NIFS-Repository (National Institute for Fusion Science)
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Formation Mechanism of Nano-Strengthening Particles in Dispersion Strengthened W-Ti Alloys
Get PDF0000-0001-9016-2318The realization of advanced fusion reactors rests upon improvements of plasma-facing materials for divertors. The toughness of tungsten is a critical issue for these improvements. As a solution, we developed dispersion-strengthened (DS) W-Ti alloys using a new process which is based on a combination of mechanical alloying (MA) and hot isostatic pressing. In this paper, the microstructure of new DS-W-Ti alloys and the formation mechanism of nano-strengthening particles are investigated. The produced DS-W-Ti alloys are dispersed with TiO2. The chemical analysis shows that new precipitation processes occurred during the MA based on redox reactions between carbon and oxygen.journal articl
Eddy current analyses for vacuum vessel of CFQS quasi-axisymmetric stellarator
Get PDFThe design activity on the CFQS quasi-axisymmetric stellarator which is conducted as a joint international project is ongoing by National Institute for Fusion Science (NIFS) in Japan and Southwest Jiaotong University (SWJTU) in China. The CFQS magnetic field generating coils consist of sixteen modular coils (MCs) in total with four different types, four poloidal field coils (PFCs), and twelve toroidal field coils (TFCs). In designing fusion devices, eddy currents and electromagnetic (EM) forces on the vacuum vessel (VV) have to be evaluated to verify the mechanical confidence and reliability. In this work, we consider the following three issues, (1) Influence of eddy current induced by external coil current change on the CFQS magnetic confinement, (2) Evaluation of EM force on VV by eddy current under a typical current of the MC and the PFC, and (3) Feasibility check of whether it is possible to heat the CFQS VV by using induction current.journal articl
Local axisymmetry-breaking–induced transition of trapped-particle orbit and loss channels in quasi-axisymmetric stellarators
Get PDFThe transition of trapped-particle orbit topologies has been investigated in quasi-axisymmetric (QA) configurations, such as the Chinese First Quasi-axisymmetric Stellarator (CFQS). It is found that the axisymmetry-breaking phenomenon in QA configurations is of great significance at some specific locations, which could easily induce blocked particles to transit into localized particles. A novel aspect is presented to interpret the transition mechanism of trapped-particle orbit topologies in this paper, i.e., as the amplitudes of non-axisymmetric field increase along the radius direction, the region of large toroidal inhomogeneity is gradually generated, which makes the length of the trapped-particle trajectory substantially short, and hence, may restrict particles to a single helical field period. Meanwhile, at such locations the "pseudo-axisymmetric" field results in coupling of the maximum radial drift and the minimum poloidal drift, which enables the transition of trapped-particle orbit topologies considerably and forms specific loss channels, degrading plasma confinement. These results may shed light on the optimization of QA configurations via avoidance of such coupling with respect to energetic particle confinement. Moreover, this work is also relevant to the generation of inhomogeneity of particle flux deposition on the devertor plates.journal articl
Discrimination of Partial Discharges in Gaseous and Liquid Nitrogen by Using Waveform Characteristics
Get PDFThe cryogenic coolant is important as an electrical insulation in an immersion cooling. One of the dielectric breakdown mechanisms of the cryogenic coolant is via bubbles, caused by partial discharge (PD). The insulation performance has been discussed with PD parameters and breakdown voltage. However, the essential PD mechanism has not been clarified in detail. In this paper, PDs in gaseous and liquid nitrogen were discriminated based on characteristics of the original PD waveform. It was shown that the difference in charge behavior appears significantly in the fall time of the original PD waveform.journal articl
The physics of the mean and oscillating radial electric field in the L–H transition: the driving nature and turbulent transport suppression mechanism
Get PDF0000-0001-5669-1937The low-to-high confinement mode transition (L–H transition) is one of the key elements in achieving a self-sustained burning fusion reaction. Although there is no doubt that the mean and/or oscillating radial electric field plays a role in triggering and sustaining the edge transport barrier, the detailed underlying physics are yet to be unveiled. In this special topic paper, the remarkable progress achieved in recent years is reviewed for two different aspects: (i) the radial electric field driving procedure and (ii) the turbulent transport suppression mechanism. Experimental observations in different devices show possible conflicting natures for these phenomena, which cannot be resolved solely by conventional paradigms. New insights obtained by combining different model concepts successfully reconcile these conflicts.journal articl
Comparative Studies on the Control Algorithm for the High-Density Ignition Regime in FFHR-d1
Get PDF0000-0002-6111-6790In the Force Free Helical Reactor FFHR-d1 (R∼15.7 m, a∼2.5 m, Bo ∼4.5 T, 〈β〉∼5 % and the fusion power of 3 GW) [1, 2], it is demonstrated that the thermally unstable operation is better to achieve the higher density and lower temperature plasma (n(0)∼ 9 × 1020 m−3 and T(0)∼7 keV) than the impurity injection method. One drawback of the thermally unstable operation is a possibility of the thermal runaway when fueling systems have failures. However, we have found that the inherently safe function exists owing to the plasma outward shift during the thermal runaway. Thus we continue to pursue this high-density operation scenario for FFHR-d1. Preliminary estimation of the alpha energy loss fraction of ∼3 % is obtained for a broad density profile in the high-density ignition regime.journal articl
Study of the Transient Behavior of Detached Plasma during Xe Gas Injection into the D-Module of GAMMA 10/PDX
Get PDF0000-0003-2019-4001The transition phenomenon of detached plasma during additional heating effects has been studied by applying a short pulse (25 ms) of Electron Cyclotron Heating (ECH) at the east plug-cell of the tandem mirror device GAMMA 10/PDX. In this paper, the plasma parameters on the target plate of the D-module are studied for understanding the impact of transient heating pulse on the sustainability of the detached plasma. The heat flux increases on the target plate for the additional heating case in comparison to without heating pulse. The ion flux also increases when the ECH heating is activated. The electron density enhances significantly during the ECH heating pulse application period. Furthermore, the electron density and the ion flux increase according to the increment of Xe plenum pressure in the case of ECH heating condition. The experimental results clarified that ECH heating pulse can drive the detachment state to attached state in spite of strong effect of Xe gas for generating the detached plasma.journal articl
Measurement of Electron Density Fluctuations Using O-Mode Microwave Imaging Reflectometry in a TST-2 Spherical Tokamak
Get PDFAn O-mode microwave imaging reflectometry system has been installed in a spherical tokamak named TST-2. The illumination wave frequency is 23 - 32 GHz, which corresponds to the cutoff electron density of 0.65 - 1.3 × 1019 m−3. The microwave image of the scattered wave is formed on an imaging detector named horn-antenna millimeter-wave imaging detector by imaging optics that consist of an ellipsoidal aluminum mirror, a Teflon lens, and a dielectric plate. The detected channel numbers are 6 (poloidal) × 6 (toroidal) × 2 (radial). The data of power and phase of the scattered wave are sampled every 0.5 μs and are stored by the LABCOM system at the National Institute for Fusion Science via a private network named SNET. The scattered wave quickly fluctuates (∼5 μs) and has a large amplitude (>100 times). From the time evolution of the phase at the internal reconnection event, it is inferred that the reduction in amplitude may be owing to the phase mixing of scattered wave in each detector channel.journal articl
Asymmetry of Quadratic Energy Transfer Between Ion Cyclotron and Whistler Modes in Fully Developed Hall Magnetohydrodynamic Turbulence
Get PDF0000-0002-7876-8373This study investigated the energy transfer of a fully developed freely decaying homogeneous isotropic turbulence of a Hall magnetohydrodynamic medium from the perspective of the interaction between the ion cyclotron (IC) and whistler (Wh) modes. The variables were decomposed using generalized Elsässer variables (Galtier, J. Plasma Phys. 72, 721 (2006)), which are given by a combination of velocity and magnetic fields. To analyze the energy transfer between these coupled variables, an analytical-mechanical approach was introduced that guaranteed both the Galilean covariance and the detailed energy balance for each decomposed mode. A remarkable asymmetry of the energy transfer between the IC and Wh modes was found. In the present study, almost one-sided energy transfers from the IC to the Wh modes were observed. In addition, a shell-to-shell transfer analysis revealed relatively weak inter-mode interactions with nonlocal features, while the intra-mode interactions were both intense and local.journal articl
Preliminary design and analysis of the CFQS supporting structure
Get PDF0000-0002-5601-7106The Chinese First Quasi-axisymmetric Stellarator (CFQS) is now under design and construction. It will be the first quasi-axisymmetric (QA) configuration device to be operated in the world. The main parameters of the CFQS are as follows: the toroidal periodic number Np = 2, major radius R = 1.0 m, aspect ratio Ap = 4.0 and magnetic field strength Bt = 1.0 T. The low Ap makes it quite challenging to design a supporting structure because of the limited space and strong electromagnetic (EM) force. In this paper, a cage-like supporting structure is proposed for the CFQS modular coil (MC) system to sustain the EM force and the weight of entire device. A finite element analysis is carried out for ensuring the reliability of the supporting structure. The analysis results of the CFQS global model indicate that the cage-like supporting structure can basically satisfy the requirement.journal articl