NIFS-Repository (National Institute for Fusion Science)
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ECE Imaging System in LHD
No full textORCID 0000-0002-8302-8986Electron Cyclotron Emission Imaging(ECEI) is a powerful tool for investigating MHD instabilities and turbulence in magnetically confined plasma. In the LHD, the ECEI system has been developed and successfully obtained two-dimensional images of temperature fluctuations. This paper describes the current V-band and Q-band ECEI systems including developments in their key components. The initial results of the observation of geodesic acoustic mode (GAM) are presented.journal articl
Monte Carlo simulation method for incoherent Thomson scattering spectra from arbitrary electron distribution functions
No full textORCID 0000-0001-9879-9532We developed a Monte Carlo simulation method to calculate incoherent Thomson scattering spectra in high temperature plasmas. The basic idea is to treat the entire scattering process as the superposition of individual photon-electron interactions. We introduce macro-particles, referred from particle-in-cell simulations, to reduce the computational cost, and obtain scattered spectra within a reasonable computational time. Since the velocity of the interacting electron is randomly sampled from an electron distribution function, the method can be applied to arbitrary electron distribution functions provided an appropriate sampling scheme is available. We present simulation results for relativistic Maxwellian and kappa distribution functions, and compare them with both analytical and numerical spectra for validation. The simulated spectra show good agreement with both analytical and numerical results, demonstrating that the Monte Carlo simulation method can reliably reproduce incoherent Thomson scattering spectra.journal articl
Visualization of fusion plasmas
No full textORCID 0000-0002-6383-6425This paper discusses the importance and advancements of visualization technology in fusion science research. First, visualization is an essential process for analyzing experimental and simulation data, aiding in the understanding of complex phenomena such as plasma. It emphasizes that, instead of conventional two-dimensional graphs, virtual reality (VR) technology enables researchers to observe data in three dimensions. This approach leads to a more intuitive understanding of complex phenomena. Additionally, VR technology provides an environment where multiple researchers can simultaneously discuss and analyze plasma physics, making it highly useful for research. Furthermore, VR plays a crucial role in effectively communicating research findings to the general public in an accurate and accessible manner. At the National Institute for Fusion Science, a VR visualization system has been established to efficiently analyze large-scale simulation data using CAVE-type VR devices. The latest technology, such as head-mounted displays (HMDs), has also been introduced. The applications of visualization technology are not limited to fusion science but are expected to expand into other fields as well, making it a promising area of ongoing development. This paper presents key visualization research achievements conducted at the National Institute for Fusion Science. We are also developing new capabilities to display both CAD and simulation data on HMDs as we port the VR software originally designed for large CAVE-type systems. These developments will also be described.journal articl
Parity Transition of MHD Instabilities in LHD Experiments
No full textORCID 0000-0003-3754-897XThis study reviews experimental observations of parity transitions in the mode structures of magnetohydrodynamic (MHD) instabilities in magnetically confined plasmas. The transition between even and odd-parity modes, which is closely related to changes in the topology of a magnetic vessel, suggests the formation and healing of the magnetic island. While the transitions from even to odd parity have been reported in several tokamak devices, recent experiments in the Large Helical Device (LHD) have revealed conventional even-to-odd parity transitions due to various instabilities and a novel transition from odd to even parity. These include transitions associated with energetic ion-driven resistive interchange instabilities, locked-mode-like instabilities, ”Edge” MHD instabilities, and double-odd-parity modes. These findings highlight the need for advanced diagnostics and theoretical modeling to further clarify the mechanisms governing parity transitions.journal articl
Neutron Camera in Large Helical Device
No full textORCID 0000-0003-4555-1837Three sets of neutron cameras have been developed in Large Helical Device to measure the spatial profile of energetic particles, since deuterium-deuterium neutrons are mainly produced by the reaction of thermal deuteron and energetic deuteron. One of neutron cameras is based on a stilbene scintillation detector operating in the pulse-counting mode with pulse shape discrimination ability for medium to high neutron emission discharges. The other two neutron cameras are based on an EJ-401 fast neutron scintillator detector operated in the current mode for low to medium neutron emission discharges. Neutron cameras have been a powerful tool for understanding beam ion confinement as well as beam ion transport during the beam ion-driven magnetohydrodynamic instabilities. The development and details of the neutron cameras in the Large Helical Device are described.journal articl
MHS Equilibria in the Non-Resistive Limit to the Randomly Forced Resistive Magnetic Relaxation Equations
No full textWe consider randomly forced resistive magnetic relaxation equations (MRE) with resistivity κ > 0 and a force proportional to √κ on the flat d-torus Td for d ≥ 2. We show the path-wise global well-posedness of the system and the existence of the invariant measures, and construct a random magnetohydrostatic (MHS) equilibrium B(x) in H1(Td) with law D(B) = µ0 as a non-resistive limit κ → 0 of statistically stationary solutions Bκ (x, t). For d = 2, the measure µ0 does not concentrate on any compact sets in H1(T2) with finite Hausdorff dimension. In particular, all realizations of the random MHS equilibrium B(x) are almost surely not finite Fourier mode solutions.journal articl
Conceptual design of Thomson scattering system with high wavelength resolution in magnetically confined plasmas for electron phase-space measurements
No full textORCID 0000-0001-9879-9532We discuss the conceptual design of a spatially-resolved spectroscopy system of Thomson scattering with high wavelength resolution capable of measuring the shape of electron velocity distribution functions in magnetically confined plasmas. We design a spatially-resolved spectrometer with 2560 wavelength channels. The estimated number of scattered photons in a single spectrometer channel is much larger than unity under the experimental setup and plasma parameters of the compact Helical device (CHD), indicating sufficient photon statistics for single-shot measurements. Simulations of the scattered spectra show that the signal-to-noise ratio exceeds 5 even under the most unfavorable conditions expected in CHD at full spectral resolution, and further improves with post-processing pixel binning. Bayesian inference applied to the simulated spectra demonstrates that the inferred plasma parameters agree with the input values within the estimated uncertainties. Comparisons between spectra generated from non-Maxwellian electron velocity distribution functions and their Maxwellian fits indicate that deviations from Maxwellian distributions can be identified using the proposed system.journal articl
A collision operator for field-mediated interactions in general relativistic kinetic theory
No full textWe develop a Hamiltonian framework for general relativistic kinetic theory on the cotan-gent bundle T∗M of a Lorentzian (pseudo–Riemannian) manifold. Starting from the geodesic Hamiltonian H, we derive a Landau-type collision operator for self-gravitating particles under-going binary interactions mediated by an arbitrary potential energy V, and couple the resulting kinetic stress–energy to the Einstein field equations to obtain the Landau–Einstein system. In the presence of a coordinate-time Killing symmetry we find a family of stationary states of the form f ∝ γ exp[−β(H + Φ)] ζ(p0), where Φ is the mean field, γ = dt/dτ , β is an inverse-temperature parameter, and ζ encodes symmetry-induced degeneracy.journal articl
Threshold analysis of lower hybrid and ion Bernstein wave instability driven by ring-like energetic ions
No full textThis paper numerically investigates the instability of lower hybrid and ion Bernstein waves driven by ring-like energetic ions, focusing on the energetic-ion density for their excitation. We systematically analyze how this threshold density depends on crucial parameters, namely, the lower hybrid resonance frequency (ωLH ) and the velocity of energetic ions. It is shown that the threshold density tends to decrease signifi-cantly when ωLH approaches integer multiples of the ion cyclotron frequency. However, this tendency depends on the energetic-ion velocity.
We also find that a higher energetic-ion velocity does not always result in a smaller threshold density; depending on the value of ωLH , the threshold density can be smaller when the energetic-ion velocity is small or intermediate. We discuss the instability mechanism theoretically and compare the theoretical predictions with the numerical results.journal articl
Study of Leakage Electromagnetic Field Monitoring with a Wide Frequency Range on Safety Management in a Magnetic Fusion Facility
No full textWith respect to considerations of nuclear fusion systems, since the electromagnetic field in the wide frequency range from the static magnetic field to extremely high frequency was utilized for the plasma confinement, heating at the magnetic confinement fusion facilities, not only ionizing radiation but also non-ionizing radiation should be investigated and discussed in terms of occupational protection as one of the issues in fusion facilities. To clarify the behavior of non-ionizing radiation, leakage from electric and magnetic fields has been observed at the Large Helical Device (LHD), because a strong magnetic field generated by large superconducting magnet coils for plasma confinement, in addition to high-frequency and high-power oscillators for ion cyclotron range of frequencies (ICRF) and electron cyclotron resonance heating (ECRH) for plasma heating, are used. In the LHD experiments, magnetic field leakage has been observed at extremely low frequencies (ELFs) around a power supply system for superconducting magnet coils. The magnetic field outside the experimental hall and the intensity of electric field leakage from the oscillators for ICRF and ECRH are significantly below the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines and radio frequency exposure protection standards in Japan (RCR-STD38). The ICNIRP and RCR-STD38 provide guidelines for limiting exposure to electric and magnetic fields to protect against established adverse health effects. The guidelines for safety management to protect the working people in fusion facilities should be made clearer with respect to occupational protection from non-ionizing radiation.journal articl