We investigated the interaction of a laser ablation plasma with a longitudinal magnetic field, intending to create a directional moving plasma for laser ion source. To study the plasma dynamics, time-of-flight measurements of ion flux were made as a function of laser intensity and the magnetic field. The results indicate that the ion current density in the forward direction is strongly affected by a moderate (∼0.2 T) magnetic field. Results also indicate that the longitudinal magnetic field can control the ion flux as well as degree of ionization of the moving plasma by reducing the transverse expansion and setting on the recombination process during the interaction.

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Intermittent pulse events have been observed in the periphery of the linear cylindrical plasmas produced in Plasma Assembly for Nonlinear Turbulence Analysis (PANTA). A novel combination of 32 azimuthal and 3 radial probes enables us to obtain 32 × 3 = 96 pairs of correlations simultaneously. The two-dimensional (2D) correlation analysis shows that the intermittent structure is well correlated with quasi-periodic bursts inside the plasma, and it reveals that the intermittent pulse is caused by the passage of a radially elongated and azimuthally distorted localized structure rotating in the ion diamagnetic direction.

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The static and dynamic behaviors of detached plasmas have received considerable attention because the use of a detached divertor is thought to provide a promising method for reducing the heat flux to plasma-facing components. In this study, fluctuations were measured with an electrostatic probe as the plasma was changed from attached to detached states by increasing the neutral gas pressure. The transition from an attached plasma to a detached plasma was found to change the phase relation between the density and the potential.

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Visible transitions of highly charged tungsten W^q+ have been observed with a compact electron beam ion trap for the charge-state range of q = 8 - 28 and the wavelength range of 365 - 475 nm. More than a hundred previously-unreported lines are presented, and the charge state of the ions emitting the lines is identified from the electron energy dependence of the spectra.

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Effects of a radial magnetic perturbation (MP) on electron cyclotron (EC) driven fast electrons are investigated in the Low Aspect ratio Torus Experiment (LATE) device. When a weak MP is applied to plasmas maintained solely by EC heating and current drive, plasma current and electron density increase, whereas energetic trapped electrons located outside the last closed flux surface (LCFS) on the low field side (LFS) decrease. The effect of MPs on energetic trapped electrons has an important role in improving EC-driven plasma discharges.

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Disruption is one of the most critical issues in tokamaks. A resonance magnetic perturbation (RMP) coil system will be installed in future tokamaks such as the International Thermonuclear Experimental Reactor to mitigate edge localized modes. In this study, the effect of RMP on tokamak disruption was investigated using the small tokamak device HYBTOK-II. It was found statistically that an externally applied RMP leads to faster current quench during disruption.

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Plasma breakdown using electron cyclotron waves with toroidally inclined launch has been studied experimentally in the stellarator/heliotron device, Heliotron J. The experimental results show that the second harmonic X-mode breakdown has a weak dependence on the parallel refractive index, N_ll, for the small toroidal injection angle range. When the fundamental resonance exists inside the vacuum chamber, the breakdown occurs quickly, independent of the toroidal injection angle.

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The interaction of an intense electron beam with a neutral background material is studied. The neutral material is ionized by the electrostatic field generated by the intense electron beam and electron impact ionization. The structure of the ionization front is analyzed using a one-dimensional model. The structure is determined primarily by electron impact ionization of the ionized background electrons. In addition, the field ionization contributes to the generation of the ionization front by increasing the density of the electron beam.

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This paper describes the design of a magnetron-injection gun for a 100 kW, 300 GHz gyrotron. With an increase in power and frequency, performance of the gyrotron becomes quite sensitive to the quality of the electron beam. Formation of a laminar electron flow is essential for the realization of a high quality beam with small velocity spread. In this study, a new method is proposed for the evaluation of the laminarity, and applied to the design optimization of the electrodes. It is found that the laminarity depends not only on the conventional design parameter of the cathode slant angle, but also on the spatial distribution of the electric field inside the beam.

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High power and high efficiency operation of 77 GHz gyrotrons was achieved by the stepwise raising of the anode voltage. In particular, a stable MOU output power of 1.8 MW was obtained for 1 s. The effect of beam-charge neutralization on the oscillation characteristics was examined. The intended beam acceleration voltage was not initially reached due to the space-charge effect but was achieved over time through the charge neutralization process. By applying the stepped anode voltage, the gyrotron operational parameters were able to be optimized for the sufficiently accelerated electron beam, leading to the improvement of the output power and the electric efficiency.

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High-power sub-terahertz pulse gyrotrons are under development in FIR FU for application to collective Thomson scattering (CTS) measurement on fusion plasmas, especially on high-density plasmas such as those produced in LHD. Recently, we achieved a new power record of 62 kW at approximately 388 GHz with second-harmonic (SH) oscillation. Following this result, we modified the electron gun of the gyrotron to couple the electron beam more strongly to another oscillation mode that has a peak coupling coefficient two times as large as that of the 62 kW mode. Oscillation tests with the new mode attained higher power of 83 kW at about 389 GHz. These results constitute new second-harmonic-oscillation power records for sub-terahertz gyrotrons.

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Microwaves generate ordered motion in material structures. Microwaves can transfer energy while maintaining coherency; this is the origin of the microwave effect. We suggest that the non-thermal energy path during microwave heating really exists and that a plasma model can explain the non-thermal enhanced-reduction in microwave heating.

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The new phenomenon of a spontaneous magnetic field in spatially inhomogeneous plasma is found. The criteria for instability are determined, and both the linear and nonlinear stages of the magnetic field growth are considered; it is shown that the magnetic field can reach a considerable magnitude, namely, its pressure can be comparable with the plasma pressure. Especially large magnetic fields can arise in hot plasma with a high electron density, for example, in laser-heated plasma. In steady-state plasma, the magnetic field can be self-sustaining. The considered magnetic fields may play an important role in thermal insulation of the plasma.

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We report the observation, in a particle-in-cell simulation, of a phenomenon in which a magnetized plasma absorbing an electron cyclotron wave is rendered transparent by a pump wave, which is a classical analog to electromagnetically induced transparency (EIT) in quantum physics. Evident mode coupling between the probe electron cyclotron wave and the pump wave driving the longitudinal plasma oscillation, which is required for the realization of EIT, is identified in addition to an observation of the electron cyclotron wave recapturing propagation in the magnetized plasma.

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In plasmas immersed in the toroidal field B_ϕ, electrons drift downward while ions drift upward due to the field gradient and curvature (B_ϕ > 0 is assumed). The plasma is usually bounded at the top and bottom by the conducting vessel walls. The same amount of current must flow into and out from the walls at the top and bottom, respectively, to complete the current circulation via the vacuum vessel. In an ECR plasma in the LATE device radial profiles of vertical charge separation currents have been for the first time measured by radially aligned multi-electrodes fabricated at the top and bottom. Both the profiles at the top and bottom are nearly the same as the profile 2P_e/RB_ϕ in the plasma as theoretically predicted. Current characteristics upon the sweep of external voltage onto the top ion collectors reveal that the current is due to inflow of ions with no secondary electron emission.

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The outward drift displacement of the pellet ablated material is studied for low-field side injection in the Large Helical Device (LHD). Stopping of the drift acceleration is shown to be mainly due to the formation of an internal current circuit owing to helical variation of the magnetic field gradient. This process is the most efficient for stopping the cross-field motion of the ablatant in the LHD because, in helical configurations, the parallel scale length of the gradient variation is shorter than in tokamaks. Simulated ablation and deposition profiles are shown to compare well with the H_α emission and post-injection density and temperature profiles.

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The particle kinetic effects play an important role in breaking the frozen-in condition and exciting collisionless magnetic reconnection in high temperature plasmas. Because this effect is originating from a complex thermal motion near reconnection point, it is very important to examine particle trajectories using scientific visualization technique, especially in the presence of plasma instability. We developed interactive visualization environment for the particle trajectories in time-varying electromagnetic fields in the CAVE-type virtual reality system based on VFIVE, which is interactive visualization software for the CAVE system. From the analysis of ion trajectories using the particle simulation data, it was found that time-varying electromagnetic fields around the reconnection region accelerate ions toward the downstream region.

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Following the atomic model based on the microfield in a plasma for bound states [Astrophysical Journal 532, 670 (2000)], I have considered an atomic modeling for computing the free-electron state-density based on the plasma microfield. In the atomic model based on the plasma microfield, it is considered that an ion in plasma is immersed in a uniform electric field that is the contribution of field values averaged over the other ions in the plasma. It has been expected a modeling for the free-state density consistent with its bound state, because the resulting free-state densities by the simple atomic model based on the plasma microfield has been found to be invalid. In this study, I have obtained a physically appropriate free-state density under the assumption that the large electric field component can be considered to exist due to the electric field originating from the nearest neighboring ion and the resulting potential around the ion shows mirror symmetry about the saddle point. The resulting state density is consistent with the experimental results. The inclusion of the free-state density has caused a slight deviation in the values of the average ionization degree of hydrogenic plasmas.

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This paper describes design and installation of a charge-exchange recombination spectroscopy (CXRS) system in Heliotron J. In this system, two tangential-heating neutral beams are used for plasma heating as well as the diagnostic beams. The sightlines are set to be parallel to the magnetic axis to achieve high spatial resolution. The spatial resolution is Δ‹r/a› ∼ 0.05 for a measurement area of 0.37< ‹r/a› < 0.79. We adopted a Czerny-Turner monochromator, whose dispersion is 0.74 nm/mm. We applied this system is applied to an NBI plasma and the time evolution of the ion temperature and the toroidal rotation velocity profile are successfully obtained.

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Characteristics of quasi-single-helicity (QSH) states have been studied in a low-aspect-ratio reversed field pinch (RFP) machine RELAX mainly by magnetic diagnostics. Internal profiles of the fluctuating radial, poloidal and toroidal magnetic fields have shown good agreement with eigenfunctions of a single helical mode. The edge magnetic fluctuation spectra are somewhat broader than what are expected from internal magnetic field profiles. In spite of these slight discrepancies, the usual measure for the QSH, the spectral index N_S lower than 2, still provides a reasonable measure for QSH states in RELAX. The QSH persistence has been improved in RELAX by the reduction of the poloidal resistance of flanges at poloidal gaps, mainly due to the improved axisymmetry of the toroidal magnetic field. QSH persistence more than 30% of the flat-topped current phase has been realized with current density lower than in other RFP, and probability of spontaneous QSH is 12.8 ± 7.3% which is higher than high-aspect-ratio RFP. It suggests the advantage of low-aspect-ratio configuration in attaining to the QSH. Comparison with recent experiments using active resonant perturbation may suggest the importance of further reduction of field errors to improve the quality of QSH.

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We have analyzed ion saturation current fluctuation measured by a fast scanning Langmuir probe (FSP) in edge region of the Large Helical Device (LHD). Positive and negative spikes of the ion saturation current were observed in the private region and on the divertor leg, respectively. It was found that the boundary position between these regions corresponds to the low-field side (LFS) edge of the divertor leg where the gradient of the ion saturation current profile was the maximum. Such a positional relationship resembles that near the separatrix in the LFS in tokamaks, where blobs and holes are generated. Statistical analysis indicates similar fluctuation characteristics among different magnetic devices.

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Low beta MHD equilibria including static magnetic islands generated by an external field are studied by using a two-step approach. The equilibria correspond to the reduced MHD equations in a straight heliotron configuration. In the first step, a diffusion equation parallel to the field line is solved with the magnetic field fixed for a solution of the pressure constant along field lines. In the second step, the equilibrium equation corresponding to the vorticity equation is solved with the pressure fixed for a solution of the poloidal magnetic flux. The two steps are iterated until the width of the island is converged. The equilibrium pressure profile as a result of the method is locally flat at both the O-point and the X-point of the magnetic island. Effects of a pressure diffusion perpendicular to the field are also studied. In this case, an equilibrium pressure of which the profile is flat at the O-point and steep at the X-point is obtained.

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Bootstrap current calculations with the neoclassical SPBSC and VENUS+δf codes have been performed on experimental Large Helical Device (LHD, NIFS, Japan) configurations with different magnetic axis positions and simplified plasma density and temperature profiles. In this paper, we use experimentally obtained electron density and temperature profiles for the LHD discharges #61863 and #82582 to compute the corresponding magnetohydrodynamic equilibrium states and collisional frequency. An improved collisional operator has been implemented into the VENUS+δf code. The comparison between the measured LHD bootstrap current and that expected from neoclassical simulations is discussed.

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In non-inductive current drive steady state operation of tokamak reactor, it is desirable that the Q_cd value determined by the plasma current balance is the same as the Q value determined by the plasma energy balance [Progress in the ITER Physics Basis, Nucl. Fusion 47, S285 (2007)]. The more quantitative scaling laws of Q and Q_cd are derived from the scaling laws of electron density, beta ratio, energy confinement time and current drive efficiency. The reduced scaling laws of Q and Q_cd are examined by comparison with the data of the standard scenario of inductive operation and the reference scenario of non-inductive operation of ITER. Sensitivities of Q and Q_cd on the plasma parameters are studied and requirement is examined to satisfy Q_cd ≈ Q.

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Effects of a static magnetic island generated by an external magnetic field on the linear stability and the nonlinear dynamics of resistive interchange modes are numerically studied by means of the reduced magnetohydrodynamic (MHD) equations in a straight heliotron configuration. Equilibria consistent with the static magnetic island are examined, where the pressure profile is locally flat inside the separatrix. The linear growth rate of the interchange mode is decreased with the increase of the static island width. The mode is completely stabilized when the static island width exceeds a threshold value. The threshold width is almost the same as the half-width of the eigenfunction of the stream function obtained for the equilibrium without the static island. The saturation level of the kinetic energy in the nonlinear evolution is also decreased with the increase of the static island width.The island width and the pressure profile are also affected by the nonlinear saturation of the interchange mode.

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We have investigated the feasibility of a thermionic energy converter (TEC) system using the high heat flux from divertor plasmas in fusion devices. If the kinetic energy of particles can be converted to electricity by using a TEC, the efficiency of fusion reactors could be improved. The basic properties of the TEC were analyzed by a two-dimensional particle-in-cell simulation, including the current-temperature characteristics and potential profile between the electrodes and the magnetic field effect. Verification experiments on the TEC system under divertor-relevant conditions were also conducted in the Active Cooling Test device, in which the heat load is produced by an electron beam.

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A pulsed discharge was superimposed to a steady state arc discharge in the toroidal divertor simulator NAGDIS-T. The dynamic response of the plasma was observed with an electrostatic probe and fast framing camera. In the first loop plasma, which is close to the plasma source, the density becomes higher in response to the pulsed discharge, and the emission from the plasma significantly increases. On the other hand, in the second loop plasma, where a recombining plasma is formed, the emission from the plasma disappears in response to the pulse. Just after the pulsed discharge, plasma instabilities were formed and they were propagated from the upstream to downstream at the velocity of ∼10 km/s. After the series of pulsed plasma experiments, arc trails were recorded around the cathode area. On a molybdenum cover of the cathode, unipolar arcing was initiated on the surface. It is likely that the pulsed discharge leads to instabilities and initiate the unipolar arcing consequently.

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Deuterium retention after low energy plasma exposure was investigated using the polycrystalline tungsten samples of which grain elongation directions are parallel and perpendicular with respect to the surface. The deuterium retention calculated from TDS spectra is 2-10 times larger in the sample with perpendicular grain elongation direction than in the parallel one for the irradiation temperatures of 450-750 K. Removing of the grain elongation anisotropy by the recrystallization resulted in disappearance of the difference in the deuterium retention of the both kinds of samples. No bubbles and no blisters have been observed in the near surface layer of the samples. Plasma irradiation should produce high energy defects that are responsible for the deuterium retention. The difference in the retention of the both kinds of samples seems to be attributed to the different effective diffusion coefficients depending on the grain elongation direction.

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A simulator has been developed to control the position and shape of plasmas. It consists of an equilibrium solver and an “isoflux" controller. The equilibrium solver identifies an equilibrium under the specified poloidal field (PF) coil current and incorporates the effect of eddy currents. The plasma position and shape are obtained as a result of the equilibrium calculation by an introducing the imaginary magnetic field. The controller enables the simulation of the control of the position and shape using the isoflux technique and optimizes the control logic of the coil current in JT-60SA. It also controls the PF coil currents such that the poloidal flux remains equal at all specified locations. The simulation of the control of the position and shape in response to prescribed changes in the configuration, internal parameters, poloidal beta, and internal inductance is demonstrated. The transition from a limiter to a divertor configuration is also simulated.

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This summary is based on 240 papers presented at the 21st Toki conference held on November 28th to December 1st 2011. It registered a record level of attendance and dealt with a wide range of fusion science and technology aspects for steady state operation which constitutes the greatest challenge on the way to the production of fusion energy on an industrial scale. Prominent aspects reported include the physics scenarios for SSO (Steady State Operation) which has to integrate in a coherent way challenging technologies. These include the technology of materials for structural and first wall components, superconductors for magnetic confinement, plasma heating systems. A considerable effort is being made to integrate the knowledge acquired in these fields into large experiments. The construction and operation of a new generation of fusion machines based on superconducting magnets has made good progress and long pulses operation demonstrate the progress made on SSO in an integrated way. They now reach the GJ level in extracted energy at plasma parameters significantly more relevant than previously. However, the ITER goal of 500 GJ is still far away and in view of the distance to travel, the level of activity in this area of research is expected to grow considerably in the years to come. The projection of present day knowledge to a DEMO fusion reactor is now being addressed by newly formed research groups.

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Stable plasma productionin a radio-frequencyplasma source and steady-state high-neutral-pressure environment in a test region has been achieved simultaneously with backflow suppression components such as orifices and a differential pumping system. Neutral pressure in the test region is changed by two orders, resulting in changes in the electron temperature and density in the test region. Conversely, neutral pressure, electron temperature, and electron density near a plasma production region showed little change. The result indicates that the experimental device provides us independent control knobs of source plasma and neutral pressure in the test region.

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We have solved the two-dimensional time-dependent Schrödinger equation for a single particle in the presence of a non-uniform magnetic field for initial speed of 10-100 m/s, mass of the particle at 1-10 m_p,where m_p is the mass of a proton. Magnetic field at the origin of 5-10 T, charge of 1-4 e, where e is the charge of the particle and gradient scale length of 2.610 × 10⁻⁵ - 5.219 m. It w as numerically found that the variance, or the uncertainty, in position can be expressed as dσ²_r /dt = 4.1ħv₀/qB₀L_B, where m is the mass of the particle, q is the charge, v₀ is the initial speed of the corresponding classical particle, B₀ is the magnetic field at the origin and L_B is the gradient scale length of the magnetic field. In this expression, we found out that mass, m does not affect our newly developed expression.

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Preliminary observation results are reported for a new discharge regime in the Plasma Assembly for Nonlinear Turbulent Analysis (PANTA), where spontaneous transitions in the equilibrium profile and fluctuation spectra occur. Two different states are defined by using the mean density value. Axial and radial profiles are observed for the two states, and large profile changes are found. The spatiotemporal evolution of the transition front is measured. Changes in the fluctuation spectrum are evaluated using conditional average and lock-in average.

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A three dimensional electrostatic plasma particle code has been developed to investigate blob dynamics. Some results of preliminary simulations are in agreement with previous studies based on a two-dimensional reduced fluid model. When periodic boundary condition is applied in the ambient magnetic field direction, it was observed that a blob evolves to a mushroom-shaped structure. Furthermore, the relation between the observed blob propagation speed and the initial blob size is consistent with the expectation by the fluid theory.

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Charge-transfercross sections of the ground state He⁺ ions in collisions with He atoms and simple molecules (H₂, D₂, N₂, CO and CO₂) have been measured in the energy range of 0.20 to 4.0 keV with the initial growth rate method. Since previously published experimental data are scattered in the low energy region, the present observations would provide reasonably reliable cross section data below 4 keV. The charge transfer accompanied by dissociation of product molecular ion can be dominant at low energies for molecular targets. In He⁺ + D₂ collisions, any isotope effect was not observed over the present energy range, compared to H₂ molecule.

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We experimentally investigated hydrogen combustion by atmospheric pressure plasma generated by a 2.45 GHz microwave discharge. Small amounts of hydrogen and oxygen were mixed in the operational argon gas during discharge. To clarify the details of combustion, optical emission was measured. The constituents of combustion-processed gases were observed by a quadruple mass spectrometer. The degree of hydrogen oxidation, the so-called conversion rate, increased with input microwave power. The maximum hydrogen conversion rate was greater than 80% under these experimental conditions. During discharge, an optical emission peak due to OH radicals was observed.

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The influence of the isotope effect (mass dependence) on the charge exchange process in low-energy collisions of ions and atoms sputtered from Plasma Facing Components (PFC) with hydrogen isotopes (H, D and T) is studied using the adiabatic theory of transitions in slow collisions developed by E. Solov'ev [Sov. Phys. Uspekhi 32, 228 (1989) [1]]. Results of the numerical calculations are presented for the charge-exchange cross sections of Li, Be, C ions colliding with hydrogen isotopes and for the inverse reactions.

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A new 17-channel soft X-ray diagnostic system was developed for a study of magnetohydrodynamics (MHD) fluctuations and installed on the Large Helical Device (LHD). The Absolute X-ray Ultraviolet Photodiodes (AXUV diode) with a large sensitivity area 10 mm × 10 mm were adopted as the detectors. The sightlines were designed to cover the whole plasma with 3.8 cm space separation and the expected radial resolution was 7 cm at the equatorial plane of LHD. The toroidally elongated pin hole (25 mm × 7 mm) was used to increase the signal to noise ratio and a Be foil of 15 µm in thickness was used to shut the visible light. The detector array was placed inside the vertically elongated section of the LHD vacuum vessel, being shielded by an aluminum box. In the experimental campaign of LHD, this fiscal year 2011, various kinds of MHD fluctuations excited in core and edge plasma regions have clearly been detected by this newly installed diagnostic system. The characteristic behaviors of the ELM activity in H-mode plasmas and the “Fishbone”-like instabilities induced by the perpendicular neutral beam injection (NBI) were derived from the soft X-ray data.

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The BIA (Binary Interaction Approximation) formulation in the presence of neutralizing immovable background ion is presented for analysis of multiple electron motion. Such a BIA scheme is applied to electrons in plasmas. A test calculation shows that 1) the plasma oscillation and its frequency are successfully detected, 2) the CPU time for the BIA are less than 1.5 sec and 1 hour for two and three dimensional analysis, while 127 sec and 13 hours for the direct integration method (DIM) by using a Runge-Kutta-Fehlberg integrator with an absolute error tolerance of 10⁻¹⁶, and 3) the number of time steps for the DIM, in such a case, are as many as 5.8 × 10⁴ and 3.6 × 10⁶, while those for the BIA are only 256 and 512.

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Two accuracy assurance schemes are combined into the Binary Interaction Approximation (BIA) to N-body problems. The first one is a sort of variable time step (VTS) scheme for a given error tolerance. Since this scheme sometimes does not converge, an error-tolerance-adjusting (ETA) scheme is also introduced. With these two schemes combined into the original BIA, a significant improvement in terms of numerical error is obtained.

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We have solved the two-dimensional time-dependent Schödinger equation for a magnetized proton in the presence of a fixed field particle with an electric charge of 2×10⁻⁵e, where e is the elementary electric charge, and of a uniform megnetic field of B = 10 T. In the relatively high-speed case of v₀ = 100 m/s, behaviors are similar to those of classical ones. However, in the low-speed case of v₀ = 30 m/s, the magnitudes both in momentum mv = |mv|, where m is the mass and v is the velocity of the particle, and position r = |r| are appreciably decreasing with time. However, the kinetic energy K = m〈v²〉/2 and the potential energy U = 〈qV〉,where q is the electric charge of the particle and V is the scalar potential, do not show appreciable changes. This is because of the increasing variances, i.e. uncertainty, both in momentum and position. The increment in variance of momentum corresponds to the decrement in the magnitude of momentum: Part of energy is transfered from the directional (the kinetic) energy to the uncertainty (the zero-point) energy.

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Electron temperature fluctuation amplitude is evaluated with a single probe using conditional sampling technique on the PANTA (linear magnetic configuration device). Electron temperature fluctuations are extracted as a function of floating potential. Normalized temperature fluctuation amplitude of about 15% is observed in low neutral pressure discharge in PANTA.

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Ion flow is studied in the cylindrical magnetized Argon plasma of the PANTA device using several experimental methods. Time delay estimation technique (TDE) is used to measure the azimuthal propagation of density fluctuations between two electrostatic probes. Ion flux is also studied in azimuthal and axial direction using a newly installed Mach probe. TDE shows velocity profiles qualitatively consistent with expected drift wave propagation and E ×B rotation. The Mach probe shows a maximum azimuthal flow and an axial flow shear close to the maximum density gradient and maximum fluctuation position. Striking differences exist in the detected axial ion flows depending on the discharge parameters. In high neutral pressure conditions reversal of axial ion flow was detected in the outer part of the plasma column. Temporal evolution of flows and fluctuations are compared as a first step to assess interplay between plasma turbulence and flows in the radial, axial and azimuthal directions.

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The chemical states of impurities such as oxygen and carbon and their effects on hydrogen retention behavior in pure boron films exposed to hydrogen plasma in the Large Helical Device were investigated by X-ray photoelectron spectroscopy and thermal desorption spectroscopy. The atomic concentrations of the boron films after hydrogen plasma exposure changed from 94% B, 4% C, and 2% O to 68% B, 20% C, and 12% O. B-C bonds and free oxygen were the major chemical states of impurities in the boron films after hydrogen plasma exposure. The hydrogen isotope retention behavior of a hydrogen-plasma-exposed boron film and a D⁺₂ -implanted one clearly differed, and the retention of hydrogen as B-H bonds was reduced by hydrogen plasma exposure owing to the chemical sputtering of hydrogen with free oxygen. In addition, the hydrogen desorption stage was observed as B-C-H bonds at approximately 900 K, although the amount of retention as B-C-H bonds was 10% of the total for a pure boron film exposed to hydrogen plasma.

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Electron density measurement remains indispensable to control fueling on a DEMO reactor. For steady-state operation of the DEMO reactor, density measurement should be highly reliable and accurate. A dispersion interferometer and a Faraday polarimeter are free from measurement errors caused by mechanical vibrations. Hence combination of the two diagnostics yields a suitable system for density measurement on future steady-state fusion reactors. A wavelength around 1 µm is one of the desirable candidates in terms of the fringe shift and the Faraday rotation angle, the variety of optical components, and the efficiency of frequency doubling for the dispersion interferometer. This paper presents a conceptual design for the dispersion interferometer and Faraday polarimeter with a 1 µm light source.

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Beam ion losses from Large Helical Device (LHD) plasmas caused by magnetic field ripples or Coulomb collisions are measured using a scintillator-based lost fast-ion probe (SLIP). The gyroradius and pitch angle distribution of beam-ion losses as well as to the total-beam losses arriving at the SLIP are measured in various plasma parameter ranges. The SLIP reveals that most lost beam ions consist of a pitch angle of 50°- 60° at relatively high toroidal magnetic field strength (B_t). These ions consist of a transition orbit with a large deviation from the flux surface. The beam ion losses arriving at the SLIP (Γ_{SLIP_SUM}) depend on the changes in the line-averaged electron density in a manner analogous to the behavior of beam ion components created by co-going neutral beam injectors. Γ_{SLIP_SUM} normalized by the beam ion components decreases as the magnetic axis position in a vacuum (R_ax) shifts inward at B_t of 0.90 T. Not only beam ions having transition orbit but also those having co-going orbit are measured at the relatively low B_t experiments at R_ax = 3.60 m. The loss domain corresponding to the co-going orbit disappeared at B_t = 0.75 T. Beam ions having transition orbit as well as those having passing orbits normalized by the beam ion components are suppressed with increasing B_t.

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Electron cyclotron current drive (ECCD) is an attractive tool for controlling plasmas. In the large helical device (LHD), ECCD experiments have been performed by using an EC-wave power source, gyrotron, with a frequency of 84 GHz. The maximum driven current was ∼9 kA with 100 kW injection power to plasma and 8s duration of EC-wave pulse. These years, high-power and long-pulse 77 GHz gyrotrons were newly installed. An ECCD experiment with 775 kW injection power was performed. The 77 GHz waves of 8 s pulse duration sustained the plasmas. The EC-wave beam direction was scanned toroidally, keeping the beam direction aiming at the magnetic axis in X-mode polarization. In spite of the change in the EC-wave beam direction, plasma parameters such as the line-average electron density, the central electron temperature and the plasma stored energy were kept nearly the same values for the discharges, ∼0.3×10¹⁹ m⁻³, ∼3 keV and ∼30 kJ, except for the plasma current. The plasma current showed a systematic change with the change in the beam direction for ECCD, and at an optimum direction with N_// ∼ −0.3, the plasma current reached its maximum, ∼40 kA. Also, current drive efficiency normalized with density and power was improved by 50% compared with that at the former 84 GHz ECCD experiment.

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We developed a polarization-based beam-combining method for multiple lasers to improve the time resolution and accuracy of a large helical device (LHD) Thomson scattering system. We combined two or more beams from different apertures of the laser heads in series into a coaxial beam line with the use of polarization optics. We have implemented this beam-combining method in an LHD Thomson scattering system from the 15th experimental campaign. We also propose a method of combining three or more laser beams by the use of a Pockels cell.

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Observations of the spectral broadening of lower hybrid (LH) waves in Alcator C-Mod are presented. We report the dependenceof the pump broadeningon two plasma parameters: plasma density and magnetic topology. As the plasma density was raised, we observed a significant increase of the pump broadening at 10 dB below the maximum peak. However, at the constant plasma density, when the magnetic topology was changed from the diverted plasma to the inner wall limited plasma, the spectral broadening was reduced. In an attempt to understand the causes of this variation, we report the change of density profiles in front of the LH launcher in two different magnetic topologies. The model analyses indicate that both the scattering process and parametric decay instability can contribute to the spectral broadening. Further experimental investigations are necessary to identify the causality between spectral broadening and the degradation of lower hybrid current drive efficiency at high density plasmas.

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Hishinuma et al. has established a new Powder-In-Tube (PIT) process using a high Ga content Cu-Ga compound in order to improve the superconducting property of the V₃Ga compound wire. In this study, we investigated the microstructure of this high Ga content Cu-Ga/V composite superconducting wire. Three different contrasts of the matrix, V-Ga phase and Cu-Ga core were observed by SEM observation in the cross section of 19-multifilament wire. The V-Ga phase was confirmed by SEM mapping. The area fraction of V-Ga phase increased when Ga content increased from 30% to 50%. Thin film sample with V-Ga phase for TEM observation was fabricated by FIB. Selected area diffraction pattern was obtained for V matrix, V-Ga phase and Cu-Ga core. The ratio of V to Ga in V-Ga phase was close to V₃Ga according to the EDS result. The interface between V matrix and V-Ga phase was linear, while the interface between Cu-Ga core and V-Ga phase was not linear. On the other hand, there were some granular grains observed in V-Ga phase around Cu-Ga core.

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