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Detection of electron temperature anisotropy by an x-ray crystal spectrometer in the Large Helical device
Full text linkORCID 0000-0002-1021-0322We examine Ar XVI spectra measured by an x-ray crystal spectrometer to estimate electron temperature anisotropy in high-temperature electron cyclotron heated plasmas in the Large Helical Device (LHD). We calculate the atomic structure and electron impact excitation cross section between magnetic sublevels of Ar XVI. By assuming a bi-Maxwellian electron distribution function and considering the mount parameters of the x-ray spectrometer in LHD, we model the expected intensity ratio q/r as a function of Te⊥B and Te∣∣B, where ⊥B and ∣∣B denote perpendicular and parallel components to the magnetic axis of toroidal plasma, respectively. The calculation results show that the intensity ratio of q and r, which are formed by inner-shell excitation from the ground state of Ar15+, is sensitive to electron temperature anisotropy. We apply the calculation results to the LHD experiments. The ratio of q/r changes with variations in electron density and collision frequency. In the core and low νe region, Te⊥B is predominant, and the electron temperature becomes isotropic above νe > 104 Hz. By combining electron temperature measurements from Thomson scattering and radial profile of Ar15+ ions estimated using the extreme ultraviolet spectrometer, local values of Te⊥B/Te∣∣B are quantitatively estimated using the q/r ratio. The derived Te⊥B/Te∣∣B is compared with collision frequency, radial electric field, and effective helical ripple, and the experimental results are explained qualitatively.journal articl
Progress of “Plasma and Fusion Cloud” research data platform towards “Open Science”
Full text linkORCID 0000-0001-6388-4489A new cloud platform to realize the plasma and fusion experimental data ecosystem, named the “Plasma and Fusion Cloud,” has been technically verified on some fundamental issues. Such an enormous amount of diagnostic data requires a high-performance computing (HPC) platform not only for LHD physics data analyses but also for next-generation experiments, such as ITER and JT-60SA. Performance evaluation has been done at NIFS by using the HPC supercomputer “Raijin” and the LHD primary data storage system, both of which are directly connected by the 100 Gbps Ethernet optical link. The test results show that almost a full bandwidth can be used by means of multiple parallel streams.
In order to make plasma and fusion diagnostic data “FAIR”, all the LHD’s diagnostic data objects are now under way to be registered with digital object identifiers (DOI) for each acquisition node and plasma pulse. In 2023, more than 1.2 million DOIs were issued for the LHD diagnostic data. Cloud technology is also very promising as a high-performance data computing platform, not only for physics data analyses but also for real-time plasma and plant controls. AWS (Amazon Web Service) S3 cloud storage has accepted a proposal to store all 2.0 peta-byte of compressed LHD physics data for open access, under the AWS Open Data Sponsorship Program (ODP). AWS is also one of the commercial providers of computing clouds in the framework of NII’s Research Data Cloud (RDC) in Japan, allowing LHD data users to increase or decrease the computing power they need on demand.
The demonstrations and technical verifications done in this study suggest that a next-generation fusion data research center could be based on cloud technology.journal articl
Review of Contributions of Image Observations Using Visible Cameras to Advancements in Sustaining Long-pulse Discharges in LHD
No full textORCID 0000-0003-0655-7347This paper reviews the contributions of image observations to extending the duration of Ion Cyclotron Range of Frequencies (ICRF)-heated long-pulse discharges in the Large Helical Device (LHD). The plasma discharges were monitored using over 25 visible cameras, three fast-framing cameras, and various advanced plasma diagnostics, which revealed that most long-pulse discharges were interrupted by the following four events: termination of ICRF plasma heating due to arcing events in antennas, uncontrollable plasma density rise by outgassing from divertor plates, iron influx from plasma-facing components in the vacuum vessel, and carbon influx originating from the divertor regions. Image observations played a crucial role in mitigating the above four events that restricted the duration of long-pulse discharges by implementing appropriate countermeasures such as enhancing the cooling efficiency of the divertor plates, adopting new operational techniques to disperse the heat-load distribution, improving the ICRF antenna configurations, installing new additional ICRF antennas, and modifying the divertor configuration. Interruptions in long-pulse discharges were statistically analyzed using experimental data in three previous experimental campaigns, demonstrating a history of continuous efforts to extend the plasma discharge duration. This paper highlights the contributions of image observations over the past two decades, which have revealed inherent limitations in conventional magnetic plasma confinement devices that utilize carbon and iron plasma-facing components in sustaining steady-state plasma discharges. Knowledge obtained from statistical analysis provides valuable information for optimizing next-generation plasma confinement devices aiming at steady-state operation.journal articl
Spectroscopic Diagnostics for Highly Charged Iron Ions Observed in Solar Corona and LHD
No full textThe EUV Imaging Spectrometer (EIS) on board the Hinode mission is capable of observing solar coronal plasma possibly in non-ionization-equilibrium. EUV emission lines from highly charged Fe ions observed in the solar corona are also produced in the Large Helical Device (LHD) and the compact electron beam ion trap (CoBIT). Time-dependent collisional-radiative model (CRM) for Fe ions is developed to diagnose those plasmas in the Sun and the laboratories by adopting the best available theoretical calculations of atomic parameters, as well as generating the experimental data.journal articl
Sintering Method of Pure Boron Pellet for Advanced Fusion Reactor
No full textORCID 0000-0001-9016-2318The proton-boron (B) fusion reaction, which has recently garnered increasing attention, requires pure boron-11 as fuel. In previous boron injection experiments in fusion reactors, small cylindrical capsules containing boron powder were used as pellets. However, this injection method posed several critical challenges, including contamination of capsule elements and low injection volume. To address these issues, our group developed capsule-free pure boron pellets, specifically for plasma experiments using boron. This paper presents the fabrication processes and mechanical properties of the pure boron pellets, which were successfully injected into the Large Helical Device (LHD).journal articl
Verification of Background Noise Estimation Method in W-Band Millimeter-Wave Back-Scattering System
No full textORCID 0000-0002-9172-7381Millimeter-wave back-scattering (BS) system is a diagnostics system capable of measuring electron-scale turbulence, which is expected to have a large influence on future burning plasma confinement. The measured frequency band overlaps that of the electron cyclotron emission (ECE), resulting in comparable background noise. We have developed a method to estimate this background noise by implementing linear regression between the BS signal intensity and that of the ECE when the probing beam is turned off, and have verified the method in LHD experiments.journal articl
Advanced Multi-Step Brazing (AMSB) for fabrication of new type of W/stainless steel first-wall component with ODS-Cu intermediate layer
Full text linkORCID 0000-0002-3744-2481The novel method “Advanced Multi-Step Brazing (AMSB)” has been developed to fabricate a new type of “divertor” and “first-wall” heat removal component in a fusion reactor. This study is focused on the latter component, in which a tungsten (W) sheet is jointed through AMSB to a stainless steel (SUS) substrate via an oxide dispersion strengthened copper (ODS-Cu) intermediate layer. The principle of AMSB is a repetitive application of the advanced brazing technique (ABT). The initial purpose of the ABT was to braze W to ODS-Cu (GlidCop®) with the Ni-11 %P filler material. Later, we confirmed that the ABT is able to produce a very tough GlidCop® and SUS (GlidCop®/SUS) joint. One of the major advantages of GlidCop®/SUS joints is physically strong tolerance against the repetitive brazing heat-cycle. Thus, a repetitive application of the ABT does not cause any negative effects against post-brazed GlidCop®/SUS joints, and hence AMSB can be applied for fabricating a single heat-removal component with multiple joint interfaces. A small-scale sample of the new type of first-wall component was fabricated through two-step brazing. At first, the GlidCop® plate was jointed to the SUS by the ABT, and then a thin W-sheet with the thickness of 0.254 mm was jointed to GlidCop®/SUS by the ABT. If a large area of the first-wall surface in the fusion reactor can be covered with such a thin W-sheet, the amount of hydrogen isotopes trapped on the first-wall surface could be significantly reduced, compared to other fabrication methods such as a vacuum plasma spray W (VPS-W). The VPS-W is theoretically less dense than a W-sheet and often contains pore structures, which could act as effective trapping sites for hydrogen isotopes. A small-scale sample of the new type of first-wall component with a W-sheet (W/GlidCop®/SUS) was successfully fabricated to overcome the above disadvantages. In addition, the W surface of the component showed low retention characteristics of hydrogen isotopes compared with other W surfaces, e.g., atmospheric plasma-sprayed W (APS-W).journal articl
Self-field measurements of a REBCO Rutherford-type cable
No full textORCID 0000-0002-1989-867XSelf-field measurements of a Rutherford-type cable immersed in liquid nitrogen have been performed. A 1-meter straight Rutherford-type cable composed of 60 REBCO tapes (12 sub-cables) was tested. The measurement results indicated that the self-field was unaffected by the cable energization ramp-rate. Based on the measurement results, a current center position within the cable cross-section was investigated during cable energization. The current center position remained unchanged during the charging. Consequently, the current distribution of the Rutherford-type cable was stable.
In addition, the 1-meter straight Rutherford-type cable was compared to other cable samples composed of stacked REBCO tapes. Under the same sample shape and cooling condition, the Rutherford-type cable and the stacked assembled in rigid structure (STARS) cable have similar characteristics with respect to the current center position during cable excitation.journal articl
Single-pixel edge enhancement of object via convolutional filtering with localized vortex phase
No full textIn this paper, we propose and numerically demonstrate an all-directional edge-enhanced microscopy based on single-pixel imaging using convolutional filtering with a localized vortex phase. The edge-enhanced microscope with a vortex filter is of particular interest for optical information processing, as it can highlight both amplitude and the phase edges of complex objects in all directions. While Fourier-domain single-pixel imaging has been proposed as a cost-effective approach for edge enhancement in non-visible wavelengths, it requires three or four times more measurements due to the need for three- or four-phase shifting to detect optical complex amplitudes in the Fourier domain. Our method implements convolutional filtering with a localized vortex phase in the spatial domain, eliminating the need for these extra measurements required by phase-shifting methods. Simulations demonstrate a high correlation coefficient of 0.95 between ideal and enhanced edges, offering significant improvements for edge enhancement in various invisible-wavelength imaging applications.journal articl
Microstructure of reduced activation ferritic/martensitic steel after transformation super plasticity
No full textORCID 0000-0001-9016-2318Reduced activation ferritic-martensitic (RAFM) steels are considered promising candidates as structural materials for fusion blankets. However, the poor processability of RAFM steels is one of the critical issues in their application to fusion blankets. To address the issue, a new material processing based on transformation super plasticity (TSP), a type of superplastic forming (SPF), has been investigated for advanced fusion engineering. Our previous studies have demonstrated that the total elongation of a RAFM steel (JLF-1) reached 170 % through tensile tests under constant load and cyclic heat treatment, suggesting that the TSP-type SPF can be used for the macroscopic deformation processing of RAFM steel. For the application of TSP-type SPF, it is essential to study the microstructure before and after TSP. In this study, the effect of cyclic heat treatment on the microstructure before and after deformation in tensile tests using the TSP was investigated. Microstructural TEM observation of M23C6 and MX precipitates showed that only M23C6 disappeared after tensile tests with cyclic and isothermal temperature and precipitated again by the following tempering. The results indicate that the MX alone remaining at the lath boundaries cannot inhibit the TSP creep deformation, and the combination of dissolution of M23C6 at the block boundaries and cyclic phase transformation may enhance the TSP creep deformation. These results will contribute to the development of advanced processing technology for fusion blankets.journal articl