Plasma and Fusion Research Vol. 12(2017)

Bispectrum and bicoherence analysis is a powerful method to analyze nonlinear interaction of turbulent plasmas. In this review, we explain the difference of the bispectrum and bicoherence analysis and possible research that can be pursued with these methods. Basic concept is explained by using several examples from previous successes brought by bicoherence and bispectrum analysis, such as drift wave-zonal flow interaction. Open questions are discussed that can be challenged by these methods. Problems such as spatial transport of fluctuation energy (i.e. turbulence spreading), momentum transport by the triplet correlation, and so on, are treated. Bispectrum and bicoherence analysis can be utilized to expand the forefront of plasma turbulence research.

Experimental plasma discharges in linear PANTA device are studied by Mach probe measurements, providing floating potential, ion saturation current, and parallel flow velocity time evolution, at different radii of the device. Spectral analysis indicates that drift waves and D'Angelo modes exist simultaneously in the plasma. A discrimination study shows they are located at different positions in radius and frequency.

Charge flows were induced from a laser ablation plasma to a grounded target. The current signals, which were directly measured using a current monitor, developed from negative to positive depending on the dynamically evolving plasma. The results showed that, initially the current was induced by an electron flow from the plasma plume to the surrounding wall and, after a transient phase, the current was replaced by an ion flow to the wall. The results suggest that there was a breaking of the quasi-neutral state of the ablation plasma and that an ambipolar electric field (double layer; DL) was generated during the ablation process.

The density fluctuation was measured with a 70-GHz microwave interferometry system in the linear divertor plasma simulator NAGDIS-II while changing from the attached to the detached states. In the detached plasma, a large amplitude fluctuation of ≤ 10 kHz appeared in the peripheral region. The fluctuation has a maximum value at the neutral gas pressure of 30 - 34 mTorr. Further, the measurement of the light emission associated with the plasma recombination indicates that the large amplitude fluctuation is located at just downstream of the recombination front, in which the recombination process is the most enhanced.

Coaxial Helicity Injection (CHI) has now been implemented in QUEST. The goals for the first transient CHI experiments were to establish reliable gas breakdown conditions, and to measure CHI-produced toroidal current generation. Both these objectives were successfully met. Toroidal currents up to 29 kA were measured. Interestingly, these first plasmas on QUEST also suggest the formation of small amounts of closed magnetic flux surfaces.

In magnetic confinement plasmas, plasma rotation plays an important role in the penetration and propagation processes of external RMP fields. To investigate the relation between the penetration of RMP fields and plasma flow, we have created a new type of Mach probe array to measure plasma flow profiles. The experimental results show that plasma flow around the q = 3 resonant surface decreases with increases in RMP fields.

Magnetic reconnection in the presence of a high-guide-field is utilized to heat the plasma in the merging start-up of a spherical tokamak configuration. The reconnection current layer between two spherical tokamaks was gradually compressed, and a quasi-steady current-sheet thickness was obtained. During the compression phase, the current layer split transiently into two separated layers, which provided a flattened magnetic-field-region near the X-point. This modification may affect the electron-energization mechanism in the merging start-up of a spherical tokamak in a high-guide-field.

The importance of the parallel momentum balance on the bootstrap current evaluation in non-axisymmetric systems is demonstrated by the benchmarks among the local drift-kinetic equation solvers, i.e., the Zero-Orbit-width (ZOW) model, DKES, and PENTA. The ZOW model is extended to include the ion parallel mean flow effect on the electron-ion parallel friction. Compared to DKES code in which only the pitch-angle-scattering term is included in the collision operator, PENTA code employs the Sugama-Nishimura method to correct the momentum balance. The ZOW model and PENTA codes, both of which conserve the parallel momentum in like-species collisoins and include the electron-ion parallel frictions, agree each other well on the calculations of the bootstrap current. The DKES results without the parallel momentum conservation deviates significantly from those from the ZOW model and PENTA. This work verifies the reliability of the bootstrap current calculation with the ZOW model and PENTA for the helical plasmas.

Large-scale reactor-relevant fusion plasmas are likely to operate near marginal stability. In this regime, we show clear evidence of interplay between core and edge regions of the plasma. This result illustrates aspects of the controversial ‘shortfall problem' in the far-core, near-edge so-called ‘No Man's Land' region and a possible route to resolve this issue. More generally, it emphasises global-scale organisation of turbulence and relevance of edge dynamics to core confinement.

The characteristics of input energy control with an intense pulsed power device was investigated to explore the properties of warm dense matter in an implosion time scale of inertial confinement fusion. An electron beam diode, which served as an impedance controller and could suppress the prepulse, was placed on the output terminal of the intense pulsed power device. The anode (output) current was regulated by changing the gap distance of the electron beam diode. The input energy into the dummy load representing the sample was controlled with the electron beam diode.

Hydrogen isotopes plasma-driven permeation (PDP) through F82H coated with two different types of tungsten coatings, i.e., sputter-deposited tungsten (SP-W) and vacuum plasma-sprayed tungsten (VPS-W) has been studied in the temperature range of 300 - 550 ^◦C. It has been found that hydrogen isotopes PDP fluxes through VPS-W coated F82H are reduced compared to that through bare F82H. However, the PDP fluxes through SP-W coated F82H are enhanced compared to bare F82H. Reduced or enhanced PDP fluxes are related to the different microstructure of tungsten coatings and its surface recombination characteristics.

A 300 GHz pulse gyrotron has been developed for use in collective Thomson scattering (CTS) diagnostics in the Large Helical Device (LHD). Single-mode oscillation power of more than 320 kW was produced. The radiation beam has a Gaussian pattern. The frequency spectrum is very narrow and stable across the pulse width. No competing mode was observed in the oscillation pulse, including during the turn-on and turn-off phases.

Two plasma-focus electrodes, consisting of a pair of counter-facing center electrodes surrounded by six outer electrodes, can sustain a long-life, high-energy-density (HED) plasma initiated by laser-triggered multi-channel discharges. With the goal of making a high-average-power plasma device, we investigated the repetition rate of this device. Double-pulse experiments showed that when the device was driven with positive polarity, in which the center electrodes were biased at a positive voltage, the time required for the device to return to a non-conducting state was less than 100 µs, regardless of the operating voltage. The device thus has a repetition rate of more than 10 kHz.

Distributed instabilities can successively change one after another to give accelerated radial propagation. The response of the linearly unstable distributed instabilities is identified in a gradual evolution phase as well as in a phase just after impact of modulation. Global nonlinear simulations of drift-interchange modes in helical plasmas are carried out with source modulation using a reduced MHD model. Conditional average of long time-series data with the modulation period reveals characteristic responses of the plasma. Smaller-scale variations comparable to the micro-temporal scale in this simulation are also included in the gradual evolution phase. The correlation analysis shows that the evolution of the mean pressure is strongly correlated with the strength of the nonlinear coupling. The evaluation of the energy balance to decompose the energy transfer into contribution from each three-wave coupling clarifies that a single mode coupling at each location has the dominant contribution to the smaller-scale pressure evolution in spite of self-organized mechanism with a wide range of comparable magnitude modes. Comparison of mode amplitudes does not define the dominant one, so identification of the active mode is useful for understanding the causality. This selection suggests the mechanism that gives the spreading effective in the quasi-steady state as for the ballistic propagation in the self-organized critical state.

The absorbed dose rate in air was measured at the National Institute for Fusion Science (NIFS) site before the deuterium plasma experiment in the Large Helical Device (LHD). A pocket survey meter was used for the measurement of 1cm dose equivalent rates in units of µSv h⁻¹ and these results were converted to absorbed dose rates in air (units: nGy h⁻¹) using a conversion factor. The arithmetic mean of the absorbed dose rates in air based on 257 measurement points at NIFS site was 45 nGy h⁻¹. The result of this study suggests that the building material and/or paving stone enhance the dose rates in air at the NIFS site.

The current radioactive cesium limit in food in Japan was established on the basis of a permissible dose relating to the food regulation in Japan (1 mSvy⁻¹) on April 2012. The current limit only accounts for the influence of ¹³⁴Cs, ¹³⁷Cs, ⁹⁰Sr, ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, and ¹⁰⁶Ru; however, the contribution due to other radionuclides such as ³H is not included. In this study, the principle focus was the influence of ³H and the necessity for establishing permissible ³H food contamination limits in the light of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. It was found that no adjustment to ³H limits in food was required due the FDNPP accident. This is because effective doses that include the contributions of ³H would not exceed the permissible dose for food regulation in Japan as long as regulatory controls are performed using the current limits. The results and concepts in this study will prove helpful for food regulation when fusion reactors with ³H are operational.

Three-dimensional particle-in-cell simulations are performed to investigate the mixing process of low energy electrons and positrons for the confinement in a simple magnetic mirror with and without the plug potentials. According to the results of simulations, it is suitable to confine low energy electron-positron plasmas in a compact magnetic mirror trap.

The nonlinear phase of magnetic reconnection is investigated by numerically solving a gyrofluid model. The scaling law for the explosive reconnection rate, which has been recently derived for an ideal two-fluid model [Hirota et al., Phys. Plasmas 22, 052114 (2015)], is found to consistently hold when either the ion-sound gyroradius ρ_S or the ion gyroradius ρ_i is comparable to the electron skin depth d_e, even in the presence of finite resistivity η. In this explosive phase, a local X-shaped current layer is spontaneously generated, in which the reconnection speed is closely related to the macroscopic shape of the layer and is almost independent of the layer width. The reconnection speed is therefore insensitive to the size of the microscopic scales, ρ_S, ρ_i, d_e and η. On the other hand, in the cold plasma limit, where ρ_S = ρ_i = 0, the intermittent acceleration of the reconnection speed is caused by the plasmoid instability. This also seems to be explosive on average, but the rate always falls below the explosive scaling law. The reconnection time extrapolated from this scaling law is shown to be fast enough to explain the time scale of solar flares.

Electron density (n_e) and electron temperature (T_e) measurements were performed via the plasma assembly for nonlinear turbulence analysis (PANTA) using the laser Thomson scattering technique. The second harmonic of Nd:YAG laser (λ = 532 nm) and an intensified charge-coupled device were used as a light source and a detector, respectively. Plasmas in PANTA were generated with Ar gas in a pressure range of 1 - 5 mTorr. The range of the applied magnetic field was 600 - 1500 G. At the center of the plasma, n_e and T_e ranged (4 - 20) × 10¹⁸ m⁻³ and 0.8 - 3 eV, respectively. Further, n_e monotonically increased and T_e monotonically decreased with the increasing gas pressure and magnetic field.

Changing the experimental conditions such as the neutral pressure and the magnetic field strength provides a drastic change in the turbulence states in linear magnetized helicon plasmas. In order to define the turbulence states and their occurrence region a throughout parameter scan in the two-dimensional parameter space was performed. The classification of the turbulence states is carried out phenomenologically based on the turbulence spectrum and the waveform.

Plasma rotation plays an important role in the suppression of turbulence, leading to an increase in energy and particle confinement. Significant rotation also leads to a stabilisation of the resistive wall mode. The external momentum input from Neutral Beam Injection (NBI) in current generation fusion plasmas may not be available for future self-heated fusion reactors. Therefore it is important to analyse the phenomenon of spontaneous rotation. At NIFS plasma rotation and momentum transport of the Large Helical Device (LHD) plasma is analysed using a code suite called TASK3D-a. In this work neoclassical effects, which can be especially significant in non-axisymmetric plasmas, were implemented in TASK3D-a. Initial analysis of neoclassical radial momentum flux profiles shows that in NBI-driven momentum input neoclassical effects, especially neoclassical damping, become dominant in the non-center plasma region. It was also found that during and after pellet-injection the neoclassical damping force becomes strong. With the implementation of neoclassical effects new features can be examined in the momentum flux-gradient relations; in the damping-dominated situation following pellet injection a large excursion in momentum flux is found. This work can aid in the search for neoclassical transport-optimised configurations for enhanced (spontaneous) plasma rotation.

In order to clarify the mechanism responsible for slowing down the precursor of an instability in Large Helical Device (LHD), whose behavior is similar to the locked mode instability in tokamaks, the spatial structure of the precursor of the locked-mode-like instability, and the relationship between the rotation of the precursor and the E × B rotation were experimentally investigated. The precursor rotates together with the E × B rotation at the resonant surface, and the precursor rotation slows down because of a decrease of the E × B rotation. The multi-channel fluctuation measurement of the precursor suggests that the precursor has a magnetic island, which may be related to the decrease of the E × B rotation. In addition, the reason for the appearance of the precursor with a magnetic island is discussed. The precursor appears when a magnetic island grows initially without rotation but then shrinks and begins to rotate.

A compact hard X-ray measurement system was developed to measure bremsstrahlung of fast electrons generated by lower hybrid wave (LHW) in the TST-2 spherical tokamak. The system, which consists of an NaI scintillator, a photomultiplier tube, a curved acrylic lightguide, a lead collimator, and linear and rotational stages,enables us to measure the energy flux profile on the midplane with the energy resolution of 10 - 15 eV at 122 keV and with the energy range from a few tens to several hundreds of keV. Preliminary hard X-ray measurements were performed for LHW-driven TST-2 plasmas, and energy spectra were attained successfully. The typical effective temperature of the spectra is in the order of 10 keV, and we obtained the time evolutions of energy flux and effective temperature with a time resolution of 10ms. Profile measurement showed that the flux at the inboard side is higher than that at the outboard side.

We have extended “numerical matching method” to weakly nonlinear regime, which is relevant for the Rutherford regime of magnetic island evolution in normal magnetic shear plasmas as well as for reversed magnetic shear plasmas to which the Rutherford theory does not apply. The numerical matching method was developed for linear stability analyses of resistive magnetohydrodynamics (MHD) modes by utilizing an inner region with a finite width, that removes difficulties inherent in its numerical applications of the traditional matched asymptotic expansion. The extended method is applied to low-beta reduced MHD simulations of magnetic island evolution in cylindrical plasmas with normal and reversed magnetic shear profiles. The numerical results agree well with fully nonlinear simulation without using the matching method from the linear to weakly nonlinear regimes continuously. Since the nonlinear equation is solved only in the inner region of a finite width, the computational cost is reduced, which enables us to include more detailed physics effects. Our extended method therefore makes a significant contribution in the MHD analysis of magnetic island evolution beyond the restriction in the conventional Rutherford theory.

The effects of impurities on runaway electron generation are studied using a zero-dimensional disruption simulation code. For describing collisions between fast electrons and partially stripped ions, a charge-resolved expression of the Coulomb logarithm is employed. Numerical analysis of the avalanche growth rate using the adjoint Fokker-Planck method is compared with two existing semi-analytic models, showing (i) the convergence of the growth rate to strong electric field limit of the Rosenbluth-Putvinski (R-P) model and (ii) the cancellation of the effect of second-order collisional diffusion for intermediate electric fields. Using the developed current quench (CQ) simulations, the parametric study is performed with the aid of the power balance analysis, which characterizes the onset of strong avalanche amplification in the presence of low-Z and noble gas species. Thermal quench (TQ) simulations are also developed for self-consistent evaluation of hot-tail seed electrons. The deposition timescale of impurity neutrals is shown to have significant impacts on hot-tail seeds, depending non-monotonically on the pre-TQ temperature and the injected impurity density.

In this study, we newly propose two types of water-cooled divertor devices called EVAPORON-3 using high thermal conductivity porous media that could be applicable especially to an enlarged heat transfer surface such as the divertor plate. Each device has a liquid-vapor separating plate (LVS plate) on the top surface of the porous medium. The LSV plate of the Type-1 device has 9 inlet holes for liquid supply and 12 outlet holes for vapor discharge against a heat transfer surface with the diameter of 30 mm. On the other hand, the LSV plate of the Type-2 device has just one liquid inlet hole at the center of the plate and 12 outlet holes for the vapor discharge. The diameter of each hole is 2.6 mm. The introduced porous medium is a particles-packed bed with the particle diameter of 1.0 mm. In order to reduce pressure loss of the liquid and vapor flows in the porous medium, two layers of the particle are set between the heat transfer surface and the LSV plate. The thickness of the porous bed is 1.82 mm. The result showed that each device enabled a high heat removal of over 5 MW/m² and, in particular, the Type-2 device succeeded in the heat flux removal of approximately 11 MW/m² at the water flow rate of 2 L/min, although the pressure loss increased by decreasing the number of the inlet holes.

The required pumping power for the liquid metal ergodic limiter/divertor REVOLVER-D (Reactor-oriented Effectively VOLumetric VERtical Divertor), which is considered as an optional divertor design for the LHD-type helical fusion reactor FFHR-d1, has been estimated with a consideration of the change in the strength of transverse magnetic field along the flow channel. It is found that the pressure drop and the required pumping power can be suppressed to an acceptable level by using insulated ducts.

A new cartridge-type blanket named the CARDISTRY-B is proposed for the helical fusion reactor FFHR-d1.This blanket is composed of the neutron shield and the tritium breeder using molten salt. Both of these are toroidally segmented every two degrees. At each toroidal angle, the segmented parts are divided further into several cartridges in order to make it possible to assemble these cartridges after completion of the superconducting magnet coils. The neutron shield is basically assembled by using mortise and tenon prepared on each of the cartridges and the lower port, instead of the wide area welding. After assembly, the plasma side of the neutron shield is welded to form a vacuum vessel. Another side of the neutron shield facing on the superconducting magnet coils is covered with the thermal shield, which was already attached before assembly. The tritium breeder cartridges can be replaced without cutting or welding of cooling pipes inside the vacuum vessel, where severe radiation dose is expected. Details of the CARDISTRY-B, including the results of motion analysis for all cartridges and estimation of the cartridge weight, are discussed.

For developing the high temperature fatigue life evaluation method using small specimen, the effect of specimen size and test environment on the high temperature fatigue life of the reduced activation ferritic/martensitic steel, F82H-IEA, was investigated at 550 ^◦C under the total strain range of 0.5 - 1.2 % using a new high temperature low cycle fatigue testing machine for the small round-bar specimen. No significant effect of the test environment (oxidation) on the fatigue life was observed in the standard-size specimen, whereas the slight reduction of the fatigue life due to that was indicated in the small-size specimen, especially at the final fracture stage of the test. It could be concluded that the new high temperature low cycle fatigue testing machine of the present study could satisfy the minimum requirements for the high temperature low cycle fatigue life evaluation using small specimen because the fatigue life data obtained using this testing machine could be acceptable based on the comparison with the previous studies data. To improve the reliability and the applicability of this test technology, further evaluations under the wider test temperature and strain conditions are expected in future.

Recent progress of the physics and engineering design study for the 8m-sized DEMO is reported. Parametric study for the divertor of the compact DEMO (a machine size ∼ 5.5 m) by using the SONIC code shows that the target heat load less than 10 MW/m² around the fusion power of ∼1.5 GW and the impurity radiation fraction of more than 80%. In the 8 m sized DEMO with these parameters, the partial detachment is obtained at the outer divertor, even in the low SOL density, due to the large impurity radiation in the SOL and divertor region. The SONIC simulation shows the peak of the target heat load is 7 MW/m². However, the peak of the ion temperature at the target is considerably high, which causes significant erosion of the target. The divertor power handling and decrease in the ion temperature have to be proceeded by the scenario development of the divertor plasma operation as well as the core plasma design and the engineering design. In the engineering study side, the tungsten monoblock target with the water cooling and the Cu-alloy cooling tube is designed. The MCNP-5 neutronics analysis shows applicability of the Cu-alloy cooling tube for the divertor unit on the high heat flux region. Also the divertor cassette with a heat removability of peak heat load of 10 MW/m² is studied. The heat transport analysis shows the maximum temperature of 1021 ^◦C at the tungsten surface and 331 ^◦C at the Cu-alloy pipe, which are acceptable level for mechanical toughness and thermal fatigue.

The non-thermal electrons (NTE) in tokamak plasma have a strong effect over the plasma properties and on the plasma facing components. This establishes an importance of NTE to be studied in both flux and energy space. The NTE was studied for Aditya tokamak by monitoring X-ray spectrum using a Silicon Drift Detector (SDD) based X-ray spectroscopic diagnostic for the ECR pre-ionization experiments. The spectra shows X-ray line radiation band on top of the bremsstrahlung. Line radiation is primarily the combination of multiple characteristics lines existed between 5 keV to 7 keV and mostly associated with multiple ionized states of iron, tokamak wall material. This is likely due to the interaction between the NTE, generated during ECR pre-ionization experiment, with the stainless steel vessel wall of the Aditya Tokamak. The behavior of NTE was studied as a function of vertical field and pre-filled pressure. With the application of vertical field, energy carried by the NTE is reduced due to higher vertical drift velocity in comparison to the case of without one. Significant reduction of the X-ray line radiation strength with higher pre-filled pressure also indicates the reduction of NTE due to the enhanced electron neutral collision.

The effects of oxygen pick-up in an irradiation environment in a vacuum chamber during ion- and neutron-irradiation of V-4Cr-4Ti alloys were studied. The density and size of titanium oxides in the alloys were found to drastically increase because of the oxygen pick-up, especially for cases of higher irradiation dose and specimen temperature. Using Zr foil as a diffusion barrier and doping the V-4Cr-4Ti alloys with Y can be considered as effective techniques for reducing the reaction of Ti with C, O, and N to form Ti(C,O,N) because of oxygen-pick up from irradiation environment.