Improved measurement of the effective ion charge (Zeff)
We have improved the measurement of the effective ion charge (Zeff), which represents the amount of impurities in the plasma in LHD. In previous measurements, Zeff was evaluated using signal obtained by the visible spectrometer for a long horizontal plasma cross-section, but an overestimated value was calculated due to the effect of light in scrape of layer which is around the plasma. In order to avoid this effect, the measurement line-of-sight was changed to a plasma cross section with a long vertical direction, which led to an improvement in the Zeff evaluation.
A new method for simultaneously injecting different impurities into the plasma
In a tracer-capsulated solid pellet (TESPEL) scheme, which is a technique for locally depositing impurities to the desired location in high-temperature plasmas, we have developed a new method for injecting multiple impurities, which is completely different from a previous method. This achievement will greatly advance the research on the control of impurities in fusion plasmas.
Observation of damage on the wall without plasma experiments
Arc trails induced by Glow Discharge Conditioning (GDC) before the plasma experiments was observed on a part of diagnostic installed in LHD. On the other hand, other part of the diagnostic made of different material survived the arcing damage. Current situation of LHD GDC might be on the border that divides the conditions of arcing ignition between different materials. This result contributes to the selection of the material to be installed in LHD in the future.
Analysis of NB Beam-Ion Loss Mechanisms in LHD
The beam ion loss mechanism in LHD has been analyzed quantitatively by using the neutron measurement and the integrated simulation*. It has been known that the "neo-classical transport"** is dominant as the beam ion loss mechanism in large size tokamaks. Contrary to tokamak cases, the neo-classical simulation can not reproduce the experimental result in LHD. This result indicates that the other mechanism is dominant as the beam ion loss mechanism in LHD.
Identification of the criteria for the plasma collapse discharge in helical plasmas
In the LHD, the plasma discharges, with the cases predicted to be unstable in MHD instabilities before the construction, are maintained without any collapse phenomena. On the contrary, in the discharges predicted to be much unstable by a theory, the collapse phenomena are observed. We investigate the criteria of the collapse discharges using the measured plasma pressure and current profiles, and identify the criteria leading to the collapse that Mercier parameter >0.3 and tearing parameter >0.
Remote Experiment from the Opposite Side of the Earth
Technical verification has been progressing for high efficiency data replication between ITER and the Remote Experimentation Centre (REC) in Japan. Transferring a huge amount of data simultaneously to multiple destinations may cause excessive loads and network bandwidth on the sender so that daisy-chained relay transfer would be a considerable solution. This study demonstrates how efficiently the replication relay could be realized for the next-generation fusion experiments, such as ITER and JT-60SA.
Data location informing service to quickly find out the data distributed in remote sites
ITER UDA structure and the replicated repository: If there is only one indexing DB, it would be a single point of failure (SPOF). Remote site’s independence against accidental loss of long-distance network connectivity or planned power outages can be improved if the replicated indexing DB can continue operation independently from the primary indexing DB even though the real-time synchronization is temporarily lost. The indexer process should always register a new data entry synchronously with the data migration process making a new copy of the data or moving to other place, locally or remotely.
Construction of the measuring system for plasma heat flux to the wall
Significant heat flux is expected at the wall called “the divertor plate” in the fusion device. The present study constructed the system for measuring the divertor heat flux by 2 dimensional (2D) thermography and thermal conduction analysis in LHD. The development of the system accelerates the detailed understanding of the divertor heat flux in the LHD and development of a numerical simulation code with high accuracy.
Slowing-down of rotating fluctuation in magnetic confinement plasmas
In LHD a rotating fluctuation is often observed and the rotation frequency of the fluctuation sometimes decreases to almost zero. After the decrease of the frequency, the fluctuation rapidly grows and the plasma confinement performance deteriorates. In this study, the slowing-down mechanism of the rotating fluctuation is experimentally clarified. Since the slowing-down is closely related to the stability of the fluctuation, this result will accelerate the research to avoid or suppress the rapid growth of the fluctuation, which causes serious degradation of the core plasma performance.
Reproduction of Impurity accumulation profiles in the Large Helical Device
In LHD, impurity layers have been accumulated on the in-vessel components every plasma discharge. The layers are very fragile and are characterized by the exfoliation property. Plasma discharges can be interrupted by the exfoliated impurity layers entering the plasma. Our simulation has successfully reproduced the areas where the impurity layers are accumulated. It will contribute to the stable operation of future nuclear fusion reactors.
Tritium in divertor tiles were analyzed by using a combustion method
To reveal the triton transport and the tritium migration in a deuterium plasma experiment in LHD, remaining tritium in divertor tiles made of graphite was investigated by using a combustion method. Obtained remaining tritium profile in two divertor tiles were in consistent with the profile of lost-points of promptly lost high energy triton on the divertor tiles calculated by using a Lorentz orbit following code (LORBIT) with taking into account divertor components.
Discovering Turbulence to Help Plasma Mixing
The mixing state of hydrogen and deuterium in plasma was measured for the first time in the world in a mixed plasma experiment of hydrogen and deuterium in LHD. They found that the hydrogen and deuterium plasmas do not mix when the size of the plasmas is small but mix when they are large, which is caused by the heating process. This achievement gives a guideline for the "mixed state" necessary for hydrogen isotope mixed plasmas used in nuclear fusion power generation.
World's First Detection of Anisotropy in Electron Motion by Spectroscopic Measurement
In the Large Helical Device (LHD), the polarization of light emitted by hydrogen atoms has been measured with an accuracy of 1%. The analysis of the polarization angle showed that the motion perpendicular to the magnetic field is more dominant than that parallel to the magnetic field for electrons at the plasma edge, which supports our intuitive understanding of the plasma condition in that region.
Let the accumulated data talk about the state of plasma
We have developed a method for grasping the state of plasma using a large amount of data accumulated in experiments in LHD and have succeeded in demonstrating its practicality. This result provides an example for methods to make the best use of the data accumulated in fusion research, and greatly advances research toward real-time control of plasma.
Estimation of the beam ion confinement time in LHD
The confinement time of fast ions, which are generated by the neutral beam injection, has been estimated quantitatively in the LHD deuterium experiment by using neutron measurement and the integrated simulation. This estimation is the first achievement in large size helical systems, to our knowledge. This research can reduce ambiguities from the plasma heating profile and can contribute to the clarification of the physics phenomena in the LHD.
First observed tritium distribution on plasma facing surface in stellarators
In LHD, tritium distribution on plasma facing surfaces was investigated for the first time in stellarator/heliotron devices by using the tritium imaging plate technique after the first deuterium plasma experimental campaign. In-vessel components such as divertor tiles and first wall panels and long-term material probes retrieved from the vacuum vessel were analyzed. It was revealed that tritium remained most densely in the baffle part of inner divertor tiles made of graphite.
Mechanism of plasma radiation concentration at “knot”of magnetic field lines
It has been reproduced in the newly developed simulation code that plasma radiation concentrates at the “knot”of the magnetic field lines, and its mechanism is recognized as the same as thermal condensation instability, which also occurs at star formation in the universe. The achievement advances the research on optimization and control of plasma radiation in future devices.
Real-time control of mm-wave propagation for highly efficient plasma heating
We have developed a real-time control system for the millimeter-wave injection system for electron cyclotron heating and have shown that the system can heat the plasma efficiently by optimizing the injection for the time-varying plasma in LHD. We have also succeeded in improving high electron-temperature operations and high-density operations by using this control system. This achievement will contribute to the sustainment of high-efficiency heating during high-power and long-pulse operations and improve the accuracy of transport studies to evaluate the performance of fusion plasmas.
Ion measurement by millimeter and sub-millimeter in plasma
In high-temperature plasmas, it is not easy to diagnose the state of ions and fast ions because they cannot be measured directly by a thermometer. As a non-contact measurement method to solve this problem, we are developing a Collective Thomson Scattering (CTS) diagnostic using electromagnetic scattering phenomena. In addition to the 77 GHz and 154 GHz bands, we have developed the world's first submillimeter-wave scattering measurement system in the 300 GHz band (in collaboration with Fukui Univ.).
Experimental database of emission spectra from various elements
Various elements with atomic numbers from 36 to 83 have been injected into LHD plasmas to observe their emission spectra. Spectra for various atomic numbers and electron temperatures have been systematically investigated by comparisons with theoretical calculations. As a result, some spectral lines have been newly identified. The present study has contributed to the development of an experimental database, useful not only for fusion, but also for atomic physics and plasma applications.
Explore the behavior of hydrogen isotopes using a tracer
Hydrogen isotopes' behavior in a fusion test device has been clarified by use of a small amount of tritium, that was produced in deuterium plasma using the LHD as a tracer. Furthermore, it was clarified that the tritium released from the plasma-facing materials in the LHD is governed by hydrogen isotope exchange reactions and diffusion in the material. These results provide valuable knowledge on the safe handling of tritium and the fuel cycle system in future fusion reactor systems.
Turbulence permeating the inner part of a plasma magnetic island
The "instantaneous heat propagation method" devised for LHD at the National Institute for Fusion Science was applied to a tokamak device (DIII-D) in the United States. Together with collaborators in the United States, we observed the phenomenon of turbulence propagating into the inner part of the magnetic island. It is well known that turbulence is generated as temperature and density increase in magnetic confinement plasmas for nuclear fusion, but this is the first experimental discovery that turbulence has propagation property.