The topological and transport properties in the edge plasma of the dynamic ergodic divertor is studied to clarify the functionality of this type of helical divertor. The heat and particle fluxes at the DED target plates were measured with Langmuir probes. Peak fluxes are found where field lines end, which penetrate deep into the plasma. The comparison of the measured target profiles to the magnetic topology shows, that heat and particles are mainly transported to the target plates via flux tubes of short connection length. About 65% of the fluxes are found in areas, where field lines with a connection length of 1-2 poloidal turns connect to the target. Analysis of the source distribution shows, that about 40% of the ion sources lie inside the downstream area of the divertor. The high fraction of convective heat flux prevents from establishing high recycling in the divertor for the pulse type discussed here.

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Theoretical cross sections values for electron capture by proton impacting on methane and silane molecules are presented, based on the multi-reference single- and double-excitation configuration interaction calculation of electronic structures of collision intermediates, and compared with the available semi-empirical formulas recommended for fusion plasma simulations. The current results may apply not only to the simulations of fusion edge plasmas, but also plasmas of technological interest.

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Passive spectroscopy is a standard diagnostic to observe the boundary layer of fusion plasmas. This method of visible spectroscopy is focused on the measurement of deuterium recycling flux and on the monitoring of impurity fluxes of O, He, etc., or W, C, etc., which result from erosion of plasma-facing components. In addition, ro-vibrational analysis of molecular transitions provides information about the molecular break-up in the plasma. Spectrometers used for studying the plasma boundary have to fulfill high demands with respect to the spectral, spatial, and time resolutions, the observable wavelength range, to the sensitivity and dynamic range of the detector in order to simultaneously observe and analyze the emission of both, atomic and molecular species present in the observed region. We present an overview spectrometer system that is able (a) to simultaneously measure strong atomic lines (e.g. D_α) and weak molecular bands (e.g. C₂ Swan band), (b) to resolve narrow molecular lines (e.g. D₂ Fulcher-α band) and allow their ro-vibrational analysis, and (c) to provide information about the spatial distribution in the plasma boundary. A full characterization of the custom-made system in a cross-dispersion arrangement with respect to resolving power, simultaneous wavelength coverage, sensitivity, etc., is presented. Examples of spectra (“footprints”) taken from an injection of C₃H₄ into TEXTOR are presented.

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A spectroscopic measurement is performed for a plasma in a rotating radiation belt, which accompanies the complete divertor detachment recently realized in the Large Helical Device (LHD) [J. Miyazawa et al., Nucl. Fusion 46, 532 (2006)]. The population distribution over the exited levels is experimentally determined from the Balmer series line intensities and is compared with the result of the collisional-radiative model calculation to determine the electron temperature T_e and density n_e. No reasonable pair of T_e and n_e is found when either the ionizing plasma or the recombining plasma is assumed. A good fitting is obtained under an assumption of the ionization balance plasma with T_e = 1.8 eV and n_e = 2 × 10²⁰ m^-3. These parameters are confirmed through analysis of a high-wavelength-resolution measurement for the Balmer series lines which clearly exhibit the Stark broadening effect.

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Carbon emissions of CIII-CVI during the recombining phase of the Large Helical Device (LHD) plasma are studied to understand their behaviors. To achieve this, four resonance transitions of CIII (977 Å: 2 s^{2 1}S-2s2p ¹P), CIV (1548 Å: 2s ²S-2p ²P), CV (40.27 Å: 1s^{2 1}S-1s2p ¹P), and CVI (33.73 Å: 1s ²S-2p ²P) are observed using absolutely calibrated VUV monochromators and EUV spectrometers. A one-dimensional impurity transport code has been used to calculate spectral emissivity under the consideration of measured n_e and T_e profiles. The temporal evolution of line emissions has been calculated and compared with the measured data. The comparison shows that the carbon density is 3 % of the electron density. However, a discrepancy between the calculated and measured values has been noticed in CIII and CIV emissions. It has been argued that three-dimensional structures of CIII and CIV emissions are likely to be the reason for this difference, which is based on the presence of relatively high-density and high-temperature plasmas in edge ergodic layer in LHD. It is also found that the CV and CVI emissions during the recombining phase increase with electron density, whereas these are nearly constant during steady state phase. This strongly suggests the disappearance of the edge particle screening effect in the recombining phase.

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A concise systematic study of impurity parameter profiles (impurity ion temperatures and poloidal velocities) measured under different plasma conditions has been carried out in the TJ-II stellarator using an active spectroscopic diagnostic system covering a large section of the central plasma minor radius, from r/a = 0.3 to 0.85, with high spatial resolution. For this, plasmas using hydrogen as the working gas were created and maintained using electron cyclotron resonance heating (from 400 to 500 kW) and line-averaged densities between 0.4 and 0.9 × 10¹⁹ m^-3 were achieved. In the first instance, it is noted that ion temperature profiles tend to be flat across the accessible plasma radius, whilst a transition in the poloidal velocity from the ion to the electron diamagnetic direction is observed with increasing density. At the same time, the shear point for impurity velocity is seen to propagate outwards towards the plasma edge. Similarly, at a fixed density, a similar tendency is observed when raising the injected heating power. These results together with the behaviour of the impurity temperatures for the same conditions, suggest that collision frequency ν_E^b/e ∼ n_eT_e^-3/2 can be considered as an important controlparameter of plasma dynamics.

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A data analysis technique for microwave imaging reflectometry (MIR) in the Large Helical Devices (LHD) and TPE-RX plasmas has been investigated. In LHD, the fast Fourier transform (FFT) is employed. The statistical properties of the fluctuation spectra on MIR signals are quantified by the time-frequency analysis by the ensemble average technique. Statistical analyses using cross-correlation and coherence spectra reveal the characteristics of MHD modes, such as wave numbers, mode numbers, and phase velocity. In TPE-RX, the wavelet analysis is more useful because the phenomena are transient in TPE-RX plasma.

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Quasi-isodynamic configurations offer the possibility of very good energetic particle confinement. They seem to offer the possibility of achieving very high plasma β, too.

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Abrupt flushing of central density occurs in internal diffusion barrier (IDB) plasmas in the Large Helical Device (LHD), where a super dense core (SDC) of the order of 10²⁰ m^-3 is formed inside; this event is called “core density collapse (CDC).” CDC must be suppressed as further increase of the central pressure is inhibited. Since CDC is always accompanied by a large Shafranov shift of the plasma center, it has been supposed that mitigation of the Shafranov shift might affect CDC. Vertical elongation of the plasma shape (κ > 1) is effective in mitigating the Shafranov shift, and κ can be controlled with the quadrupole magnetic field B_Q. To examine the impact of plasma shape control on CDC, B_Q scan experiment has been performed in the LHD. The large Shafranov shift in IDB plasmas is mitigated by increasing κ. As a result, CDC is suppressed and high central β values of approximately 7% have been achieved in vertically elongated plasmas. The optimum κ varies with the magnetic configuration. Beta gradients greater than those at CDC in the κ = 1 configuration are observed without CDC in vertically elongated plasmas.

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In the future fusion reactor, plasma density peaking is important for increase in the fusion power gain and for achievement of confinement improvement mode. Density control and internal transport barrier (ITB) formation due to pellet injection have been simulated in tokamak and helical reactors using the toroidal transport linkage code TOTAL. First, pellet injection simulation is carried out, including the neutral gas shielding model and the mass relocation model in the TOTAL code, and the effectiveness of high-field side (HFS) pellet injection is clarified. Second, ITB simulation with pellet injection is carried out with the confinement improvement model based on the E × B shear effects, and it is found that deep pellet penetration is helpful for ITB formation as well as plasma core fuelling in the reversed-shear tokamak and helical reactors.

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Large-current-capacity high-temperature superconducting (HTS) conductors using YBCO tapes are being considered as an option for the LHD-type fusion energy reactor FFHR. The typical operating current, magnetic field, and temperature of such conductors in FFHR are 100 kA, 13 T, and 20 K, respectively. A preliminary design of the HTS conductor has been proposed for the FFHR helical coils. Analyses have been performed on the proposed HTS conductor regarding thermal properties, mechanical structures, AC losses, and quench detection and protection. It is suggested that stainless steel might be a better choice for the outer jacket of the HTS conductor compared to aluminum alloy. Due to increased specific heats of conductor materials at 20 K, HTS magnets are supposed to be operated more stably compared to low-temperature superconducting (LTS) magnets operated at ∼4 K. The required refrigeration power is also reduced. Therefore, using HTS conductors, it is considered to be viable to assemble the continuous helical coils in segments with joints of conductors, as additional heat generation at the joints can be taken care by utilizing the surplus refrigeration power. According to these analyses, HTS conductors seem to be promising for the FFHR coils.

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LHD-type reactors have attractive features for fusion power plants, such as no requirement of a current drive and a wide space between the helical coils for the maintenance of in-vessel components. One disadvantage was considered the requirment of a large major radius to attain the self-ignition condition with a sufficient space for blankets. According to the recent reactor studies based on experimental results in LHD, the major radius of plasma is set at 14 to 17 m with the central toroidal field of 6 to 4 T. The stored magnetic energy is estimated at 120 to 130 GJ. Both the major radius and the magnetic energy are about three times as large as those for ITER. We intend to summarize the requirements for superconducting magnets of the LHD-type reactors and propose a conceptual design of the magnets with cable-in-conduit (CIC) conductors based on the technology for ITER.

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FFHR is a concept design of a steady-state fusion reactor that has been studied to demonstrate a large helical device (LHD)-type fusion power plant. The weight and thermal contraction of cryogenic components are sustained by support posts. A folded multi-plate-type post adopted in the LHD could be feasible for the FFHR. The dimensions of the post were determined according to the buckling load estimation against a gravitational load. Using this fundamental design, the flexibility and stress distribution of the post were calculated by a finite element method. The flexibility against the radial displacement was 24 kN/mm, and the maximum stress for a carbon-fiber-reinforced plastic and stainless steel plate was 155 and 544 MPa, respectively, which were below their allowable levels. The heat loads were calculated as 10.5 kW at 80 K and 0.34 kW at 4 K; the results revealed that the heat load at 4 K was almost 1/20 compared with that on a post made entirely of stainless steel. Natural frequencies were analyzed to assess safety against external loads such as an earthquake. The results showed that the LHD-type support post was suitable for the FFHR from the mechanical and thermal viewpoints.

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For conducting numerical simulations of plasma dynamics consisting of multiple phases, a new immersed interface method (IIM) scheme to solve multiphase flows with different viscosities and densities is being developed. The jump conditions for velocity, pressure, and their derivatives necessary for the finite difference approximations in the IIM are derived. The derivation results in sets of coupled equations that can be solved numerically by an iterative method.

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Hall effects on small-scale turbulence are studied by means of direct numerical simulations (DNS) of both, the incompressible single-fluid equations and the incompressible Hall magnetohydrodynamics (MHD) equations. It is shown that the energy spectrum of the velocity and magnetic fields coincide with our earlier results of shell model simulations. A direct comparison between the numerical results of incompressible Hall MHD turbulence and those of incompressible single-fluid MHD turbulence reveals that the Hall term excites the small scales in the magnetic field by strengthening energy transfer from/to the scales comparable to the ion skin depth.

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Nonlinear development of collisionless driven reconnection is investigated by making use of the electromagnetic particle simulation code “PASMO.” This code is developed for an open system, which is subject to an external driving source. The electric field at the reconnection point increases, and approaches the external driving field as time goes on. After the formation of x-shaped field structure around the reconnection point, the length of the electron dissipation region continues to increase for a short time. Finally, it stops to grow and relaxes to a steady state when the ratio of the width to length is constant. Thus, Sweet-Parker-like electron dissipation region is formed in a steady state, while the reconnection rate is controlled by the driving electric field.

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The ohmic heated plasma in TU-Heliac was biased by a hot-cathode to drive E × B poloidal rotation. Coincident measurements of the line density and ion saturation currents revealed the existence of density collapse accompanied by fluctuation in the poloidally rotating plasma. The radial density profiles just before and after the collapse were estimated. The steep density gradient vanished with the density collapse. The power spectra of the fluctuation were calculated from the ion saturation current obtained with a high-speed triple probe. The frequency of fluctuation was compared to the E × B poloidal rotation frequency, where the fluctuation appeared with density collapse. The fluctuation frequency was 2 to 3 times the rotation frequency. This suggested that the poloidal mode number of fluctuation was m =2 or 3.

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High ion temperature (T_i) hydrogen plasmas were successfully demonstrated in the LHD in an experimental campaign (FY2006). The power increase of the perpendicular neutral beam injections (NBIs) has mainly contributed to this achievement. T_i exceeded 5 keV at the average plasma density (n_e) of 1.2 × 10¹⁹ m^-3 and also achieved 3 keV at n_e > 3 × 10¹⁹ m^-3. Neoclassical (NC) analysis was performed for such plasmas. We confirmed that even if T_i or the plasma density became higher, NC transport was remained to a certain value because of the existence of the ambipolar radial electric field (E_r), which was roughly two orders of magnitude smaller for cases not involving the existence of ambipolar E_r. Besides, a T_i and n_e scan of discharge 75235 was also performed to consider the plasma parameter dependence of NC transport. Using these calculations, it is shown that NC ion thermal diffusivities are reduced to a small percentage of NC electron thermal diffusivites even at the fusion reactor relevant parameters such as n_e ∼ 1 × 10²⁰ m^-3 and T_i ∼ T_e ∼ 10 keV.

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In TJ-II plasmas, the perpendicular rotation velocity of the turbulence changes from positive to negative (from ion to electron diamagnetic direction) inside the Last Closed Magnetic Surface (LCMS) when the lineaveraged plasma density exceeds some critical value, this change being dominated by the inversion in the radial electric field. In this work we study the parameters that control the inversion in the perpendicular rotation. A parametric dependence of the critical density has been obtained studying plasmas confined in different magnetic configurations (different rotational transform and/or plasma volume) and heated with different ECH power levels. The studied data set shows a positive exponential dependence on heating power and a negative one on plasma radius, while the dependence on rotational transform has low statistical meaning. Besides, analysis of local plasma parameters points to plasma collisionality as the parameter that controls the inversion of the perpendicular rotation velocity of the turbulence.

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Numerical analysis is performed for a low beta neutral beam heated plasma and a superdense core plasma in the Large Helical Device (LHD) to investigate the time evolution of the rotational transform profile, which is calculated consistently with the three-dimensional magnetohydrodynamics equilibrium. Although there is a considerable amount of noninductive current flow, such as the bootstrap and Ohkawa currents, the inductive current component prevents a rapid change of the rotational transform profile in these plasmas.

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Previously reported benchmarking examples of the analytical formulae of neoclassical viscosities were presented implicitly assuming applications in a future integrated simulation system of the Large Helical Device (LHD). Therefore, the assumed toroidal period numbers were mainly N = 10. However, in this type of calculation, an implicit (or sometimes explicit) assumption of ι/N « 1 is sometimes included. This assumption is included not only in simplified bounce-averaged drift kinetic equations for ripple diffusions, but also in the equation before the averaging for non-bounce-averaged effects determining neoclassical parallel viscosity and banana-plateau diffusions. For clarifying the applicability of the analytical methods for configurations with extremely low toroidal period numbers (required for low aspect ratios), we show recent benchmarking examples in the National Compact Stellarator Experiment (NCSX) with N = 3 and the Quasi-Poloidal Stellarator (QPS) with N = 2.

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It is shown in Large Helical Device experiments that transport modeling based on only fluid description is insufficient to express edge transport phenomena in and around a magnetic island with lower collisionality. Furthermore, in recent tokamak experiments, it is found that the so-called stochastic diffusion theory based on “field line diffusion” overestimates the radial energy transport in collisionless edge plasma affected by resonant magnetic perturbations, though the perturbations induce a chaotic behavior in the field lines. These results imply that conventional modeling of edge transport should be reconsidered for a lower collisionality case. A simulation study of neoclassical transport in magnetic islands and ergodic regions is attempted for understanding the fundamental properties of such collisionless edge plasma. By using a drift kinetic equation solver without the assumption of nested flux surfaces (the KEATS code), it is possible to conduct the investigation. In this paper, we report the simulation study of ion transport in the ergodic region, neglecting the effects of an electric field and neutrals. The simulation results are interpreted through the discussion based on statistical studies.

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NEO-2 is a code for the computation of neoclassical transport coefficients and current drive efficiency in toroidal devices which is based on the field line integration technique. The possibility to use the complete linearized collision integral is realized in this code. In this report the results of comparison of the full matrix of transport coefficients in a tokamak with analytical models are presented. Effects of simplifications of the linearized collision model (e.g., reduction to a Lorentz model) are studied in order to provide a comparison with various momentum correction techniques used for the computation of transport coefficients in stellarators.

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The progress in a 3-dimensional, non-local neoclassical transport simulation code “FORTEC-3D” is described. The main purpose of the code is to solve the drift-kinetic equation in general a 3-dimensional configuration using the δf Monte Carlo method, and to calculate neoclassical fluxes and the time evolution of the ambipolar radial electric field simultaneously. This article explains new numerical schemes adopted in FORTEC-3D in order to overcome numerical problems, which happen especially in the cases where the bifurcation of radial electric field occurs. Examples of test simulation for an LHD magnetic field configuration with a bifurcated electric field are also shown. With improved numerical schemes, FORTEC-3D can calculate neoclassical fluxes and trace the time evolution stably for several ion collision times, which is sufficiently long to observe GAM damping and formation of the ambipolar electric field.

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The present status of the development of a hierarchy-integrated simulation code for toroidal helical plasmas, TASK3D, is reported. TASK3D is developed by extending the integrated modeling code for tokamak plasmas, Transport Analyzing System for tokamaK (TASK) [A. Fukuyama et al., Proc. of 20th IAEA Fusion Energy Conf. (Villamoura, Portugal, 2004) IAEA-CSP-25/CD/TH/P2-3]. In order to extend TASK to be applicable for threedimensional configurations, a new module for the radial electric field in general toroidal configurations has been developed and implemented. As a first test for this implementation, numerical simulations for the time evolution of temperature and electric field are conducted on the basis of an LHD experimental result, by a successful combination of a diffusive transport module and the implemented electric field module.

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For the preparation study of a future burning plasma, experimental information about the internal structure of energetic particle-driven MHD instabilities, such as toroidal Alfvén eigenmodes, is important. In the Large Helical Device, three microwave reflectometer systems have been recently installed for measuring the density fluctuation in the wide radial region with a high spatial resolution. One of the three systems has three channels with fixed frequencies. The others are operated in a frequency-hopping mode. These systems are used for measuring the internal structures of an MHD mode and its temporal behaviors. Using these systems, strongly localized MHD instabilities driven by the energetic particles are measured in the plasma core region, and the measured values are consistent with the theoretical expectation.

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Mode behaviors including non-linear development and pressure profiles during internal reconnection events are clarified in the MAST spherical tokamak. In a q₀ > 1 discharge, the tearing mode is a trigger; however, non-linearity of modes is not confirmed. On the other hand, in a q₀ < 1 discharge, the harmonics of a m/n = 4/1 mode of ∼22 kHz are confirmed. Method for identification of poloidal mode in ST configuration is also given.

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Two-dimensional two-fluid flowing equilibria of helicity-injected spherical torus with non-uniform density and both toroidal and poloidal flows for each species have been numerically determined by the nearby-fluids procedure. It is found from the numerical results that the equilibrium for the driven λ (≡ μ₀ j·B/B² ) profile exhibits a diamagnetic toroidal field, high-β (toroidal beta value, β_t = 32%), hollow current profile, and centrally broad density. In contrast, the decaying equilibrium exhibits a paramagnetic toroidal field, low-β (β_t = 10%), centrally peaked current profile, and density with a steep gradient in the outer edge region. In the driven case, the toroidal ion and electron flows are in the same direction, and two-fluid effects are less important since the E × B drift is dominant. In the decaying case, the toroidal ion and electron flows are opposite in the outer edge region, and two-fluid effects are significant locally in the edge due to the ion diamagnetic drift.

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The effect of toroidal current on the rotational transform was investigated in Heliotron J by measuring magnetohydrodynamic (MHD) activities at two configurations with a rotational transform (ι/2π) close to 0.5. The resonant m/n = 2/1 mode was observed in an ECH + co-NBI plasma in the configuration with ι/2π = 0.48 at ρ = 0.7. Here, m and n are the poloidal and toroidal mode numbers, respectively. ρ is the normalized minor radius. The rotational transform is increased presumably due to the toroidal current. The location of the ι/2π = 0.50 rational surface was determined to be ρ = 0.8-0.9 by soft X-ray (SX) fluctuations related to the MHD mode. An equilibrium calculation considering the toroidal current showed that the increase in the rotational transform due to the toroidal current was consistent with experimental results. The resonant mode structure was also investigated in an ECH + counter-NBI plasma at the ι/2π = 0.50 configuration. The location of the ι/2π = 0.50 rational surface, as determined by SX signals, did not change significantly compared with that obtained under a vacuum configuration. There is no significant difference between the rotational transform profile that considers toroidal currents by equilibrium calculation and that of the vacuum configuration. These results suggest that the change in the rotational transform profile caused by the toroidal current was small, owing to the balance between the bootstrap current and counter-flowing NB driven current.

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Plasma dynamics and structure are studied using a one-dimensional theoretical model for the anomalous transport diffusivities. In this analysis, the high collisional Pfirsch-Schlüter regime is examined and the anomalous particle diffusivity is employed. The reduction of the anomalous particle diffusivity and a steep gradient in the density profile can be obtained. This prediction may be the theoretical explanation for the internal diffusion barrier observed in super dense core plasmas of large helical device.

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Density profiles in LHD were measured and particle transport coefficients were estimated from density modulation experiments in LHD. The dataset used in this article included a wide range of discharge conditions, e.g., for different heating powers, magnetic axes, and toroidal magnetic fields scanned to cover wide regions for neoclassical transport. The minimized neoclassical transport configuration in the dataset (R_ax = 3.5 m, B_t = 2.83 T) showed peaked density profiles, and its peaking factors increased gradually with decreasing collisionality. These results are similar to those observed in tokamaks. At some other configurations, peaking factors were reduced with decreasing collisionality and a larger contribution of neoclassical transport produced hollow density profiles. Comparison between neoclassically and experimentally estimated particle diffusivities showed different minimum conditions. This suggests that the condition for neoclassical optimization is not the same as that for anomalous optimization. A clear difference in spatial profiles of turbulence was observed between hollow and peaked density profiles. A major part of the fluctuations existed in the unstable region of the linear growth rate of the ion temperature gradient mode and trapped electron mode.

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Profile stiffness is quantified using a simple technique. The approach is tested on a paradigmatic numerical stiff transport model for one field (particles). The stiffness is found to exhibit radial structure and to depend on collisionality, which might help explaining the observed lack of stiffness in stellarators, as compared to tokamaks. The extension of this approach to heat transport requires some care. A proposal for a stiffness quantifier for heat transport is presented, and tested on data from the TJ-II stellarator.

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Density fluctuations are analyzed in high volume average beta and high core density discharges in the Large Helical Device (LHD) using a 2D phase contrast imaging system and far infra-red interferometer. Both these regimes share similarly high beta gradients and evidence of pressure driven MHD modes is presented. In high volume average beta plasmas, both large and ion-gyro scale density fluctuation levels increase with beta and, in the edge, compare favorably with growth rate of resistive interchange modes, showing additional dependence on density at fixed β. In high core density plasmas with internal diffusion barrier, intermittent fluctuation bursts around mid radius are observed which are triggered when the normalized density gradient exceeds a certain threshold. The intermittent character is stronger for outward shifted plasmas and there appears to be a fluctuation suppression mechanism, possibly related to temperature gradient.

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A computer code, which was written to investigate flute modes and related transport, can be applied to a magnetic shearless confinement system as well as a tandem mirror. Computer simulation conducted in a modeled magnetic divertor with initial rigid plasma rotation shows that flute modes enhance radial transport during its growing phase, and shear (zonal) flows appear in the final stage.

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Here we propose an algebraic analysis approach for multibody Coulomb interactions. The momentum transfer cross section calculated by the algebraic approximation is close to the exact one. The CPU time required for the algebraic approximation is only about 20 min using a personal computer, whereas the exact analysis requires 15 h to integrate the entire set of multibody equations of motion, in which all the field particles are at rest.

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The creation of holes and clumps in an energetic ion energy spectrum associated with Alfvén eigenmodes was examined using the neutral particle analyzer (NPA) on the LHD shot #47645. The difference in slowingdown times between the holes and clumps suggested that the energetic ions were transported over 10% of the plasma minor radius. The spatial profile and frequency of the Alfvén eigenmodes were analyzed with the AE3D code. The phase space structures of the energetic ions on the NPA line-of-sight were investigated with Poincaré plots, where an oscillating Alfvén eigenmode was employed for each plot. The phase space regions trapped by the Alfvén eigenmodes appeared as islands in the Poincaré plots. The radial width of the islands corresponded to the transport distance of the energetic ions. Since island width depends on Alfvén eigenmode amplitude, it was found that Alfvén eigenmodes with amplitude δB_r/B ∼ 10^-3 transported energetic ions over 10% of the minor radius.

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Ion cyclotron range of frequency (ICRF) wave propagation and absorption are investigated using TASK/WM, in which Maxwell's equation for an RF wave electric field with a complex frequency is solved as a boundary value problem. The wave propagation is solved in the tokamak (JT-60U) and helical (LHD) configurations in the minority ion heating regime. Magnetic flux coordinates based on the MHD equilibrium in LHD were obtained using the VMEC code. A new model for the radial extension of the magnetic coordinates is applied to improve the numerical error near the plasma-vacuum boundary. The ICRF wave propagation and absorption are clearly seen at the ion cyclotron resonance layer and two-ion-hybrid layers with a high spatial resolution.

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In the compact helical system (CHS), electron heating by electron Bernstein waves (EBWs) was experimentally investigated to study a technique for high-density plasma heating over cutoff density. The EBWs are excited through a mode conversion process by X-mode waves injected to plasmas from the high-field side. In the experiment, within the range of an oblique EC-wave beam injection angle, evident heating effect was observed. The dependences of the heating effect on the wave's toroidal injection direction and polarization show that the absorption of the mode-converted EBWs should be the cause of the plasma heating effect.

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An active current drive is a promising technique for improving plasma performances by controlling rotational transform and/or magnetic shear profiles in helical devices. A current drive based on electron cyclotron resonance heating is the most appropriate scheme for this purpose in terms of locality of driven current. Optimum conditions for an efficient electron cyclotron current drive (ECCD) in Large Helical Device (LHD) are being investigated using three-dimensional ray-tracing code, which can simulate propagation and power dissipation of electron cyclotron waves with large parallel refractive index. In the present experiment, inversion of directions of driven plasma current corresponding to injected ECCD modes was demonstrated successfully, and the results could be elucidated by the Fisch-Boozer theory. In addition, clear shifts of rotational transform were observed by motional Stark effect polarimetry. Our findings verified that the ECCD can be used as an effective actuator for controlling the rotational transform and magnetic shear profile in LHD.

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Studies on slow-wave devices with a novel disk cathode and two types of rectangular corrugation are reported. The beam voltage is weakly relativistic, which is less than 100 kV. A disk cathode can generate a uniformly distributed annular beam in the weakly relative region. Rectangular corrugation having the ratio of the corrugation width to the periodic length of 50 or 20% is used. Using the former, output powers of about 200 kW are obtained at around 100 kV. For the latter, the effect of slow cyclotron resonance is observed in the low-energy region of around 30 kV. Output powers of slow cyclotron masers are in the range of a few hundred W. The operation mode of a slow cyclotron maser can be controlled between axisymmetric and nonaxisymmetric modes by changing the end condition of rectangular corrugation.

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Poloidal and toroidal electric fields generated by electron cyclotron heating are studied in a helical plasma. A linearized Fokker-Planck equation is solved by the adjoint method, assuming a helically symmetric configuration for simplicity. It is found that the poloidal and toroidal electric fields are generated near the bottom of the magnetic ripple, and that the larger radial flux is enhanced in a helical plasma compared with that in a tokamak plasma.

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Efficient heating at the third-harmonic electron cyclotron resonance was attained by injection of millimeterwave power with 84 GHz frequency range at the magnetic field strength of 1 T in LHD. The electron temperature at the plasma center clearly increased, and the increment in the temperature reached 0.2-0.3 keV. The dependence of the power absorption rate on the antenna focal position was investigated experimentally, showing that the optimum position was located in the slightly high-field side of the resonance layer. Ray-tracing calculation was performed in the realistic three-dimensional magnetic configuration, and its results are compared with the experimental results.

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Time-dependent simulation of neutral-beam-heated LHD plasmas has been carried out using the Toroidal Transport Analysis Linkage (TOTAL) simulation code focusing on the time evolutions of beam energy and kinetic energy. This code consists of three-dimensional equilibrium VMEC with bootstrap currents and a onedimensional transport HTRANS with neoclassical loss determined by ambipolar radial electric field as well as anomalous transport. Neutral beam deposition is calculated using the Monte Carlo code HFREYA, and the slowing down process was calculated using the Fast Ion Fokker-Plank code FIFPC. The simulated time evolution of total energy, including beam energy, roughly agrees with the time evolution of the experimentally measured energy. The temporal change in the beam velocity distribution is also clarified.

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A scintillator-based lost fast-ion probe (LIP) was designed and constructed to measure energetic-ion-losses caused by energetic-ion-driven MHD instabilities such as Alfvén eigenmodes in the Large Helical Device (LHD). It provides information about the pitch angle and gyroradius of the lost energetic ions simultaneously. It is installed at the outboard side of a horizontally elongated poloidal cross section to detect co-going energetic ions. Two apertures of the LIP were designed to detect energetic ions, whose pitch angle and gyroradius are 35-50 degrees and 1.5-15 cm at the detector position, respectively. The scintillator P46 was adopted because it has high luminosity even in a high temperature environment. The scintillation light emitted from the scintillator surface by the impact of lost energetic ions is detected using an array of photomultiplier tubes with high time-resolution (up to 200 kHz) and an image-intensified C-MOS camera (up to 2 kHz).

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A reliable method to evaluate the probability density function of escaping atom kinetic energies is required for analyzing neutral particle diagnostic data used to study the fast ion distribution function in fusion plasmas. In this paper, digital processing of solid state detector signals is proposed as an improvement of the simple histogram approach. Probability density function of kinetic energies of neutral particles escaping from plasma has been derived in a general form, taking into consideration the plasma ion energy distribution, electron capture and loss rates, superposition along the diagnostic sight line, and the magnetic surface geometry. A pseudorandom number generator has been realized to simulate a sample of escaping neutral particle energies for given plasma parameters and experimental conditions. Empirical probability density estimation code has been developed and tested to reconstruct the probability density function from simulated samples assuming Maxwellian ion energy distribution shapes for different temperatures and classical slowing down distributions with different slowing down times. The application of the developed probability density estimation code to the analysis of experimental data obtained by the novel Angular-Resolved Multi-Sightline Neutral Particle Analyzer has been studied to obtain the suprathermal particle distributions. The optimum bandwidth parameter selection algorithm has also been realized.

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In higher harmonic ion cyclotron resonance heating using the fast wave, a helium resonance layer appears near the plasma core. It is very important to detect the helium ions to investigate the confinement of α particles, which are produced by a nuclear reaction in ITER or a fusion reactor. In Large Helical Device (LHD), we attempt to observe the charge-exchange helium particles using a compact neutral particle analyzer (CNPA). Helium acceleration below 5 keV can be confirmed by comparing the signal ratio of helium in adjusted plate voltages of the CNPA to that of hydrogen. Successful helium measurement in LHD will lead to the development of α particle measurement.

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Carbothermic reduction of magnetite in a 2.45-GHz microwave multimode furnace was investigated with multipoint pyrometric and spectroscopic measurements. Both experimental results emphasize the importance of surface heating of the specimen by microwave-generated plasma for reducing iron oxide. The pyrometric observation shows a shift of the heating mode from the direct volumetric heating by microwave to the surface heating by microwave-generated plasma, when the temperature of the material suddenly rises from ∼800 to ∼1000 °C accompanied by light emission from plasma. The emission spectrum in the near-UV range (240-310 nm) changes drastically from a continuous spectrum to a line emission spectrum of iron, representing progress of carbothermic reduction of iron oxide. The multipoint spectroscopic observation indicates extensive carbothermic reduction occurring on the entire upper surface of the specimen.

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Radial profiles of K_α X-ray spectra of metallic impurities have been measured using an assembly of soft X-ray pulse-height analyzers in Large Helical Device. The assembly is quipped with a radial scanning system, which gives us detailed profile information, especially in long pulse discharges. The local emissivity profiles of the Fe-K_α X-ray spectra have been quantitatively obtained by an Abel inversion technique. The density profile of the iron impurity in each charge state is analyzed using the energy shift profiles of the Fe-K_α spectra with the help of an impurity code analysis. As an example of the present method, the iron density of 1.2× 10⁹ cm^-3 in the plasma center is obtained at the line-averaged electron density of 2.6× 10¹³ cm^-3.

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