The Large Helical Device (LHD) is equipped with two types of antennas for Ion Cyclotron Range of Frequencies (ICRF) heating. One is the high-power-oriented antenna called the FAIT antenna, and the other is called the HAS antenna, where the wavenumber parallel to the magnetic field lines can be controlled. The performance of these antennas has been drastically improved by utilizing two types of impedance transformers.
Heat transport in fusion plasmas is often modeled as a temperature gradient. In contrast, when a narrow region away from the plasma center is heated to create a plasma with a peculiar hollow temperature profile, and then a narrow region near the center is additionally heated, a transient phenomenon was observed in which the heat pulse propagates against the temperature gradient. Furthermore, by switching the heating on and off, it was experimentally evaluated that the driven heat flux is not determined by the temperature gradient alone.
In plasma experiments in the Large Helical Device (LHD), we discovered that the controllability of plasma particles can be improved by compressing fuel particles in a region called the divertor and by pumping them out strongly with a cryogenic vacuum pump. This achievement is expected to greatly advance research on particle control in fusion power generation.
Good core plasma performance realized in deuterium plasmas compatible with heat load mitigation at the plasma facing components
In a deuterium plasma experiment in large helical device (LHD), we have succeeded in reducing the excessive heat load on the device wall while maintaining the performance of the confined plasma. It was shown that, in addition to the isotope effects of the deuterium plasma, the magnetic field structure in the peripheral region plays an important role in separating the high-temperature confined plasma from the low-temperature plasma in the peripheral region. These results provide a bright prospect for the operation of mixed deuterium/tritium plasmas in future fusion power reactors.
Stable and effective heat load reduction on the wall was realized using neon and krypton injection compared with single-species injection in the Large Helical Device (LHD). Since heat load on the wall should be reduced in fusion reactors, an operational method is investigated to disperse the heat load as plasma radiation using impurity gas injection into the plasma (divertor detachment). This achievement will contribute to develop the operational method of stable divertor detachment.
We have shown that the magnetic field oscillations caused by energetic beam ions induce the transport of energetic ions exciting the oscillations. In this study, we developed a new detector to measure high-energy neutrons with high sensitivity and observed that energetic particles that are not the cause of the oscillation also escape from the plasma. This achievement has advanced our understanding of energetic ion transport due to magnetic field oscillations, which is a concern in future fusion reactors.
In magnetic confinement plasmas, the rapid growth of temperature fluctuations after the rotation of its fluctuation stops is a problem, and a new measurement technique at LHD has successfully measured the fine structure of temperature fluctuations during the slowing-down of the rotation. We found for the first time that the radial profile of the temperature around the location of the fluctuation changes with time, either flattening or tilting, and that some of the region are constantly flattens. This result will contribute to the understanding of the above fluctuations and will advance the study of other fluctuations using this measurement method.
We analyzed extreme ultraviolet emission spectra of highly charged praseodymium and neodymium ions introduced into high-temperature plasmas produced in the Large Helical Device (LHD). Based on the recently published list of lines of neodymium ions in an electron beam ion trap (EBIT) experiment, spectral lines of praseodymium ions were identified as well from the similarity of the spectral features of both the elements. Some of them have been identified experimentally for the first time in LHD.
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.
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.
In plasma experiments at the Large Helical Device (LHD), unusual emission of visible light inside the plasma vessel has been observed. This emission must be predicted to avoid unexpected damages on the plasma vessel. This research experimentally shows that the unusual emission of visible light inside the plasma vessel can be predicted using a Support Vector Machine, a machine learning method.
In the deuterium plasma experiment at the Large Helical Device (LHD), the spatial distribution of energetic particles confined in the plasma was measured by using a neutron profile diagnostics to measure the neutron emissiomn profile. A comparison with the neutron emission profile predicted by the energetic particle orbit calculation shows good agreement with the experimental results under high and medium field strength conditions. This has led to a qualitative understanding of the confinement of energetic particles in the LHD.
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.
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.
Layers of neutral gas and ions are formed around a hydrocarbon pellet injected into a high-temperature plasma due to interaction with the high-temperature plasma. In this study, we succeeded in deriving a scaling law that determines the region where the light emitted from the pellet is observed. This result enables us to predict the spatial distribution of the electron density and electron temperature around hydrocarbon pellets with high accuracy. Thus, it is expected to advance the study of high-temperature plasmas using hydrocarbon pellets.
We have developed a new scattering instrument using millimeter wave to observe micro-scale turbulences of about 1 mm in high-temperature plasmas. Although strong intensity of the micro-scale turbulence is thought to have a significant impact on the confinement of high-temperature plasmas, it has been difficult to directly observe it. In this study, we developed a special metal lens and other equipment for use in large helical devices (LHD) and succeeded in detecting the signal caused by turbulence in plasma.
Development of High-Performance Millimeter-Wave Filters for Diagnostics of Internal Structure of Ultra-High-Temperature Plasmas
In fusion reactors, millimeter-wave megawatt electromagnetic waves are used to heat the plasma to 100 million degrees Celsius, where the fusion reaction occurs. However, the powerful electromagnetic waves cause damage and noise to the plasma diagnostic system, making it difficult to measure the plasma correctly. To solve this problem, we developed a high-performance notch filter in the millimeter-wave band, which removes only specific frequencies for the millimeter-wave electron temperature diagnostic, and succeeded in measuring the electron temperature. This developed filter is one of the fundamental technologies required for plasma diagnostics using millimeter-waves and has a wide range of applications in industry, communications, and other fields.
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.
We applied the regression analysis, which shows the dependence of one parameter on the other parameters, to investigate the relationship between the neutron emission rate and externally controllable parameters in the LHD deuterium experiment. We found that the neutron emission rate can be expressed by the formula consisting of plasma density and heating power. Hence, we obtained a method to control the fusion energy in a future burning plasma. A trial of extending the total neutron emission rate record in steady-state discharge was performed based on a data-driven approach. We successfully updated the total neutron emission rate record supported by the regression expression.
We have developed a neutron flux monitor equipped with the latest digital circuit technology to accurately measure and control neutrons generated in deuterium plasma experiments at the Large Helical Device (LHD). The monitor has a wide measurement range, fast response time, and high immunity to electromagnetic noise and is in stable operation at LHD. It is expected to play an active role as a standard control and measurement device in fusion plasma experiments such as JT-60SA and ITER..
First observation of degradation of confinement and reversal of plasma flow due to broken confinement magnetic field
In the Large Helical Device (LHD), the leakage of the plasma particles and the reversal of the plasma flow due to the broken confinement magnetic field are observed. The broken confinement magnetic field is produced by changing the direction of the plasma current which is driven by the electromagnetic wave. These findings show that the direction of the plasma current driven by the electromagnetic wave is important for the plasma confinement and the braking of the plasma flow, which was observed in the previous research, is caused by the reversed force due to the breaking magnetic field.
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.
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.
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.
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
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.
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.
The mixing state of hydrogen and deuterium in plasma was measured for the first time in the world in a mixed plasma experiment using hydrogen and deuterium in a large helical device (LHD). It was found that the hydrogen and deuterium plasmas do not mix when their size of the plasmas is small but do when they are large, 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.
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.
We established the pulse shape discrimination method for evaluating thermal neutron flux using artificial diamond detector. By this method, the detection efficiency of thermal neutron increased with 1.5 times compared to the previous work. This work can improve the neutron control and measurement system in the fusion reactor.
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.
In high-ion temperature plasma discharges in the Large Helical Devices, magnetic field oscillations, caused by energetic ions, limit the sustainment of high-ion temperature state. In this study, the energetic ions escaping from the plasma due to the magnetic field oscillation has been observed by the neutron emission profile diagnostics in a deuterium plasma experiment. Based on this result, it is expected to develop a method to maintain a high-ion temperature state for a long time.
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.
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.
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.
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.
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.).
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.
Experiments in the Large Helical Device (LHD) have shown that heavy hydrogen (deuterium) plasma with neutrons in the nucleus has better confinement than light hydrogen (hydrogen) plasma without neutrons in the nucleus, due to reduced plasma turbulence. Future nuclear fusion experiments will use deuterium and tritium with two neutrons in the nucleus, which is even heavier than deuterium. Thus, further improvement of confinement will be expected.
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.
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.