A high ion temperature (T_i) was achieved using a combination of perpendicular and parallel injected neutral beams in the Large Helical Device (LHD). Microturbulence spatial profiles in a high-T_i discharge were measured by two-dimensional phase contrast imaging (2D-PCI) through almost the entire vertical central chord. The 2D-PCI microturbulence spectral ranges covered wavenumbers (k) of 0.1-1 mm⁻¹ and frequencies (f) of 20-500 kHz. The ion thermal conductivity (χ_i) increased in the entire region with increasing T_i. However, the difference between the experimental and neoclassical values of χ_i became smaller at ρ < 0.5, where ρ is the normalized position, in the high-T_i phase. Increasing fluctuation was not observed at this location, suggesting improved ion energy transport in this region. On the other hand, at ρ > 0.5, χ_i deviated from the neoclassical value due to enhancement of the experimental χ_i and reduction in the neoclassical χ_i by a positive radial electric field. Increasing turbulence was observed at ρ = 0.6-0.8, with fluctuations likely propagated to the ion diamagnetic direction in the plasma frame, suggesting that the observed turbulence degrades the ion energy transport at this location in the high-T_i phase.

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The toroidal magnetic flux through the plasma column is calculated analytically for current-free stellarators of arbitrary geometry without assumptions on the plasma shape, aspect ratio, etc. This is done with accuracy sufficient for extracting the contribution due to the finite plasma pressure from this flux. The final result is a formula relating the measured diamagnetic signal with β, the ratio of the plasma pressure to the magnetic pressure. This formula is obtained assuming small β and the relative depth of the magnetic well. These are natural conditions for stellarators, therefore the final result can be recommended for magnetic diagnostics without practical limitations.

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Kink instability and the subsequent plasma flow during the sustainment of a coaxial gun spheromak are investigated by three-dimensional nonlinear magnetohydrodynamic simulations. Analysis of the parallel current density λ profile in the central open column revealed that the n = 1 mode structure plays an important role in the relaxation and current drive. The toroidal flow (v_t ≈ 37 km/s) is driven by magnetic reconnection occurring as a result of the helical kink distortion of the central open column during repetitive plasmoid ejection and merging.

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Magnetic reconnection dynamics due to the nonlinearly destabilized double tearing mode (DTM) is simulated, focusing on the nonlinear growth phase in the framework of reduced resistive magnetohydrodynamics (MHD). The nonlinearly explosive growth of the DTM accompanying fast magnetic reconnection is found to result from a secondary instability, the mechanism of which consists of the sequential unstable modulation due to two- dimensional distortion of magnetic islands and modification of the nonlinear current profile. The trigger dynamics of the nonlinear growth phase is illustrated via the investigation of the evolution of both the kinetic and magnetic energies of the secondary instability.

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Nonlinear dynamics of a magnetic island is simulated using a set of reduced two-fluid equations, and the excitation of the electric field in the vicinity of the magnetic island is observed. An impact of the error field (external symmetry-breaking magnetic perturbation) on the electric field is examined, and its global structure changes with a small error field. A numerical measurement simulating the heavy ion beam probe is carried out on the field given by the nonlinear simulation. An effect of the finite width of the injected beam is taken into account, and the spatial resolution is evaluated. It is found that the measuring error due to the finite beam width is significant when the radial gradient scale length of the electrostatic potential is comparable to the beam width.

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A spatial distribution of toroidal flow in a field-reversed configuration plasma generated by the field-reversed theta-pinch method has been measured by an ion Doppler spectroscopy (IDS) system with a line-spectrum of impurity carbon (CV: 227.2 nm) and a Mach probe. With the IDS system, the axial profile of toroidal flow was observed. The observed results show different time evolutions of toroidal spin-up at around the midplane and end regions of the FRC. The radial profile of toroidal flow has also been observed on the weakly ionized plasma outside the separatrix with the Mach probe method. These results indicate propagation of toroidal momentum to the scrape-off plasma outward from the center.

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The effect of a sharp short-wave truncation on Hall magnetohydrodynamic (MHD) turbulence is studied numerically to obtain basic information for constructing sub-grid-scale models of the Hall MHD equations. Hall MHD turbulence is found to be less sensitive to truncation than MHD turbulence, because the Hall term suppresses energy transfer in the magnetic field at relatively low wave numbers.

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In order to explain the difference between the motion of plasmoids created by pellet injection in tokamak and helical plasmas, magnetohydrodynamic (MHD) simulations including ablation processes have been performed in tokamak and Large Helical Device (LHD) plasmas. In the LHD, plasmoid motion depends on the initial location of the plasmoid, whereas in a tokamak, the plasmoid always drifts in the direction opposite to that of the curvature vector. It is verified that there are two main forces acting on the plasmoid, and the connection length determines the dominant force.

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The line-averaged electron density has been measured over a wide range of discharge conditions in a low-aspect-ratio reversed field pinch (RFP) machine RELAX. A 104 GHz heterodyne interferometer was mainly used throughout the experiments. The measured electron density n_e varied from ∼0.3 × 10¹⁹ m⁻³ to ∼3 × 10¹⁹ m⁻³, depending upon the discharge conditions. A new 60 GHz homodyne interferometer, characterized by simplicity and low cost, has been developed for the density measurement particularly in low density regime. The results from the new homodyne interferometer have shown good agreement with those from the heterodyne system in low-density regime of n_e up to ∼10¹⁹ m⁻³.

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The magnetohydrodynamic (MHD) simulation code MHD Infrastructure for Plasma Simulation (MIPS) was benchmarked on ballooning instability in the Large Helical Device (LHD) plasma. The results were compared to the results of linear analysis by using the CAS3D code. Both the linear growth rates and the spatial profiles were found to be in good agreement. An extended MHD model with finite ion Larmor radius effects was implemented into the MIPS code. Ballooning instabilities were investigated using the extended MHD model, and the results were compared with those using the MHD model. Ion diamagnetic drift was found to reduce the growth rate of the short-wavelength modes; hence, modes with a diamagnetic drift frequency comparable to the ideal MHD growth rate are the most unstable. The most unstable toroidal mode number of ballooning instability in the LHD is reduced to |n| ≤ 5 for hydrogen plasma with ion number density n_i ≤ 10¹⁹ m⁻³.

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Recent results from a low-aspect-ratio (low-A) reversed field pinch (RFP) machine with an aspect ratio of 2 (R/a = 0.51/0.25) are reported. The discharge characteristics of low-A RFP plasmas are described. The discharge regime in F − Θ space indicates that lowering the aspect ratio expanded the operational F − Θ space to the extremely high-Θ (> 3), deep-reversal (F < −1) region and to the non reversal (F > 0) region. A rotating helical Ohmic equilibrium RFP state could be realized in shallow-reversal discharge on REversed field pinch of Low Aspect eXperiment (RELAX). A preliminary feedback system is developed that results in plasma current with a longer discharge duration of ∼2 ms.

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Multi-dimensional effects on electron motion in a magnetosonic shock wave propagating obliquely to an external magnetic field are studied by means of a two-dimensional (two space coordinates and three velocities), relativistic, electromagnetic particle code. The simulations demonstrate that after trapping and energization in the main pulse of the shock wave, some electrons are detrapped from it while maintaining their ultrarelativistic energies. The detrapping is caused by magnetic fluctuations propagating along the wave front. Furthermore, some of the detrapped electrons are found to be accelerated by the shock wave to much higher energies because they can enter and exit the shock wave several times as a result of their gyromotions.

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A magnetosonic shock wave propagating obliquely to an external magnetic field can repeatedly accelerate thermal ions if θ₀≈45°, where θ₀ is the angle between the wave normal and the external magnetic field. The ion energy gains in this process are theoretically analyzed, and an expression for the maximum energy is derived. This theory is verified using a two-dimensional, electromagnetic particle code. Furthermore, whistler wave instabilities generated by the accelerated ions are studied. The simulation demonstrates that whistler waves are excited in both the upstream and downstream regions, but that the whistler waves in these two regions have different frequencies and wavenumbers. It is shown that the characteristics of these waves can be explained by linear theory.

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Nonneutral plasmas confined in the helical magnetic surface (HMS) region can be produced by injecting electrons from outside the last closed flux surface (LCFS). Recently, we numerically calculated outward electron orbits that extend to the inner HMS region. Once it penetrates the HMS region, the injected electron is never lost to the chamber wall, because the negative self-electric potential φ_s in the stochastic magnetic region (SMR) acts as a potential barrier. Remarkably, during the reflection process at the potential barrier, the electron is still trapped in the foot of the negative φ_s region, although it completely overcomes the SMR. The electron then resumes inward movement across the HMS.

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High-frequency fluctuations in the ion cyclotron range of frequency (ICRF) are excited in magnetically confined plasmas because of the distortion of velocity distribution. In deuterium plasma experiments in JT-60U, ion cyclotron emission (ICE) detected as magnetic fluctuations is observed using ICRF antennas as pickup loops. The toroidal wave-numbers can be estimated using the phase differences between the signals from antenna elements arrayed in the toroidal direction. In this manuscript, ICE due to fusion product (FP) H ions, ICE(H), which is identified separately from the second-harmonic ICE caused by D ions, is newly reported. ICE is considered to result from spontaneous excitation of magnetosonic waves associated with FP high-energy ions. ICE caused by ³He ions and T ions has already been identified and confirmed to have finite toroidal wave-numbers. In contrast, ICE caused by ions originating in neutral beam injection has no toroidal wave-numbers. It is suggested that the appearance of ICE(H) depends strongly on the plasma density, and weak magnetic shear operation is one of the possible conditions for the observation of ICE(H).

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Wave-particle interactions can produce non-Maxwellian momentum distribution functions, which modify the propagation and absorption of the wave. Kinetic analysis including deformation of the momentum distribution function is required for quantitative wave heating analysis. The deformation strongly depends on the loss mechanism and radial transport of energetic ions. In our previous work, we studied time evolution of the momentum distribution function by local ICRF heating analysis. In the present analysis, we describe global analyses of multi-species heating including loss mechanisms and radial transport.

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The fast disruption of the runaway electrons-beam in a high-voltage picosecond atmospheric-discharge [G. A. Mesyats and M. I. Yalandin, IEEE Trans. Plas. Sci. 37 785 (2009)] is considered. The plasma electrostatic instabilities are proposed as a mechanism of such disruption. Strong over-voltage (more than 1 MV/cm at gas pressure 1 atm) provides an intense electrons-acceleration in the runaway regime. It is shown that neither collisions nor a static potential profile could deliberately prevent this runaway regime. Whereas, the cathodeanode bridging impossible within tens of pisoseconds. It is obtained that the characteristic times of a simplest electrostatic-instabilities build-up are consistent with the observed runaway-electrons beam-duration at the certain plasma density, which was considered as a parameter. The agreement our estimation results and the measurements [G. A. Mesyats, V. G. Shpak, S. A. Shunailov and M. I. Yalandin, Technical Phys. Lett. 34 169 (2008); Mesyats et al., ibidem. 32 18 (2006)] confirms that collective plasma processes indeed can provide the observed picosecond termination of the fast-electrons beam in a high-voltage gas-filled diode within the tens of picoseconds.

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A possibility of distortion in bulk-electron velocity distribution function due to injection of intense laser-induced (LI) fast electrons is examined. The Fokker-Planck (FP) equations for LI-fast and bulk electrons are simultaneously solved consistently considering the time evolution of Rosenbluth potentials in two-dimensional (2D) velocity space. The FP simulation shows that an asymmetric distortion of bulk-electron distribution function becomes appreciable when number density of the fast electrons relative to bulk plasma is increased, and that when bulk electrons are assumed to be Maxwellian, the propagation distance of LI-fast electron tends to be shorter due to overestimated power deposition compared with the case when distortion process of the bulk electron distribution is considered.

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Laser-irradiated microclusters can generate energetic ions that produce fusion reactions. The amount and spectrum of these ions depend on the cluster-size distribution, electron heating mechanism, and cluster expansion dynamics. This paper describes recent physics results pertinent to the items listed. It is shown that the size distribution of large clusters can be determined from absorption measurements in a pump-probe experiment. It is also shown how a laser can create a two-component electron distribution with a hot minority whose energies exceed the ponderomotive potential. The heating rate and the limitations on electron energy are examined. The hot electron component expands with an equal number of ions. A first-principle model is presented that describes ion acceleration by the hot electron pressure together with adiabatic cooling of the hot electrons.

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By a classical molecular dynamics (CMD) simulation with a modified Brenner's reactive empirical bond-order (REBO) potential, we found that graphite with zigzag (1010) and armchair (1120) edge states is destroyed more easily than other structures, i.e., graphite with the (0001) surface, and diamond with the (100), (111), (120), and (110) surfaces. Experimental results indicated that graphite is eroded under hydrogen atom injection with E_in = 0.3 eV, and that diamond is not eroded under the same conditions. Our simulation results are consistent with these experimental results. We also reveal the temperature and saturation dependence of the surface structure of carbon crystals.

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In order to investigate the dependence of effective inverse photon-efficiency D/XB values on the plasma parameters, we have been performed calculations of effective D/XB values by Monte Carlo method. Photon fluxes are converted into particle fluxes with aid of D/XB values. A D/XB value is critical factor in the study of chemical erosion by spectroscopic measurement. In modeled tokamak edge plasma, transfer of hydrocarbon molecules (CH₄, C₂H_x (x = 2,4,6), C₃H_y (y = 4,6,8)) and CH and C₂ emissions have been simulated. The plasma temperature ranged from 1 eV to 100 eV and the plasma densities are 10¹⁸ m⁻³, 10¹⁹ m⁻³ and 10²⁰ m⁻³. In the temperature region of less than 3 eV, the calculated D/XB values increase with decreasing temperature due to decreasing of emission counts. In the high temperature region (≥ 10 eV), the D/XB values increase with a rise in the temperature due to decrease of number of fragment of type CH and C₂.

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In GAMMA 10, Electron Cyclotron Heating (ECH) is used for the formation of the axial confining potential (P-ECH) and for the bulk electron heating in central-cell (C-ECH). The paper describes the dependence of the diameter of the iris limiters on the plasma stored energy and the hydrogen recycling. In this experiment, it was observed that the plasma diamagnetism was degraded with the ECH injection and Hα emission was increased near the limiters at the same time. In the case that the diameter of the iris limiters shrunk to 350 mm, the plasma diamagnetism decreased with each ECH by the plasma-limiter interaction. On the other hand, in the case that the diameter of the iris limiters widened to 380 mm, the diamagnetism of the plasma increased with P-ECH, however the plasma collapsed with C-ECH. The above results indicated the close relationship between the iris limiters and the plasma stability. In this paper, we discuss the mechanism of the degradation of the plasma performance and the optimal limiter condition for effective operation of ECH.

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The first measurement of neutral gas pressure modulation in the range of 10 kHz was performed using a piezoelectric transducer near the cylindrical inner wall surrounding microwave discharge plasma in the HYPER-I device. The signals from the transducer output were investigated with the ion saturation current signals from two Langmuir probes located at the plasma boundary layer.

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The incident angle dependence of reactions between graphene and hydrogen atoms is obtained qualitatively by a classical molecular dynamics simulation under the NVE condition, in which the number of particles (N), volume (V), and total energy (E) are conserved, with a modified Brenner's reactive empirical bond-order potential. The chemical reactions depend on two parameters, the polar angle θ and azimuthal angle φ of the incident hydrogen. The simulation results showed that the reaction rates depend strongly on polar angle θ. The reflection rate increase with increasing θ, and the adsorption rate also depends on θ. The θ dependence is the result of the three-dimensional structure of a small potential barrier covering adsorption sites. The penetration rate also depends on φ for large θ.

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This paper presents a scheme for combining a Langmuir probe with a fast camera for measurement of the behavior of blob-like structures in the boundary region of an electron cyclotron resonance (ECR) heating plasma in Q-shu University Experiment with a Steady-State Spherical Tokamak (QUEST, major radius, R = 0.68 m, minor radius, a = 0.40 m, and toroidal magnetic field, B_t = 0.25 T at R = 0.64 m). The frame rate of the camera was typically set to 40,000 frames per second (FPS) with 192 × 144 pixels per frame. Radial motion of blob-like structures was observed in the half of the plasma space where the probe head was located. A radially movable and rotatable probe system was used to measure the floating potential from single unbiased tips, the potential of the positively biased tip, and the ion saturation current in two orthogonal directions in the outboard midplane region. Time series of the ion saturation current measured by the Langmuir probe and of pixels in a 40,000 FPS movie were compared and cross-correlated. The results of the two diagnostics agreed well, and the spatial scale was found to be of the same as the size of the probe head. The ion saturation current was asymmetric in terms of the time the blob-like structure was passing; fast camera imaging also clearly demonstrated the blobs' filamentary structures and radial motion at the edge of QUEST. This means that plasma in the blob-like structure hunches over, like blobs in other devices. The typical radial velocity of the structures is ∼1 km/s, and the structures were accelerated along their path of radial motion from the inner to the outer parts of the vacuum vessel.

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In edge plasma in the Large Helical Device (LHD), incoherent fluctuations with intermittent density bursts were observed with a two-dimensional lithium beam probe that can measure electron density and its fluctuation two-dimensionally. Statistical analysis using a probability denstiy function (PDF) showed that those bursts have the characteristics of blobs, which have been observed in tokamaks. Two-dimensional PDF analysis indicates that the blobs originate in the ergodic layer outside the last closed flux surface and move to the outer region. Wavelet analysis showed that the direction of blob propagation is not always outward but is sometimes in arbitrary directions. These characteristics may be a result of the complicated magnetic field structure of the heliotron configuration.

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We have developed a spectrometer specialized for a simultaneous high-resolution measurement of emission spectra of the hydrogen atomic Balmer-α, -β, -γ lines and molecular Fulcher-α (v' = v'' = 0) and (v' = v'' = 2) ro-vibronic bands, where v' and v'' is the vibrational quantum number. The instrumental widths for the respective wavelength ranges are 0.008, 0.009, 0.010, 0.008, and 0.007 nm. We apply the spectrometer to the observation of emissions from a LHD plasma. The observed profiles of these emission lines and bands show polarization dependence. The absolute intensities of these emissions are also estimated.

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A 3D visualization of argon and nitrogen-argon plasma electron density (n_e) and effective temperature (T_eff) distributions were constructed using the I-V curves from Langmuir probe traces at specific discrete positions in the extraction region of a magnetized sheet plasma ion source. Argon and mixed N₂-Ar sheet plasmas are characterized using the calculation of the electron energy distribution function (EEDF). By taking the current vs. voltage reading of a single probe in 68 discrete locations of the plasma, a 3-dimensional map of the electron density and electron temperature were constructed. The map was constructed to understand the global condition of the extraction region of the source. The technique can be applied to the determination and understanding of edge plasma parameters in magnetic confinement devices.

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The first lower hybrid (LH) full wave simulation of an ITER-scale plasma is presented. LHEAF [O. Meneghini et al., Phys. Plasmas 16, (2009)], an efficient LH full wave solver based on Finite Element Method (FEM) was used. In this study the scalability of the LHEAF approach was investigated, and the possibility of using massive parallel computer for solving extremely large problems was shown. In reactor scale plasmas, LH waves having a typical n_|| ≈ 2 are expected to be absorbed in the periphery of the plasma. In order to exploit the spatial localization of the LH waves, LHEAF is modified to consider only the region of plasma where the wave fields are non-zero. By this approach, the size of the computational domain was reduced by more than a factor of 10. In this simulation, the magnetic equilibrium and the density and temperature profiles proposed for AT operation scenario on ITER are used. In addition, the wide SOL is supposed to play an important role in the propagation of the LH waves on ITER, and its presence was included in the simulation. For a Maxwellian plasma the power deposition profile is narrow and peaks at r/a ≈ 0.7.

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A bright polychromator for compact, efficient YAG Thomson scattering measurement was designed on the basis of ray-tracing calculations. The optical input of the polychromator is a 2-mm-diameter optical fiber with a numerical aperture of 0.37. High refractive index glass lenses with monolayer anti-reflection coatings were used to reduce aberration effects and achieve good efficiency. A fast, low-noise detection system consisting of an avalanche photodiode and operational amplifiers was designed and tested using YAG laser pulses; the measured full width at half maximum (FWHM) of the laser pulse was 10 ns. The measured noise was dominated by the thermal noise of the input resistance R (= 470 Ω) when the signal was low, and the shot noise became dominant when the number of detected photons was greater than about 500.

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It is important to determine the best method for reconstructing the magnetic flux when eddy currents are significantly induced during magnetic measurement in spherical tokamaks (STs). Four methods for this reconstruction are investigated, and the calculated magnetic fluxes are compared to those measured in the cavity of a vacuum vessel. The results show that the best method is the one that uses currents from virtual coils for reconstruction. In this method, the placement of the virtual coils is optimized with numerical simulations using the Akaike information criterion (AIC), which indicates the goodness of fit of models used to fit measured data. The virtual coils are set on a line 15 cm outside the vacuum vessel.

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Neutral beam injection heating has been studied for high-density mode plasma in the GAMMA 10 tandem mirror. During NBI heating, the increase in diamagnetism was found to be larger for the high-density mode plasma than the hot-ion mode plasma. A numerical model using zero-dimensional particles and energy balance equations was developed to investigate NBI heating in the central cell area. The evolution of total energy density was found to be consistent with that of diamagnetism in the low-density hot-ion mode. The initial ion temperature is a key parameter for enhancing beam heating. In high-density mode, discrepancies were found between the measured diamagnetism and the calculated total energy density. It is probably because that time behavior of the electron temperature is not suitable.

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Charge exchange-neutral particle analyzer diagnostics are used to obtain majority ion temperature profiles for plasmas created in the TJ-II stellarator. However, because of technical limitations, measurements are limited to two radial positions per discharge. Therefore plasma reproducibility for a large number of discharges is essential for performing scans across the plasma minor radius and hence for obtaining ion temperature profiles. Such conditions have recently been achieved in the TJ-II by the implementation of a lithium wall coating which has significantly facilitated this task. In this paper, ion temperature profiles obtained under electron cyclotron resonance (ECR) and neutral beam injection (NBI) plasma heating conditions are compared with electron temperature and density profiles obtained using a Thomson Scattering diagnostic. From this, similarities in the forms of the ion temperature and electron density profiles are noted for ECR heated plasmas whilst for NBI heated plasmas similarities are observed between ion temperature and electron temperature profiles. Finally, after explaining the different behaviours of the profiles in terms of ion power absorption profile, we demonstrate the possibility of determining ion temperature profiles using two localized ion temperature measurements plus Thomson Scattering profiles.

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In positive ion based Neutral Beam Injectors (NBI), generally corona model is used in analyzing the Doppler shifted spectroscopy diagnostics data for estimating the ion species mix in the ion-source, ion and beam species fractions in extracted beam and power fractions injected into Tokomak. At the beam energies 10-60 keV/amu, the non-radiative processes such as collisional quenching of the excited neutrals affect these estimations when background pressure is ≥ 1 mTorr. We present here a modified corona model that takes into account the effect due to collisional quenching. We describe the application of the present model to a typical Doppler shifted spectral data obtained in SST-1-NBI injector.

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To expand the application range of heavy ion beam probe measurement in the Large Helical Device (LHD), it is important to increase the probing beam current. The current can be increased by improving the charge exchange efficiency in a gas cell. Experiments were conducted on a tandem accelerator at Kobe University, 5SDH-2, which allows various experimental conditions such as gas species, gas pressure, and beam energy to be selected. Initially, the gas thickness was determined by a beam attenuation method with a He⁺ ion beam. Next, the dependence of the positive beam current on the gas thickness was measured. The ionic charge fraction of the accelerated beam was measured and compared with the value calculated using the ionization cross section calculated in [M. Nishiura et al., NIFS Report, NIFS-884 (2008)]. The dependence of the Au⁺ fraction on the target gas thickness shows good qualitative agreement with the experimental value. However, the cross sections obtained experimentally are several times larger than those calculated.

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A method for neutral beam measurement by using a Faraday cup is proposed in this paper. The method enables us to detect neutral beams by controlling secondary electrons by using a biasing grid in front of the Faraday cup. A procedure is also proposed for in situ determination of the secondary electron emission coefficient of the Faraday cup. Experimental results show that appropriate emission coefficients are determined for helium beams with energies of 2-10 keV. The neutral flux charge-exchanged from a helium ion beam passing through a helium gas is also measured. Saturation of the neutral flux is observed above the pressure expected from the solution of a rate equation. The method is promising for neutral beam measurement, especially for small current-neutral beams.

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A detailed spectral profile of the Lyman-α line of neutral hydrogen, i.e., the transition from n = 2 to the ground state, where n is the principal quantum number, was measured with a vacuum ultraviolet spectrometer for the plasma in the Large Helical Device. Self-reversal was observed in the spectral profile when the plasma density was increased with repetitive injection of hydrogen pellets. A one-dimensional radiation transport model was used for creating the Lyman-α spectral profile, for which an emission and absorption medium with a slab geometry and constant plasma parameters were assumed. The population density of the n = 2 level generally has a peaked spatial profile even with constant ground state density because of the reabsorption effect which is essential for the emergence of self-reversal in the spectral profile. We used the n = 2 level population distribution in the medium by Molisch et al. [Radiation Trapping in Atomic Vapours, Oxford University, Oxford, 1998] and evaluated the Lyman-α spectral profile as a function of the optical thickness. The observed line profile was found to be well fitted, for example, with a ground state density of 5.2 × 10¹⁸ m⁻³ and a medium thickness of 10 cm when a Lorentzian profile having a full width at half maximum of 0.0018 nm is adopted for the emission and absorption coefficients.

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In order to excite fast wave in the ion cyclotron range of frequencies (ICRF) using multiple antennas with the phase difference in the Large Helical Device (LHD), a controllable wavenumber antenna that consists of two single-strap antennas is designed, and the electrical characteristic features of the antenna are estimated by using a three-dimensional electromagnetic commercial code and the simplified antenna model. Controlling the radio-frequency (RF) phase difference between these two single-straps, reverse-phase excitation can be achieved in order to reduce the impurity production. According to this estimate, the RF current profile on the strap surface is strongly concentrated on the both horizontal strap edges, and the electrical strap length at frequency of 85 MHz is longer than the quarter wavelength of 85 MHz (λ_{85 MHz}/4). Excitable wavenumber spectra are different in various RF phases and frequencies. At the low frequencies (< 60 MHz) with in-phase, effective wavenumbers between k = 0 m⁻¹ and k = 10 m⁻¹ can be excited. During the reverse-phase excitation, large (k = 6, 15 m⁻¹) wavenumber spectra with low (k = 0 m⁻¹) wavenumber kept small are obtained.

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The dispersion characteristics and slow-wave instabilities of a sinusoidally corrugated waveguide with coaxial slow-wave structure (SWS) are analyzed. In SWS, a central cylindrical conductor is surrounded by an outer cylindrical conductor. Sinusoidal corrugation is formed on either conductor or both conductors. The corrugation parameters are those used for an X-band SWS. The relative phase between the sinusoidal corrugations on the inner and outer conductors affects the dispersion characteristics. Instabilities due to beam interactions with the slow -waves are examined by considering three-dimensional beam perturbations. The slow cyclotron instability occurs in addition to the Cherenkov instability, since transverse as well as longitudinal perturbations are included.

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The Thomson scattering system in the TST-2 has been upgraded to improve the reliability and accuracy of measurements. The signal intensity increased because of a new high-energy (1.6 J) laser. A large-numerical-aperture (N.A.) fiber was tested, and it was found that a 6-m-long fiber can be used without significant transmission loss. With the upgraded system, the typical central electron temperature and the electron density for ohmic discharges (with a plasma current of 60 kA) are 150 eV and 1.5 × 10¹⁹ m⁻³, respectively. The temperature profile has a maximum near the center of the plasma.

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Studies of coaxial oversized backward wave oscillators (BWOs) are reported. The beam voltage is weakly relativistic (less than 100 kV). The slow-wave structure consists of a periodically corrugated oversized waveguide and periodically corrugated inner conductor, whose target operating frequency due to Cherenkov interaction is in the K-band. A starting energy exists for the coaxial oversized BWO as it does for a hollow oversized BWO. The coaxial slow-wave structure has two surface wave modes caused by the inner and outer corrugations. Operation based on these surface modes can be controlled by the beam diameter. The phase difference between the inner and outer corrugations has little effect on operation of the oversized BWO.

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A preliminary simulation study of Doppler reflectometry is performed. The simulations solve Maxwell's equations by a finite difference time domain (FDTD) code method in two dimensions. A moving corrugated metal target is used as a plasma cutoff layer to study the basic features of Doppler reflectometry. We examined the effects of the full width at half maximum (FWHM) of the electromagnetic waves and the corrugation depth of the metal target. Furthermore, the effect of a nonuniform plasma is studied using this FDTD analysis. The Doppler shift and velocity are compared with those obtained from FDTD analysis of a uniform plasma.

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Infrared Imaging Video Bolometers (IRVB) provide an image of the plasma radiation by using a thin metal foil to absorb the radiation, which is then imaged by an IR camera. In LHD IRVBs at Ports 6-T and 1-O have been upgraded. Both of the ∼ 1 micron × 70 mm × 90 mm gold foils have been replaced with ∼ 2.5 micron Pt foils of the same area for greater sensitivity (Pt vs Au) and absorption of higher energy photons estimated up to 8.2 keV. Also the IR cameras have been replaced; an FLIR/SC500 (∼ 100 mK, 60 fps, 320 × 240 pixels) IR camera at Port 1-O with a FLIR/Phoenix (∼ 16 mK, 345 fps, 320 × 256 pixels) IR camera at Port 10-O and an AGEMA/THV 900 LW (∼ 190 mK, 15 fps, 136 × 272 pixels)] with a FLIR SC-4000 (∼ 19 mK, 327 fps, 320 × 256 pixels) IR camera at port 6-T. In the current campaign these IRVBs will use 8 mm × 8 mm apertures resulting in 12 × 16 channels on the foil. The noise equivalent power density for these two new IRVBs ranges from ∼ 360 µW/cm² for 327 fps operation to 3.6 µW/cm² for 15 fps operation, which is over two orders of magnitude of improvement over the previous IRVB at Port 6-T.

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A multi-channel version of the multi foil thermal detector (MFTD) prototype has been developed and tested at the FNL ion accelerator in Tohoku University, which uses an imaging bolometer as a thermal sensor and 2-dimensional set of the foils with the different thicknesses as an energy discriminator for alphas. A Phoenix IR camera from FLIR Systems had been used for monitoring the temperature profile variations under the ion beam irradiation. The results obtained showed that the tested foils are applicable for the MFTD design.

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At Japan Atomic Energy Agency (JAEA), a MeV accelerator has been developed to demonstrate acceleration of H⁻ ion beams at the ITER-relevant power density. After long pulse beam acceleration tests of up to 10 s, molten areas were observed around the apertures of the grids due to excess heat load on the grids. To identify the cause of the melting, a three-dimensional (3D) beam analysis was performed. The stripping loss of negative ions and magnetic fields in the accelerator were included in the calculation to examine the beam trajectories precisely. It was clarified that the beamlet deflection angle was larger than 10 mrad due to space charge repulsion among the beamlets and magnetic fields, which resulted in excess heat loads of more than 20 kW/cm² at the grounded grid. To compensate for beamlet deflections, an aperture offset and field shaping plate were designed.

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We have developed a compact electron spectrometer (ESM) to investigate ignition mechanism in the Fast Ignition Realization Experiment project. Hot electrons, produced by the irradiation of a gold target by using a compressed chirped pulse laser, are used for auxiliary heating of the imploded core. An imaging plate is used as the electron detector for medical purposes. However, the relationship between the beam intensity and the detector signal is not clear. The ESM should be calibrated because it is highly compact and has a complex magnetic field for bending caused by installation in a limited space. We have performed the calibration by using an L-band LINAC at the Institute of Scientific and Industrial Research, Osaka University, in order to obtain an accurate electron spectrum. The calibration used a single electron pulse at two different energies, 9.5 MeV and 27.1 MeV, with 0.1-10 pC. The energy spreads are 0.2 MeV at 9.5 MeV and 0.3 MeV at 27.1 MeV. The analyzer has been tested to measure energetic electrons from plain and integrated targets irradiated by the Laser or Fast Ignition Experiment (maximum energy of 10 kJ) up to 800 J.

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A fast-ion charge exchange spectroscopy (FICXS) diagnostic was recently installed on the Large Helical Device (LHD). The aim of this diagnostic is to evaluate fast-ion confinement property by measuring energy-resolved fast-ion spatial profiles in LHD. To accurately evaluate the FICXS profile, the effect of halo neutrals, which are created by the charge exchange between bulk ions and injected neutral beams (NBs), must be treated properly in the current analysis of the FICXS spectra in the LHD. We have estimated the effect of halo neutrals on the measurement from the shapes of Doppler-shifted H-alpha spectra. The ratio of halo neutrals to the injected NBs on the lines of sight (LOSs) is estimated by using the EIRENE-3D Monte Carlo code. The results of these two methods agreed well. This indicates that the effect of halo neutrals should be considered when interpreting the FICXS diagnostic data as well as that from the injected NB, when the probing NB is radially injected and the emitted photons are observed from perpendicular LOSs.

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The University of Tokyo Spherical Tokamak (UTST) was constructed to explore the formation of ultrahigh-beta spherical tokamak (ST) plasmas using double null plasma merging. The main feature of the UTST is that the poloidal field coils are located outside the vacuum vessel to demonstrate startup in a reactor-relevant situation. Initial operations used partially completed power supplies to investigate the appropriate conditions for plasma merging. The plasma current of the merged ST reached 100 kA when the central solenoid coil was used to assist plasma formation. Merging of two ST plasmas through magnetic reconnection was successfully observed using two-dimensional pickup coil arrays, which directly measure the toroidal and axial magnetic fields inside the UTST vacuum vessel. The resistivity of the current sheet was found to be anomalously high during merging.

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Ion cyclotron range of frequencies (ICRF) heating is used to heat plasma in magnetically confined fusion plasma experiments. ICRF heating has been used in the Large Helical Device (LHD) and contributes to high-power steady-state experiments. Antenna loading resistance is important in ICRF heating; a high loading resistance is required for high-power injection. Many elements influence the antenna loading resistance. Here, the dependence of the loading resistance on various parameters is investigated. The loading resistance is very low at lower wave frequencies. High-power injection using such frequencies was difficult in plasma heating experiments in the LHD. The loading resistance increases with the plasma density. The distance between the antenna and the plasma boundary is closely related to the plasma edge density. It is important to keep the antenna away from the plasma and also keep the loading resistance at a certain level in steady-state operation for the various types of plasma. The effect of additional heating and magnetic field strength are also investigated. These results will contribute to the design of new ICRF antennas, the ICRF heating experiment in the LHD, and ICRF heating in future plasma devices.

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Optical emission in a negative hydrogen ion source for the Large Helical Device Neutral Beam Injector (LHD-NBI) has been measured to investigate the behavior of Cs. Two optical sight lines exist parallel to the plasma grid, in the discharge area and in the magnetic filter area near the plasma grid. In the discharge area, the spectrum intensity from Cs⁺ ions is considerably increased during 20 s of the beam extraction. This indicates a considerable increase in the Cs⁺ density inside the plasma due to the impact of back-streaming H⁺ ions. A strong neutral Cs spectrum is observed in the magnetic filter area, where the electron density is lower than in the discharge area. The rate of increase of neutral Cs is much enhanced after t = 30 s, probably because the Cs adsorbed on the cooled region inside the arc chamber evaporates because its temperature increases during the long pulse discharge.

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