Keiji Sawada

Effective ionization and dissociation rate coefficients of molecular hydrogen in plasma

A simplified collisional‐radiative model has been constructed for the system of the ground state, electronically excited stable states, and the ionic state of molecular hydrogen in plasma. Effective rate coefficients have been calculated for production of electrons, molecular ions, protons, and hydrogen atoms from molecular hydrogen. The ratio of the effective ionization rate of molecular hydrogen to the Balmer α photon emission rate and the effective rate coefficients for radiation and energy losses are also presented.

Design and optimization of tapered structure of near‐field fibre probe based on finite‐difference time‐domain simulation

The finite‐difference time‐domain method was employed to simulate light propagation in tapered near‐field fibre probes with small metal aperture. By conducting large‐volume simulations, including tapered metal‐cladding waveguide and connected optical fibre waveguide, we illustrated the coupling between these guiding modes as well as the electric field distribution in the vicinity of the aperture. The high collection efficiency of a double‐tapered probe was reproduced and was ascribed to the shortening of the cut‐off region and the efficient coupling to the guiding mode of the optical fibre. The dependence of the efficiency on the tapered structure parameters was also examined.

Relations between the ionization or recombination flux and the emission radiation for hydrogen and helium in plasma

On the basis of the collisional-radiative models for neutral hydrogen, and neutral and ionized helium, the relationship between the ionization flux or the recombination flux and the photon emission rate of a representative visible line of each species is investigated. It is found that both fluxes are proportional to the photon emission rate and that the proportionality factor depends rather weakly on the plasma parameters in the ranges of practical interest. This implies that the observed emission line intensity can be a good measure of the ionization flux or the recombination flux. The relation between the total radiation power rate and the ionization or recombination flux is also considered. For a hydrogen plasma in ionization balance the Balmer-α line intensity takes the maximum value near the optimum temperature of 1.3 eV, while for plasmas out of ionization balance it takes the minimum near that temperature. This latter characteristic corresponds to the recently observed “inverse edge-localized mode”...

Determination of the hydrogen and helium ion densities in the initial and final stages of a plasma in the Large Helical Device by optical spectroscopy

Emission line intensities in the UV and visible wavelength range have been measured in the initial and final stages of a discharge in the Large Helical Device (LHD) [O. Motojima et al., Phys. Plasmas 8, 1843 (1999)]. From these results combined with Te and ne, which are determined spectroscopically, the ionization and recombination fluxes of hydrogen and helium are estimated in the region of high line emissivities. The temporal behavior of line-averaged ne which is evaluated from these fluxes is found to be consistent with the results of the interferometer measurement. This implies that in such transient stages of the plasma the temporal variation of ne is dominated by the atomic ionization-recombination processes rather than the particle transport or diffusion. On the basis of that characteristic in LHD, the line-averaged ion densities for hydrogen and helium just before the termination of the plasma heating are determined. Thus, a method of the density determination of the fueling gas and of the ash is ...

Electrical and Optical Properties of Poly(3,4-dialkylthiophene)

Poly(3,4-dialkylthiophene) films prepared by both electrochemical and chemical methods utilizing FeCl 3 as catalyst exhibit much larger band gap compared with non-substituted and 3-substituted polythiophene films. Poly(3,4-dialkylthiophene)s with long alkyl chains are soluble in several solvent. However, thermochromism has not been found contrary to poly(3-alkylthiophene), which are explained in terms of large torsion angle between neighbouring thiophene rings due to the steric hindrance by bulky alkyl group. Poly(3,4-dimethylthiophene) with large band gap also demonstrates drastic spectral, electron spin resonance (ESR) and conductivity changes upon doping due to an insulator-metal transition.

FDTD simulation of tapered structure of near-field fiber probe

The finite-difference time-domain method was employed to simulate light propagation in tapered near-field fiber probes with several small metal apertures. By conducting large-volume simulations, including tapered metal-cladding waveguide and connected optical fiber waveguide, we illustrated the coupling between these guiding modes as well as the electric field distribution in the vicinity of the aperture. The dependence of the spatial resolution and the collection efficiency on the aperture diameter were reproduced.

Effect of Initial Vibrational Excitation of Molecular Hydrogen on Molecular Assisted Recombination in Divertor Plasmas

We have calculated the effective rate coefficient for molecular assisted recombination (MAR) by using our collisional-radiative model for atomic and molecular hydrogen with recent atomic and molecular data incorporated. The result for the vibrational ground state molecule agrees very well with the original rate coefficient proposed by Krashennikov et al. The rate coefficient depends strongly on the initial vibrational excitation of the molecule released from the wall.

A study on temperature effects on hydrogen recycling and molybdenum impurity emission from a movable limiter in TRIAM-1M Tokamak

In order to investigate the surface temperature effects on plasma fuel recycling and impurity release from the plasma facing components, plasma discharges have been performed under selected plasma–wall interaction (PWI) conditions in the high-field superconducting tokamak, TRIAM-1M. By moving a water-cooled molybdenum movable limiter (ML) beyond the last closed flux surface, as defined by poloidal limiters, the surface temperature profile on it is varied. Hot spots have been observed on the ML surface in such conditions. The release behaviour of fuel as well as impurity particles from the ML surface has been studied as a function of hot spot temperature (Thot) by means of wide range spectroscopy (200–1600 nm). A critical Thot is found to be ~2100 K above which the emission of both hydrogen and impurity particles enhances significantly. This is indicative of some thermally activated process playing an important role in PWIs between the limiter and the edge plasma. With the rise in hot spot temperature localized PWI at the ML is found to dominate the global recycling even when external fuelling is stopped.

Spatial Resolution of Near-Field Scanning Optical Microscopy with Sub-Wavelength Aperture

The finite-difference time-domain (FDTD) method is employed to solve the three dimensional Maxwell equation for the situation of near-field microscopy using a sub-wavelength aperture. Experimental result on unexpected high spatial resolution is reproduced by our computer simulation.

Evaluation of hydrogen atom density in the plasma core region based on the Balmer- line profile

The Balmer-α line profile is measured with high wavelength resolution for a discharge in the Large Helical Device. The line profile is regarded as a superposition of continuously varying Doppler broadened components and is expressed as the Laplace transform. Numerical Laplace inversion of the measured line profile gives the distribution function of line emissivity in terms of atom temperature. The temperature dependence of the line emissivity is interpreted as spatial dependence so that the ionization rate and atom density of neutral hydrogen are determined. The temperature range of the detected atoms extends beyond 2 keV which corresponds to a penetration depth of about 1 m in the plasma, or the location at ρ ~ 0.3, where ρ is the normalized minor radius. The atom density of approximately 1013 m−3 is derived in the plasma core region which is more than four orders smaller than that at the plasma boundary. Calculation of neutral transport with a Monte-Carlo simulation code gives satisfactory consistency with the experimental results.