A new method is presented for identifying waveforms of fluctuations in turbulent plasmas. The method is based on heartbeat analysis in which the convolution of a waveform is obtained by employing the phase tracking method. Phase tracking is performed by correlating raw time-series data with a template waveform; the template is evaluated through iteration procedure. The method is applied to fluctuations in a PANTA plasma, and the nonlinear waveform and its distribution of periods are obtained.

View full abstract

A multi-pass Thomson scattering (TS) system based on confining laser pulses in an optical cavity was constructed for measuring very low-density plasma in the TST-2 spherical tokamak device. This paper describes the setup of the optical system, injection of the laser pulse into the cavity, and properties of the confined laser pulse. A combination of Pockels cell plus polarizer, which serves as an optical shutter, allows us to inject and then confine intense laser pulses in the cavity. A photodiode signal monitoring the very weak light leaking from the cavity mirrors demonstrated that the laser pulse makes many round trips, with a round-trip efficiency of approximately 0.73. The effective number of round trips (i.e., the signal enhancement factor) is approximately 3.7. For an injection efficiency of approximately 0.69, a cavity-confined laser pulse, applied to Thomson scattering, will yield a scattered signal that is five times larger than that from a single-pass laser pulse.

View full abstract

We analyzed the higher harmonics of temperature perturbation in the modulated electron cyclotron heating experiment on TJ-II plasma. The higher harmonics (e.g., 5th harmonic) exhibited significantly weaker decay in amplitude as they propagated in radius as compared with the prediction by diffusive model. The change in the time derivative of temperature at the onset (and turning-off) of the heating power propagates in radius with very little temporal smoothening.

View full abstract

The plasma in Tohoku University Heliac (TU-Heliac) is biased by a hot cathode that drives the J × B poloidal rotation. In the biased plasma, a density collapse accompanied by high-frequency bursting density and potential fluctuations was observed. To eliminate the effect of potential fluctuations and to measure the magnetic fluctuation, a high common mode rejection ratio (CMRR) magnetic probe with a preamplifier at the probe head in a vacuum vessel was designed and installed. Owing to the absolute calibration, the magnetic probe has a high magnetic sensitivity of 10⁴ V/T (@100 kHz), high CMRR of −75 dB (@500 kHz), and frequency bandwidth DC −1 MHz. In the biased plasma, the power spectrum of the normal mode signal was 10⁴ times larger than that of the common mode signal in all frequency ranges, which indicates that the probe can successfully minimize the capacitive pickup noise relative to the actual magnetic fluctuation signal. In addition, we successfully observed the broad spectrum (100 kHz < f < 300 kHz) of the magnetic fluctuation and MHD instability in the biased plasma in TU-Heliac.

View full abstract

The hysteretic magnetic island response to an externally applied resonant magnetic perturbation (RMP) field is observed in the LHD. Thresholds of the amplitude of the RMP for the growth/healing transition of the magnetic island differ. In the case that the RMP is ramped up, that field is initially shielded and the magnetic island is healed. After that, when the increasing RMP exceeds a threshold, that field penetrates into the plasma and the magnetic island appears. In the case that the RMP is ramped down, the threshold of the RMP for island healing is smaller than that for growth.

View full abstract

A Tracer-encapsulated Solid Pellet (TESPEL) injection method has been shown to be useful for many fields of plasma diagnostics. In order to provide more flexibility for such plasma diagnostics, we are developing various types of TESPELs. Here we report a new type TESPEL with a thin shell made of poly-dichlorostyrene useful for the impurity transport study. The new multilayer TESPEL configuration is also discussed. This type of TESPEL is not only useful for impurity transport study but also for the neutron observation due to the transient increase of the deuterium in the local position in the plasma for the upcoming LHD D-D experiment.

View full abstract

A multi-pass Thomson scattering (TS) system has the advantage of enhancing scattered signals. We constructed a multi-pass TS system modeled on the GAMMA 10 TS system; the new system has a polarization-based configuration with an image relaying system. For the first time, we used the new system to measure electron temperatures in the GAMMA 10 plasma. By using the multi-pass TS system with four passes, the integrated scattering signal was magnified by approximately a factor of three.

View full abstract

For successful plasma current (I_p) ramp-up by the lower hybrid wave (LHW), the plasma density must be kept at a low level. In the TST-2 I_p ramp-up experiment using the LHW, the application of the electron cyclotron wave (ECW) was found to reduce the plasma density by about 10%. This allowed for further I_p ramp-up later in the discharge.

View full abstract

Utilizing the advantage of the Tracer-encapsulated Solid Pellet (TESPEL) injection method, the radiation power emitted from heavy atoms such as tungsten is measured knowing the total amount of the heavy atoms. The experimentally obtained radiation power is compared with the theoretical calculations. The TESPEL method is found to be useful for evaluating the absolute radiation power from the heavy atoms, although the present status is preliminary and further data should be accumulated.

View full abstract

The kinetic ballooning mode (KBM) turbulence in Tokamak plasma is investigated by electromagnetic gyrokinetic simulations. From the entropy balance analysis, it is revealed that the field-particle interactions transfer a significant fraction of the ion entropy produced by the instability to electrons. Then, the produced ion entropy balances to the sum of the ion and electron dissipations at the saturation of the KBM instability growth, in contrast to ITG turbulence where ion entropy production mostly balances to ion dissipation.

View full abstract

The nonlinear simulation of resistive ballooning turbulence is performed in tokamak edge geometry. The spatiotemporal autocorrelation is calculated for the gradient of turbulent heat flux. The typical ballistic nature in the correlation plot is introduced by the “Lagrangian correlation,” which has spatial and temporal dependence. Propagation distances of the ballistic pulses of the gradient of turbulent heat flux are quantified and are about four times the characteristic size of the front.

View full abstract

Ion temperature gradient (ITG) driven turbulence simulation for a transient edge temperature sink localized in the poloidal plane is performed using a global Landau-fluid code in the electrostatic limit. Pressure perturbations with (m, n) = (±1, 0) are induced by the edge sink, where m and n are poloidal and toroidal mode numbers, respectively. It was found in the previous simulation [M. Yagi et al., Contrib. Plasma Phys. 54, 363 (2014)] that the nonlinear dynamics of these perturbations are responsible for the nonlocal plasma response/transport connecting edge and core in a toroidal plasma. Present simulation shows, however, that the ITG turbulence in the core region dissipates the large-scale (m, n) = (±1, 0) perturbations and weakens the edge-core connection observed in the previous simulation.

View full abstract

Sequential phenomena of fusion reactor at anomaly events, e.g., sudden loss of coolant due to leaks or guillotine breaks at ex-vessel, should be carefully studied before the construction and operation of the fusion power demonstration reactor (DEMO). While a possibility of plasma termination due to impurity ejection from the wall by the loss of its cooling ability is usually considered, there might be a possibility of a plasma disruption induced by the thermal quench of the toroidal field (TF) coil due to the decrease of nuclear radiation shielding performance. Neutronics analyses taken for the Slim-CS DEMO designs with and without coolant water in the shield blanket showed that the nuclear heating rate at its TF coil increases more than 100 times by the coolant loss. Thermal calculation with the calculated heating rates indicated that the TF coil would reach the critical temperature within several seconds. Thus, a passive shutdown sequence other than the impurity from the wall sequence possibly occurs in case of the loss-of-coolant accident (LOCA) by a large penetration of neutrons onto the TF coils, a quench of coil current, and a termination of plasma confinement.

View full abstract

To improve the thrust efficiency of a pulsed magnetohydrodynamic (MHD) accelerator, we perform numerical calculations that simulate the experimental conditions, which were used earlier, in an apparatus that include a model rocket engine. The one-fluid one-dimensional-MHD simulation results show agreement between the experimental and numerical results. We discuss simulation results for temporal and spatial distributions of the electrical conductivity and current density.

View full abstract

A broadband continuously frequency tunable gyrotron with a triode-type magnetron injection gun was developed as power source for analysis of protein structures. The TE_7,3 oscillation mode was selected to avoid mode competitions in the high magnetic field side. Axial modes of the TE_7,3,−10 were sequentially excited by changing the cavity magnetic field, and frequency tuning of about 4 GHz around 395 GHz was observed with output power greater than 50 W. The frequency also varied about 1GHz as the anode-cathode voltage varied. Thus, the broadest tuning bandwidth in the 400 GHz band gyrotrons was achieved.

View full abstract

The effect of ion impact on the incubation time of single-walled carbon nanotubes (SWNTs) during plasma chemical vapor deposition (CVD) has been studied. Based on the systematic investigations, the incubation time of large-diameter SWNTs was found to be sensitive to the ion impact during plasma CVD. The incubation time difference (Δt_i) between small- and large-diameter SWNTs can increase up to 120 s with the introduction of appropriate ion impact during the growth process of the SWNTs. The selectivity of the effects of ion impact on the incubation time of small- and large-diameter SWNTs was also found to be sensitive to the hydrogen concentration during plasma CVD. This indicates that the hydrogen ion can play an important role during the nucleation period in the development of SWNTs with plasma CVD. Since the increase in Δt_i for each chirality is important for the realization of chirality-selective growth of SWNTs, we believe that our findings can contribute to this aspect, one of the main goals in the science and application fields of SWNTs.

View full abstract

The electrical device performances of graphene directly grown on a SiO₂ substrate have been improved through the precise adjustment of growth conditions such as growth temperature and growth time in plasma chemical vapor deposition (CVD). Only at the suitable combination of growth time and temperature, high quality and uniform graphene sheet can be directly grown on a SiO₂ substrate. Forward and reverse sweeps of source-drain current (I_ds) vs. gate bias voltage (V_gs) showed small hysteresis, possibly caused by the clean surface of the graphene device fabricated by plasma CVD, a technique that did not involve any transfer. Four-point probe measurements to evaluate the intrinsic sheet resistance of the fabricated graphene showed its value to be 170 - 200 Ω/sq, a value much lower than that of graphene directly grown on SiO₂ substrate by other techniques. This low sheet resistance possibly originated from the high quality of graphene obtained by plasma CVD. These observations suggest that graphene directly grown on SiO₂ substrate by plasma CVD should be a very promising candidate for fabrication of graphene-based high-performance electrical devices.

View full abstract

A capacitive-coupled power extraction magnetohydrodynamic (MHD) generator has electrodes outside a channel, that carries a combustion gas; the channel functions as a capacitor. By applying an alternating magnetic field, this MHD generator can extract power from the capacitors. Because the configuration prevents the electrodes from exposure to the gas, this kind of generator offers the advantage of being able to operate over a long time. Based on a theoretical model, the performance of the generator is evaluated when a sinusoidal alternating magnetic field is applied. From the results, we assess the feasibility of obtaining electrical power from this device.

View full abstract

The initial stage of fiber-form nanostructure growth on tungsten and molybdenum surfaces with helium plasma irradiation is investigated by a technique that allows time evolution of nanostructure growth to its spatial variation. The pitting of the original base surface resulting from successive hole formation is clearly demonstrated. The surface fine-grained miniatuarization proceeds with increasing hole area. Then, loop-like nano-scale structures appear with some coherency, which are thought to be precursors of fiber-form tendrils. Loop raptures and branching out are considered as possible candidates of the initial stage of nano-fiber growth.

View full abstract

A Thomson scattering diagnostic system has been developed for measuring electron temperatures in low-aspect-ratio reversed-field pinch (RFP) plasmas in REversed-field pinch of Low Aspect ratio eXperiment (RELAX). In the range of plasma currents I_p = 50 - 80 kA, the central electron temperature was around 100 eV and, showed a weakly increasing trend as I_p increases. To estimate the central electron pressure p_e0, a density calibration was performed from simultaneous measurements with a 104-GHz microwave interferometer. The maximum p_e0 increased with I_p up to ∼ 70 kA to preserve the approximate relation p_e0/B_θa² ∼ constant, where B_θa is the edge poloidal field. In the higher current region, p_e0 tended to saturate, which may be improved by optimizing minute control of equilibrium in the higher current region.

View full abstract

A feedback control system for the stabilization of resistive wall mode (RWM) was applied to a low-aspect-ratio reversed field pinch (RFP) with minimum power supply capabilities to control the single mode. The system consists of 64 saddle coils (4 and 16 in poloidal and toroidal direction, respectively) in the actuator covering the whole torus on the outer surface of the vacuum vessel. The sensor coils also have the same structure. The saddle coils are connected in series to control the single m/n = 1/2 mode, which has the largest growth rate in RELAX. The radial component of the magnetic field from the sensor coils was suppressed to the preset level and the m/n = 1/2 magnetic mode, which otherwise grows with field penetration time of the vessel, was reduced to 0.1% of the edge poloidal field throughout the discharge. The RFP discharge duration has been extended to ∼3.5 ms, the upper bound determined by the saturation of the iron core. Finally, the MHD control issues in a low-A machine are discussed.

View full abstract

A statistical approach is proposed to predict thermal diffusivity profiles as a transport “model” in fusion plasmas. It can provide regression expressions for the ion and electron heat diffusivities (χ_i and χ_e), separately, to construct their radial profiles. An approach that this letter is proposing outstrips the conventional scaling laws for the global confinement time (τ_E) since it also deals with profiles (temperature, density, heating depositions etc.). This approach has become possible with the analysis database accumulated by the extensive application of the integrated transport analysis suite to experiment data. In this letter, TASK3D-a [M. Yokoyama et al., Plasma Fusion Res. 9, 3402017 (2014)] analysis database for high-ion-temperature (high-T_i) plasmas [H. Takahashi et al., Nucl. Fusion 53, 073034 (2013)] in the LHD (Large Helical Device) [O. Kaneko et al., Nucl. Fusion 53, 104015 (2013)] is used as an example to describe an approach.

View full abstract

A new design of a cylindrical discharge-type fusion device is proposed to achieve the precise alignment of components and prevent abnormal discharges. Cylindrical insulators that act as parts of the vacuum boundary and fix the relative position between cathode and anodes were used. The configuration of the electrodes was optimized to maintain stable discharge at low pressures and high discharge voltages, which enhances the neutron yield. The optimization was performed via analysis of the electric field and ion movements by developing a 2D code, and the results were experimentally verified. The results show that grounding the anodes is better for stable discharge at lower pressures. The optimized combination of the anode height and cathode overhang is 3 cm and 1 cm, or 2 cm, respectively. The discharge characteristics and neutron production rate were then experimentally measured for the optimized configuration. The attained highest discharge voltage (instantaneous) was ∼50 kV and the neutron production rate of 3 × 10⁴ n/s was obtained by continuous discharge with an applied voltage of 30 kV and discharge current of 30 mA.

View full abstract

Spatial profiles of the atomic (H⁰) density and the resultant H_α line intensity are investigated in a large negative ion source (the Japan Atomic Energy Agency (JAEA) 10 A negative ion source). The H⁰ density analysis has been done in the present study with the effects of production, transport, and ionization processes by taking into account the non-Maxwellian component of electron energy distribution function (EEDF). The H⁰ density profile shows a non-uniform spatial profile due to the local enhancement of the H⁰ production rate even with the flattening effects by the ionization and the transport processes. The H_α line intensity observed from the viewing ports in the spectrometry is compared with the line intensity in the calculation to validate the numerical results. The both results show a good agreement in the spatial profile. It has been shown that the non-Maxwellian component of the EEDF plays an important role to determine the profile of the H_α line intensity in the plasma production region.

View full abstract

We measured extreme ultraviolet spectra of Fe ions for plasmas produced in the Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS). Iron was injected into the plasmas by using a tracer-encapsulated pellet. By controlling the neutral beam injection pattern, we could produce plasma with a central electron temperature of approximately 500 eV, which was suitable for producing Fe XVII ions. We measured seven Fe XVII lines. The intensity ratio for λ of 20.468 to 25.493 nm was consistent with the theoretically calculated value of 1.1. This calculated value was determined purely from the branching ratio due to the common upper level of these transitions, although Warren et al. [Astrphys. J. 685, 1277 (2008)] reported a larger ratio of approxinately 2 from Hinode EIS measurements. The other five ratios for Fe XVII lines in our LHD measurements were also consistent with the theoretical ratios calculated with a collisional-radiative model. A preferred atomic dataset for Fe XVII is suggested to obtain better agreement between the measured and calculated ratios.

View full abstract

The influence of a guide field on collisionless driven reconnection is investigated by means of two-dimensional electromagnetic particle simulation in an open system. In a quasi-steady state when reconnection electric field evolves fully, a current layer evolves locally in a narrow kinetic region and its scale decreases in proportion to an electron meandering scale as the guide field is intensified. Here, the meandering scale stands for an average spatial scale of nongyrotropic motions in the vicinity of the reconnection point. Force terms associated with off-diagonal components of electron and ion pressure tensors, which are originating from nongyrotropic motions of charged particles, becomes dominant at the reconnection point and sustain the reconnection electric field even when the guide field is strong. It is also found that thermalization of both ions and electrons is suppressed by the guide field. For the weak guide field, an electron nonthermal component is significantly created through a fast outburst from the kinetic region, while for the strong guide field, an ion nonthermal component is generated through the acceleration by an in-plane electric field near the magnetic separatrix.

View full abstract

The diagnostics method of a tracer-encapsulated solid pellet (TESPEL) has been developed. TESPEL consists of polystyrene as an outer shell and of specific material as a tracer in the core. Owing to the advantages of the TESPEL, the following results have been successfully obtained: (1) distinctive different feature of impurity transport between the plateau and Pfirsch-Schlüter regimes depending on the impurity source location which is analyzed with the STRAHL code, (2) specific feature of a non-local thermal transport such as abrupt increase of electron temperature in the plasma core in case of plasma cooling in the plasma periphery due to a small TESPEL injection, (3) spatially resolved energy distribution of the high energy particles obtained by a pellet charge exchange method, (4) longer impurity containment inside the magnetic island which is observed by depositing the tracers in the magnetic island by means of the TESPEL, and (5) identification of new spectral lines using interested atoms contained in the core of the TESPEL.

View full abstract

Thomson scattering system is absolutely calibrated by anti-Stokes rotational Raman scattering from nitrogen gas filled in HuanLiuqi-2A (commonly referred to as HL-2A) chamber. Since the intensity of strayed laser light from the lower and upper divertor throats is very strong, additional transimission-type short pass filters are directly piled up on each interference filter to block the laser wavelength. The obtained data of electron density show good reliability, and they are in time trace coincident with the measured results of HCN laser interferometer. The uncertainties of measured results and the improving methods of the system are discussed.

View full abstract

For the MHD equilibrium reconstruction of a reverse field pinch device, it is a big issue to identify accurately the strong eddy current flow on the shell. In the present work, boundary integrals of the eddy current along the shell are added to the conventional Cauchy-condition surface method formulation. The eddy current profile is unknown in advance but straightforwardly identified using only the signals from magnetic sensors located outside the plasma. Two ideas are introduced to overcome the numerical difficulties encountered in the problem. One is an accurate boundary integral scheme to damp out the near singularity occurring at the sensor position very close to the shell. The other is the modified truncated singular value decomposition technique to solve an ill-conditioned matrix equation when a large number of nodal points exist on the shell. The capability of the new method is demonstrated for a test problem modeling the RELAX device.

View full abstract

A new effective wall-conditioning technique was proposed for realizing high ion temperature (T_i) plasmas in the Large Helical Device using the ion cyclotron range of frequency (ICRF) wave under the established magnetic confinement field. A series of ICRF heating discharges using He as the working gas was conducted ahead of the high-T_i plasma discharges. After sufficient repetitive wall-conditioning discharges, we observed a decrease in the line-averaged electron density, the formation of a peaked electron density profile, a reduction in the Hα emission, and an increase in the central T_i. The results suggest that the stored hydrogen inside the vacuum vessel structures, such as the first wall, the diverter, and other components, sputtered out owing to the He plasmas of the ICRF wall-conditioning discharges and the hydrogen recycling was decreased. Consequently, the ion heating power of NBI increased in the plasma core region, leading to higher central T_i.

View full abstract

A ka-band multi-channel Doppler reflectometer system was constructed for the Large Helical Device (LHD) using a comb frequency generator as a source. A filter bank system is utilized for precise quadrature phase detection, and preliminary back-scattered waves were obtained in LHD plasma experiments. In addition, a direct digital signal acquisition system was successfully demonstrated for providing a greater number of multi-channel measurements.

View full abstract

A two-dimensional transport modeling applicable to a whole tokamak plasma is proposed. The model is derived from the multi-fluid equations and Maxwell's equations and the moment approach of neoclassical transport is employed as fluid closures. The multi-fluid equations consist of the equations for particle density, momentum, energy and total heat flux transport for each plasma species. The expressions of the parallel viscosity and heat viscosity are extended in order to be applicable to both inside and outside of the last closed flux surface. It is confirmed that our neoclassical transport model is consistent with the ordinary flux-surface-averaged one-dimensional neoclassical transport model. Our transport equations are coupled with the electromagnetic equations in order to describe the time evolution of tokamak plasmas. The procedure for coupling a transport solver based on our transport model with an equilibrium solver is also briefly described.

View full abstract

A local gyrokinetic Vlasov simulation code GKV is extended to incorporate realistic tokamak equilibria including up-down asymmetry, which are produced by a free-boundary 2D Grad-Shafranov equation solver MEUDAS. By using a newly developed interface code IGS, two dimensional rectangular equilibrium data from MEUDAS is converted to straight-field-line flux coordinates such as Hamada, Boozer, and axisymmetric coordinates, which are useful for gyrokinetic micro-instability and turbulent transport analyses. The developed codes have been verified by a cross-code benchmark test using Cyclone-base-case like MHD equilibrium, where good agreement in the dispersion relation of ion temperature gradient (ITG) driven mode has been confirmed. The extended GKV is applied to two types of shaped plasmas expected in JT-60SA tokamak devices, i.e., ITER-like and highly-shaped plasmas, and ITG-mode stability and residual zonal-flow level are investigated. Through the detailed comparisons, more favorable stability properties against the ITG mode are revealed for the highly-shaped case, where the lower ITG-mode growth rate and higher residual zonal-flow levels compared to the ITER-like case are identified.

View full abstract

Effects of the Hall term and the gyro-viscosity on the Rayleigh-Taylor instability in a 2D rectangular slab are studied numerically. Nonlinear magneto-hydrodynamic (MHD) simulations with these effects reveal that the combination of the Hall term and the gyro-viscosity causes the lower growth rates and the lower saturation level of unstable modes relative those in the single-fluid MHD case, while neither the gyro-viscosity nor the Hall term shows a strong stabilization effect only by itself. It is also shown that the mixing width of the density field can grow as large as that in the single-fluid MHD case, even though the saturation level of the kinetic energy is lowered and the detailed density profile becomes sharper. These numerical results suggest that the extension of the MHD equations can bring about a growth of unstable modes in a lower level, although it does not necessarily mean a weaker impact of the instability to the equilibrium.

View full abstract

Temperature Screening Effect (TSE) on the impurity transport in fusion plasmas has been studied by a kinetic Monte Carlo simulation model [Y. Homma and A. Hatayama, J. Comput. Phys. 250, 206 (2013)]. The TSE drift is induced by the background-plasma tempetature gradient across the magnetic field, and is dependent on various parameters of the impurity species as well as on the background-plasma condition. A series of test simulations has been performed in wide range of parameter space. It has been confirmed that the parametric dependences of the TSE drift are correctly reproduced by our kinetic model.

View full abstract

Numerical methods for solving the monoenergetic drift kinetic equation (DKE) are powerful tools for obtaining viscosity coefficients. However, these methods do not apply when the collision frequency and radial electric field become large. For example, when the radial electric field becomes large, poloidal resonance effect occurs and degrades the accuracy of the numerical solutions to DKE. But when we calculate the neoclassical viscosity in Heliotron J, which is an L = 1 helical-axis heliotron device, we cannot neglect the resonance effect in the presence of high-Z ions. In this study, we combine viscosity coefficients calculated by the numerical method with those obtained from analytical solutions that take the effects of the first poloidal resonance into account. We use this method to obtain monoenergetic viscosity coefficients for arbitrary collision frequencies and radial electric fields in the L = 1 heliotron device.

View full abstract

Nonlocal electron heat transport in magnetized dense plasmas is studied numerically using a nonlinear Fokker-Planck (FP) model with self-consistent electric fields. The nonlocal effect in electron heat transport is evaluated by comparison with the effective mean free path and the scale length of the temperature gradient. The dependence of the nonlocal electron heat transport on the effective mean free path is shown in this study. Under a very strong magnetic field, the effective electron mean free path becomes shorter than the scale length of the temperature gradient and the results of the FP and linear models agree well. Under a very strong magnetic field, the electron distribution is described by the Maxwell-Boltzmann distribution.

View full abstract

Implosion and heating experiments with the scheme of fast ignition (FI) have been performed on Gekko-XII and LFEX laser platform at the Institute of Laser Engineering in Osaka University. A cone-guided CD-shell has been used as a base-line target for the fast ignition realization. The information about energy transfer from LFEX petawatt (PW) laser is quantitatively studied with an absolute Kα diagnostics and a fast electron trajectory simulation. The transfer efficiency (η_TE) is estimated for a planar interaction as a reference and for several types of guiding-cone. As a general trend, the guiding-cone enhanced the η_TE by a factor of 3 comparing with the planar case.

View full abstract

In this work we present the technical implementation of a digital VERSA Module Eurocard (VMEbus) system used to detect and track, in real-time, magneto-hydrodynamic instabilities on the JET tokamak. This VMEbus system runs on a 1 ms clock cycle and performs the unsupervised detection and real-time tracking of the individual components in a multi-harmonic spectrum of coherent electro-magnetic instabilities, actively driven by a set of in-vessel antennas. Its main real-time output signals are the frequency, amplitude, toroidal mode number and damping rate of such modes. Moreover, this controller also provides some of the protection and control tools for the antenna system, such as the reference for the voltage and current control waveforms, and a trip signal related to the shorted-turn protection of the antennas. Current applications of this novel controller focus on the measurement of the damping rate of Alfvén Eigenmodes with different toroidal mode numbers. The successful technical implementation and scientific exploitation of this innovative VMEbus system opens possibilities for the real-time detection and the ensuing control of electro-magnetic instabilities in other present and future fusion devices.

View full abstract

The electrical insulator and hydrogen permeation barrier coatings are important materials to realize the liquid metal and molten-salt typed breeding blanket systems. We found that erbium oxide (Er₂O₃) is one of the promising materials as the electrical insulator and hydrogen permeation restraint coatings. Establishing the mechanical property evaluation method for these coating is extremely important to certify the durability of coating material in the blanket systems. The adhesion strength property, which is one of the key mechanical properties of coating materials, was investigated using the nano-scratch method. From the results, it was found that the nano-scratch test was able to evaluate the adhesion strength of the Er₂O₃ coating synthesized by the Metal Organic Chemical Vapor Deposition (MOCVD) process with high reproducibility. Furthermore, the adhesion strength of the Er₂O₃ coating before and after thermal cycling was evaluated using this method. The adhesion strength after 50 thermal cycles at 700^◦C was kept around 70% compared with that before thermal cycling.

View full abstract

To reduce maintenance workers' dose rate caused by activated dust adhering to the ITER blanket remote handling system (BRHS), dust must be removed from BRHS surfaces. Dust that adheres to the top surface of the BRHS rail from cyclic loading of the vehicle manipulator is considered to be the most difficult dust to remove. Dust removal experiments were conducted to simulate the materials, conditions, and cyclic loading of actual BRHS operations. The tungsten powder used to simulate the dust was squashed, and the area of contact by cyclic load was increased, but the powder was not embedded into the matrix. The increase in the area of contact increased the total intermolecular force between a tungsten particle and the surface, which was considered the main force adhering dust to the test piece surface. A combination of dust removal methods, including vacuum cleaning and brushing, was applied to the simulated dust that adhered to the test pieces. The results showed that vacuum cleaning is effective in removing dust from the non-cyclic loaded surface. The combined methods were highly efficient in removing the dust that strongly adhered to the rail surface.

View full abstract

Conceptual design studies are being carried out on the application of high-temperature superconducting (HTS) conductors and coils to the magnet systems of fusion reactors. A 100-kA-class HTS conductor is required to be applied at high magnetic fields of > 12 T. A simple stack of YBCO tapes embedded in copper and stainless-steel jackets is found to be a practical approach to producing large-scale conductors that exhibit high cryogenic stability and mechanical rigidity. The feasibility of the segmented fabrication method for large complex HTS coils, such as the helical coils in the LHD-type helical fusion reactor FFHR-d1, is being investigated by developing mechanical bridge-type lap joint technology of HTS conductors.

View full abstract

High-temperature superconductor technology has become remarkably advanced, and components such as current leads and magnets have been explored for their applicability to fusion. Especially, REBa₂Cu₃O_7-δ (RE = rare-earth: REBCO) tape has been developed in research fields other than fusion, because of superior performance of REBCO in high magnetic fields. Here, we fabricated a REBCO coil with a radius of 150 mm and a total coil current of 55.2 kA, to be applied as an internal floating coil in Mini-RT, a plasma experimental device. This newly fabricated REBCO coil has replaced the Bi₂Sr₂Ca₂Cu₃O₁₀ (BSCCO) coil developed in 2003, and has improved coil performance and plasma operation. A REBCO tape of width 4.3 mm was employed, and a 0.1 mm-thick copper laminate was attached for the countermeasure at the quench. In addition, the REBCO tape was wrapped in a polyimide sheet for electric insulation, increasing the total thickness of the REBCO tape to 0.27-0.28 mm. A persistent current switch was constructed from a small-turn REBCO winding with a heater formed from a stainless steel sheet. Solder joint of two REBCO tapes over a 30 mm length were employed, by overlapping their copper laminates. Persistent current, with a decay time ∼320 h, has been successfully achieved at 25 K operation temperature. This decay time corresponds to a resistance of ∼125 nΩ, roughly consistent with the total resistance of the seven joint sections in the persistent current circuit.

View full abstract

In future fusion reactors, plasma control can be anticipated to be quite complicated because actuators and diagnostics would be limited because of extreme environmental conditions, such as high neutron fluxes. In addition, control parameters and actuators are not in simple one-to-one correspondences (e.g., NBI power affects not only plasma current but also fusion power). This results in the need of using multi-input multi-output control systems. To confront this problem, we have developed a control system design that involves an state equation. In this research, simulations were performed in which three plasma parameters (fusion power, plasma current, and plasma density) were controlled using three actuators (NBI power, amount of gas puff, and inductively driven current). Parameters for these actuators were determined from the state equation, and the plasma parameters were simultaneously controlled with sufficiently high accuracy.

View full abstract

Nanostructure formation on molybdenum surfaces due to helium plasma exposure is investigated, with a focus on determining the temperature band for the growth of fiber-form nanostructures. Precise temperature measurements can be obtained using thin thermocouples inserted into sheet specimens. The temperature range for nanostructure growth was determined to be 800 ∼ 1050 K under incident helium ion energies of 50 ∼ 100 eV and an ion flux of 2×10²¹ m⁻² ·s⁻¹. Surface morphologies near the upper and lower boundary temperatures differ from those of a standard fiber-form nanostructure. In a standard case, nano-fibers of molybdenum were found to have diameters of approximately 50 nm, whereas those of tungsten are half of the molybdenum fibers.

View full abstract

Key aspects of the safety study of a water-cooled fusion DEMO reactor is reported. Safety requirements, dose target, DEMO plant model and confinement strategy of the safety study are briefly introduced. The internal hazard of a water-cooled DEMO, i.e. identification of hazardous inventories, identification of stored energies that can mobilize these hazardous inventories and identification of accident initiators and scenarios, are evaluated. It is pointed out that the enthalpy in the first wall/blanket cooling loops, the decay heat and the energy potentially released by the Be-steam chemical reaction are of special concern for the water-cooled DEMO. An ex-vessel loss-of-coolant accident (ex-VV LOCA) of the first wall/blanket cooling loop is also quantitatively analyzed. The integrity of the building against the ex-VV LOCA is discussed.

View full abstract

The first wall of a fusion reactor blanket faces the core plasma directly. The first wall endures high heat loads that lead to high thermal stresses. To ensure the reliability of the first wall structure, it is desirable to reduce the thermal stress. In this study, structural optimization of the blanket first wall was carried out using the Taguchi method. The finite element method was used to conduct a numerical simulation to investigate the thermo-mechanical responses of the blanket first wall. The optimal configuration of the blanket first wall was derived.

View full abstract

Magneto Plasma Sail (MPS) is one of the next generation space propulsion systems which generates a propulsive force using the interaction between the solar wind plasma and an artificial inflated magnetosphere generated by a superconductive coil. In the MPS system, the magnetosphere as a sail must be inflated by the plasma injection from the spacecraft in order to obtain the thrust gain. In the present study, the magnetic inflation concept is numerically tested by so-called ion one-component plasma model. As a simulation result, the magnetic moment of the system is drastically increased up to 45 times that of the coil current at plasma-β = 20 and r_Li/L (radius of gyro motion / characteristics length of the magnetic field) = 0.01, and this is the first successful magnetosphere inflation obtained by numerical simulation. Corresponding maximum thrust gain is also estimated to be about 45.

View full abstract

Experiments on a recombination plasma with pulse plasma flow have been performed in the linear divertor simulator TPD-SheetIV. The pulse plasma flow was generated using a switching circuit controlled by the electric potential of the adjacent floating electrode in the plasma source. The duration of the pulse was 0.3 ms at a frequency of 50 Hz. The time dependence of the electron density n_e , temperature T_e , and energy distribution function f_e(E) were measured using a Langmuir probe. The ionization and recombination events are analyzed using the Collisional-Radiative model, taking into account high-energy electrons.

View full abstract

A hybrid code which uses the spectral method for an incompressible fluid and the combined compact finite difference method for passive scalar is developed and applied to compute the spectrum of the passive scalar variance in turbulence at very high Schmidt numbers up to 1000. The accuracy and efficiency of the hybrid code are found to be very satisfactory when compared to the full spectral computation. The scalar spectrum in the viscous-convective range by direct numerical simulation is found to obey k⁻¹ power law and to exponentially decay in the far diffusive range, and compared to Kraichnan’s spectrum. It is argued that the exponential decay of the spectrum in the far diffusive range is due to the intermittency effect of the velocity field.

View full abstract