Gyrokinetic theory and simulation results are presented to investigate regulation of ion temperature gradient (ITG) turbulence due to E × B zonal flows in helical systems. In order to examine effects of changes in helical magnetic configuration on anomalous transport and zonal flows, magnetic field parameters representing the standard and inward-shifted configurations of the Large Helical Device (LHD) [O. Motojima, N. Ohyabu, A. Komori, et al., Nucl. Fusion 43, 1674 (2003)] are used. The linear gyrokinetic analyses show that the largest growth rate of the linear ITG instability for the inward-shifted configuration is slightly higher than that in the standard one while, as theoretically predicted, zonal flows generated by given sources keep larger values for longer time for the inward-shifted case because of a smaller safety factor, a lower aspect ratio, and slower radial drift velocities of helical-ripple-trapped particles. It is shown from the gyrokinetic Vlasov simulation of the ITG turbulence that, in spite of the higher ITG-mode growth rate, the inward-shifted plasma takes a smaller average value of the ion thermal diffusivity in the steady turbulent state with a higher zonal-flow level. These results imply that neoclassical optimization contributes to reduction of the anomalous transport by enhancing the zonal-flow level and give a physical explanation for the confinement improvement observed in the LHD experiments with the inward plasma shift. When equilibrium radial electric fields produce poloidal E × B rotation of helically-trapped particles with reduced radial displacements, further enhancement of zonal flows and resultant transport reduction are theoretically expected.

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We have developed a novel critical current and stability measurement experimental setup, which utilizes a closed electric circuit with a multi-strand superconducting cable. The feature of this setup is mechanical loading applied to the multi-strand cable in the transverse direction. It was reported that Lorentz forces caused degradation in the critical current of the ITER-TFMC conductor. Furthermore, these phenomena were mainly observed in the ITER full-size conductors with large Lorentz forces under high magnetic fields. The advantage of our setup is critical current measurement under mechanical stresses comparable to those in the full-size conductor under high magnetic fields. By employing an inductive critical current measurement technique, we conducted an experiment with a transport current of about 10 kA without any power supply or current leads. In our experiments, we observed significant degradation in critical currents due to a compressive stress of about 30 MPa. We applied an innovative technique to mitigate the critical current degradation in mechanically loaded Nb₃Sn superconducting multi-strand cables. We molded one such cable with ice and tested it. No degradation occurred in the icemolded cable. In addition, stability was also ensured due to the large thermal conductivity of ice. Thus, we have successfully mitigated the degradation in the critical current of the Nb₃Sn conductor by ice molding.

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We investigated the plasma conditions for obtaining highly efficient extreme ultraviolet light from laserproduced tin plasmas for lithography of next generation semiconductors. Based on accurate atomic data tables calculated using the detailed configuration accounting code, we conducted 1-D radiation hydrodynamic simulations to calculate the dynamics of tin plasma and its emission of extreme ultraviolet light. We included the photo-excitation effect in the radiation transport. Our simulation reproduced experimental observations successfully. Using our verified code, we found that a CO₂ laser can be useful in obtaining higher conversion efficiencies up to 4%.

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Linear plasmas allow multi-point, low-temperature measurements with Langmuir probes. We measured ion saturation-current fluctuations of a Large Mirror Device-Upgrade linear plasma using a poloidal Langmuir probe array. By varying the discharge conditions, the spatiotemporal behavior showed a change from a coherent sine wave to a turbulent waveform through a periodic, modulated sine wave. The two-dimensional (poloidal wave number and frequency) power spectrum for each regime showed a single fluctuation peak, a peak and its harmonics, and a number of peaks in the poloidal wave number-frequency space. Bi-spectral analysis was performed for the turbulent regime, and showed the existence of nonlinear couplings among fluctuation peaks and broadband components.

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Experimental parameters of fine particle plasmas, densities and temperatures of components and the fine particle size, are explicitly expressed in terms of dimensionless characteristic parameters, such as Coulomb coupling and strength of screening, and typical examples are given. Though characteristic parameters are readily obtained from experimental parameters, the reverse is not trivial and it is shown that the charge neutrality condition limits the characteristic parameters realized by fine particle plasmas within the domain (a/λ)² /(Γ/A) ≥ 3. Here, a and λ are the mean distance between fine particles and the screening length, respectively, Γ = (Qe)² /ak_BT_p is the Coulomb coupling of fine particles, and A = (n_eT_e + n_iT_i)/n_pT_p is the ratio of the ideal gas pressures of ambient plasma (of ions and electrons) and fine particles.

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Alfven resonance effects on the evolution of a magnetic island driven by an externally applied perturbation are investigated for rotating plasmas. The simulation results show the importance of Alfven resonance for obtaining a perturbed current profile and estimating a critical value of the external perturbation, beyond which the magnetic island grows rapidly. The nonlinear evolution of the externally driven magnetic island is also investigated at low and high viscosities ν. It is shown that the transition phase accompanying the secondary reconnection at the initial X-point in the driven magnetic island evolution occurs in low resistivity and viscosity plasmas.

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Radial density profiles are obtained by applying the database slice and stuck density reconstruction technique to measured line-integrated densities, using multi-channel far-infrared laser interferometer installed on the Large Helical Device (LHD). A novel method that considers flux surfaces in the reconstruction process has been developed and applied successfully to a pellet-injected discharge in the LHD, enabling the identification of transient density peaking due to an inward pinch and enhanced diffusion in the edge region. The relationship between the change in particle transport and measured turbulence information obtained using reflectometry was also studied.

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Ionization balance from F-like to bare ions of In and I is investigated as a function of trapping time by measuring extracted ions from an electron beam ion trap, the Tokyo-EBIT. The present experiment is the first clear demonstration of the temporal behavior of the very highly charged ions that have been produced in the EBIT. The growth rate of extracted H-like ions is compared with that of the intensity of the x-ray emission resulting from radiative recombination into the K-shell of H-like ions. The absolute number density of trapped ions was estimated from the intensity of the x-ray emission.

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Microwave imaging reflectometry (MIR) was developed in TPE-RX, one of the world's largest reversed field pinch (RFP) devices. The system optics are made of aluminum mirrors, Teflon lenses, and Plexiglas plates in order to reduce size. In this system, frequencies are stabilized so that noise can be reduced using narrow bandpass filters. A 4×4 2-D mixer array and phase detection system have also been developed. With this system, density fluctuations in the high-Θ RFP plasma, pulsed poloidal current drive (PPCD) plasma, and quasi-single helicity (QSH) plasma are observed in TPE-RX. This is the first MIR experiment in an RFP device.

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The conjugate variable method, an essential ingredient in the path-integral formalism of classical statistical dynamics, is used to apply the Hamilton-Lie perturbation theory to a system of ordinary differential equations that does not have the Hamiltonian dynamic structure. The method endows the system with this structure by doubling the unknown variables; hence, the canonical Hamilton-Lie perturbation theory becomes applicable to the system. The method is applied to two classical problems of plasma physics to demonstrate its effectiveness and study its properties: a non-linear oscillator that can explode and the guiding center motion of a charged particle in a magnetic field.

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A high density plasma source using helicon discharge was designed for the measurement of the population ratio of the metastable state and ground state in a helium neutral beam. The required density of the plasma source was investigated in terms of the rate coefficient relevant to the beam-plasma interaction. The charge-exchange for the metastable state is a dominant process in a beam-energy range of 30-300 keV for the beam attenuation, promising the measurement of the metastable fraction in the helium neutral beam using this plasma source. The first plasma with the electron density of 3.4 × 10¹² cm^-3 and the electron temperature of 10 eV was successfully produced in argon gas.

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Benchmark tests of two simulation codes used for studying microturbulence and energetic-particle dynamics in magnetic fusion plasmas are conducted on present-day parallel supercomputer systems. Both the codes achieved high efficiency on the Earth Simulator with vector processors, and showed good performance scaling on massively parallel supercomputers with more than 10,000 commodity processors. The benchmark results obtained indicated high adaptability of fusion plasma simulation codes to state-of-the-art supercomputer systems.

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A diagnostic method for measuring nonlinear evolution of a laser wakefield by multiple sidebands of Raman scattering using probe laser light has been reported. In this paper, particle-in-cell simulations are used to demonstrate the validity of this probing method. The influence of plasma density, pump laser intensity, propagation length, and nonlinearity of the wakefield on probe laser light has been investigated. In particular, when trapping and acceleration of electrons occurs, the wing structure of the spectrum of probe laser light indicates the existence of highly relativistic electrons from which the injection fraction of the accelerated electrons can be obtained. Thus, this diagnostic method can be employed to measure laser wakefields conveniently for various purposes.

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Recent progress in plasma performance and the understanding of the related physics in the Large Helical Device is overviewed. The volume-averaged beta value is increased with an increase in the neutral beam injection (NBI) heating power, and it reached 5.0% of the reactor-relevant value. In high-β plasmas, the plasma aspect ratio should be controlled so that the Shafranov shift would be reduced, mainly to suppress transport degradation and the deterioration of the NBI heating efficiency. The operational regime of a high-density plasma with an internal diffusion barrier (IDB) has been extended, and the IDB, which was originally found using the local island divertor, has been realized in the helical divertor configuration. The central density was recorded as high as 1 × 10²¹ m^-3, and the central pressure reached 130 kPa. Based on these high-density plasmas with the IDB, a new ignition scenario has been proposed. This should be a scenario specific to the helical fusion reactor, in which the helical ripple transport would be mitigated. A low-energy positive-NBI system was newly installed for an increase in the direct ion heating power. As a result, the ion temperature (T_i) exceeded 5.2 keV at a density of 1.2 × 10¹⁹ m^-3 in a hydrogen plasma. Transport analysis shows improvement of ion transport, and the T_i-increase tends to be accompanied by a large toroidal rotation velocity of the order of 50 km/s in the core region. The plasma properties in the extended operational regime are discussed from the perspective of a steady-state helical fusion reactor.

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This paper discusses the results from experiments in HSX testing the properties of a quasisymmetric stellarator. HSX is a quasihelical stellarator with minimal toroidal curvature and a high effective transform. The high effective transform was verified from passing particle orbits as well as from the magnitude of the Pfirsch-Schlüter and bootstrap currents. The passing particle orbit shift, the helical structure of the Pfirsch-Schlüter current and the direction of the bootstrap current were all consistent with the lack of toroidal curvature. Good agreement was observed between data from plasma currents obtained by a set of magnetic pick-up coils and the results of the V3FIT code. Good confinement of trapped particles was observed with quasisymmetry. These particles may be responsible for a coherent global MHD mode that was detected during ECH at B = 0.5 T. It was found that the breaking of quasisymmetry increased the hollowness of the density profile and the damping of plasma flow while decreasing the core electron temperature, in good agreement with neoclassical models. At 0.5 T, anomalous transport appeared unaffected by the degree of quasisymmetry; more work is need to understand if this still holds at 1.0 T. The experimental energy confinement time and electron temperature profile could be reproduced reasonably well with a combination of neoclassical transport and a modified Weiland model for ITG/TEM turbulence.

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This work surveys the main results concerning the effects of the rotational transform, its low order rational values and its shear on the confining properties of low shear devices. It is meant to promote further studies aimed at clarifying their role in future, reactor grade, devices. 1-D transport studies are encouraged as the effects of rotational transform on confinement appear to be of local nature. Low order rational values of the rotational transform are associated with both degraded and improved confinement, being the magnetic shear a plausible cause for the difference. Very small shear values are enough to avoid deletereous effects of the low order rationals in high rotational transform discharges, but further experiments are needed to elucidate whether there is a threshold shear that depends on the rotational transform itself.

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The impurity transport characteristics in the ergodic layer of the Large Helical Device (LHD) are analyzed using the 3D edge transport code (EMC3-EIRENE), in comparison with the experimental data. The 3D modeling predicts the impurity retention (screening) in the ergodic layer at the high-density plasma. It is found that the edge surface layer plays an important role in impurity retention, where the friction force significantly dominates over the thermal force. The line intensity measurements of CIII to CVI show consistent behavior with the modeling, indicating impurity retention in the ergodic layer. The applicability of the model for high-Z impurity is also discussed, and it is found that the experimental data is consistent with the results of edge transport modeling.

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Spatiotemporal correlation of heat transport and micro- to meso- scale or macro-scale coupling of plasma turbulence are investigated in LHD plasmas, where fast propagation of a cold pulse and a non-local temperature rise are observed. Evidence of a timescale shorter than the diffusion time and a spatial scale longer than the micro-turbulence correlation length between core heat fluxes and edge temperature gradients is found. At the same time, an envelope of turbulent density fluctuations is also found to be modulated. Observation of low frequency (≤ 2 kHz) amplitude modulation of density fluctuation suggests the existence of meso- to macro-scale turbulent structures in the plasma, in which a non-local temperature rise takes place. Relationships between the turbulence with long radial correlation length and the edge-core coupling of heat transport are discussed.

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The roles of momentum transport on the toroidal rotation velocity (V_t) profile and the properties of transport coefficients are found by transient momentum transport analysis. The perturbation technique enables us to evaluate the momentum diffusivity (χ_φ) and the convection velocity (V_conv), and to calculate V_t profiles driven by external torque input by neutral beams (NBs). The measured V_t profiles with and without the external torque input are almost reproduced by χ_φ and V_conv in low-β (β_N < 0.4) plasmas. At higher β, the local pressure gradient plays a role in determining the local value of intrinsic rotation velocity. Concerning the momentum transport, χ_φ increases with increasing heating power, and decreases with increasing plasma current (I_p). In H-mode plasmas, χ_φ is smaller than that in L-mode plasmas under similar experimental conditions. It is found that χ_φ, which is separated from the non-diffusive term increases with increasing heat diffusivity (χ_i), χ_φ/χ_i ∼ 1-3, and -V_conv increases with increasing χ_φ, V_conv/χ_φ ∼ -2.5 to -0.7 m^-1 , in H-mode plasmas.

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Electron cyclotron current drive (ECCD) experiments were conducted in the stellarator/heliotron (S/H) devices, such as Heliotron J, TJ-II, CHS, and LHD. Experimental results show that ECCD can be controlled by the power injection angle, absorption position and magnetic field structure. The current drive efficiencies are similar, γ = n_eI_ECR/P_EC = 8 - 16×10¹⁶ A/Wm² , ζ = 32.7n_eI_ECR/P_WT_e = 0.03-0.05, when the magnetic field ripple ratio is 0.93 < B_min/B_max < 1.0. The reversal of driven current direction is observed depending on the magnetic field ripple structure, indicating that the amplitude and direction of EC current is determined by the balance between the Fisch-Boozer effect and the Ohkawa effect, and that the Ohkawa effect is stronger in S/H devices compared with tokamaks. Control of net toroidal current by using ECCD is demonstrated; a net zero current state is attained by cancelling the bootstrap current.

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We explore the possibility of having a flux-expansion divertor in TJ-II. As a first step, the three-dimensional map of the particle flux has been obtained for two different plasma regimes using the code ISDEP, which computes the ion guiding-centre trajectories. We consider the particle trajectories rather than the field lines due to the fact that, in TJ-II, common ion orbits can separate from the field lines, and moreover the plasma electric field and the collisionality must be considered. We have chosen a configuration that presents flux expansion at given toroidal positions. We have estimated the heat and particle fluxes and checked that it is possible to reduce them strongly by intersecting the trajectories at a given zone of the space. Future studies, maybe including the creation of an ergodic zone, will determine the best strategy for intercepting the trajectories.

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The coherent MHD oscillation, which consists of the fundamental frequency of several kilohertz and its higher harmonics, (harmonic oscillation: HO) has been observed in Compact Helical System. HO consists of two pairs of harmonic series. One is located in the core region near the ι = 0.5 rational surface (denoted as “HO (core)”), the other is located in the edge region near the ι = 1.0 rational surface (denoted as “HO (edge)”). In the present study, bispectral analysis is applied to the fluctuation data, for which HO is measured by beam emission spectroscopy (BES) and using magnetic probes. The analysis has revealed that fundamental mode of HO in both the magnetic and core density fluctuations have phase correlation with the harmonics including fundamental oscillation, while HO in edge density fluctuation does not have such phase correlation. Mode numbers of HOs are identical for harmonic components having different frequencies, i.e., m/n = -2/1 for HO (core) and m/n = -1/1 for HO (edge). It suggests that the generation of harmonics cannot be interpreted simply as mode coupling because the summation rule for the wavenumber is not satisfied, even though the bicoherence value is significant. The bicoherence value and relative amplitude of higher harmonics correlate with each other, which suggests that bicoherence indicates the degree of distortion of the signals.

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Plasma flow velocity shears parallel and perpendicular to magnetic field lines are independently controlled and superimposed by a modified plasma-synthesis method with concentrically three-segmented electron and ion emitters. The fluctuation amplitude of a drift wave with an azimuthal mode number m = 3 is observed to increase with increasing parallel shear strength in the absence of a perpendicular shear. When the perpendicular shear is superimposed on the parallel shear, the drift wave with m = 3 is found to transform into that with m = 2. Furthermore, the parallel shear strength required for excitation of the drift wave increases with a decrease in the azimuthal mode number. Based on these results, superposition of the parallel and perpendicular shears can affect characteristics of the drift wave through variation of the azimuthal mode number. These phenomena can be verified by theoretical calculations of the growth rate of the drift wave using an eigenmode analysis.

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We estimate the electron angular velocity shear ∂_rω_θo, which can be formed by plasma heating near the low-order rational surface with a poloidal chain of magnetic islands. We suppose that the plasma is heated sufficiently that its electrons start to miss the magnetic islands during their radial collisional shift and movement along the toroidal surface. This provides an ion volume charge in some regions of magnetic islands, which leads to shear formation. The time taken for shear formation is short. The conditions for magnetic island width leading to the shear are derived. It is shown that even narrow magnetic islands can lead to the shear. The shear can damp instabilities with a growth rate smaller than the ion cyclotron frequency. The spatial structures of convective vortical cells are described. We derive inverse dependences of the radial width of excited vortices on ∂_rω_θo and radial gradient of plasma density ∂_rn_0e. Amplitude of electron radial oscillations is smaller for larger ∂_rω_θo and ∂_rn_0e. These dependences promote a steep radial distribution of the plasma density and internal transport barrier.

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A perpendicular neutral beam (P-NB) injector was installed in the Large Helical Device (LHD) and utilized for high-power ion heating and measurement of the ion temperature (T_i) profile by charge exchange spectroscopy. Significant progress was achieved in ion heating experiments using P-NB, and a high T_i of 5.2 keV was achieved at the plasma center. A T_i peak profile with a steep gradient in the core region was observed, indicating an improved mode of ion transport. The enhancement of toroidal flow in the core region was observed with the T_i rise. Transport analysis shows improvement in anomalous transport in the core region, and the neoclassical transport remains unchanged when the high T_i is realized.

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In order to investigate the formation of toroidal flow in helical systems, both NBI driven flow and spontaneous toroidal flow were observed in Large Helical Device (LHD). The toroidal flow driven by NBI is dominant in plasma core while its contribution is small near plasma edge. The spontaneous toroidal flow changes its direction from co to counter when the radial electric field is changed from negative to positive at plasma edge. The direction of the spontaneous toroidal flow due to the radial electric field near plasma edge is observed to be opposite to that in plasma core where the helical ripple is small.

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Plasma rotation or its shear is important in the formation of a transport barrier. It is believed that a rotating helical magnetic field generated by a dynamic ergodic divertor (DED) can generate a rotational torque in tokamak plasmas, and therefore control the rotation profiles. To measure the plasma rotations and investigate the effect of DEDs on them, we developed a passive spectroscopic measurement system for the small tokamak HYBTOK-II. A spontaneous toroidal plasma rotation in the co-current direction and poloidal plasma rotation in the electron diamagnetic drift direction have been observed without using a DED. Considerable changes in the plasma flow have been obtained using a DED for reducing toroidal and poloidal rotation velocities near the resonant magnetic surface. The change in the plasma rotation velocity was found to couple with the change in the radial electric field.

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In H-mode plasmas, two-dimensional steep structures of the electrostatic potential and density are formed when a large poloidal flow exists, whose formation mechanism has been studied to obtain a quantitative understanding of the particle transport in H-mode transport barriers. The previous two-dimensional model is extended to investigate parallel flow dynamics when potential and density distributions do not satisfy the Boltzmann relation. The extended model includes the generation mechanism of a poloidal shock structure and geodesic acoustic mode, whose competitive formation can be studied.

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Geodesic acoustic modes (GAMs) in the multi-ion system are investigated. It was found that the high-frequency branch decreases with increase in effective ion mass. The low-frequency branch (ion sound wave, ISW) of the damping rate also becomes small. The ratio between the damping rate of GAM and ISW is found to become order of unity in the region of q < 4 (q is safety factor).

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Fast-flowing plasmas in supersonic and super-Alfvénic regime are generated in combined experiments of ion cyclotron resonance heating (ICRH) and acceleration in a magnetic nozzle. During radio-frequency (RF) wave excitation in a fast-flowing plasma produced by a magnet-plasma-dynamic arcjet (MPDA), strong ion cyclotron heating is clearly observed. Thermal energy in the heated plasma is converted into flow energy in a diverging magnetic nozzle, where the magnetic moment μ is nearly kept constant. Plasma flow energy can be controlled by changing the input RF power and/or modifying the magnetic nozzle configuration. In a strongly diverging magnetic nozzle, an Alfvén Mach number as well as ion acoustic Mach number are more than unity, that is, supersonic and super-Alfvénic plasma flow is realized.

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The high-order structure functions have been analyzed to characterize the edge plasma intermittency in fusion devices. The scaling properties of edge turbulence have shown a strong deviation from a prediction of the Kolmogorov's K41 model. The turbulent fluctuations demonstrate a generalized scale invariance and log-Poisson statistics.

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The cherenkov and slow cyclotron instabilities driven by an axially injected electron beam in a cylindrical waveguide are studied using a new version of the self-consistent linear theory considering three-dimensional beam perturbations. There are three kinds of models for beam instability analysis, which are based on a cylindrical solid beam, an infinitesimally thin annular beam, and a finitely thick annular beam. Among these models, the beam shape properly representing the often used actual annular electron beams is the finitely thick annulus. We develop a numerical code for a cylindrical waveguide with a finitely thick annular beam. Our theory is valid for any beam velocity. We present eigen-modes of the cylindrical system with the plasma and beam. Instabilities driven by the annular beam in a dielectric-loaded waveguide are also examined.

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In the LMD-U linear magnetized plasma, fluctuation measurements have been performed with multi-channel poloidal Langmuir probe arrays to investigate the spatiotemporal behaviors of drift wave turbulence. Two-dimensional (poloidal mode number-frequency) power spectrum showed not only fluctuation peaks but also the existence of a broadband fluctuation. The broadband fluctuation developed at high poloidal mode number and high frequency regions and not along the linear dispersion relation curve of drift wave modes. It showed shorter correlation time and poloidal length than the peaked fluctuations. Two-dimensional axial coherence was measured with two poloidal probe arrays separated in the axial direction. The axial coherence was strong for both the broadband fluctuation and peak fluctuations, suggesting the quasi-two-dimensional structure of the drift wave turbulence.

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Pure electron plasmas and electron plasmas with a finite ion fraction have been studied in the Columbia Nonneutral Torus (CNT) since the end of 2004. Results from the first three years of operation are summarized. Stable, small Debye length pure electron plasmas are routinely created, and have confinement times up to 20 msec. The confinement is limited by radial transport caused by internal rods, as well as electron-neutral collisions. The neutral driven transport rate is indicative of poor particle orbits in CNT, despite the strong radial electric field. Numerical simulations shed light on this issue, demonstrating the detrimental effects of variations in the electrostatic potential on a magnetic surface. With the installation of a magnetic surface conforming electrostatic boundary and the transition to external diagnostics, significantly longer confinement times should be possible. Also presented are observations of sudden confinement jumps that have a hysteretic behavior, and observations of an ion driven instability.

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Using a fast camera with tangential view, the motion of the filamentary structures in peripheral regions was observed. In the L-mode, the filamentary structures are relatively thinner than those of the H-mode. The direction of filamentary structure motion in the H-mode plasma was changed opposite to that in the L-mode plasma, and the motion speed was also doubled. During L-H transition, this motion was stopped. If this motion is poloidal rotation due to E_r × B drift, E_r should be positive in the L-mode plasma and negative in the H-mode plasma.

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Identification of radionuclides obtained from deuterium experiments and evaluation of dose rate level were performed on the structural materials of the Large Helical Device and the Experimental Hall. Energies of neutron sources are 2.45 MeV (D-D reaction) and 14 MeV (D-T reaction). Neutron fluence was calculated using the two-dimensional transport code DOT-3.5. Generation of radionuclides was calculated using the CINAC code. Radionuclides of ^93mNb, ⁶³Ni, and ⁶⁰Co for helical coils, ⁵⁵Fe and ⁶⁰Co for stainless steel, ⁵⁵Fe, ⁶⁰Co, and ^93mNb for poloidal coils, and ⁴⁰K and ⁵⁵Fe for floor concrete were dominant after a series of experiments with deuterium gases. Evaluation of dose rate level for the structural materials and air were performed taking into account a current experimental schedule.

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A Magnetron is a high efficiency microwave source, although the energy conversion efficiency of a pulsed-power magnetron with a relativistic electron beam is less than that of a non-relativistic magnetron. We studied the possibility of increasing the energy conversion efficiency of a high-power magnetron using a transparent cathode. The conversion efficiency is controlled by the resonance efficiency between the electron beam and microwave oscillation, and initial rise time of the oscillation. In particular, the initial rise time of interaction with the pulsed-power generator is important. The transparent cathode can lower start-up times and enhance the oscillation efficiency. It consists of independent cathode strips, each of which produces an azimuthal magnetic field. The radial drift velocity of electrons emitted from this cathode is accelerated more than that of electrons emitted from a normal cathode. In particle-in-cell electron simulation, the availability of the transparent cathode was indicated. We investigated the experimental effect of the transparent cathode. The experimental setup of the relativistic magnetron is operated with “ETIGO IV,” which is a 400-kV-class repetitive pulsed-power generator. The start-up time of magnetron with the transparent cathode is shorter than that with a traditional cathode. We expect that the transparent cathode method will be advantageous over the current method.

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Radial electric fields are closely related to the confinement of plasma. Electrode biasing experiments were conducted to actively control the radial electric field by ramping up/down the electrode current in the Tohoku University Heliac (TU-Heliac). An emissive probe array consisting of three filaments was designed as a new type of measurement equipment. Using this probe, time evolutions of the spatial structure of the radial electric field were measured in the biasing experiments. The radial electric field tended to extend from the inner region to the outer region, while ramping up the electrode current. In the plasma inner region, the radial electric field was maintained for a longer time than in the outer region, while ramping down the electrode current. The radial electric field, poloidal flow of plasma, and fluctuation level of the ion saturation current changed appreciably in the period when the electrode voltage showed negative resistance against the electrode current. Therefore, the period when the plasma shows negative resistance agreed with the transition region to the improved confinement mode.

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A new method for rotating magnetic islands by external perturbation fields is proposed. In the experiments, perturbation fields were produced using four pairs of cusp field coil, in which alternating currents having a π/2 phase shift flowed. A phase shifter for the coil currents was designed and constructed. The phase difference in the floating potential signals measured using two Langmuir probes confirmed that the magnetic islands rotated in the counterclockwise direction (c/cw). The clockwise (cw) rotation was also observed in the plasma biased by the hot cathode electrode. These experimental results suggest the ability of producing plasma poloidal rotation driven by rotating islands.

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Density clamping or pump-out phenomena is observed both in tokamaks and helical systems during additional heating, particularly high power density heating as electron cyclotron resonance heating (ECRH). Enhanced electron convective flux induced by a perpendicular acceleration of electrons is one of the candidates of the mechanism of these phenomena. The mechanism of the enhancement of the electron flux is investigated experimentally by comparing the density profile evolutions due to second harmonic X (X2) mode ECRH at the ripple top and bottom. The result suggests that ECRH-induced electron flux is one of the dominant mechanisms of density clamping, which can be used as a powerful tool for controlling the particles and the heat transport.

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In nuclear fusion plasmas, both thermal energy and particle transports governed by plasma turbulence are anomalously enhanced above neoclassical levels. Plasma turbulence induces various complex phenomena in transport processes, such as nonlinearity and nonlocality. Therefore, it is very important to clarify the relationship between plasma turbulence and anomalous transports. We have approached these complicated problems by analyzing the dynamics, which are recognized as temporal trajectories in a flux-gradient space, rather than using conventional power balance. In particular, in fusion research, it is critical to elucidate the mechanism of electron thermal energy transport, because the incoming burning plasmas are sustained by the heating of alpha particles. In Large Helical Device (LHD), the dynamic relationships between electron thermal fluxes and electron temperature gradients are investigated using modulated electron cyclotron heating and modern electron cyclotron emission diagnostic systems. Some trajectories, such as a hysteresis loop and a line segment with a steep slope, are observed in high-temperature LHD plasmas. Strong nonlinear properties in the transport are revealed by studying the dynamics.

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In helical systems, neoclassical transport is one of the important issues in addition to anomalous transport, because of a strong temperature dependency of heat conductivity and an important role in the radial electric field determination. Therefore, the development of a reliable tool for the neoclassical transport analysis is necessary for the transport analysis in Large Helical Device (LHD). We have developed a neoclassical transport database for LHD plasmas, DCOM/NNW, where mono-energetic diffusion coefficients are evaluated by the Monte Carlo method, and the diffusion coefficient database is constructed by a neural network technique. The input parameters of the database are the collision frequency, radial electric field, minor radius, and configuration parameters (R_axis, beta value, etc). In this paper, database construction including the plasma beta is investigated. A relatively large Shafranov shift occurs in the finite beta LHD plasma, and the magnetic field configuration becomes complex leading to rapid increase in the number of the Fourier modes in Boozer coordinates. DCOM/NNW can evaluate neoclassical transport accurately even in such a configuration with a large number of Fourier modes. The developed DCOM/NNW database is applied to a finite-beta LHD plasma, and the plasma parameter dependences of neoclassical transport coefficients and the ambipolar radial electric field are investigated.

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An increase in ion temperature has been observed with superposition of centrally focused electron cyclotron resonance heating (ECRH) to plasmas heated by high-energy neutral beam injection (NBI) in Large Helical Device. The ion-temperature (T_i) rise is accompanied by the formation of electron internal transport barrier (ITB). A transport analysis shows that ion transport as well as electron transport is improved with the reduction of anomalous transport. A neoclassical ambipolar flux calculation shows a positive radial-electric field (E_r) in the region of the T_i rise, and E_r should suppress the enhancement of ripple transport due to the T_i-rise. These analyses indicate the ion transport improvement in the core electron-root confinement plasmas. Toroidal rotation is driven in the co-direction by applying ECRH, and the toroidal rotation velocity is increased with the T_i rise. A correlation between the T_i rise and toroidal rotation is suggested.

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Effect of ellipticity on thermal transport was investigated in ECH plasmas in LHD. Ellipticity κ is scanned from 0.8 to 1.4 by controlling the quadrupole magnetic field. Experimental data of energy confinement time align with the scaling for all configurations; however, there exist systematic offsets. Performance τ_E^exp /τ_E^ISS04 is summarized as 0.94 ± 0.02 for κ = 0.8, 1.41 ± 0.07 for κ = 1.0, and 0.91± 0.03 for κ = 1.4. Local transport analysis based on power balance indicates that the anomalous transport predominates the plasma transport. However, the observed anomaly shows correlation with the change in the effective helical ripple ε_eff. Physical background of this correlation and the dependence on the poloidal viscous damping rate C_p is discussed. The present experimental comparison suggests a negative evidence for the relevance of C_p.

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A stable path to high-beta plasmas is investigated for an inward-shifted LHD configuration. An improvement of Mercier stability is observed due to the nonlinear saturation of the resistive interchange modes. For this study, a multi-scale numerical scheme is used. In this scheme, the beta value is increased by adding a small pressure increment to the background pressure. We focus on the dependence of the Mercier stability on the profile of the pressure increment. It is found that the total pressure profile approaches a profile marginally stable to the Mercier criterion when the pressure increment has a fixed profile.

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Nonlinear magnetohydrodynamics (MHD) simulations are conducted for a helical plasma with a large pressure gradient to investigate the collapse event induced by MHD instabilities. The simulation results show that the ballooning-like instabilities on an intermediate spatial scale induce disordering of structures in the barrier region and a drop in the central pressure. It was revealed that the core pressure fall is related to disordering of the magnetic field structure. The simulation results are compared qualitatively with the experimental observations of the collapse events in the superdense core state of the Large Helical Device (LHD). Compared with our previous simulation studies of similar situations in the spherical tokamak (ST) plasma, the results for the helical cases show milder collapses than those in the ST case.

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Multidirectional diagnostics employing high-resolution atomic energy spectrometers [1, 2] are being used to study the ion component heating mechanisms and fast ion confinement in helical plasmas. Since the natural atomic flux source is not localized in contrast to the pellet charge exchange [3,4] or diagnostic neutral beam methods [5], the correct interpretation of such measurements in a complex toroidally asymmetric geometry requires careful numerical modeling of the neutral flux formation and knowledge of the charge exchange target distributions, relevant cross-sections, and the magnetic surface structure. The measured neutral flux calculation scheme for LHD geometry was given in [6], and the influence of the geometry effect on the interpretation of measured data was shown. The current method was applied for the simulation of the experimental signal of the angular-resolved multi-sightline neutral particle analyzer (ARMS-NPA) [1] along it's 20 sightlines in the LHD geometry configuration. In order to check the influence of the geometry effect on the interpretation of experimental results, calculations were conducted for the isotropic Maxwellian plasma-ion-energy probability density function. The behavior of the calculated and experimental ion spectra from neutral beam injector (NBI) is discussed.

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In this paper, pig iron production by microwave heating is experimentally investigated. In order to explore possible effects of the microwave frequency, identical mixtures of magnetite and carbon powder were processed in different microwave systems operating at 2.45 GHz and 30 GHz, respectively. The weight ratio of magnetite and carbon in the powder mixture was 89:11 weight%. According to the corresponding chemical equation, this should allow to produce pig iron that includes 2 weight% of carbon. High-quality pig iron was obtained in the 30 GHz heating system in air. At 2.45 GHz heating in nitrogen gas, pig iron was obtained. However, under similar conditions of 30 GHz heating system in air, FeO was mainly obtained. This result suggests that the chemical reduction of magnetite is more efficient at higher microwave frequencies.

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In order to observe edge neutral recycling for fusion machine operation, the absolute values of H_α and HeI emissions have been usually analyzed with measurement of edge density and temperature. On the other hand, it is known that the temperature and density dependences of each line can be negated by taking the ratio between the two lines. The intensity ratio of H_α (6563 Å) to HeI (5876 Å or 6678 Å) visible spectral lines is adopted, and the results are presented here instead of the method usually used for monitoring the edge particle recycling and the effect of wall conditioning in LHD. The ionization events per photon were calculated for both emissions using a collisional-radiative model, and the ratio of H⁺ flux (≡ H⁰ influx) to He⁺ flux (≡ He⁰ influx) was obtained. The H_α and HeI (5876 Å) emissions in LHD have been measured using a monitor assembly with an interference filter and optical fiber array. The H⁺ /He⁺ flux ratio has been thus evaluated by integrating the H_α and HeI emissions from a 10-channel toroidal array. As an example of the measurement, all discharges performed during the past 9 years in LHD have been analyzed. The present method shows that the aftereffects of the H₂ and He glow discharge cleaning for vacuum wall conditioning on the LHD discharges become clearly visible. The replacement of deposited atoms on the carbon divertor plates was also analyzed. It is found that the replacement is completed by 40-50 shot repetition of discharges. These results indicate the effectiveness of the present method as a good monitor for fusion machine operation.

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The distribution of strike points in a closed divertor configuration with additional baffle plates installed in the toroidal ends of closed divertor components near lower/upper ports is calculated by tracing magnetic field lines from the last closed flux surface in various magnetic configurations (radial position of the magnetic axis, R_ax = 3.50 ∼ 3.90 m). The calculation shows that the ratio of the number of strike points on additional plates is maximum for R_ax = 3.60 m, showing that the plates are effective in this magnetic configuration. Neutral particle transport is investigated using a fully three-dimensional code EIRENE with one-dimensional plasma fluid analysis of divertor legs, where the plasma parameter profiles on the legs are obtained by an iterative calculation including interaction processes between the plasma and neutral particles. The plates raise the pressure of molecular hydrogen locally near the baffle plates to more than 0.2 Pa, which is enough for efficient particle control using vacuum pumping systems installed near the plates. The simulation proposes one possible candidate of optimized closed divertor configuration for the three-dimensionally complicated magnetic structure in the LHD plasma periphery.

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