NIFS-Repository (National Institute for Fusion Science)
Not a member yet
13159 research outputs found
Sort by
Pulse height analysis and pulse shape discrimination of pure LaCl3 scintillation crystal across a broad neutron energy range
No full textA pure LaCl3 crystal scintillation detector has been developed and shows strong potential for fast neutron detection, particularly via the 35Cl(n,p)35S and 35Cl(n,α)32P reactions. These reaction channels make it a promising candidate for neutron spectroscopy applications in nuclear fusion research. In this study, a comprehensive characterization of the pure LaCl3 scintillation detector was performed in a mixed neutron and γ-ray radiation field. Pulse height analysis and neutron–γ pulse shape discrimination were systematically evaluated over a wide neutron energy range, from approximately 2.46 MeV to 16.89 MeV, using both a 252Cf spontaneous fission source and a mono-energetic neutron beam at the Fast Neutron Laboratory, Tohoku University. The results reveal a clear separation between neutron- and γ-induced signals, with figure-of-merit values exceeding 1.18 at specific energies, demonstrating the detector’s excellent pulse shape discrimination performance. The pulse height response associated with charge generation in the pure LaCl3 scintillation crystal was systematically characterized. Energy calibration and linearity for neutron energies up to 5.61 MeV were verified through the peak corresponding to the 35Cl(n,p)35S ground-state reaction. These findings confirm that pure LaCl3 scintillators exhibit excellent neutron–γ discrimination, enabled by differences in the pulse height and decay time characteristics of the induced signals, making them well-suited for high-resolution neutron spectroscopy in mixed radiation fields typical of magnetic confinement fusion environments.journal articl
Axisymmetric dynamos sustained by a modified Ohm’s law in a toroidal volume
Full text linkORCID 0000-0002-2973-0635This work tackles a significant challenge in dynamo theory: the possibility of long-term amplification and maintenance of an axisymmetric magnetic field. We introduce a novel model that allows for non-trivial axially symmetric steady-state solutions for the magnetic field, particularly when the dynamo operates primarily within a ‘nearly spherical’ toroidal volume inside a fluid shell surrounding a solid core. In this model, Ohm’s law is generalised to include the dissipative force, arising from electron collisions, that tends to align the velocity of the shell with the rotational speed of the inner core and outer mantle. Our findings reveal that, in this context, Cowling’s theorem and the neutral point argument are modified, leading to magnetic energy growth for a suitable choice of toroidal flow. The global equilibrium magnetic field that emerges from our model exhibits a dipolar character. The central insight of the model developed here is that if an additional force is incorporated into Ohm’s law, symmetric dynamos become possible.journal articl
Z-dependent three-level simultaneous-crossing of excited states in germanium-like highly charged atomic ions
No full textSimultaneous-crossing of three energy-levels has been found for the first time in the atomic number -dependent
wavelength curves at = 66 of Ge-like highly charged atomic ions. Large Helical Device (LHD) and EBIT are used for obtaining the wavelength of EUV emissions from the transitions of 4 or 4 excited states into their ground state ([Ni]4242−)0. Multi-Configuration Dirac-Fock calculation has been carried out for analysis. Interactions between the three excited-state configurations ([Ni]424−4−)1, ([Ni]44−(42+)0)1, and ([Ni]44−(42+)2)1 lead to the three-dimensional rotation of atomic state-vectors in the configuration space on the way for to pass the crossing point. -evolution of the atomic states has been analyzed employing the quaternion algebra. The spectral anomalies near the level-crossing are simulated by the atomic state-vector rotation. The optically invisible states are thus become visible by sharing the intensities of optically allowed configuration ([Ni]424−4−)1.journal articl
First Confinement Time Evaluation for Particles Axially Injected into a Non-Adiabatic Trap
Full text linkThis 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
History, present status, and future directions of vanadium alloys for fusion reactors
No full textORCID 0000-0001-5598-1895Since the need for low-activation fusion reactor materials was recognized in the mid-1980s, vanadium alloys have been researched and developed as promising candidates for blanket structural materials. Vanadium alloys are non-ferromagnetic and ductile materials and thus have advantages different from other candidates. However, since vanadium alloys are still at a premature industrial stage, research and development have been carried out not only on issues specific to fusion reactors but also on those related to industrial materials. Blankets using vanadium alloys as structural materials and liquid Li as a tritium breeder and coolant (self-cooled V/Li blankets) have unique characteristics, and research and development of issues specific to V/Li blankets have been conducted in conjunction with the development of vanadium alloys. In this paper, the progress of this research and development is reviewed, and the remaining issues are pointed out. Furthermore, the future direction of vanadium alloy research and development is discussed, considering recent changes in the environment surrounding fusion energy.journal articl
MHD analysis of electromagnetic GAMs in up-down asymmetric tokamaks
Full text linkWe 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
液体窒素中における水平面上の遷移沸騰熱伝達 ―遷移沸騰域における定常的な沸騰曲線とその曲線に現れるヒステリシスの観測―
Full text linkORCID 0000-0002-5682-3343An open question has been existing whether hysteresis appears in transition boiling heat transfer. Herein, steady-state experiments were conducted using liquid nitrogen as the test fluid to obtain boiling curves for film and transition boiling regimes. In the experiments, steady-state boiling curves were obtained not only in the film boiling regime but also in the transition boiling regime, and hysteresis in the transition boiling regime was clearly observed. Steady transition boiling was observed when the superheat gradually decreased from the film boiling regime, whereas a higher heat flux was observed when the superheat increased. This hysteresis continued until the minimum heat flux temperature.journal articl
Azimuthal mode decomposition of millimeter-wave integer and non-integer optical vortices generated by a spiral mirror
Full text linkORCID 0000-0001-6636-8500We investigated the characteristics of both integer and non-integer optical vortices (OVs) in the millimeter-wave regime generated by a spiral mirror. An OV, characterized by the phase term eiℓϕ, also known as a Laguerre–Gaussian (LG) beam, is a solution to the wave equation in cylindrical coordinates under the paraxial approximation in free space. This typically restricts the azimuthal mode number ℓ to integer values.
However, the spiral mirror enables the generation of OVs with expected azimuthal mode numbers ¯ℓ, which can take either integer or noninteger values in the reflected wave. We employed a field reconstruction method to analyze both integer and non-integer OVs with ¯ℓ generated by the spiral mirror, decomposing them into their constituent integer ℓ components. Our results showed that when ¯ℓ is specified as an integer, the generated OV primarily contains that single ℓ. In contrast, specifying a non-integer ¯ℓ results in an OV composed of a superposition of multiple integer ℓ values. In other words, non-integer OVs are superpositions of LG beams with different integer azimuthal mode numbers.
These OVs are generated by the spiral mirror with a focus function, allowing for the specification of arbitrary waist sizes and focal positions, as designed for this research. Furthermore, we revealed that this focus spiral mirror can also avoid the separation of singularities that were present in OVs generated by conventional flat spiral mirrors.journal articl
Reactive molecular dynamics simulations of the intra- and intermolecular reactions of hydrogen-abstracted polyethylene chains
Full text linkPolymers can be radicalised by radiation, and the reaction of the generated radicals with each other could result in structural changes in the original material. Our previous reactive molecular dynamics (MD) simulations showed that several types of reactions occur at specific locations in polymer structures at low temperatures. However, not much is known about the occurrence of other types of reactions for these materials. In this study, reactive MD simulations were performed on hydrogen-abstracted polyethylene chains to clarify the conditions under which other types of reactions, such as chain scission and crosslinking, occur. Our simulation results showed that chain scission occurred only at a specific location at temperatures of 348 K or higher. Intermolecular crosslinking occurred only when two CH radicals were fixed at a distance of approximately 2.0 Å apart. However, our simulations also indicated that such movement does not occur freely owing to repulsive forces. These findings provide a new understanding of the structural changes and chemical reactions of polymers caused by radiation damage.journal articl
On electromagnetic perturbations of geodesic acoustic modes in anisotropic tokamak plasmas
No full textElectromagnetic 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