arXiv subjects

Masahiro Hasuo

Publications and source records attributed to Masahiro Hasuo.

6 recordsLinked to original sources

arXiv2024

A scaling law of the neutral opacity and Balmer-$α$ 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-$α$ 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-$α$ 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↗

arXiv2023

Plasma-Parameter Dependence of Ro-Vibrational Temperatures for $\mathrm{H}_2$ in LHD Divertor

We analyzed a thousand visible spectra of Fulcher-$α$ band measured for divertor plasmas in Large Helical Device. With a coronal model and Baysian inference, the population distribution of hydrogen molecule in the electronical ground state were estimated. The non-thermal population distribution was recovered with a two-temperature model, which has two sets of rotational and vibrational temperatures, as well as their mixture coefficient. The lower rotational temperature significantly changes according to the plasma parameters. Its nearly linear dependence on the electron density was found, which is consistent with previous works. The lower vibrational temperature also shows a small density dependence, as reported by a previous work. On the other hand, the higher rotational and vibrational temperatures as well as the mixture coefficient only show slight changes over the broad range of plasma parameters. These population parameters show a significant correlation; with higher electron density, all the temperatures and the fraction of the higher-temperature component increase simultaneously. This suggests that the electron-impact plays an important role to determine the population distribution.

physics.plasm-ph↗

arXiv2021

Stable Energy Distribution of Weakly Dissipative Gasses under Collisional Energy Cascades

Collisional thermalization of a particle ensemble under the energy dissipation can be seen in variety of systems, such as heated granular gasses and particles in plasmas. Despite its universal existence, analytical descriptions of the steady-state distribution have been missing. Here, we show that the steady-state energy distribution of the wide class of collisional energy cascades can be well approximated by the generalized Mittag-Leffler distribution, which is one of stable distributions. This distribution has a power-law tail, as similar to Levy's stable distribution, the index of which is related to the energy dissipation rate. We demonstrate its universality by comparing Mont-Carlo simulations of dissipative gasses as well as the spectroscopic observation of the atom velocity distribution in a low-temperature plasma.

physics.plasm-ph↗

arXiv2020

Population kinetics of many-electron atoms in ionizing plasmas studied using a continuous collisional radiative model

Collisional--radiative (CR) models based on \textit{ab initio} atomic structure calculation have been utilized over 20 years to analyze many-electron atomic and ionic spectra. Although the population distribution of the excited states in plasmas and their emission spectra are computed using the CR models, systematic and analytical understanding of the population kinetics are still lacking. % In this work, we present a reduced model of the population dynamics in many-electron atomic ions, in which we approximate the dense energy structure of complex many-electron atoms by a continuum, a continuous CR model (CCRM). Using this simplification, we show an analytical population distribution of many-electron atoms in plasmas and its electron-density and temperature dependence. % In particular, the CCRM shows that the population distribution of highly excited states of many-electron atoms in plasmas resembles a Boltzmann distribution but with an effective excitation temperature. We also show the existence of three typical electron-density regions and two electron-temperature regions where the parameter dependence of the excitation temperature is different. Analytical representations of the effective excitation temperature and the boundaries of these phases are also presented.

physics.atom-ph↗

arXiv2019

Robust Regression for Automatic Fusion Plasma Analysis based on Generative Modeling

The first step to realize automatic experimental data analysis for fusion plasma experiments is fitting noisy data of temperature and density spatial profiles, which are obtained routinely. However, it has been difficult to construct algorithms that fit all the data without over- and under-fitting. In this paper, we show that this difficulty originates from the lack of knowledge of the probability distribution that the measurement data follow. We demonstrate the use of a machine learning technique to estimate the data distribution and to construct an optimal generative model. We show that the fitting algorithm based on the generative modeling outperforms classical heuristic methods in terms of the stability as well as the accuracy.

physics.plasm-ph↗

arXiv2016

Data-Driven Sensitivity Inference for Thomson Scattering Electron Density Measurement Systems

We developed a method to infer the calibration parameters of multichannel measurement systems, such as channel variations of sensitivity and noise amplitude, from experimental data. We regard such uncertainties of the calibration parameters as dependent noise. The statistical properties of the dependent noise and that of the latent functions were modeled and implemented in the Gaussian process kernel. Based on their statistical difference, both parameters were inferred from the data. We applied this method to the electron density measurement system by Thomson scattering for Large Helical Device plasma, which is equipped with 141 spatial channels. Based on the 210 sets of experimental data, we evaluated the correction factor of the sensitivity and noise amplitude for each channel. The correction factor varies by $\approx$ 10\%, and the random noise amplitude is $\approx$ 2\%, i.e., the measurement accuracy increases by a factor of 5 after this sensitivity correction. The certainty improvement in the spatial derivative inference was demonstrated.

stat.AP↗