NIFS-Repository (National Institute for Fusion Science)
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Comprehensive Assessment of Tritium to the Environment Discharged By Deuterium Plasma Experiment Using Large Helical Device, Japan
No full textAt the National Institute of Fusion Science (NIFS), the deuterium plasma experiment was conducted using the large helical device (LHD) from 2017 to 2022 for the high performance of the plasma experiment. Through this experiment, a small amount of tritium was produced by D-D fusion reaction and released to the atmospheric environment through the stack. To understand the impact of tritium on the environment, environmental tritium monitoring was conducted before, during, and after the experiment for public acceptance and in accordance with local governments. From this monitoring, no impacts were observed on monthly precipitation and pine needle samples at the NIFS site. As the result of a comprehensive assessment combined with atmosphere and environmental water monitoring, it was concluded that the impact of discharged tritium from the stack of LHD to the surrounding environment would be none and/or negligibly small.journal articl
Impact of Edge Magnetic Island on Thermal Instability, Equilibrium, and Detachment Dynamics Induced by Impurity Radiation in LHD
No full textThe impact of edge magnetic islands on divertor detachment in LHD is investigated, with emphasis on thermal instability and thermal equilibrium. During a density ramp-up, as the edge plasma temperature is reduced, radiation is enhanced in cases with an edge magnetic island compared to those without one, and the detached plasma state remains stable. In contrast, in the absence of the island, the increase in radiation becomes uncontrollable, ultimately leading to a radiation collapse of plasma. Analysis of thermal instabilities indicates that the X- and O-points of the island are particularly susceptible to thermal instability due to their distinct magnetic topologies. Impurity radiation measurements reveal that, during density ramp-up, radiation initially emerges around the island’s X-point. Following the detachment transition, the location of peak radiation shifts to the O-point, where signatures of volume recombination are observed. Numerical simulations of edge plasma transport reproduce these dynamic trends, which reinforces the experimental interpretation. Thermal instability growth rates estimated from experimental data indicate that, within the island’s O-point, the growth rates decrease as detachment deepens; in contrast, in the absence of the island, the growth rates continue to increase, approaching radiation collapse. Additional aspects of thermal instability and radial thermal equilibrium are discussed to elucidate the factors contributing to detachment stabilization and to outline remaining challenges for future investigations.journal articl
Implementation and Technical Evaluation of Shot-by-Shot Data Transfer from GAMMA 10/PDX To LHD LABCOM Via SNET
No full textWe report the implementation and technical evaluation of a shot-by-shot data transfer system from the GAMMA 10/PDX tandem mirror device to the LHD LABCOM system via SNET, as part of the Fusion Virtual Laboratory project. Since 2008, we have transferred approximately 1.5 TB of experimental data annually. In 2023, we successfully established real-time CAMAC data transfer on a per-shot basis. This paper details the network architecture, data acquisition systems, transfer protocols, and operational reliability. The system enables remote access and supports collaborative research by providing timely and structured data to the LABCOM database. The technical performance and future development plans are also discussed.journal articl
Recipe of Helium Neutral Beam Injection for Studying Helium Transport in LHD
No full textTo advance the understanding of helium transport physics, helium neutral beam (He-NB) experiments were initiated in the Large Helical Device (LHD) during the FY2021 experimental campaign. To facilitate these investigations, modifications were implemented in both the impurity exhaust system and the neutral beam (NB) injection system, enabling the use of the helium beam in a typical fusion experimental environment. In the positive-ion-source-based NB system—originally designed for hydrogen and deuterium beams—the gas supply lines were adapted to introduce helium and argon into the ion sources, while argon was injected into the neutralizer cells. These modifications allowed for the injection of a high current and high focused 78 keV He-NB into the plasma. To improve helium exhaust efficiency at main vessel, turbo-molecular pumps were installed, increasing the effective pumping speed by approximately 1.4 times. During He-NB operation, a gradual decrease in the arc current of the NB ion source was observed with each discharge. This effect was attributed to the formation of helium bubbles on the tungsten filament surface, and a recovery method using argon gas discharges was successfully demonstrated. Plasma heating by He-NB injection was confirmed, and both energetic helium ions and bulk helium ions were successfully measured. Their spatial distributions were also observed. These results contribute to a deeper understanding of helium behavior in fusion plasma and offer valuable insights for future helium transport studies.journal articl
Scattering theory in noncanonical phase space: A Drift-Kinetic collision operator for weakly collisional plasmas
No full textORCID 0000-0002-2973-0635After developing a scattering theory for grazing collisions in general noncanonical phase spaces, we introduce a guiding center collision operator in five-dimensional phase space designed for plasma regimes characterized by long wavelengths (relative to the Larmor radius), low frequencies (relative to the cyclotron frequency), and weak collisionality (where repeated Coulomb collisions induce cumulatively small changes in particle magnetic moment). The collision operator is fully determined by the noncanonical Hamiltonian structure of guiding center dynamics and exhibits a metriplectic structure, ensuring the conservation of particle number, momentum, energy, and interior Casimir invariants. It also satisfies an H-theorem, allowing for deviations from an equilibrium Maxwellian distribution due to the nontrivial kernel of the noncanonical guiding center Poisson tensor, spanned by the magnetic moment. We propose that this collision operator and its underlying mathematical structure may offer valuable insight into the study of turbulence, transport, and self-organizing phenomena in both laboratory and astrophysical plasmas.journal articl
Aneutronic Fusion Study in Large Helical Device
No full textORCID 0000-0003-4555-1837A study of aneutronic fusion, which refers to fusion reactions that do not produce neutrons, has been conducted on the Large Helical Device (LHD). For the D–³He study, feasibility investigations of detecting charged fusion products have been performed. High-energy neutral beams, lost proton detectors, and gamma-ray detectors were planned for installation to validate the D–³He fusion reactions. Numerical calculations show that the expected D–³He fusion rate is 2.7 × 1016 s-1 with most of the protons being lost immediately from the plasma. For the p–¹¹B study, the total alpha particle emission rate was estimated to be 1014 s-1, and the loss points of alpha particle distribution were calculated. Based on these numerical simulations, we installed an alpha particle detector on a manipulator and positioned it at the bottom of the LHD. The time evolution of the alpha particle detection rate, measured by the detector, was found to be consistent with the predictions from the numerical simulations, demonstrating the first successful observation of the p–¹¹B reaction in a magnetic confinement system.journal articl
Density Dependence of Low-Frequency Edge Harmonic Oscillations in LHD H-Mode Plasmas
No full textThis paper investigates the density dependence of low-frequency edge harmonic oscillations (LF-EHOs) in the Large Helical Device (LHD) H-mode plasmas. A series of discharges with varying density settings reveal that the L-H mode transition occurs above a certain density threshold, and LF-EHOs appear only above an even higher density threshold. In discharges exhibiting LF-EHOs, the cross-coherence between density and magnetic fluctuations for the fundamental frequency remains consistently high across the entire density range. In contrast, the cross-coherence for the second harmonic exhibits an increasing trend with higher plasma density. These findings suggest that the LF-EHO potentially plays a noticeable role in edge profile saturation.journal articl
High Power Long Pulse Experiment by ICRF Heating in LHD
No full textORCID 0000-0003-2718-4930Long pulse discharge experiments have been conducted at LHD as one of the challenges in the quest for a fusion reactor. The plasma could be sustained for 48 min, using 1.2 MW of heating power from the ion cyclotron range of frequencies (ICRF) and electron cyclotron heating (ECH) power. The line-averaged electron density was 1.2 × 1019m-3, the ion and electron temperature was 2 keV, and the input energy reached 3.36 GJ. The ICRF antennas used were the HAS (handshake form) and the FAIT (field-aligned-impedance-transforming) type, which upgraded and replaced the existing PA (poloidal array) antennas, in addition to the existing PA antenna. These antennas had the following characteristics. The PA one had a Faraday shield on one antenna strap removed. The HAS antenna’s straps were aligned in the toroidal direction and was phase controllable in the toroidal direction. The FAIT antenna had a built-in impedance transformer to obtain high plasma loading resistance. Long pulse discharges are often terminated by an influx of impurities, which are the result of exfoliation of deposits left during the discharges. Overcoming this impurity problem is one of the challenges of a steady state operation.journal articl
Effect of Nuclear Elastic Scattering on the Fusion Reaction Rate in the LHD Plasmas
No full textThe enhancement of the D(d, n)3He fusion reaction rate, due to the production of energetic deuterons (referred to as the knock-on tail) via nuclear elastic scattering of energetic beam protons, was observed on the large helical device (LHD). The enhanced reaction rate was measured as an increase in the neutron generation rate by more than one order of magnitude. This observed phenomenon was explained through both the Boltzmann–Fokker–Planck analysis and particle simulations that consider three-dimensional particle motion in beam-heated deuterium plasma. The simulations demonstrated a good agreement with the observed phenomenon, leading to the conclusions that the knock-on tail is formed in the deuteron velocity distribution function.journal articl
光渦ビームを用いた分光技術による粒子計測
No full textORCID 0000-0002-0602-0665本稿では，螺旋(らせん)状の波面を持つ光渦ビームを用いた新たなプラズマ流速計測法を紹介する．従来法では原理的に不可能であった光軸と直交する流れの測定を，光渦による方位角ドップラー効果を利用することで可能にした．RFプラズマ中のアルゴン準安定原子の流れに対して，光渦レーザー吸収分光（OVLAS）法によって流速を定量的に評価した．さらに，光渦の強度分布に非対称性を導入した非対称光渦レーザー誘起蛍光（aOVLIF）法により，1方向からのレーザー入射のみで3次元流速ベクトルを決定できる可能性を示した．journal articl