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Investigation of modular coil misalignment on magnetic flux surface in the CFQS quasi-axisymmetric stellarator
Full text linkThe CFQS is a quasi-axisymmetric stellarator of which the concept combines the advantage of good confinement of a tokamak and the steady-state operation capability of a stellarator. The CFQS has 16 modular coils (MCs), which consist of four independent shapes. Accurate positioning of MCs is one of the most important requirements, in particular, in stellarator since coil misalignment may have a significant effect on the quality of magnetic flux surfaces. Assuming some cases of displacement of MCs, we have investigated the effect on rotational transform profile and magnetic flux surfaces in the vacuum field. According to the results, it was found that MC misalignment has no significant effect on the CFQS plasma if the displacement is within a range expected in the actual MC installation.journal articl
Thermal Runaway of a REBCO Coil Co-Wound With a Copper Tape Immersed in Liquid Nitrogen/Hydrogen
Full text linkORCID 0000-0003-1454-8117Aiming at the establishment of design criteria for cryostable HTS magnets, we have investigated the behavior of thermal runaway of REBCO three-turn coils immersed in liquid nitrogen and liquid hydrogen. Sample 1 consisted of a REBCO tape with a copper layer 0.1 mm thick and a co-wound copper tape 0.2 mm thick, and Sample 2 consisted of the REBCO tape only. Each conductor was wound in a groove of a G10 plate, and only an upper side of the coil was cooled with cryogen. In order to simulate local degradation, the testing part, 0.1 m long, was damaged by bending, and critical currents at 77 K were reduced to less than 1/10 of the original value of 120 A. In liquid nitrogen, thermal runaway of Sample 1 occurred at 107 A after wide propagation of a normal zone, while that of Sample 2 occurred at 57 A before propagation of a normal zone. In liquid hydrogen, thermal runaway of both samples occurred before propagation of a normal zone, and the amount of heat generation of Sample 1 when starting thermal runaway was 1.5–1.8 times as high as that of Sample 2, which was lower than the ratio of the wetted areas. The co-wound copper tape was less effective for the short normal zone, and it worked effectively as a bypass-current path after wide propagation of a normal zone.journal articl
Surface modifications on unirradiated and ion-irradiated tungsten after exposed to deuterium plasma at LHD divertor-leg position
Full text linkORCID 0000-0001-5089-3642In order to clarify the divertor plasma-induced tungsten (W) surface modifications as well as the irradiation defects effect, two kinds of ITER grade W were exposed to the large helical device deuterium (D) plasma at the divertor-leg position. One was the iron (Fe) ion irradiated W to produce irradiation defects, and the other was the unirradiated W. The distributions of divertor plasma-induced surface modifications on these two kinds of W were clarified by scanning electron microscope and transmission electron microscopy. A co-deposition layer which was mainly made up of carbon (C) and Fe was formed at the private flux region (>21 mm). No significant surface change was observed at the strike point region (14–16 mm). The oxygen-enriched amorphous W structures (OEAWs) caused by plasma surface interactions were observed at the 2–14 mm (scrape-off layer region) and 20 mm. At the 2–14 mm and 20 mm, the OEAWs density on the pre-irradiated W sample is lower than that on the unirradiated W sample. On the other hand, the OEAWs size on the pre-irradiated W sample is larger than that on the unirradiated W sample at the 8–12 mm and 20 mm. While, the OEAWs size on the pre-irradiated W sample is smaller than that on the unirradiated W sample at the 2–6 mm. This study implies the possibility of forming OEAWs on the surface of W divertor. And the irradiation defects affect the density and size of OEAWs.journal articl
Nonlinear Gaussian process tomography with imposed non-negativity constraints on physical quantities for plasma diagnostics
Full text linkORCID 0000-0002-6479-049XWe propose a novel tomographic method, nonlinear Gaussian process tomography (nonlinear GPT), that uses the Laplace approximation to impose constraints on non-negative physical quantities, such as the emissivity in plasma optical diagnostics. While positive-valued posteriors have previously been introduced through sampling-based approaches in the original GPT method, our alternative approach implements a logarithmic Gaussian process (log-GP) for faster computation and more natural enforcement of non-negativity. The effectiveness of the proposed log-GP tomography is demonstrated through a case study using the Ring Trap 1 device, where log-GPT outperforms existing methods, standard GPT, and the minimum Fisher information methods in terms of reconstruction accuracy. The results highlight the effectiveness of nonlinear GPT for imposing physical constraints in applications to an inverse problem.journal articl
Numerical analysis of miter bend with spiral phase mirror
Full text linkTo excite optical vortices using high-power millimeter waves, we use a miter bend with a spiral phase mirror. Through numerical simulations, we demonstrate that vortex beams can be successfully excited by employing a spiral phase mirror that appropriately accounts for the phase difference between the input and output modes. The simulations also reveal the generation of higher-order modes caused by diffraction inherent to the miter bend structure and unintended reflections arising from the singularity at the optical axis of the spiral phase mirror. Additionally, we propose a method to estimate the topological charge, which corresponds to the vorticity, from real-valued data. The simulation results confirm that vortex beams are successfully excited as the dominant mode.journal articl
Numerical Analysis of Electrode Geometry Effects on Proton Generation and Acceleration in Pyroelectric Crystal Electrostatic Fields
Full text linkTo support the development of a compact experimental system for the proton-boron (p-11B) fusion reaction, we performed numerical simulations of proton generation and acceleration in the electrostatic field produced by a pyroelectric crystal. Electrostatic potential distributions were calculated by solving the Laplace equation for three electrode configurations: a disk electrode alone, a disk with a needle electrode, and a disk with a cylinder electrode. The proton impact rate on a boron target placed opposite the electrodes was evaluated for each configuration. It was found that the disk and needle electrodes achieved a maximum impact rate of approximately 37% at a crystal heating temperature of 5 K, while the cylinder electrode achieved a comparable impact rate at a lower temperature of 1 K. These results indicate that the cylinder electrode configuration can achieve efficient proton acceleration at reduced heating temperatures.journal articl
Relating Hamiltonian systems with multiple invariants to generalized Hamiltonian mechanics via multisymplectic geometry
Full text linkORCID 0000-0002-2973-0635Classical Hamiltonian mechanics, characterized by a single conserved Hamiltonian (energy) and symplectic geometry, ‘hides’ other invariants into symmetries of the Hamiltonian or into the kernel of the Poisson tensor. Nambu mechanics aims to generalize classical Hamiltonian mechanics to ideal dynamical systems bearing two Hamiltonians, but its connection to a suitable geometric framework has remained elusive. This work establishes a novel correspondence between generalized Hamiltonian mechanics, defined for systems with a phase space conservation law (invariance of a closed form) and a matter conservation law (invariance of multiple Hamiltonians), and classical Hamiltonian mechanics via multisymplectic geometry. The key lies in the invertibility of differential forms of degree higher than 2. We demonstrate that the cornerstone theorems of classical Hamiltonian mechanics (Lie-Darboux and Liouville) require reinterpretation within this new framework, reflecting the unique properties of invertibility in multisymplectic geometry. Furthermore, we present two key theorems that solidify the connection: i) any classical Hamiltonian system with two or more invariants is also a generalized Hamiltonian system and ii) given a generalized Hamiltonian system with two or more invariants, there exists a corresponding classical Hamiltonian system on the level set of all but one invariant, with the remaining invariant playing the role of the Hamiltonian function.journal articl
Molecular dynamics study on clustered DNA damage: AP sites on the same strand
Full text linkAlthough radiation-induced clustered DNA damage can have critical biological consequences, the underlying molecular mechanisms remain unclear. To explore the effect of clustered DNA damage on DNA structure and dynamics, we performed molecular dynamics simulations on damaged DNA with two AP sites on the same strand, that is, a tandem AP cluster. The results showed that the cluster insertion of the two AP sites had a significant impact on the DNA's local and global structures. Local structural deformations as well as the extrahelical form, AP-base pairs, and irregular base pairs were frequently observed. Unlike a single AP site, the tandem AP cluster revealed that these local structural features occurred simultaneously within a small separation. Moreover, we found that the presence of tandem AP sites induced global bending of DNA. This suggests that the present case with tandem AP sites may have a non-negligible impact on the biological function of damage repair.journal articl
Hybrid simulation study on ion heating by low-frequency wave excited in a field-reversed configuration
Full text linkIn this paper, we use hybrid simulation to elucidate the plasma heating mechanism due to waves excited in Field-Reversed Configuration (FRC) plasma. The plasma parameters are a separatrix radius of 0.16 m and a separatrix length of 1.16 m (x-point position is z = ± 0.57 m). The wave excitation antenna consists of two loop antennas with a radius of 0.3 m and is placed at a position of z = ± 0.5 m. The current waveform of the antenna is a sine wave with a maximum current value of 30 kA and a frequency of 160 kHz. The simulation results showed that the excited waves caused compression/expansion of the plasma, and at the same time, the temperature of the plasma increased or decreased at the compressed/expanded position. When waves are applied, a 23% increase in the volume-averaged ion temperature in the separatrix is observed compared to the case without waves applied. On the other hand, no increase in electron temperature is observed. For the electron fluid, the adiabatic condition is well established, and temperature changes are observed as the plasma compressed and expanded. On the other hand, for ions, kinetic energy perpendicular to the magnetic field lines increases during compression, and part of this energy is transferred to the energy of the parallel component by collisionless pitch angle scattering, resulting in heating due to the so-called magnetic pumping.journal articl
Heat flux mitigation characteristics in the radiative divertors with multi-peaks of heat flux in the large helical device
Full text linkORCID 0000-0001-6090-5010In this study, the effects of impurity seeding on the one- and two-dimensional heat flux profiles of the large helical device divertor with a multi-peaks of heat flux were investigated, focusing on the heat-flux-reduction characteristics at the two peaks. The divertor heat flux profile was reconstructed using finite element analysis based on the temperature distribution measured via infrared thermography. The divertor heat flux profiles were calculated using the EMC3-EIRENE code considering the impurity seeding. In general, the calculated results were consistent with the experimental observations, including the heat flux value and reduction via impurity seeding. The reduction in the divertor heat flux due to impurity seeding was almost similar at different heat flux peaks and exhibited little dependence on the heating power variance. The difference between the low- and high-heating-power cases is discussed by tracing the magnetic field lines from the divertor plate and considering the penetration depth of the impurity particles into the scrape-off layer plasma.journal articl