Zakharov [R.B. White and L.E. Zakharov, Phys. Plasmas 10, 573 (2003)] is applied to axisymmetric equilibria with slowly varying magnetic-flux functions.

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Deuterium retention and trapping in tungsten plasma sprayed layers after low energy plasma irradiation was investigated by means of thermal desorption technique. Plasma irradiation at low temperatures (< 650 K) showed the retention and trapping similar to polycrystalline tungsten material. On the other hand, irradiation at high temperatures (> 650 K) showed significant increase of the retention in the plasma sprayed layers contrary to the polycrystalline material. Thermal desorption spectra showed extremely high temperatures of the peak positions which are non-typical for the tungsten materials. The possible reasons for such high deuterium retention in the plasma sprayed layers after irradiation at high temperatures and the possible trapping mechanisms are discussed.

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Tungsten is one of the candidate materials being considered for fabricating the plasma facing components (PFCs) of fusion reactors. When used in fusion reactors, PFCs are exposed to high heat loads from the fusion plasma, resulting in the PFCs being subjected to induced thermal stresses. This study focused on the creep deformation of pure tungsten under such a deformation-controlled thermal stress. Bend stress relaxation (BSR) tests were performed on specimens of as-received pure tungsten at temperatures of 600 - 1000^◦C for 0.1 - 1 h in vacuum. The BSR ratio decreased with an increase in the temperature, decreasing sharply in a short period of 0.1 h and then decreasing gradually. Most of the stress was relieved during the BSR test performed at 1000^◦C for 1 h. The activation energy of stress relaxation, calculated using the cross-cut method, was similar to that of the Peierls mechanism. This suggested that the mobility of the dislocations controls the stress relaxation mechanism. Most of the stress was relieved quickly because the activation energy of stress relaxation was relatively small.

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The key to improving the heat load of vacuum plasma sprayed tungsten coatings on low activation ferritic steel maintained at low temperatures is elimination of stratified low-density layers with many large pores, in which thermal cracks propagate preferentially. The low-density layers are formed owing to the deposition of large solidified tungsten particles, which remain mainly at the periphery of the spray stream. In this study, by shading this periphery, partially homogeneous tungsten coatings without large pores were successfully obtained. The coatings are expected to show good heat load, which is feasible for nuclear fusion applications.

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Langmuir probes installed in the lower divertor region under high heat flux have been developed and manufactured for JT-60SA. A probe electrode with a head having a rooftop shape is made of a carbon fiber composite and can withstand heat fluxes of up to 10 MW/m² for 5 s and 1 MW/m² for 100 s. This has been achieved by increasing the volume of the probe electrode that is not directly exposed to the plasma. To minimize the reduction of the heat removal performance of the divertor, the Langmuir probes are installed in toroidal gaps with widths of 10 mm between the divertor cassettes, without embedding them in the divertor tiles. Aluminum oxide coatings have been applied to insulate the probe electrodes from the divertor cassettes and to limit the toroidal thickness to 8 mm. Brazing of the nickel connectors to the probe electrodes has reduced the toroidal thickness of the Langmuir probes. A minimum spatial resolution of 13.5 mm has been achieved to the Langmuir probes installed on the inner and outer divertor targets.

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Carbon migration in the tungsten coating layer exposed to JT-60U divertor plasmas has been investigated by analysis of chemical binding states of the carbon atoms. More than 1% of carbon atoms were accumulated as graphitic carbon, amorphous carbon and/or carbon-deuterium bonds. This concentration was more than five orders of magnitude higher than the solubility of carbon atoms in tungsten lattice. Up to 20% of ditungsten carbide (W₂C) was also formed in the tungsten coating layer. These findings suggested the following carbon migration mechanism in the tungsten coating layer. The incident carbon migrates along grain boundaries and defects such as pores over the depth which is evaluated by the carbon diffusion coefficient in tungsten lattice. The carbon atoms trapped on grain surface penetrate and diffuse in the grains. The carbon atoms exceeded the solubility of carbon atoms in tungsten lattice chemically bind to tungsten atoms and form W₂C.

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The corrugated horn antenna is widely used for the evaluation of transmission lines in electron cyclotron resonant heating systems, but little is known about the intensity and phase of its radiation profile over a wide frequency range. This paper is concerned with the development of a full D-band (110-170 GHz) corrugated horn antenna. The antenna was designed based on electromagnetic simulation codes for the finite element method and the method of moment. To verify these numerical simulations, a low-power test system was established, and its dynamic power range was defined for the precise measurement of antenna radiation. The fabricated antenna was measured and analyzed with basic Gaussian optics for a number of frequencies throughout the D-band. The measured radiation profiles are Gaussian-like and agree well with the numerical simulations. The radiated intensity pattern is slightly elliptical in the lower frequency range, as unwanted higher-order modes arose at the corrugated mode conversion section and generated the elliptical radiation pattern. The evolutions of the antenna radiation do not correspond with those expected by basic Gaussian optics.

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