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Suppression of resistive interchange instability by external RMP
Get PDF0009-0001-9215-3818We experimentally investigate the effect of external resonant magnetic perturbation (RMP) on resistive interchange MHD instability which is typically observed in the Large Helical Device. We apply the m/n = 1/1 external RMP to discharges with the m/n = 1/1 interchange instability. We find that the electron density fluctuation as well as the magnetic field one is clearly reduced without a reduction of the pressure gradient by imposing the external RMP. Moreover, the achieved beta value as well as the beta gradient at the resonant surface is a little improved under certain conditions. Next, we investigate the responses of interchange instability to the external RMP under some different operational conditions. We find that the amplitude of the external RMP to completely suppress the instability with the shielding of the external RMP has a higher correlation with the volume-averaged beta value than other plasma parameters.journal articl
Fabrication of tungsten-based optical diffuser using fiberform nanostructure via efficient plasma route
Get PDF0000-0001-8616-6041Optical diffusion is an essential process used to manage photons in a wide range of photoelectric systems. This work proposes an approach to fabricate novel optical diffusers by a plasma-processing technique, using fiberform nanostructures formed by helium plasma irradiation and subsequent annealing. After an annealing procedure in the air for oxidation, the optical properties and the light-diffusing abilities of these nanostructured thin films were studied. In addition to the morphology analysis and total transmittance measurement, the diffusion efficiency of the optical diffusers was analyzed using a transmitted scatter distribution function (TDF). It was revealed that the diffusion efficiency of a device with an irradiation time of 30 minutes could reach 97%. The results demonstrate the potential of these nanostructured optical diffusers for various photoelectric applications.journal articl
Effects of annealing temperature on microstructure and hardness of cold rolled high-purity V-4Cr-xTi alloys
Get PDF0000-0001-5706-0221The microstructural evolution and Vickers hardness variations upon annealing in the temperature range of 873 – 1273 K of cold rolled high-purity V-4Cr-(0∼4)Ti alloys are investigated. Results show that partial recrystallization occurs at 1073 K, recrystallization is complete at 1273 K, and Ti addition results in grain refinement. Grains become more isotropic after annealing at 1273 K. Besides, precipitates are observed after annealing at 973 K and above when Ti concentration is 1 wt% and higher. The size of precipitates increases with increasing Ti concentration and annealing temperature. Furthermore, Vickers hardness of cold rolled V-4Cr-xTi alloy drops to a minimum with increasing annealing temperature to 1173 K due to recovery of dislocations and recrystallization process. With annealing at 1273 K, hardness does not vary for V-4Cr-0Ti alloy, whereas it slightly increases for V-4Cr-(1∼4)Ti alloys, which is presumably attributed to partial dissolution of precipitates into the matrix. Additionally, purification by further reducing the high-activation impurities and interstitial impurities dose not evidently affect the recrystallization behavior and hardness of V-4Cr-4Ti alloy after recrystallization, however, the low-activation property is expected to be enhanced.journal articl
Wave Absorption Profile during ICRF Heating in Mixed Plasma in Large Helical Device
Get PDFORCID 0000-0001-6075-1146A minority ion ratio in fast wave heating for the ion cyclotron range of frequencies affects plasma heating mechanisms and mode conversion ratios to short waves in a mixed plasma. The mode conversion region and these wave absorption profiles are functions of minority ion ratio, and direct wave absorption by electrons increases when the minority ion ratio exceeds about 6% in a simple two-dimensional wave absorption calculation with a single toroidal mode. At the minority ion ratio of 10%, the amount of absorption by electrons is comparable to that by ions. We find a significant relationship between the minority ion ratio and the wave absorption profile, exceeding the minority ion ratio of 10% for wave absorption to electrons.journal articl
Characteristics and causes of voltage observed at the current feeder of high-temperature superconducting WISE conductor
Get PDFORCID 0000-0002-3541-6298The HTS (high-temperature superconducting) conductor is a feasible candidate for constructing magnets for next-generation fusion devices because of its higher critical current in a high magnetic field. A new concept of the HTS-WISE (Wound and Impregnated Stacked Elastic tapes) conductor has been studied aiming to apply the fusion reactor magnet. Here, the WISE-U conductor is composed of stacked thirty REBCO tapes (10 mm width, 65 μm thickness, Ic = 370 A @77 K, s.f.) wrapped by a stainless-steel coil tube which is inserted into the metal pipe. The 4 m-long REBCO tapes are folded with a radius of curvature of 35 mm in a hairpin-like structure. A low-melting-point metal U-Alloy 60 whose melting point is 60°C is poured into the pipe for impregnation to make the non-insulation conductor. The REBCO tapes and the current feeder made of oxygen-free copper were also impregnated with the U-Alloy 60 to connect. This fabrication method has the advantage of being easier to fabricate than the technique of connecting each tape using indium foil. The energization test results showed that a maximum current value of 16.9 kA was recorded at B = 5 T and T = 30 K, however, a burnout occurred in the current feeder before the critical current was determined. Then, the improved WISE conductor has been designed and tested which showed a maximum of 19kA was reached in the self-field and 20K. However, burnout still occurred in the current feeder section. In those experiments, the superconducting section has not been damaged at all. If this burnout had been avoided, a higher current-carrying capacity could have been obtained. Identifying the cause of burnout and improving the current feeder is required.journal articl
Improvement of electron temperature and density accuracy in Thomson scattering diagnostics by an accumulation of 100 laser pulses within 5 milliseconds
Get PDFIn order to measure the electron temperature and density in high-electron-temperature and low-density plasmas, improvement of the signal-to-noise ratio of the Thomson scattering measurement signal in the Large Helical Device has been tried. One hundred laser pulses of about 1 J in 5 ms were injected at 50 μs intervals into plasmas in an almost steady state. By averaging the scattered signals obtained from the same plasma and the same location, the noise in the signals and the error in the background level were also reduced. When the number of signals to be averaged was large, the scattering of the electron temperature profile was small and the magnitude of the error also became small.journal articl
Harmonic Structure of Lower Hybrid Waves Driven by Energetic Ions at 4000 km Altitude: PIC Simulation
Get PDF0000-0001-8894-3430Past satellite observations in the polar region at about 4,000 km altitude have reported lower hybrid waves (LHWs) and their harmonics, but the excitation mechanism of the harmonic LHWs has not yet been clarified. We perform one-dimensional, electromagnetic, full particle-in-cell simulations by setting parameter values in the ranges observed by the satellite. The results from the two runs, that is, for ωpe/Ωe = 0.1 and 0.2, show that the energetic ions can generate the harmonic LHWs in both cases. However, in the former case, more harmonic LHWs with larger amplitudes are created and stronger background-ion acceleration is observed. Thus, our present simulations have solved a previously unsolved problem, revealing the possibility that harmonic LHWs are involved in ion acceleration phenomena commonly observed in the polar region, such as ion outflow.journal articl
A simplified model to estimate nonlinear turbulent transport by linear dynamics in plasma turbulence
Get PDFA simplified model to estimate nonlinear turbulent transport only by linear calculations is proposed, where the turbulent heat diffusivity in tokamak ion temperature gradient(ITG) driven turbulence is reproduced for a wide parameter range including near- and far-marginal ITG stability. The optimal nonlinear functional relation(NFR) between the turbulent diffusivity, the turbulence intensity T, and the zonal-flow intensity Z is determined by means of mathematical optimization methods. Then, an extended modeling for T and Z to incorporate the turbulence suppression effects and the temperature gradient dependence is carried out. The simplified transport model is expressed as a modified nonlinear function composed of the linear growth rate and the linear zonal-flow decay time. Good accuracy and wide applicability of the model are demonstrated, where the regression error of σmodel=0.157 indicates improvement by a factor of about 1/4 in comparison to that in the previous works.journal articl
Molecular Dynamics Simulation on Hydrogen Trapping on Tungsten Vacancy
Get PDF0000-0002-0593-8810The effect of hydrogen on the structural change of vacancies in tungsten is analyzed by molecular dynamics in order to clarify the interaction between vacancies and hydrogen in tungsten. Simulations are performed at different temperatures (573, 773, 1073 K) and with different numbers of hydrogen in the vacancies (0, 18, 36, 54). Evaluating (1) isopotential surface for the increase of total potential energy, (2) the root mean square deviation (RMSD) of tungsten atoms, (3) the root mean square fluctuation (RMSF) of tungsten atoms and (4) density distribution in radial direction, we found that the presence of a large number of hydrogen in a vacancy at each temperature caused changes in the structure. This fact partially supports the experimental fact that in the coalescence of two tungsten vacancies, the retention of hydrogen activates them.journal articl
Basic Research on a Magnetic Refrigeration System for Cooling to Liquid Hydrogen Temperature
Get PDF0000-0002-1565-8353Magnetic refrigeration technology is expected to be a highly efficient process at a temperature of around 20 K, which is the hydrogen liquefaction temperature. In magnetic refrigeration, a magnetic field change is applied to a magnetocaloric material (MCM) to obtain a magnetocaloric effect. In addition, since the greater the magnetic field change, the better the cooling characteristics, the magnetic field strength of permanent magnets is insufficient, and the use of superconducting coils that can generate strong magnetic fields with low power consumption is essential. We are researching and developing a static magnetic refrigeration system (SMRS) that does not have moving parts to obtain magnetic field changes, and generates them by altering the energizing current to the coil. The key to devising this system is an AC loss of the superconducting coil. In this study, we measured the AC loss under energization conditions of multiple palm-sized REBCO coils at liquid nitrogen temperature, and calculated the efficiency of the SMRS by improving the accuracy of the analysis method for evaluating the AC loss, based on the results. We report the results and discuss the technical feasibility of SMRS.journal articl