Kinetic estimation of tritium and neutron yields in all LHD deuterium experiment campaigns

ORCID 0000-0003-3293-488XTritium yields in all six Large Helical Device deuterium experiment campaigns, or 41 064 discharges, have been numerically estimated. Usually, tritium yields are estimated from neutron yields by assuming that they are the same. In present fusion devices, this assumption is inaccurate because a fusion reaction between thermal-deuterons and fast-deuterons is dominant. In this paper, the energy distribution of fast-deuterons injected by neutral beam injectors is taken into account for the estimation of the ratio of the tritium yields to the neutron yields, Yt/Yn. By integrated simulation, the Yt/Yn is approximately 0.94 in each campaign. Because assumptions applied in the simulation aim to avoid under-estimation of the tritium yields compared to the actual value, the Yt/Yn should be 0.85 < Yt/Yn < 0.94.journal articl

On electromagnetic perturbations of geodesic acoustic modes in anisotropic tokamak plasmas

Electromagnetic geodesic acoustic modes are analytically investigated in tokamak plasmas with anisotropy, utilizing gyro-kinetic equations and a rigorously self-consistent anisotropic distribution. When including first-order finite-orbit-width effects and first-order finite-Larmor-radius effects, it is proven that the anisotropy with an arbitrary strength does not induce the m = ±1 harmonics of A∥, where m and A∥ denote the poloidal wavenumber and the parallel component of the perturbed magnetic vector potential, respectively. The rigorously self-consistent anisotropy introduces an equilibrium electrostatic field with poloidally asymmetric structure, and consequently induces an additional ⃗E×⃗B drift term within the gyro-kinetic equation. This equilibrium electrostatic field inhibits the anisotropy from generating non-zero m = ±1 harmonics of A∥. Indeed we demonstrate that introducing anisotropy self-consistently into the equilibrium quantitatively influences m = ±1,±2 harmonics of the perturbed electrostatic potential, but only the m = ±2 harmonics of A∥.journal articl

MHD analysis of electromagnetic GAMs in up-down asymmetric tokamaks

We analytically investigate geodesic acoustic modes (GAMs) in tokamak plasmas with up-down asymmetric and non-circular cross-sections using magnetohydrodynamics (MHD) and a Miller-like flux surface model. Explicit expressions for GAM frequency, magnetic field perturbations, and Lagrangian displacement are presented. Our results reveal that (I) up-down asymmetry (σ) slightly increases the GAM frequency and introduces additional sin or cos components (opposite to the dominant component) to the perturbations; (II) the inverse aspect ratio (ε), the gradient of the Shafranov shift (Δ′), triangularity (δ), and its gradient (sδ) can induce additional subdominant components of perturbations. The poloidal mode numbers of the dominant and subdominant components differ, and in certain cases, the amplitude of the subdominant component can approach or even exceed that of the dominant component. These results provide analytical explanations for previous MHD and gyro-kinetic simulation outcomes, and offer useful guidance for measuring multiple components of perturbations.journal articl

Progress in the development of radiation detectors for fusion neutron source A-FNS

ORCID　0000-0003-4555-1837The time evaluation of the intensity and spectrum of a neutron source provides the fundamental data for neutron irradiation experiments. Therefore, the study and development for measurement of radiation flux and spectrum with high accuracy is necessary. The development of fast neutron detectors has been advanced through a collaborative research undertaking between the National Institute for Fusion Science and the National Institutes for Quantum Science and Technology for Fusion Neutron Source, A-FNS, which is a high-energy and high-intensity neutron source based on the deuterium-lithium stripping reaction with its energy spectrum extending up to 55 MeV. d-D and d-T neutron spectrometers based on scintillation and diamond detectors were developed in the accelerator-based d-D and d-T neutron sources. The initial radiation measurements using the developed detectors were conducted in the Linear International Fusion Materials Irradiation Facility Prototype Accelerator (LIPAc). The d-D neutron spectrum and source emission rate were measured by a high-pressure 3He proportional counter and stilbene scintillation detector, respectively. Accordingly, the fast-neutron spectrum from the LIPAc beam dump was measured using an EJ-301 scintillation detector. Furthermore, an initial test of a single-crystal chemical-vapor-deposition diamond detector and the gamma ray spectroscopy using large-volume LaBr3:Ce scintillation detector were also conducted. The time evolution of the gamma ray pulse count rate was consistent with the beam injection pattern, i.e., 3 ms injection every 100 ms. The demonstration of the performance of the developed detectors at the current stage of LIPAc provided the basic experimental evidence necessary for designing a radiation monitor system for A-FNS.journal articl

In-vessel colorimetry of Wendelstein 7-X first wall components after OP2.1

ORCID　 0000-0001-5522-3082Colorimetry has been continuously utilized for the estimation of deposition layer distribution on the first wall panels and divertor target elements in each Operation Phases (OP) of Wendelstein 7-X (W7-X). In OP2.1, significant achievements were made in plasma-wall interaction studies and divertor performance, completed by the installation of actively cooled divertors together with the change of material from fine grain graphite to Carbon Fiber-reinforced Composite. This upgrade enabled the achievement of long-duration plasma discharges, reaching up to 500 s with a total injected energy of 1.3 GJ. Compared to a factor of 2.5 increase in the net average deposition thickness between OP1.2a and 1.2b, no significant change was observed between OP1.2b and OP2.1. The balance of deposition and erosion of deposition layer on the first wall panels is presumably changed in OP2.1. The considerable reasons are discussed qualitatively from the point of possibilities such as lower surface temperature of the divertor target elements, lower impurity level of bulk plasmas, and difference of the location of sputtered carbon source. Moreover, the clear pattern of the colorimetry on the divertor target elements is seen in OP1.2a and OP2.1 not in OP1.2b. However, it is not currently conclusive whether changes in reflectivity and optical properties on the graphite divertor surface are due to erosion or deposition.journal articl

First Confinement Time Evaluation for Particles Axially Injected into a Non-Adiabatic Trap

This study presents the first detailed investigation of confinement times in non-adiabatic traps during the axial injection of thermal plasma from the mirror edge, using particle trajectory calculations. Plasma is supplied from a coaxially positioned plasma source through an orifice with a ring-shaped aperture. The results of the analysis show that the longest confinement time occurs when the ring radius of the aperture is approximately equal to the ion Larmor radius at the mirror region. Under these conditions, the confinement time is found to be approximately four times the time it takes for particles to traverse the device length at thermal velocity.journal articl

Consideration on relation between penetrated power and topological charge of millimeter-wave vortex in magnetized plasma

ORCID　0009-0009-9605-513XIt was demonstrated that the vortex field of a hybrid mode can propagate in the magnetized plasma region where plane waves are unable to propagate due to the cut-off condition. In this study, the dependence of the penetrated power of injected as millimeter-wave vortices of the hybrid mode in magnetized plasma is analyzed using the Finite-Difference Time-Domain (FDTD) method. The effect of the radius size of the corrugated waveguide on the penetrated power is described, revealing its significant contribution to reducing the deviation of topological charge in the hybrid mode. Furthermore, it was found that the penetrated power of the vortex field in magnetized plasma strongly depends on the topological charge l and the deviation of topological charge.journal articl

Irradiation Effects of As-Fabricated and Recrystallized 12Cr ODS Steel Under Dual-Ion Beam at 973 K

ORCID　 0000-0001-5706-0221The microstructure evolution and hardness variations of as-fabricated and recrystallized 12Cr oxide dispersion strengthened (ODS) steel after dual-ion (6.4 MeV Fe3+ and energy-degraded 1 MeV He+) irradiation at 973 K up to 10.6 displacements per atom (dpa) at peak damage and 8900 appm He are investigated. Results show that the oxide particles slightly shrink in the as-fabricated specimen, while they are stable in the recrystallized specimen. Furthermore, larger helium bubbles are trapped at the grain boundaries in the as-fabricated specimen, and the size of helium bubbles in the grains is almost the same for both as-fabricated and recrystallized specimens, indicating that reduction of grain boundaries would reduce the potential nucleation sites and suppress the helium segregation. Moreover, no obvious hardening occurs in the as-fabricated specimen, whereas the hardness increases a little in the recrystallized specimen. Based on the barrier model, the barrier strength factor of helium bubbles is calculated. The value is 0.077, which is much smaller and suggests that helium bubbles seem not to significantly induce irradiation hardening.journal articl

Spatial Structure of Low-Frequency Hall-MHD Wave Propagation in a Field-Reversed Configuration Plasma

A linear Hall-MHD simulation code is developed to investigate the spatial structure of two-dimensional wave propagation excited in a field-reversed configuration (FRC) equilibrium plasma. A low-frequency oscillating magnetic field at 160 kHz is externally applied by a ring coil installed concentrically with the device axis in the open-field region. The generated toroidal magnetic field propagates primarily along magnetic field lines at a phase velocity comparable to that of shear Alfvén waves. Due to the Hall effect, toroidal magnetic fluctuations penetrate into the closed-field region near the separatrix over a distance on the order of the ion skin depth. In the core region, where magnetic fluctuations vanish, ion density oscillations become dominant.journal articl

Linear Landau damping, Schrödinger equation, and fluctuation theorem

ORCID　0000-0001-5444-1758A linearized Vlasov–Poisson system of equations is transformed into a Schrödinger equation, which is used to demonstrate that the fluctuation theorem holds for the relative stochastic entropy, defined in terms of the probability density functional of the particle velocity distribution function in the Landau damping process. The difference between the energy perturbation, normalized by the equilibrium temperature, and the entropy perturbation constitutes a time-independent invariant of the system. This invariant takes the quadratic form of the perturbed velocity distribution function and corresponds to the squared amplitude of the state vector that satisfies the Schrödinger equation. Exact solutions, constructed from a discrete set of Hamiltonian eigenvectors, are employed to formulate and numerically validate the fluctuation theorem for the Landau damping process. The results offer new insight into the formulations of collisionless plasma processes within the framework of nonequilibrium statistical mechanics.journal articl