A tungsten (W) deposition layer is formed while a sample is exposed to a helium (He) plasma at the surface temperature of 473 or 573 K. The formed He-W co-deposition layer was composed of fine grains, the size of which increased with the surface temperature. The samples with the co-deposition layer was exposed to a deuterium (D₂) plasma, and the D retention was investigated using thermal desorption spectroscopy. It was found that the co-deposition layer has quite different D retention characteristics compared with bulk W.

The magnetic field dependence of a linear argon plasma is examined with a tomography system in PANTA. It is found that a plateau region exists around a particular region of magnetic field (∼600 G), below and above which the plasma changes the properties of emission and its fluctuations. A model is proposed to explain the observed dependence, and the comparison demonstrates that the dependence should be ascribed to the change in the Lamor motion inside the plasma production source using helicon wave and the plasma transport after the production.

Research on a new type of wire-array z pinch based on a slow generator was conducted. When a current was applied to divergently aligned two Cu wires of 25 µm in diameter, the wires were interrupted near the inner electrode and contracted locally. A soft x-ray pulse was generated, and a spatially isolated hot spot was observed near the inner electrode. A new method for converging a wire array to a single point was presented.

The dependence of hydrogen plasma parameters on magnetic field configuration and neutral pressure was investigated in the radio-frequency (RF) plasma source DT-ALPHA. It was found that higher electron density was obtained when the lower hybrid resonance condition was satisfied near the RF antenna. It was also found that use of lower hydrogen neutral pressure yielded higher electron density plasma. By optimizing the resonance condition and neutral pressure, the hydrogen plasma of T_e ∼ 10 eV and n_e > 1 × 10¹⁷ m⁻³ was achieved.

A compact hard X-ray camera has been developed to study the behavior of fast electrons accelerated by lower hybrid waves in the TST-2 spherical tokamak. The camera consists of a pinhole made of tungsten alloy, a thin resin window, a thin LYSO scintillator disk, 5 fiber optic light guides and 5 photomultiplier tubes and shields. The camera is installed on a tangential port, and two-dimensional images are obtained by rotating the system around the axis of the port on a shot by shot basis. The typical energy range is 10 - 250 keV, and the energy resolution is about 100% (FWHM) at 100 keV. The measured images of a lower hybrid wave driven plasma show bright region at the inboard side (i.e., high field side).

The radial correlation length L_r of electron density fluctuations has been measured using a two-channel O-mode reflectometer system in the helical-axis fusion plasma experimental device Heliotron J. The experimental results show that L_r is around 1.4 ± 0.7 mm in three different magnetic field bumpiness configurations (i.e. low, medium and high) for the low-density electron cyclotron heating (ECH) discharges. In high-density neutral beam injection (NBI) discharges with HIGP and pellet injection, L_r is found to be around 1 ± 0.2 mm.

A regression approach has been adopted to acquire a quantitative guidance for updating the total neutron emission rate (S_n) in the Large Helical Device (LHD) with employing the externally controllable parameters such as heating power and plasma density. A deduced regression expression is worthwhile to understand for the contribution of an individual parameter during high S_n discharges, and then to be exploited in the experiment planning to update the record S_n value in LHD in the coming campaigns. It was found that S_n in high S_n discharges in LHD is expressed as S_n = 10^14.25 × n_{e_avg}^0.52 × P_N-NB^0.69 × P_P-NB^0.37, where n_{e_avg}, P_N-NB, and P_P-NB represent the line-averaged electron density [10¹⁹ m⁻³] and the injection power of negative and positive ions based neutral beam injection [MW], respectively. This expression shows that, among three parameters, P_N-NB is essential for achieving high S_n in this employed high S_n discharge database.

A database of the total neutron emission rate (S_n) in the ion cyclotron range of frequency (ICRF) heated deuterium plasma in the Large Helical Device (LHD) was developed with changing the ICRF injection power and electron density. The S_n measured in the experiment was compared with the neutron emission rate calculated by the thermal reactivity of deuterium plasma to predict S_n in long-pulse discharge and to understand the heating scheme of ICRF. The ratio of S_n evaluated by calculation and that by measurement in the electron cyclotron heating (ECH) and ICRF heating case has almost the same tendency as that of the ECH only case in which energetic deuterons are not expected. In the first campaign of deuterium ICRF plasma discharge with injection power up to 1.5 MW, neutron emission with regard to the ICRF tail deuteron was thought to be negligible. We expanded the S_n database and started building an S_n prediction code to perform a long-pulse deuterium discharge.

We rapidly report the benchmark of density functional theory simulation on the Plasma Simulator composed of vector processor. The improved OpenMX code on the SX-Aurora TSUBASA of the vector processor achieved performance higher than the Intel Xeon Gold of the scalar processor.

The realization of advanced fusion reactors rests upon improvements of plasma-facing materials for divertors. The toughness of tungsten is a critical issue for these improvements. As a solution, we developed dispersion-strengthened (DS) W-Ti alloys using a new process which is based on a combination of mechanical alloying (MA) and hot isostatic pressing. In this paper, the microstructure of new DS-W-Ti alloys and the formation mechanism of nano-strengthening particles are investigated. The produced DS-W-Ti alloys are dispersed with TiO₂. The chemical analysis shows that new precipitation processes occurred during the MA based on redox reactions between carbon and oxygen.

Arcing experiments were conducted in the linear plasma device Pilot-PSI, where a pulsed plasma was superimposed to a steady state plasma. The arcing was observed by a fast framing camera, and the sample was analyzed with a transmission electron microscope. Observations of glowing objects released from the sample in response to the arcing and destruction of the fuzzy layer at the edge of the arc trail without significant melting suggested that dust was formed and released from the surface in response to arcing.

Liquid tin (Sn) is a promising coolant for liquid surface divertor of fusion reactors. However, material compatibility of liquid Sn with structural materials is one of the important issues. The purpose of the present study is to investigate the corrosion resistance of tungsten (W) materials such as pure W, W-5Re alloy and W based sintered alloy (HAC2) in liquid Sn. Static corrosion tests were performed at 773 K for 250 hours. Pure W and W-5Re revealed corrosion resistance in liquid Sn, though HAC2 corroded due to dissolution of sintering additives.

We have developed a new volumetric ignition method called Laser Breakdown-Assisted Long-distance Discharge Ignition (LBALDI). In LBALDI, laser breakdown plasma is utilized to assist spark discharge by facilitating formation of ionization channel. It is expected to improve lean ignitability limit by long distance discharge for extended initial flame kernel, which was proved by our previous research. The authors investigated the influence of laser incident angle on the probability of discharge by LBALDI. It was found that discharge was the most successful when the incident laser was directed perpendicular to the discharge path.

A novel inverse analysis method was developed and tested to determine the spatial distribution of the ionization rate of hydrogen gas in an inertial electrostatic confinement (IEC) fusion device. The ionization rate distributions were inversely determined so that an experimentally observed H_α spectrum could be reproduced by the linear combination of H_α partial spectra that were numerically predicted by one-dimensional Monte Carlo simulations. This method is useful for improving the performance of IEC fusion device as a neutron source because the ionization rate distribution greatly affects the ion energy distribution and fusion rate.

A data-driven approach called sparsity-promoting dynamic mode decomposition (SP-DMD) is applied to the plasma turbulence signals obtained with an azimuthal probe array. The spatiotemporal turbulence can be reasonably decomposed into seven modes which capture the azimuthal bunching of the turbulence. A superiority of the DMD analysis to the conventional stationary analysis is demonstrated.

The accuracy for the measurement of the plasma potential was improved using an emissive probe (EP) considering the space-charge-limited (SCL) effect. To validate the measurement method, the plasma potential measured using a method combining double probe (DP) and single probe (SP) was compared to the developed method. The two measurement methods showed a good agreement: it was confirmed that the EP considering the SCL effect was able to measure the plasma potential accurately even in detached plasmas with a high-temporal-resolution.

The feasibility of operation with high plasma density assumed in DEMO conceptual designs is one of the major concerns. Although the operation density would be reduced if peaked density profiles are realized, accumulation of impurity ions injected for divertor heat load reduction may be a problem. We analyzed the argon (Ar) ion transport in the core plasma with various density profile shapes, under the condition of the given fusion power maintained by the feedback control of injection frequency of deuterium-tritium pellets, using the TOTAL code. The ratio of the Ar density to the electron density was fixed at 0.5%, the expected value in JA DEMO, on the plasma surface. For the peaked density profiles, Ar was accumulated in the central region, which caused larger increment in the electron density, but the increment was smaller than the reduction of operation density by peaking the fuel density profile in moderately peaked cases. As a result, the line-averaged electron density and the pedestal electron density were in the feasible ranges reported in the previous experimental study, for moderately peaked density profiles. It was revealed that making the peaked density profiles can improve the feasibility of the DEMO operation density.

The stability of contact discontinuities was studied by means of one-dimensional electrostatic (ES) hybrid- and full-Vlasov simulations with the initial parameter based on observational data. The ES hybrid-Vlasov simulations show that the sharp gradient of the ion number density is generated at the early stage and is subsequently maintained for a long term. On the other hand, the sharp gradient is absent in the ES full-Vlasov simulation. The generalized Ohm's law shows that the electron pressure gradient accounts for the electric field on the ion time scale in both ES hybrid- and full-Vlasov simulations. It is shown that there is a difference in the time evolution of the electron pressure between the ES hybrid- and full-Vlasov simulations, which is mainly caused by the electron heat flux.

A 2.45 GHz microwave power injected through a quartz glass window produced a stable sheet-shaped plasma with the 11 cm wide and 4 cm high rectangular cross section in a static linear magnetic field. A thin slot gas conduit opened near the surface of the window supplied a discharge gas forming a stream of neutral atoms that intersected perpendicularly the plasma flowing toward the window along the magnetic field. The gas stream caused the change of the luminous intensity distribution of the plasma around the microwave window indicating reduction of the local plasma heat load. The window did not show any damage for a continuous operation of plasma up to 2 kW microwave input power.

An innovative concept for power and particle removal from the divertor is proposed. This scheme takes full advantage of both liquid metal convection and conduction to remove heat from the divertor, which is the most difficult issue for fusion reactor design. We propose that a liquid metal (LM) should replace the solid divertor plates on the bottom of the vacuum vessel. The LM is continuously supplied from openings located at the inner separatrix strike point on the floor of the LM container on the bottom of the vacuum vessel, and exhausted from openings located at the outer separatrix strike point on the floor of the LM container. The LM flow is guided along the field line to reduce MHD drag. In the event of a disruption, the current induced in the LM during the current quench is in the same direction of the plasma current. The induced LM current would either attract the plasma toward the LM divertor (leading to a benign Vertical Displacement Event), or force the LM toward the core plasma, providing automatic disruption mitigation, not requiring a learning process. The use of liquid tin instead of liquid lithium would provide greater stability against Rayleigh-Taylor and Kelvin-Helmholtz instabilities in quiescent plasmas.

We developed a multi-channel Hα array system to measure two-dimensional (2D) radial profiles of Hα emissivity in the central cell of the tandem mirror GAMMA 10/PDX. It consists of a 2D optical collection system, which contains twelve channels of lenses, Hα filters, and bundled optical fibers in vertical and horizontal directions. To study Hα emission behavior and its fluctuation in the hot-ion mode plasma experiments with additional plasma heating application, we used the modified Phillips-Tikhonov tomography method. After applying the fast Fourier transform analysis to the 2D Hα emissivity profiles, we could successfully obtain the 2D fluctuation images for the first time. These results are useful for the detailed study of fluctuations in magnetically confined plasma.

A 280 G linear magnetic field sustained a 2.5 mm diameter stable hydrogen plasma column produced by a duoplasmatron plasma cathode. A high-intensity magnetic field created by a pair of permanent magnets and the field compression structure realized the passage of a dense plasma flow through a 2 mm diameter hole. Both ions and electrons can be extracted from the downstream plasma where a linear magnetic field can be induced to guide the plasma for striking a tungsten target. Luminous intensity distribution around a tungsten target located at another end of the magnetic field confronting to the plasma cathode was examined. A substantial reduction in the Hα line spectral broadening was observed that enabled a precise spectroscopic study of the hydrogen particle reflection at the solid target surface.

We propose a new visualization method for large-scale computer simulations called “4D Street View”. This method uses omnidirectional in-situ visualization cameras to record a simulation from multiple viewpoints. The omnidirectional video dataset produced by the simulation is then analyzed interactively. In this paper, we apply the method to a magnetohydrodynamics turbulence simulation with 64 viewpoints to prove that it is possible to achieve interactive in-situ visualization for supercomputer simulations. Three types of in-situ visualization by omnidirectional cameras (with 4π steradian solid angle) from each viewpoint are applied. At the end of the simulation, a video dataset with 192 omnidirectional video files is produced. We demonstrate that we can change the viewing position, angle, and applied visualization method interactively using a specially designed application program.

A phase space structure of the electron distribution function is investigated by the gyrokinetic theory and numerical simulations to investigate a possible mechanism of the excitation of the beam instability which induces collisionless magnetic reconnection in a strong guide field. It is shown that the perturbed electron distribution function develops in proportion to the shifted Maxwellian distribution as the reconnection electric field accelerates electrons along the guide field at the X-point, with parity symmetry around the z- axis. The accelerated electrons are expected to excite the kinetic Alfvén waves (KAWs) when the beam velocity exceeds the Alfvén speed. The obtained results suggest a possible scenario for anomalous resistivity generation in the case with the strong guide field where the beam electrons accelerated at the X-point lose their parallel momentum through interactions with the self-excited KAWs.

Divertor detachment using higher-Z (Kr) and/or lower-Z (Ne) is investigated. Using Kr+Ne superimposed seeding, plasma radiation could be enhanced at the upstream region in the edge plasma with suppression of impurity accumulation toward the core plasma. Here, the pre-seeded Kr emission was drastically enhanced after the subsequent Ne seeding. Moreover, the detachment using Kr+Ne seeding could be stably sustained while the detachment using single species seeding is short-lived. Reduction of edge electron temperature due to Ne seeding can promote the Kr emission at the upstream region. It indicates the availability of multi-species impurity seeding with different cooling rate.

In response to recent upgrade of the raytracing code for the electron cyclotron resonance heating (ECH) in LHD, transient electron thermal transport is reanalyzed. The upgraded code LHDGauss-U takes into account the oblique injection of the ECH ray to magnetic field line. The obtained results show reduced transport hysteresis widths by up to ∼ 20% where the heating absorption is less significant, but qualitative features of the transport hysteresis reported in previous studies are found to be preserved.

The transition phenomenon of detached plasma during additional heating effects has been studied by applying a short pulse (25 ms) of Electron Cyclotron Heating (ECH) at the east plug-cell of the tandem mirror device GAMMA 10/PDX. In this paper, the plasma parameters on the target plate of the D-module are studied for understanding the impact of transient heating pulse on the sustainability of the detached plasma. The heat flux increases on the target plate for the additional heating case in comparison to without heating pulse. The ion flux also increases when the ECH heating is activated. The electron density enhances significantly during the ECH heating pulse application period. Furthermore, the electron density and the ion flux increase according to the increment of Xe plenum pressure in the case of ECH heating condition. The experimental results clarified that ECH heating pulse can drive the detachment state to attached state in spite of strong effect of Xe gas for generating the detached plasma.

We investigated effects of the local helical magnetic field on the plasma vertical position to suppress Vertical Displacement Event (VDE) in TOKASTAR-2. Conditions for VDE occurrence were investigated on the vertical plasma position and the current of coils for elongating the plasma, with and without the helical field, and no clear effects of the helical field were observed. Even though the helical coil currents were increased and the plasma vertical position were adjusted, the existing local helical coils were not effective to stabilize the plasma vertical position, resulting in the plasma current quench. We evaluated the effective radial field, which is expected to stabilize the plasma vertical position, using magnetic field line trace calculation. We found that the distribution and magnitude of the effective radial field generated by the existing helical coils were not appropriate for stabilization of the plasma vertical position. We designed new local helical coils consisting of triangular coils located on the upper and lower sides of the plasma. The new coils can generate the effective radial magnetic field, which is expected to stabilize the plasma vertical position.

We are developing a Lagrange (LG)-Monte-Carlo (MC) scheme for three-dimensional (3D) SOL/Divertor plasma fluid modeling. By using test particles, the scheme is suitable for handling 3D complex geometries. The semi-implicit treatment of the pressure gradient term enables us to improve the robustness of the coupling of the continuity and the momentum equations. Detailed numerical checks of the integrated scheme of LG-MC have been done for a simple 1D geometry. Benchmark tests between the new LG-MC and a conventional Finite-Volume scheme were carried out and good agreement was obtained. A first test calculation for a 3D cylindrical geometry has been also successfully done.

Magneto-hydro-dynamics (MHD) simulations are carried out for the first time with PLATO tokamak parameters to represent competition of plasma instabilities. The plasma equilibrium is evaluated with the vertical coil configuration in PLATO by using free boundary equilibrium code. The equilibrium is introduced from TASK/EQ to MHD simulations by MIPS code to calculate nonlinear saturation dynamics. In the simulation, a ballooning mode and kink mode both become unstable. We present the dependencies on plasma parameters to identify the instabilities. The ballooning and kink modes become unstable in the steep gradient region and at the safety factor q = 1 surface near the center of the plasma, respectively, so the nonlinear flattening of the pressure profile near the center is stronger in the kink case. The interaction between the modes affects the evolution of instabilities.

In neutral-beam-injection (NBI)-heated plasma, the emission spectra of the neutrons produced by D(d,n)³He and T(d,n)α fusion reactions are known to be distorted from Gaussian distribution functions in both the high and low energy sides along the NBI direction. This study shows that the effect of NBI heating can be applied to the central energy region, i.e., the 2.4 - 2.5 MeV range in the D(d,n)³He neutron emission spectrum. Conventionally, the intense spectrum anisotropy appears via the anisotropy in the double-differential D(d,n)³He cross-section. Herein, we have shown the anisotropy in the neutron emission spectrum of the central energy range (2.4 - 2.5 MeV) due to the alteration of the neutron emission spectrum (2.4 - 2.5 MeV). Caused by the modification of the deuteron velocity distribution function is large enough to negate the anisotropy caused by the double-differential D(d,n)³He cross-section. An application of the anisotropy effect to fuel-ion ratio diagnostics is discussed, and the attendant degree of improvement is evaluated.

We extended the global gyrokinetic code GKNET (GyroKinetic Numerical Experiment of Tokamak) to non-circular shaped plasmas with analytical magnetic equilibria, which satisfy the Grad-Shafranov equation up to the second order with respect to aspect ratio. The extended version allows us to set the equilibria with non-unity elongation and non-zero triangularity, where the finite Shafranov shift is consistently determined. The allocated mesh follows the magnetic field line with periodic boundary conditions along the poloidal and toroidal directions so that the calculation cost for solving the gyrokinetic quasi-neutrality condition can be reduced by utilizing 1D FFT and MPI_ALLtoALL transpose technique. Based on the developed code, we studied the effect of elongation and triangularity on linear Ion Temperature Gradient (ITG) instability with adiabatic electrons in a non-circular shaped Tokamak by paying attention to the symmetry breaking of mode structure due to global profile effects. It is found that elongation reduces ITG instability owing to the effective reduction of flux-surface averaged ion temperature gradient and increases the asymmetry characterized by the Bloch angle θ_b. On the other hand,when elongation is approximately unity, triangularity weakly affects the growth rate, while negative triangularity stabilizes ITG modes and increases the asymmetry in a large elongation regime.

Proliferation of dusty layers inside the Tokamak prompts deleterious effect in Tokamak operation. The stability of the fusion plasma is adversely affected by building-up of dusty layers inside the tokamak which actuate disruptions in tokamak's performance. The current study is based on probing the mechanisms of dust accumulation inside the Tokamak device by simulating the process with the support of pulsed plasma accelerator. Experimentally it was observed that upon interacting with the pulsed power plasma stream, the materials used for designing the in-vessel elements of Tokamak such as graphite, lost its crystalline structure which in turn affects its inherent properties like ductility, malleability, rust resistance etc., thus emanation of dust paricles take place as a result of collisions between plasma species such as electrons, ions and neutrals with molecules of graphite as the tokamak material, ergo causing erosion in the tokamak material. The concatenated dusty layers formed due to accumulation of dust particles inside the Tokamak can cause probable instability in fusion plasma that would mar the function of the Tokamak device, besides the generated dust would become chemically reactive, radioactive or toxic. The dynamics of dust grains can be realised by Coulomb force, ion drag force, frictional forces on ions, ion pre-sheath drifts etc. and can also be interpreted from the kinetic theory of dusty plasma, to account this the numerical studies have been carried out with DUSTT and UEDGE codes to understand the dust dynamics and transport mechanisms.

The Large Helical Device (LHD) started the deuterium plasma experiment on March 7, 2017. Approximately 6.4 GBq of tritium was produced in the first deuterium plasma experiment campaign and were utilized as tracers for the investigation of release behavior and the balance of tritium in the LHD vacuum vessel. To determine the tritium balance in LHD, the tritium release from the vacuum vessel was continually observed during the plasma experiment period and the vacuum vessel maintenance activities. The tritium exhaust rate was approximately 32.8% at the end of the plasma experiment. After the plasma experiment, the vacuum vessel was ventilated by room air for the maintenance activity and the tritium release from the in-vessel components was observed. The tritium release rate gradually decreased and became constant after four-month in spite of water vapor concentration. It is suggested that the tritium release mechanism from the vacuum vessel is a diffusion-limited process from the bulk. The tritium release amount during the maintenance activity for one year was approximately 5.0%. Considering the decrease of tritium decay for 1.5 years, tritium inventory in LHD was estimated to be approximately 3.66 GBq (57.2%) at the end of maintenance activity.

Optimization of the helically-winding structure required for the helical fusion reactor utilizing the high temperature superconductor (HTS) tape is studied by numerical calculation. When the HTS tapes are wound to be the helical structure without the torsion, a significant edgewise strain appears. This might lead to the degradation of the performance of the HTS tape. Therefore, the edgewise strain must be minimized. It is found that the edgewise strain can be suppressed by the proper determination of the torsion by the optimization utilizing the Fourier decomposition of the distribution of the edgewise strain. The HTS tapes with edgewise strain are intrinsically deformed due to the torsion to reduce the edgewise strain so to be an “edgewise-strain-free” helical shape.

Self and mutual inductances of toroidal-field (TF) coils are empirically expressed by linear combinations of three coil-shape parameters: elongation, aspect ratio, and triangularity, based on their calculation results with the Neumann formula. A regression function was also obtained for calculating rough values of self-inductances of toroidal-shape structures such as a vacuum vessel in a Tokamak-type fusion reactor. An analysis for their eddy currents induced during fast discharge of TF coils is presented for showing as an application example of these formulas.

This study investigated the energy transfer of a fully developed freely decaying homogeneous isotropic turbulence of a Hall magnetohydrodynamic medium from the perspective of the interaction between the ion cyclotron (IC) and whistler (Wh) modes. The variables were decomposed using generalized Elsässer variables (Galtier, J. Plasma Phys. 72, 721 (2006)), which are given by a combination of velocity and magnetic fields. To analyze the energy transfer between these coupled variables, an analytical-mechanical approach was introduced that guaranteed both the Galilean covariance and the detailed energy balance for each decomposed mode. A remarkable asymmetry of the energy transfer between the IC and Wh modes was found. In the present study,almost one-sided energy transfers from the IC to the Wh modes were observed. In addition, a shell-to-shell transfer analysis revealed relatively weak inter-mode interactions with nonlocal features, while the intra-mode interactions were both intense and local.

The cryogenic coolant is important as an electrical insulation in an immersion cooling. One of the dielectric breakdown mechanisms of the cryogenic coolant is via bubbles, caused by partial discharge (PD). The insulation performance has been discussed with PD parameters and breakdown voltage. However, the essential PD mechanism has not been clarified in detail. In this paper, PDs in gaseous and liquid nitrogen were discriminated based on characteristics of the original PD waveform. It was shown that the difference in charge behavior appears significantly in the fall time of the original PD waveform.

The hybrid method based on the collocation element-free Galerkin method and the boundary element method has been applied to the 2D steady-state scattering problem of the electromagnetic wave. In addition, the performance of the proposed method has been investigated numerically. In this study, the numerical solution of the proposed method has been obtained by using the GMRES(m) method for the complex linear system. The results of computations show that the relatively smooth distribution of electric field is obtained regardless of the boundary shape. Therefore, it is found that the proposed method can be used as one of the tools for solving the 2D steady-state scattering problems of the electromagnetic wave.

The use of cesium leads complicates ion source operation and requires regular maintenance for continuous operation. The development of a negative-ion source without Cs seeding is desired in neutral beam injectors. A magnetized-sheet plasma producing negative ions using volume production without Cs seeding was designed. The experiment was performed by a TPDsheet-U and tested using steady-state hydrogen plasma. Two different types of grid structures were used in the experiment to extract the negative-ion beam: single- and multi-aperture grids. The multi-aperture grids have been developed to achieve more beam current. The negative hydrogen ions were successfully extracted from the sheet plasma using both single- and multi-aperture grids. The current densities of the ion beams increased with increasing discharge current and extraction voltage. At an extraction voltage of 9.5 kV and a discharge current of 80 A, the approximate current density of the negative hydrogen ion beam was 8.4 mA/cm² for the case of single-aperture grids. At an extraction voltage of 9.5 kV and a discharge current of 50 A, the approximate current density of the negative hydrogen ion beam was 23 mA for the case of multi-aperture grids.

In order to directly measure high energy ions in a plasma and to establish a new energetic-ion-production method, we have developed an ion energy analyzer for measuring the ion energy distribution. The analyzer is a retarding potential analyzer consisting of three grids and a collector. By applying a voltage to each grid, the electrons are retarded and the ions are separated by energy. In the measurement of the plasma with the electron density of higher than 10¹⁷ m⁻³, it was found that the collector current was smaller than that deduced from the geometrical transparency of the grids. Measurement of the current flowing into each grid and a particle-in-cell simulation revealed that collector current decreased due to space charge between the 2nd and 3rd grids.

High electron density discharge mode was observed in the downstream of a magnetic beach, so called test region in a linear plasma device with an electron cyclotron resonance source. A model for formation of the high density phase based on electron motion along the field line was proposed. The model reproduced the tendency that the duration of the high density phase was longer for the higher magnetic field in the test region in the experiment. Some discrepancies between the experiment and the model were newly found in the experiment with detailed scan in the magnetic field in the test region with the modified time evolution of the magnetic field.

A new design of a multi-path Thomson scattering system with signal delay function based on a polarization control technique is proposed on Heliotron J to deal with the overlapping phenomenon of scattered light signals. By operating double Pockels cells to control the injection timing of the laser beam into plasma, the adjacent scattering signals corresponding to opposite incident laser beams are expected to be separated to some extent for better analyzing the anisotropic temperature. Two image relay systems are also designed and integrated into the system to suppress the reduction of the laser beam power during multi-path propagation. Estimations are made on residual power of laser and separation degree of adjacent scattered light signals to reveal the feasibility of this new design. This design is instructive for fusion devices that desire anisotropic measurement but also face limitations of less setting optical path.

The dynamic behavior of ionization waves in glow discharge plasma was examined, particularly with regard to periodicity. In a series of experiments, neon plasma was produced by the glow discharge between two electrodes after the glass tube was evacuated to high vacuum. Fluctuations in the light intensity were sampled with a line-scan camera and photodiodes as spatiotemporal signals for data analysis. The largest Lyapunov exponents were calculated from the time series of the experimental samples to quantitatively estimate the complexity of the system. The signal-to-noise ratio (SNR) became saturated as the intensity of the external force applied to the chaotic state was gradually increased because chaotic oscillations in the ionization wave synchronized to the external force. A periodic orbit in the chaos system was emphasized, and the oscillation became coherent as the state SNR reached its maximum value. When the periodicity was emphasized, the oscillation became coherent not only in time but also in space. Periodicity was also observed in system-induced feedback without an external force. Similar results were obtained with an external force and feedback; however, the former caused the chaos system to synchronize with the external force, while the latter yielded periodic oscillations from the chaos-chaos interaction.

A one-dimensional model for plasma sheet thinning [J. K. Chao et al., Planet. Space Sci. 25, 703 (1977)] according to the Current Disruption (CD) model of auroral breakup is extended to two dimensions. An initial disturbance generates a rarefaction wave. In the 1D model the rarefaction wave propagates tailward at sound velocity, which is regarded as a signature of the thinning. However, in the MHD simulation of the 2D model the rarefaction wave is quickly lost in the plasma sheet recompression, while the thinning continues propagating at a slower velocity. This shows that the dynamics of plasma sheet thinning may be dominated by sheet-lobe interactions that are absent from the 1D model.

We developed a Fokker-Planck solver to analyze heating and current drive with neutral beam injection in tokamak plasma and introduced it into the integral transport code TOTAL. In the developed solver, the fast ion distribution function is expanded in the eigenfunctions of the collisional pitch angle scattering operator. The time evolution of the distribution function is obtained by solving 1-D partial differential equations for velocity, so that high-speed analysis is possible. In the analyses using the point source and assuming zero toroidal drift, the results of the heating power and the driven current evaluated in steady state agreed well with the results calculated by the OFMC code and the ACCOME code. We demonstrated that the TOTAL code implemented with the solver is able to simulate a DEMO operation scenario with neutral-beam injection.

Parametric decay instabilities (PDIs) in lower hybrid (LH) frequency range were observed in TST-2 in a various parameter range. LH wave measurement has been performed using four RF magnetic probes. We have been studied experiments aiming at efficient non-inductive current drive with the outboard and the top antennas installed on the outboard side and the top side of the vacuum vessel, respectively. Since PDIs are believed to deteriorate the current drive efficiency, we investigated conditions to avoid them. Operations with various combination of antennas (outboard, top), toroidal magnetic field strength and direction (CW, CCW B_T), working gas (hydrogen, deuterium) and density (normal, high) were investigated. In the outboard launch case, PDI sideband frequency δf associated with ion cyclotron quasi-modes was observed with large degree in any B_T strength in both hydrogen and deuterium plasma. On the other hand, no clear such sideband δf peak was observed only in deuterium plasma with the top launch case. Among various scenarios, the top launch with CW and CCW toroidal field in deuterium plasma seems to be desirable in terms of avoiding PDIs.

A new Langmuir probe has been designed and installed in TST-2 for measurements of Scrape-Off Layer plasmas (SOL). Non-inductive current drive is considered essential for spherical tokamak reactors. It has previously been shown that a large amount of injected Lower Hybrid Wave (LHW) power is lost in the SOL [1]. A full density profile of SOL conditions is necessary to accurately simulate the propagation of LHW in TST-2. A new probe was designed for durability, larger signal and Mach probe measurements. The new probe has been installed in TST-2 and results have been obtained. Temperature measurements show T_e = 30 - 50 eV during RF injection and <10 eV otherwise. Density measurements show n_e = 2.0 × 10¹⁵ m⁻³ and 1.5 × 10¹⁶ m⁻³ during flat-top RF power injection from Outboard- and Top-launch antennas, respectively. This is above the cut-off density for the 200 MHz LHW (5 × 10¹⁴ m⁻³) in TST-2, thus LHW can propagate through SOL plasma.

Removal of the central solenoid is considered essential to realize a spherical tokamak fusion reactor, but non-inductive plasma start-up is a challenge. Start-up using lower-hybrid (LH) waves has been studied on the TST-2 spherical tokamak at the University of Tokyo. The equilibrium poloidal field is believed to be generated mostly by the wave driven fast electrons, which are highly non-thermal and have large orbit excursions from the flux surfaces due to low plasma current. Such an equilibrium can be qualitatively different from the Grad-Shafranov equilibrium routinely used for internal magnetic field reconstruction in a tokamak. In this work, the effect of fast electrons on the MHD equilibrium was investigated by considering the equilibrium solution of the hybrid-MHD model [Y. Todo and A. Bierwage, Plasma Fusion Res. 9, 3403068]. The fast electron distribution function was estimated using a LH current drive simulation based on ray-tracing and an orbit-averaged Fokker-Planck solver. The equilibrium solution of the hybrid-MHD model was successfully fitted to the magnetic and kinetic measurements. The resulting poloidal flux function was more skewed towards the outboard side when fast electrons were introduced, which was more consistent with the density profile measured by the Thomson scattering diagnostic.

Arc trails found in heliotron/stellarator devices Large Helical Device (LHD) and Wendelstein 7-X (W7-X) were inspected; arcing occurred on different locations at various situations. In LHD, a helium-plasma-induced tungsten nanostructure sample was installed and exposed to a LHD plasma. Many arc trails were formed only on the sample with nanostructures, suggesting an easy initiation of arcing compared to pristine tungsten. After the completion of annual campaign 2011 in LHD, arc trails appeared on a graphite divertor tile which was taken out for inspection. Because the arc trails had a linear shape, the arcing was likely caused by main discharge operation. In W7-X, some Langmuir-probes installed within the limiter tiles suffered severe damage with arc trails during the operational phase 1.1. After the operational phase 1.2b, in-vessel inspection was performed for the first time for all the plasma facing components focused on arcing . No arc trails appeared on graphite components including test divertor units. In turn, considerable number of the trails appeared on non-plasma exposed region. Most of the arc trails had no clear linearity, indicating that arcing initiated during glow discharge cleaning phase. Only a few trails seemed to be affected by the existence of an external magnetic field. A vast majority (80%) of arcs were initiated from the edge of surfaces, while half of arc trails starting on the interior region of surfaces were accompanied by deposition spots. Broad arc trails appeared on the surfaces of diagnostic ports and mirrors.