Non-neutral electron plasmas in a uniform magnetic field are investigated experimentally as a two dimensional fluid. It is confirmed that the phase space volume increases during a vortex merging process, which is consistent with the presence of viscosity. It is also suggested that the following vortex holes movement is associated with a dissipative process.
A conical target made of brass was irradiated by a transversely excited atmospheric CO2 laser having a pulse duration of ∼ µs in atmospheric air and He. A two-step structure composed of two spheroidal shock waves was observed to be driven by the irradiation. The shock front evolved at more than 7 km/s corresponding to a shock Mach number of 20. Before the two-step evolution of the shock wave, we observed a streamer-like structure in the laser-irradiated region, which indicates a phenomenon driven by energetic electrons.
A prototype system for plasma turbulence tomography was tested in linear plasma with aiming at future application to toroidal plasma. The line-integrated plasma emission lines of ArI and ArII were measured with sufficiently high signal-to-noise ratios, and the signals were reconstructed using the maximum likelihood expectation maximization (MLEM) algorithm and parallel computing. Here, the first results for local plasma emissions and fluctuation spectra obtained with the prototype system are reported.
An application of Hadamard transform method to laser-induced fluorescence spectroscopy (LIF) is proposed, in order to improve spatial resolution, signal-to-noise ratio, and experimental efficiency. The new method allows for measurement of spatial structure of ion flow and/or temperature efficiently with single detector by changing Hadamard mask configurations.
Recent advances in head mounted display (HMD) systems, specifically those demonstrated by the Oculus Rift, provide a new platform for three-dimensional scientific visualization. Taking advantage of this opportunity, we have constructed a cost-effective video see-through visualization system by combining a stereoscopic camera system and an Oculus Rift device. The see-through HMD system enables a researcher to analyze numerical data in a virtual reality space, with keeping visual communication with nearby collaborators in real space. We have ported our visualization software for CAVE systems, VFIVE, to the HMD system. The ported software enables its user to analyze three-dimensional scalar/vector fields in a virtual reality space while simultaneously being able to view the natural surroundings.
The ion temperature and flow of RF start-up plasmas in TST-2 and LATE were measured using a visible spectrometer. The plasma currents were 9 kA and 8 kA, respectively. The typical ion temperatures Ti and toroidal flow Vφ were 4 eV and 1 km/s, respectively, in the TST-2 plasma sustained by the lower hybrid wave (20 kW) and Ti ∼10 eV and Vφ ∼5 km/s in the LATE plasma sustained by the electron cyclotron wave (50 kW). The poloidal flow velocities were comparable to the toroidal velocities. The ion temperatures were relatively high and the ion orbit loss can be significant.
An innovative color measurement technique is employed in the Large Helical Device (LHD). This study provides a method for obtaining in broad spatial extent and in great detail the color information of the first wall relating to the thickness of the deposition layer. The RGB (Red, Green, and Blue) value, mainly of the stainless steel plates on the helically twisted coil, is measured by a color analyzer equipped with an integrated sphere light source. On the outer torus side, the colors of almost all stainless steel plates are close to carbon black, which suggests that deposition is dominant. On the inner torus side, all plates except for those neighboring the carbon divertor plates are almost white, as in the case of the stainless steel substrate of the first wall, which suggests that erosion is dominant. The relationship between the color and the distance from the stainless steel plates to the plasma is investigated.
In this research, the profiles of electron temperature Te and density ne for a spherical tokamak with the plasma current sustained by lower hybrid waves alone have been measured for the first time using Thomson scattering diagnostics in the TST-2 spherical tokamak device. The Te profile was hollow and the ne profile was like bell-shaped. Te and ne near the plasma center were 6 eV and 6 × 1017 m−3, respectively, leading to a pressure of 0.6 Pa. On the other hand, whole pressure at the plasma center calculated using an equilibrium reconstruction code EFIT was around 20 Pa. Therefore, it is suggested that fast electrons play an important role in the plasma equilibrium.
Pilot-PSI is a magnetized linear plasma device designed for investigating the plasma-surface interactions at ITER-relevant parameters. A frequency-multiplied microwave interferometer system was installed on the Pilot-PSI device for preliminary measurements in the divertor-relevant plasma. We report measurements of the electron line integrated density and its fluctuations. The Hα line emission was monitored using a fast visible camera and was compared with the interferometer data. Both diagnostics measured similar fluctuation frequencies. This suggests that the fluctuations of ions and electrons are well coupled in Pilot-PSI, at least in the plasma regime that was investigated.
We have tested the telegraph equation for transport dynamics in magnetized plasma by comparing its results with experimental observations in the Large Helical Device [S. Inagaki et al., Nucl. Fusion 53, 113006 (2013)]. The telegraph equation includes a finite relaxation time for turbulence intensity in response to the changes in global plasma parameters. This model was applied to nondiffusive radial heat pulse propagation under the periodic modulation of the heating power. The model showed some success in reproducing the amplitudes of higher harmonics. However, the phase relation between the temperature gradient and heat flux was opposite to that in experimental observations.
The runaway electron (RE) generation during tokamak disruptions is investigated by kinetic simulations. Specifically, three dimensional (two-dimensional in momentum space; one-dimensional in the radial direction) Fokker-Planck simulations are coupled with the self-consistent electric field caused by the disruptions. The thermal quench time is varied, and the results are compared with those of the steady-state solution of the RE generation rate. The hot-tail effect is enhanced when the thermal quench time is shorter than the electron slowing down time.
Self-healing (spontaneous shrinkage) of externally induced magnetic islands is a critical issue in helical systems, where helical ripple-induced neoclassical viscous torques play essential roles. In this study, effective helical ripple rates of magnetic fields in multi-helicity helical systems are revisited. In a typical parameter regime of the Large Helical Device, effective helical ripple rates are sensitive to magnetic axis positions. An extended theory of the self-healing taking into account effective helical ripple rates is firstly developed. It is newly found that self-healing thresholds considerably depend on magnetic axis positions, which is due to neoclassical viscous torques depending on effective helical ripple rates.
The adverse effects of DEMO on the future electrical grid due to sudden output interruptions, such as disruptions, were analyzed quantitatively. The results indicated that when considerable percentage of renewables are installed, the power system would experience serious frequency deviations as large as 0.4 Hz, which is greater than the current tolerance, 0.2 Hz. DEMO installation would need an assessment as part of the power system, together with mitigation devices, to be connected to the grid.
Formation energy of an isolated hydrogen atom in Cr23C6 has been investigated using atomisitic calculation based on the density functional theory. The lowest calculated formation energy is −0.48 eV, where a hydrogen atom is located at a trigonal bipyramidal site surrounded by five Cr lattice atoms, due to electric charge of atoms. Although it is a rough estimate, a comparison with the formation energy in Fe may imply that hydrogen retention in F82H steel can be much higher in Cr23C6-based precipitate than in Fe-based matrix.
A fast two-color pyrometer system was developed to measure the back-surface temperature of thin tungsten materials during plasma-gun generated edge localized mode-like pulsed plasma irradiation. The developed pyrometer system had a time resolution of ∼5 µs and the lowest measureable temperature was ∼1600 K. We observed that the back-surface temperature of the thin tungsten material during the pulsed plasma irradiation reached ∼3280 K. The absorbed energy density and the pulse width of the pulsed heat load estimated by the measured time evolution of the back-surface temperature and 3D heat analyses using ANSYS code were ∼0.52 MJm−2 and ∼1.6 ms, respectively.
Significantly collimated fast electron beam with a divergence angle 10◦ (FWHM) is generated through the interaction of ultra-intense laser light with a uniform critical density plasma in experiments and 2D PIC simulations. In the experiment, the uniform critical density plasma is created by ionizing an ultra-low density foam target. The spacial distribution of the fast electron is observed by Imaging Plate. 2D PIC simulation and post process analysis reveal magnetic collimation of energetic electrons along the plasma channel.
A unique linear Paul trap is designed for a systematic experimental study of nonlinear beam dynamics with the tabletop apparatus “S-POD” at Hiroshima University. S-POD is the abbreviation of “Simulator of Particle Orbit Dynamics” where we can produce a non-neutral plasma physically equivalent to a charged-particle beam in an alternating-gradient focusing channel. Unlike a regular Paul trap with four quadrupole rods, the present trap configuration includes extra electrodes that enable us to control the strengths and time structures of low-order nonlinear fields independently of the linear focusing potential. We here consider the insertion of thin metallic plates in between the quadrupole rods. The size and arrangement of those extra electrodes are optimized by using a Poisson solver. Simple scaling laws are derived to make a quick estimate of the sextupole and octupole field strengths as a function of the plate dimension. Particle tracking simulations are performed to demonstrate the controlled excitation of nonlinear resonances in the modified Paul trap.
High-ion temperature experiments in the Large Helical Device (LHD) are categorized in terms of the heating scenarios that are closely related to the development of neutral beam injection (NBI) systems. Although high-energy tangential negative-NBI heating has greatly contributed to extending the plasma parameter regime in LHD, the ion temperature does not increase because the electron heating is dominant with negative-NBIs. In the high-Z discharges, it was demonstrated that the ion temperature increased with an increasing ion heating power and achieved 13.5 keV with the negative-NBIs. Low-energy perpendicular positive-NBIs were installed for the ion heating, and the ion temperature was increased to more than 7 keV in hydrogen discharges. In the high-ion temperature plasmas, an ion internal transport barrier (ion ITB) was formed, and the impurity hole was observed in the core. Long-pulse ion cyclotron range of frequency heating (ICH)/electron cyclotron resonance heating (ECRH) helium discharges are effective for wall conditioning, leading to a decrease in the neutral density and a peaked density profile. Consequently, the ion heating efficiency increases in the core, and the central Ti is raised up to 7.5 keV. With the superposition of high-power ECRH, high-performance plasmas of Ti ∼ Te ∼ 6 keV were obtained. In the planned deuterium experiment, the ion heating power will be increased with the deuterium beam injection, and Ti = 10 keV is expected.
We have developed a double-pass Thomson scattering diagnostic system for the TST-2 spherical tokamak device. By measuring the first- and second-pass scattering signals simultaneously, we obtained the two directional pressures and we measured the pressure anisotropy (i.e., the ratio of the pressures) with an error of 5% - 10% for moderate density (≅ 2 × 1019 m−3) plasmas. We observed 30% and 100% anisotropy at the center and edge of the Ohmic-heated plasmas, respectively. We propose a three-temperature Maxwellian model, in which the fitting is better than in the shifted-Maxwellian model. The estimated plasma current density was close to the averaged current density. The results suggest that the contribution of thermal electrons to a plasma current is large in Ohmic-heated plasmas.
The influence of deuterium retention on the electron-impact secondary electron emission (SEE) is studied in isotropic graphite (ETU-10). The ETU-10 surface sheath voltage and its deuterium retention under deuterium plasma exposure were measured simultaneously. Deuterium retention was estimated using in situ nuclear reaction analysis. While deuterium retention increased with decreasing graphite sample temperature, the sheath voltage on the sample surface decreased. The sheath potential variation is considered to be due to the SEE yield variation, which was estimated using the sheath voltage. The estimated SEE yield value increased by approximately 10% as the deuterium retention rose by a factor of two.
A Tracer-Encapsulated Solid Pellet (TESPEL) was developed for promoting an impurity transport study in a magnetically-confined plasma. One of the advantages of the TESPEL is that it can make a three-dimensionally localized impurity source in the plasma. This enables us to inject the tracer impurity inside or in the vicinity of the region of interest. Recently, a new-type TESPEL with a thinner outer shell has been developed in order to achieve a shallower deposition of the tracer impurity. With the TESPEL having the thinner shell, we have achieved about 4 cm shallower deposition of the tracer impurity, compared with the case of the conventional thick-shell type TESPEL with the same outer diameter of about 700 µm. Moreover, for the achievement of the further shallower deposition of the tracer impurity, we also developed the TESPEL with a tracer-impurity-doped thin shell. After the injection of the TESPEL with the tracer-impurity-doped thin shell, the line emissions from the highly-ionized doped impurity are clearly observed with a vacuum ultraviolet spectrometer, which clearly demonstrates its ability to carry the impurity as a new tool.
Spatial distributions of electron temperature and density have been measured using Langmuir probes on a tungsten V-shaped target plate of a divertor simulation experimental module in GAMMA 10/PDX. In standard hot-ion plasmas, the electron temperature is about 30-50 eV, which is comparable to that in confined core plasmas. The electron density is on the order of 1016 m−3 but increases about 3 times owing to additional ion cyclotron range of frequency heating. In a direction along the target plate toward the corner, the electron temperature seems to slightly decrease toward the corner. In another direction perpendicular to this direction, the electron temperature profile seems to be roughly flat. The profile of electron density compensated for the expansion of magnetic flux tubes is similar to the core plasma, but the compensated density decreases near the corner. As the open angle of the target decreases, the compensated density decreases although the electron temperature does not change much. These density decreases may be due to increase in flow velocity caused by electric and pressure forces in the pre-sheath.
In order to optimize the transferred power efficiency from ICRF fast ions to bulk plasma, we have developed a code in which models of behaviors of ICRF fast ion are minimally adopted in order to save calculation time. A tendency of the transferred power efficiencies evaluated by the developed code is almost the same as that evaluated from the full analyses. In the regime with low efficiency of transferred power, the effect of the position of the resonance layer is large. The efficiency of the resonance layer through the point near the magnetic axis is found to be smaller than that of the typical ICRF resonance layer.
Nanostructured tungsten formed by the exposure to helium plasma in a linear plasma device was installed in the large helical device (LHD). After the exposure in a series of experiments in the 2012 fiscal year campaign in LHD, the samples were analyzed by scanning electron microscope (SEM), transmission electron microscope (TEM), and energy dispersion x-ray spectroscopy (EDX). It was found that part of the nanostructures was totally covered with carbon based material probably from divertor, while some other parts were eroded by sputtering. On the erosion dominant region, it was revealed that the head part of nanostructures was sputtered and the surface became rounded, but the nanostructures still remained near the surface. Optical reflectance of the material was measured, and it was found that the morphology changes increased the optical reflectivity up to ∼10% from typically less than 1%. The possibility and limitation of the nanostructured tungsten as a light absorber (viewing dump) are discussed.
In the Large Helical Device (LHD) semiconductor-based detector arrays for soft X-ray (SX) emission have been used for studying MHD instabilities. However, a semiconductor device is expected to be damaged in high neutron flux environments in the coming LHD deuterium plasma experiments. In order to measure an SX in such environments, a CsI:Tl scintillator-based diagnostic is being developed. Though CsI:Tl is sensitive to a neutron and to a gamma-ray, as well, effects on scintillation light can be reduced if very thin CsI:Tl foil (50 micrometers) is used. An SX is converted to visible light by a scintillator and led to optical fibers. The light is transferred away from LHD and detected by a semiconductor detector array set in a neutron and gamma-ray shielding box. Effects by neutrons and gamma-rays on scintillation light are quantitatively estimated based on the deuterium plasma experiment condition of LHD.
A new far infrared (FIR) laser interferometer with high time resolution has been developed in Heliotron J for measuring high performance plasmas. The FIR laser interferometer is a heterodyne-type Michelson interferometer with a 1 MHz intermediate frequency. The interferometer uses a super rotating grating and the viewing chord passes through an off-axis position. Refraction in high density plasma is estimated using the TRAVIS ray-tracing code. The results suggest that it is possible to extend the range of the interferometer up to 1.5×1020 m−3. The first line-averaged plasma density measurements using the FIR laser interferometer have been made in ECH and NBI heated plasmas. The relative change in the density profile shape is evaluated from the ratio of the line-averaged density obtained by the FIR laser and microwave interferometer with a different viewing chord. The difference in the measurements suggests that a more peaked density profile is formed in NBI plasma than in ECH + NBI plasma, which is in agreement with Nd:YAG Thomson scattering measurements.
Tokamak plasmas with an internal transport barrier (ITB) are capable of maintaining improved confinement performance. The ITBs formed in plasmas with the weak magnetic shear and the weak radial electric field shear are often observed to be modest. In these ITB plasmas, it has been found that the electron temperature ITB is steeper when toroidal rotation is in a co-direction with respect to the plasma current than when toroidal rotation is in a counter-direction. To clarify the relationship between the direction of toroidal rotation and heat transport in the ITB region, we examine dominant instabilities using the flux-tube gyrokinetic code GS2. The linear calculations show a difference in the real frequencies; the counter-rotation case has a more trapped electron mode than the co-rotation case. In addition, the nonlinear calculations show that with this difference, the ratio of the electron heat diffusivity χe to the ion's χi is higher for the counter-rotation case than for the co-rotation case. The difference in χe /χi agrees with the experiment. We also find that the effect of the difference in the flow shear between the two cases due to the toroidal rotation direction on the linear growth rate is not significant.
Nonlinear interplay of the electron temperature gradient (ETG) modes and the trapped electron modes (TEMs) was investigated by means of gyrokientic simulation. Focusing on the situation where both TEMs and ETG modes are linearly unstable, the effects of TEM-driven zonal flows on ETG turbulence were examined by means of entropy transfer analysis. In a statistically steady turbulence where the TEM driven zonal flows are dominant, it turned out that the zonal flows meditate the entropy transfer of the ETG modes from the low to high radial wavenumber regions. The successive entropy transfer broadens the potential fluctuation spectrum in the radial wavenumber direction. In contrast, in the situation where ETG modes are unstable but TEMs are stable, the pure ETG turbulence does not produce strong zonal flows, leading to a rather narrow spectrum in the radial wavenumber space and a higher transport level.
Linearized model collision operators for multiple ion species are implemented in a local flux-tube gyrokinetic code. The newly implemented collision operator satisfies the conservation properties of particles, momentum, and energy, as well as the adjointness relations for collisions between different particle species, which are numerically confirmed by the test simulations. The linear zonal flow response with finite collisionality, is also compared between the new collision operator and the simplified model collision operator.
Radiofrequency (RF) waves could be used for plasma current start-up in spherical torus (ST) reactors, where plasma formation and current drive without the ohmic heating solenoid is required. In such a plasma, the electrons can be represented by two temperature components, i.e. high-temperature low-density electrons and low-temperature high-density electrons. In order to describe the equilibrium of such plasmas, we develop a three-fluid (two electron fluids and one ion fluid) axisymmetric equilibrium model with toroidal and poloidal flows. This model has been applied for the first time to a recent TST-2 discharge, and we have obtained an equilibrium which is consistent with experimentally observed results. It is found that (1) the toroidal current density and pressure are dominated by the high-temperature low-density electron (eh-electron) fluid and (2) the radial force balance for each fluid species is quite different, i.e. the ion fluid is confined by the electric force due to the negative electrostatic potential while the eh-electron fluid pressure gradient force is balanced by the Lorentz force (its toroidal current density times the poloidal magnetic field). These results are different from previous speculations.
We have investigated preformed plasma generation inside a cone on cone-in-shell targets for fast ignition scheme. We focused on the study of preformed plasma generation via irradiation by an implosion laser. In the GEKKO-XII laser system (Institute of Laser Engineering, Osaka University), a large fraction of laser energy is frequency-doubled to a 527 µm wavelength light with KDP crystals, but unconverted fundamental light (λ = 1.053 µm) is ultimately a possible heating source for the target. We measured the temperature at the inside tip of the cone in several experimental conditions to verify the effect of unconverted light. The experimental results indicated that the direct irradiation inside the cone with the unconverted fraction is the primary source of preheating effects. The results also suggest that preheating is suppressed by “long” cone targets whose sizes are typically larger than the focal points of incident unconverted light.
Tungsten (W) is considered as primary candidates for plasma-facing materials (PFM) in current fusion reactor designs because of their high melting point and high sputtering resistance. In this study, pure W rods fabricated by a swaging process and having two different diameters (6 and 10 mm) are examined. To investigate the effect of anisotropy and recrystallization on the tensile properties of W rod, grain structure, hardness, tensile strength and elongation, and fracture surfaces are observed or measured for as-received and heat-treated materials. Based on grain structure observation and tensile tests, the axial and radial directions of W rod show different microstructures, called microstructure anisotropy due to swaging. A significant anisotropy of tensile properties is observed in as-received W rods at room temperature. Tensile strength of the rod with smaller diameter is higher than that of the one with larger diameter, and plastic deformation is observed only for the as-received smaller diameter rod, in further measurements at room temperature. However, when tested at 773 K there is no obvious anisotropy of tensile properties observed in as-received and heat-treated materials. Through these comparisons, the smaller diameter rod material shows better tensile properties than the one with larger diameter because of its higher reduction ratio. The reduction ratio plays an important role in affecting the microstructure and tensile properties of W rod material.
Bead-on-plate welds were produced on the 4 mm-thick V-4Cr-4Ti alloy (NIFS-HEAT-2), using a 2.0 kW YAG laser. The post-weld heat treatments (PWHT) were carried out in various conditions. Microstructures, Vickers hardness and Charpy impact properties were obtained for the weld metal after the PWHT. After PWHT for one hour, the hardness increased and after the peak declined with temperature. At 873 K, the hardness increased and after the peak declined with the time of PWHT. Microstructural observation showed that high density of fine precipitates formed homogeneously when the hardness increased, but the precipitate distribution changed into heterogeneous forming islands of developed precipitate aggregates, coincident with decrease in hardness and recovery of impact properties. Optical microscope observations suggested that a cellular structure of precipitate aggregate region was formed by PWHT. Microchemical analysis showed that Ti was enriched in the precipitate aggregate region. Therefore the areal oscillation of Ti concentration with cellular structures formed by melting and resolidification during the welding resulted in the heterogeneous precipitation by the following PWHT. The precipitation in the Ti-rich area purified the matrix of the weld metal and induced the recovery of hardening and impact property degradation. Optimum PWHT conditions were discussed according to the present results.
This paper presents a simulation of a three-stage cascaded staggered double vane slow-wave structure (SWS). The results suggest that >10 W of peak power can be produced between 208 GHz and 238 GHz and a maximum gain of 32.4 dB at 220 GHz, driven by three 20 mA electron beams. The proposed circuit does not require an attenuator and the length of each stage is 27.45 mm. Because of the current density and short circuit length, the structure shows application potential as a terahertz radiation source.
A gas cluster is a collection of atoms or molecules weakly bound by van der Waals forces. Gas clusters may form by the adiabatic expansion of gases. In this study, it is demonstrated by molecular dynamics simulations that a low-energy beam of oxygen gas clusters may be used to oxidize the top surface layer of silicon (Si) substrates without affecting its deeper layers. An incident oxygen gas cluster with sufficiently low incident energy may stick to the Si surface and expose a large number of oxygen molecules to the surface Si atoms for extended periods until the cluster sublimates. This may cause the formation of Si-O bonds only on the top Si surface. This is in contrast to the oxidation of Si by oxygen ion beams or plasmas, where deeper layers of the Si surface are typically oxidized by the energetic incident oxygen ions. An oxidized single Si layer may be chemically removed; therefore, this nearly single-layer oxidation process by oxygen gas cluster beams may lead to the development of a new atomic layer etching technology for Si.