Effect of vacancy, self-interstitial atoms and transmutation rhenium on lattice thermal conductivity in tungsten

Tungsten (W) is a promising candidate for plasma-facing components (PFCs) because of its excellent properties, such as a high melting point and high thermal conductivity (TC). However, neutron irradiation introduces lattice defects and transmutation, leading to the degradation of material properties. In this study, the reduction of lattice thermal conductivity (LTC) in W caused by point defects, such as vacancies, self-interstitial atoms, substitutional rhenium (Re) and interstitial Re were calculated using molecular dynamics at 300 K and 800 K using equilibrium molecular dynamics (EMD). Also, we calculate phonon density of state (DOS) and discuss how the point defects change the phonon DOS of crystalline W. It was found that interstitial Re decreased LTC most significantly, followed by random self-interstitial atoms, vacancies, and substitutional Re. Additionally, it was found that the calculation results of the phonon-phonon scattering rate by EMD deviate significantly from those by the ab initio method.journal articl

Self-field measurements of a REBCO Rutherford-type cable

ORCID　 0000-0002-1989-867XSelf-field measurements of a Rutherford-type cable immersed in liquid nitrogen have been performed. A 1-meter straight Rutherford-type cable composed of 60 REBCO tapes (12 sub-cables) was tested. The measurement results indicated that the self-field was unaffected by the cable energization ramp-rate. Based on the measurement results, a current center position within the cable cross-section was investigated during cable energization. The current center position remained unchanged during the charging. Consequently, the current distribution of the Rutherford-type cable was stable. In addition, the 1-meter straight Rutherford-type cable was compared to other cable samples composed of stacked REBCO tapes. Under the same sample shape and cooling condition, the Rutherford-type cable and the stacked assembled in rigid structure (STARS) cable have similar characteristics with respect to the current center position during cable excitation.journal articl

Spectral line identification for highly charged heavy ions in soft X-ray and EUV regions based on Z-dependence measurements

ORCID　0000-0001-6536-9034This study focuses on new identifications of discrete spectral lines of heavy ions with one or two outermost 4s/4p subshell electrons in soft X-ray and extreme ultraviolet regions. The study is basically based on the spectroscopic data systematically taken in the last decades in high-temperature plasmas produced in the Large Helical Device using multiple grazing incidence spectrometers. In addition, we have newly taken spectroscopic data using the Tokyo electron beam ion trap (EBIT) for some of the missing elements in the past EBIT experiments. Consequently, we could identify a number of discrete lines experimentally for the first time in this study based on the detailed atomic number (Z)-dependences of the wavelengths of major resonance lines. Some transitions of Ga- and Ge-like ions show irregular Z-dependences in which the transition wavelength suddenly shifts to another curve at around a specific Z due to the effect of strong configuration interaction.journal articl

Z-dependent three-level simultaneous-crossing of excited states in germanium-like highly charged atomic ions

Simultaneous-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

Azimuthal mode decomposition of millimeter-wave integer and non-integer optical vortices generated by a spiral mirror

ORCID　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

Time-dependent density functional theory simulation for analyzing the neutralization process of hydrogen ions injected onto tungsten surfaces

ORCID　0000-0001-9050-2802We conducted time-dependent density functional theory simulations to investigate the neutralization process of a hydrogen ion injected at 100 eV onto the (110) surface of tungsten. Additionally, we introduced a method for evaluating the detection probability of electrons within a localized region. This probability serves as an indicator for identifying hydrogen in its various states: positive ion, neutral atom, and negative ion. Our findings indicate that following the collision, the probabilities of detecting hydrogen as a positive ion, neutral atom, and negative ion are approximately 25%, 50%, and 25%, respectively.journal articl

Fiber-in-line infrared spectrometer fabricated by femtosecond laser structuring of microchannels in fluoride glass fiber

In this study, we developed a fiber-in-line infrared spectrometer using fluoride-glass fiber and evaluated its measurement performance. In the fabrication process, a sensor structure functioning as a sample cell for spectroscopic measurements was created in the fiber waveguide by femtosecond laser processing. Measurements of water–glucose samples confirmed the sensor’s ability to obtain transmission spectra, allowing the absorption characteristics of the samples to be determined. The decrease in transmittance at the absorption wavelength of water and glucose confirms the feasibility of an optical fiber in-line infrared absorption spectroscopy measurement system.journal articl

Steady compressible 3D Euler flows in toroidal volumes without continuous Euclidean isometries

We demonstrate the existence of smooth three-dimensional vector fields where the cross product between the vector field and its curl is balanced by the gradient of a smooth function, with toroidal level sets that are not invariant under continuous Euclidean isometries. This finding indicates the existence of steady compressible Euler flows, either influenced by an external potential energy or maintained by a density source in the continuity equation, that are foliated by asymmetric nested toroidal surfaces. Our analysis suggests that the primary obstacle in resolving Grad’s conjecture regarding the existence of nontrivial magnetohydrodynamic equilibria arises from the incompressibility constraint imposed on the magnetic field.journal articl

Measurement of Ion Temperature during the Intermittent Negative Spike in Floating Potential

The ion temperature in an ECR plasma is determined from the ratio of wavelength-integrated spectra obtained using a spectrometer with sufficient time resolution. The ion temperature during the intermittent negative spikes in floating potential is extracted using the conditional averaging method. For the first time, we have successfully observed a decrease in ion temperature during intermittent events.journal articl

プロジェクトレビュー：国際エネルギー機関（IEA）PWI協定に拠る国際共同研究 国際共同研究の入口として、そしてその成果

ORCID　0000-0003-0161-0938国際エネルギー機関（IEA）の技術協力プログラム（TCP）の一つであるプラズマ・壁相互作用（PWI）TCP （以後PWI 協定）は，日本，欧州，米国，豪州，英国が参加する多国間協定であり，日本では核融合科学研究所 が執行機関となっている．PWI 協定に拠る国際共同研究は，プラズマ・壁相互作用に関係する，材料分析，周 辺・ダイバータプラズマ物理，プラズマ計測などの国際共同研究で多くの成果を挙げている．またこれらの国際 共同研究は，学生・若手研究者が国際共同研究へ参加する入口ともなってきた．そこで本稿では，PWI 協定の概 要，同協定に拠る国際共同研究の成果を紹介するとともに，特に，学生や若手研究者が，どのように国際共同研 究を始めて，どのように外国研究機関で研究を行ったか，彼の地に滞在中にどのような経験をしたのか，も紹介 する．journal articl