NIFS-Repository (National Institute for Fusion Science)
Not a member yet
13159 research outputs found
Sort by
Gamma Ray Diagnostics in Large Helical Device
No full textORCID 0000-0003-4555-1837Gamma ray diagnostics have been developed and installed in Large Helical Device (LHD) to study plasma physics, as well as to measure neutron fluence and spectrum in the LHD torus hall. LaBr3:Ce scintillation and high purity Germanium (HPGe) detectors have been installed. LaBr₃:Ce scintillation detectors were used to measure gamma rays emitted from the plasma. Radiation shielding, composed of borated polyethylene and lead, was designed for the LaBr₃:Ce scintillation detector based on Monte Carlo radiation transport calculations. A gamma ray peak around 480 keV, believed to correspond to the 6Li(d, p’γ)7Li reaction, was observed. HPGe detectors were utilized for measuring the activity of metal foils. Using the activation analysis method, the thermal and epithermal neutron fluence distribution inside the LHD torus hall, the neutron spectrum, and the relationship between radioactivity and total neutron emission yield were determined.journal articl
Modeling particle uptake and release in porous carbon to support cryosorption pump design
No full textORCID 0000-0002-1642-1235Rational design of high-performance cryosorption pumps requires decoupling intrinsic material properties from complex geometric effects. However, predicting performance limits is challenging because the hierarchical pore structure of activated carbon complicates the relationship between microscopic adsorption and macroscopic gas release. To address this, we developed an isothermal test-particle Monte Carlo simulation framework that models particle dynamics within representative pore structures. We propose the release starting time, defined as the time to reach a specific release fraction (e.g., 10%), as a practical and robust metric. Our simulations demonstrate that this metric is largely insensitive to structural variations such as pore depth and connectivity. This robustness substantiates the utility of the release starting time as a structure-independent indicator that directly reflects the distribution of effective adsorption potential values (ΔE). Building on this insight, we present a method to es-timate the shape of this ΔE distribution by comparing activation energies at different release fractions. This approach provides a physically grounded framework to interpret observable gas release behaviors, linking a material's microstructure to its macroscopic release characteristics. By offering a new tool to support material selection and performance prediction, this study advances cryosorption pump development beyond empirical trial-and-error.journal articl
Nonlinear excitation of energetic particle driven geodesic acoustic mode by resonance overlap with Alfvén instability in ASDEX Upgrade
Full text linkThe Alfvén instability nonlinearly excited the energetic-particle-driven geodesic acoustic mode on the ASDEX-Upgrade tokamak, as demonstrated experimentally. The mechanism of the energetic-particle-driven geodesic acoustic mode excitation and the mode nonlinear evolution is not yet fully understood. In the present work, a first-principles simulation using the MEGA code investigated the mode properties in both the linear growth and nonlinear saturated phases. Here we show that the simulation successfully reproduced the excitation and coexistence of these two modes, and agreed with the experimental results well. Conclusive evidence showed that the resonance overlap is the excitation mechanism of the energetic-particle-driven geodesic acoustic mode. In the linear growth phase, energetic particles that satisfied different resonance conditions excited the Alfvén instability, which then caused energetic particle redistribution in phase space. These redistributed energetic particles caused resonance overlap, exciting the energetic-particle-driven geodesic acoustic mode in the nonlinear phase.journal articl
Response of negative ion beamlet width and axis deflection to RF field in beam extraction region
Full text linkORCID 0000-0002-5892-6047Beam-divergence characteristics of single negative ion beamlet have been experimentally investigated with a superimposition of a controlled perturbation of a radio frequency wave (RF) field in a filament-arc discharge negative ion source. Oscillations of a negative-ion beamlet width and axis responding to the RF perturbation were observed, which may be a cause of the larger beam divergence angle of the RF negative ion source for ITER. It is pointed out that the oscillation of the beamlet width depends on the perveance and on an RF frequency such that the oscillation is suppressed at perveance-matched conditions and at low RF frequencyjournal articl
Parity transition of radial structure of MHD instability in magnetically confined torus plasmas
Full text linkORCID 0000-0003-3754-897XThe study on the parity of the radial profile of radial displacements due to MHD instabilities in magnetically confined torus plasmas is a crucial subject in fusion plasma research. In some experimental devices, only the parity transition from even to odd parity profile (even-to-odd parity transition) has been observed. Now, an odd-to-even parity transition is first observed after the even-to-odd parity transition in the Large Helical Device (LHD). Moreover, the odd-to-even and even-to-odd parity transitions repeatedly occur when the resonant magnetic perturbation (RMP) applied by external coils is large. The even parity profile is driven by the resistive interchange MHD instability, while the odd parity profile is driven by the “Edge” MHD instability. The even-to-odd and odd-to-even parity transitions are inferred to occur when the “Edge” instability becomes unstable and when it stabilizes under the unstable interchange instability, respectively.journal articl
Optical design and efficiency measurement of an extreme ultraviolet high-resolution spectrometer for unresolved transition array research
Full text linkORCID 0000-0002-1021-0322Understanding the structures of the unresolved transition array (UTA) observed in extreme ultraviolet (EUV) spectra from many-electron atoms is crucial for various applications, including fusion science and nanolithography. To measure the fine structure of the UTA from tungsten and tin at around 5 and 13.5 nm at the Tokyo electron beam ion trap, we developed a high-resolution EUV spectrometer. The designed spectrometer achieves a resolving power of λ/dλ > 5000 at 5 and 13.5 nm. The fabricated large-area grating was experimentally examined at beamline BL5B of the UVSOR synchrotron facility to evaluate the diffraction efficiencies and their variation across the ruled area. The measured diffraction efficiencies are 0.65% ± 0.07% at 5 nm (second order) and 7.9% ± 0.2% at 13.5 nm (first order). The variation in the diffraction efficiency across the ruled area is 2.2%, 13.6%, and 10.0% in zeroth, first, and second order diffractions, respectively. The discrepancies in diffraction efficiencies between the experiments and the calculations were 5.2%, 29%, and 35% for the zeroth, first, and second diffraction orders, respectively.journal articl
In-vessel colorimetry of Wendelstein 7-X first wall components after OP2.1
Full text linkORCID 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
Development of Data Assimilation System for Temperature and Density Control in Helical Plasmas
Full text linkWe develop a real-time adaptive predictive control system based on data assimilation (DA) for the temperature and density of helical fusion plasmas. The DA-based control approach enables the harmonious integration of measurement, heating, fueling, and simulation and can provide a flexible platform for adaptive model predictive control. The core part of the control system, ASTI, is built upon the integrated simulation code TASK3D and a data assimilation framework DACS. DACS integrates adaptation of the predictive model (digital twin) to the actual system using real-time measurements and control estimation that is robust against model and observation uncertainties. We perform numerical experiments using ASTI to control the electron temperature profile and density of a virtual plasma generated by TASK3D. The results demonstrate that ASTI can effectively drive the virtual plasma state toward the target state while bridging the gap between the digital twin and the virtual plasma. Furthermore, the numerical experiments clarify the effects of hyperparameters in the DA-based control approach on control performance.journal articl
Progress in High-Temperature Superconducting WISE Conductors for Helical Fusion Reactors
Full text linkORCID 0000-0002-3541-6298Applying high-temperature superconducting (HTS) conductors to the magnets of Helical fusion reactors, which require a higher degree of three-dimensionality compared to the magnets of tokamak devices, can enhance plasma performance by increasing fusion power. By stacking Rare-Earth Barium Copper Oxide (REBCO) tapes, winding them into specific shapes, and then impregnating them with low-melting-point metals, the strain on the tapes can be minimized. Using this method, a 2-m-long U-shaped WISE (Wound and Impregnated Stacked Elastic tapes) conductor was subjected to current testing at temperatures ranging from 6 to 20 K and magnetic fields of 8 T. The conductor had the capability of maintaining 40 kA for 8 s at 6 K, 8 T with an average engineering current density of 31 A/mm2, without a temperature rise or quench. Repeated tests at 20 K and 8 T with a maximum current of 22 kA showed no temperature increase, confirming the conductor’s mechanical robustness. In the preliminary stages of the energization test, an increase in voltage reminiscent of a quench was observed, even though the current was lower than the critical current value. However, this voltage was suppressed as the maximum value decreased with each repetition of energization. Such behavior is considered similar to a training effect and indicates the movement of REBCO tape inside the conductor.journal articl
Electron Density Structure Measurements With Scattered Intense Laser Beam
No full textShort-pulse intense lasers have the potential to model extreme astrophysical environments in laboratories. Although there are diagnostics for energetic electrons and ions resulting from laser-plasma interactions, the diagnostics to measure velocity distribution functions at the interaction region of the laser and plasma are limited. We have been developing the diagnostics of the interaction between the intense laser and plasma using scattered intense laser. We performed experiments to measure electron density by observing the spatial distributions and the ratio of horizontal to vertical polarization components of the scattered laser beam using optical imaging. The observed ratio of polarization components is consistent with the drive laser beam, indicating the observed light originates from the drive laser. Imaging of the scattered light shows the structure of electron density, the zero moment of the electron velocity distribution function, interacting with the intense laser. We observed the change of structure due to the laser pre-pulse that destroys the target before the arrival of the main pulse.journal articl