arXiv subjects

Motoshi Goto

Publications and source records attributed to Motoshi Goto.

6 recordsLinked to original sources

arXiv2026

Polarization measurements of the $2s$--$2p_{3/2}$ VUV transition in N$^{4+}$ excited by electron collisions

We present the linear polarization of the $2s$--$2p_{3/2}$ transition in Li-like N$^{4+}$ excited by electron collisions, measured at electron energies between 85 and 1000~eV. The measured polarizations are compared with theoretical values calculated using the Flexible Atomic Code (FAC). Although the overall trend that the degree of polarization decreases with increasing electron energy is reproduced, the theoretical values are systematically larger in magnitude than the experimental values. After examining possible experimental sources of depolarization, we suggest that the discrepancy likely originates from an overestimation in the theoretical model. Further theoretical and experimental investigations along the isoelectronic sequence are needed to resolve this discrepancy.

physics.atom-ph↗

arXiv2026

A vacuum-ultraviolet spectropolarimeter for an electron beam ion trap

We have developed a vacuum-ultraviolet spectropolarimeter for an electron beam ion trap (EBIT) to measure the linear polarization of emission lines from multiply charged ions around the Lyman-$\alpha$ wavelength. The main components for polarimetry are a rotatable MgF$_2$ waveplate and a SiO$_2$/MgF$_2$ multilayer-coated fused silica plate that functions as a reflective polarizer. A grazing-incidence grating is mounted between them to provide wavelength dispersion. The polarization is determined from the intensity modulation of the spectral line as the waveplate is rotated. The performance of the spectropolarimeter was demonstrated by measuring the polarization of the $2s$--$2p_{3/2}$ transition in Li-like N$^{4+}$ (124~nm) excited by a 1000~eV electron beam in an EBIT. Clear modulation of the line intensity was observed as a function of the waveplate rotation angle. From the measured modulation amplitude, the degree of linear polarization was determined to be $P=-(0.178^{+0.014}_{-0.005})$, with the negative sign indicating that the emission is polarized predominantly perpendicular to the electron beam. This result demonstrates the capability of the present spectropolarimeter to determine polarizations with an absolute uncertainty $\Delta P$ on the order of $0.01$. This instrument provides a useful tool for benchmarking magnetic-sublevel-resolved collision theories through polarization measurements of multiply charged ions excited by a unidirectional electron beam.

physics.atom-ph↗

arXiv2025

Conceptual design of Thomson scattering system with high wavelength resolution in magnetically confined plasmas for electron phase-space measurements

We discuss the conceptual design of a spatially-resolved spectroscopy system of Thomson scattering with high wavelength resolution capable of measuring the shape of electron velocity distribution functions in magnetically confined plasmas. We design a spatially-resolved spectrometer with 2560 wavelength channels. The estimated number of scattered photons in a single spectrometer channel is much larger than unity under the experimental setup and plasma parameters of the Compact Helical Device (CHD), indicating sufficient photon statistics for single-shot measurements. Simulations of the scattered spectra show that the signal-to-noise ratio exceeds 5 even under the most unfavorable conditions expected in CHD at full spectral resolution, and further improves with post-processing pixel binning. Bayesian inference applied to the simulated spectra demonstrates that the inferred plasma parameters agree with the input values within the estimated uncertainties. Comparisons between spectra generated from non-Maxwellian electron velocity distribution functions and their Maxwellian fits indicate that deviations from Maxwellian distributions can be identified using the proposed system.

physics.plasm-ph↗

arXiv2025

Enhancement of Mid-/High-Z Impurity Transport by Continuous Li-granule Dropping in a Stellarator Plasma

An enhancement of core impurity transport is observed in high-density plasmas of the stellarator LHD heated by neutral beam injection when continuous lithium (Li) granule dropping is performed. In the experiments reported here, in which the TESPEL is employed to inject trace amounts of titanium (Ti) and molybdenum (Mo) into the plasma core, confinement times for these impurities are seen to reduce significantly when Li dropping is applied, this reduction being more notable for Mo. In order to gain some initial insight into these observations, simulations are performed using the drift-kinetic transport code SFINCS for the Mo case. These simulations indicate that, while neoclassical transport prevails for the main plasma components (electrons, majority ions and low Z impurities), the classical contribution seems to be dominant for transporting Mo impurities. In summary, this work reports the first experimental observation of the degradation of mid-Z and high-Z impurity confinement induced by the continuous dropping of Li granules into a high-density stellarator plasma. In the case of the Mo impurity, simulations suggest that classical transport is the key mechanism underlying the enhanced impurity transport.

physics.plasm-ph↗

arXiv2024

A scaling law of the neutral opacity and Balmer-$\alpha$ wing shape in high-temperature plasmas

Hydrogen atoms penetrating deep inside high-temperature magnetically confined plasmas by repetitive charge-exchange collisions result in a particle source, which affects the plasma performance significantly. In this \textit{Letter}, we present an approximate solution of the fluid equations for neutral transport and the analytical representation of the neutral opacity, in a simplified plasma geometry. This analysis predicts a power-law decay in the Balmer-$\alpha$ line wings which reflects the velocity distribution of the neutral atoms, with the power-law index analytically represented as well. These scaling laws are validated by the comparison with a simple Monte-Carlo simulation and spectroscopic observations of Large Helical Device plasmas. Since the Balmer-$\alpha$ line wings are experimentally accessible, our formulation opens the door to directly observe the neutral opacity and thus the particle source distribution in the plasma.

physics.plasm-ph↗

arXiv2024

Experimental Validation of Collision-Radiation Dataset for Molecular Hydrogen in Plasmas

Quantitative spectroscopy of molecular hydrogen has generated substantial demand, leading to the accumulation of diverse elementary-process data encompassing radiative transitions, electron-impact transitions, predissociations, and quenching. However, their rates currently available are still sparse and there are inconsistencies among those proposed by different authors. In this study, we demonstrate an experimental validation of such molecular dataset by composing a collisional-radiative model (CRM) for molecular hydrogen and comparing experimentally-obtained vibronic populations across multiple levels. From the population kinetics of molecular hydrogen, the importance of each elementary process in various parameter space is studied. In low-density plasmas (electron density $n_\mathrm{e} \lesssim 10^{17}\;\mathrm{m^{-3}}$) the excitation rates from the ground states and radiative decay rates, both of which have been reported previously, determines the excited state population. The inconsistency in the excitation rates affects the population distribution the most significantly in this parameter space. On the other hand, in higher density plasmas ($n_\mathrm{e} \gtrsim 10^{18}\;\mathrm{m^{-3}}$), the excitation rates \textit{from} excited states become important, which have never been reported in the literature, and may need to be approximated in some way. In order to validate these molecular datasets and approximated rates, we carried out experimental observations for two different hydrogen plasmas; a low-density radio-frequency (RF) heated plasma ($n_\mathrm{e}\approx 10^{16}\;\mathrm{m^{-3}}$) and the Large Helical Device (LHD) divertor plasma ($n_\mathrm{e}\gtrsim 10^{18}\;\mathrm{m^{-3}}$)... [continued]

physics.plasm-ph↗