Yohei Iida

Calculation of spatial distribution of optical escape factor and its application to He I collisional-radiative model

An integral analytical formula for a spatial distribution of the optical escape factor (OEF) in an infinite cylindrical plasma is derived as a function of an arbitrary upper state spatial density profile, the temperature ratio of the upper state to the lower state, and the optical depth of the corresponding transition. Test calculations are carried out for three different upper state profiles, i.e., uniform (rectangular), parabolic, and Gaussian upper state profiles. The OEF takes on negative values at the periphery of the parabolic and Gaussian upper state profiles. These characteristics cannot be expressed by the conventional OEF formulas derived for the center of the plasma, even though the optical depth is increased. In addition to the analytical derivation of the formula, two practical formulas are proposed: an empirical formula of the spatial distribution of the OEF for the Gaussian upper state density profile and a linear formula of the OEF distribution for upper state profiles that are expressed as linear combinations. These formulas enable us to calculate the spatial distribution of the OEF for the multiple-Gaussian upper state profile without the need for time-consuming integral calculations.

Application of optical emission spectroscopy for He I considering the spatial structure of radiation trapping in MAP-II divertor simulator

The He I optical emission spectroscopy that considers the spatial structure of radiation trapping was proposed by us and was applied to a MAP-II divertor simulator. The spatial distribution of the optical escape factor was calculated from the n  (1)P (n≥3) state profiles measured by visible spectroscopy. The profile of 2  (1)P, which is immeasurable by visible spectroscopy, needs to be broader than that of the 3  (1)P state. The sensitivity of the 2  (1)P profile to the T(e) value estimated by He I spectroscopy is investigated.

Experimental Study of the Atomic and Molecular Processes Related to Plasma Detachment in Steady-State Divertor Simulator MAP-II

Atomic and molecular processes relevant to the volumetric recombination phenomena were investigated in a linear divertor plasma simulator MAP-II. Volumetric recombination is induced in He plasma by puffing of He or H2. In the He puffing case, the reduction of the ion flux is dominated by the electron-ion recombination. In the H2 puffing case, however, it is dominated by the molecule-assisted recombination (MAR), which is characterized by the disappearance of the Helium Rydberg spectra and by the existence of the hydrogen negative ions. Current achievement and the future prospect are described.

Spectroscopic measurement of the degree of ionization in a helium electron cyclotron resonance discharge in a simple cusp field

For an electron cyclotron resonance (ECR) discharge, a simple cusp field can improve electron confinement and enhance the degree of ionization (DOI) without sacrificing accessibility to the plasma. In this study, the spatial distribution of the DOI is experimentally revealed in a helium plasma produced with widely used 2.45 GHz and 800 W microwaves. The DOI is evaluated from the electron density and ground state atom density measured using HeI emission line intensities and by collisional-radiative model analysis. It is found that the DOI increases to more than 15% within a reasonably large volume surrounded by the ECR surface and locally reaches as high as 25%.

Measurements of helium 23S metastable atom density in low-pressure glow discharge plasmas by self-absorption spectroscopy of HeI 23S–23P transition

The helium 23S metastable atom densities are experimentally evaluated by self-absorption spectroscopy of the HeI 23S–23P transition spectra in two kinds of cylindrical glow discharge plasmas, which have different radii and are operated under different pressures of 300 and 20 Pa. The spectra are measured by using an interference spectroscopy system with a wavelength resolution of about 60 pm, and the relative intensities of the fine structure transitions are analyzed. It is found that the method is in principle applicable to plasmas with the pressure up to about the atmospheric pressure and electron density on the order of up to 1022 m−3. For a plasma with an absorption length of 10 mm and a spatially uniform temperature of 300 K, the method is sensitive to the metastable atom density roughly from 1016 to 1019 m−3.

Helium atom line-intensity ratios as an integrated diagnostic tool for low-pressure and low-density plasmas

The intensity ratios between specific pairs of helium atom (HeI) emission lines are functions of the electron temperature (Te) and density (ne), and these functions have been used for the analysis of Te and ne in various types of discharge plasma. We applied this method to a low-density (ne < 1018 m−3) plasma, where the procedure of the analysis is markedly different from that of higher-density plasmas. The 21S and 23S metastable atom densities are affected by transport, making it practically necessary to set Te, ne, the metastable atom densities, and the optical escape factors, which represent the effect of photoexcitation, as unknown variables and determine them simultaneously. Conversely, the transport of metastable atoms can be evaluated from the analysis.

Measurement of a Near-Infrared Atomic Helium Line for the Evaluation of Radiation Trapping in a MAP-II Divertor Simulator

A near-infrared (NIR) spectrometer system intended to measure the He I 2S-2P radiative transition of 2058.130 nm was developed. A Czerny–Turner spectrometer with a focal length of 35 cm and an F-number of 3.8 is modified to give it the ability to measure the NIR region by installing a 256-channel InGaAs linear image sensor. The population in the 2P state measured in the MAP-II divertor simulator exhibits a significant broadening in the spatial distribution, compared with the other states, which indicates the effect of radiation trapping for the resonant state.