Katsunori Muraoka

Laser Thomson scattering for low-temperature plasmas

In recent years, low-temperature discharge plasmas with electron temperatures around a few eV have been actively studied and used for fundamental discharge physics research and industrial process applications. Here the electron density and temperature (and sometimes also the electron energy distribution function) are the most fundamental parameters that dictate the fates of these discharge plasmas and this information is of utmost importance. Laser Thomson scattering, which was developed for high-temperature plasmas in the early 1960s, has gained widespread use in the low-temperature plasma community since the late 1970s and has now matured as an established method of plasma diagnostics. Scattering diagnostic techniques for high-temperature plasmas have had to be modified to accommodate particular situations and constraints, such as laser perturbation of plasmas, low electron densities and the presence of material surfaces near to the plasmas. In this review, starting from a basic description of the technique, we outline some of the most salient results, which would not have been obtained without it, in discharges ranging from high-pressure to low-pressure gases, and near to material surfaces. Also, the signal-to-noise ratio and future potential of the method are discussed.

Laser-aided diagnostics of plasmas and gases

PART I: FUNDAMENTALS Laser-aided diagnostics of gases and plasmas Basic principles of different laser-aided measurement techniques Hardware for laser measurements PART II: APPLICATIONS AND MEASUREMENTS Plasma measurements Combustion measurements Measurements in gas flow systems Laser processing measurements Analytical chemistry Remote sensing

Performance tests of newly developed adsorption/plasma combined system for decomposition of volatile organic compounds under continuous flow condition

The adsorption/plasma decomposition with the combination of adsorption honeycomb-sheets and a plasma element is a new technology for small-sized apparatuses to decompose volatile organic compounds (VOCs) at concentrations lower than about 100 ppm. The feasibility of the prototype adsorption/plasma decomposition apparatus was evaluated with the simulated exhausts containing one VOC component and with real exhausts from a painting booth and an adhesion factory. The apparatus decomposed VOCs effectively at the painting booth exhaust but not always satisfactorily at the adhesion factory exhaust. The performance test results with real exhausts were discussed with respect to the concentration and discharge pattern of the exhausts and the basic properties of the system such as cooperation of adsorption and plasma reaction and the concentration dependence of the performance.

Comparisons of density profiles in JT-60U tokamak and LHD helical plasmas with low collisionality

In order to understand particle transport systematically in toroidal plasmas, electron density profiles were compared in JT-60U tokamak and LHD helical plasmas with low collisionality. The peakedness of density profiles increased with decreasing collisionality in ELMy H-mode plasmas of JT-60U when the collisionality at half the minor radius was in the collisionless regime. The collisionality dependence of density profiles in LHD plasmas was similar to that in JT-60U plasmas in the same collisionality regime when neoclassical transport was reduced by geometrical optimization. On the other hand, in LHD plasmas having relatively larger neoclassical transport than that in the above case, the peakedness of the density profiles decreased with decreasing collisionality. Density profiles in LHD plasmas tend to approach those in JT-60U, which are dominated by anomalous transport, as the contribution of neoclassical transport was reduced.

Temperature Dependence of Intensities of Laser-Induced Fluorescences of Ethylbenzene and Naphthalene Seeded in Gas Flow at Atmospheric Pressure: Implications for Quantitative Visualization of Gas Temperature

The present study has been carried out with the aim of developing a technique for measuring two-dimensional gas temperature profiles based on two-color fluorescence induced by a one-color laser. The laser sheet of the fourth harmonic (266 nm) from a Nd:YAG laser induced fluorescence in species doped in a nitrogen gas flow. The LIF spectra of seven fluorescent species, namely acetone, methylethylketone, acetaldehyde ethylbenzene, anisole, aniline, and naphthalene, were measured to select the best prospective pair of fluorescent species for this technique. Ethylbenzene and naphthalene show relatively high LIF intensities and their LIF spectra overlap less with each other than with other species. Also, ethylbenzene has a high temperature dependence while naphthalene has a low temperature dependence. Thus by selecting one portion of wavelengths in the range where ethylbenzene or naphthalene is dominant, the temperature of the gas can be determined using the ratio of LIF intensities of the mixture at the two wavelengths with good temperature sensitivity. In addition, a general principle is presented for finding out an optimum pair of wavelengths to obtain a good temperature sensitivity in those LIF spectra.

Laser Thomson scattering measurements of electron density and temperature profiles of a striated plasma in a plasma display panel (PDP)-like discharge

A laser Thomson scattering technique has been used to directly measure the electron density (ne) and electron temperature (Te) profiles of a striated plasma in a plasma display panel (PDP)-like discharge. A modulation in ne was observed only on the anode side of an electrode substrate; this corresponds to a feature similar to that of the emission images obtained using an intensified charge-coupled device (ICCD) camera. Also, a modulation in Te was observed and it was found to be out of phase from the modulation in ne.

Simultaneous decomposition of diesel particulate material and NOx using dielectric barrier discharge

Abstract We propose a new technique for simultaneous decomposition of diesel particulate material (DPM) and nitric oxides (NOx) exhausted from a diesel engine. DPM and NOx are collected in a reactor using an electrostatic precipitation and a honeycomb-shaped adsorbent, respectively. After the sufficient collections, a dielectric barrier discharge (DBD) is generated in the closed space to condense and decompose DPM and NOx. DPM and NOx were decomposed simultaneously and effectively by the DBD, which act as the oxidant and reductant, respectively. It is considered that this technique is the most suitable for the decomposition of DPM and NOx with small concentration and high gas flow rate.

A model for striation formation in ac PDP discharges

A model is presented to explain the mechanism of striation formation in ac plasma display panel discharges. A simple Monte-Carlo simulation code was used to follow electron motion in such discharges. The obtained features of striations, such as a striation pitch, striated accumulation of charged particles on a dielectric surface above an anode and phase oppositions of electron density and average energy are consistent with previous experiments and numerical simulations. Comparison of the experimental and theoretical space pitch of the striations allows an indirect determination of the electric field.

Temporal evolution of particle transport of Super Dense Core plasma in LHD

A highly peaked density profile was obtained in pellet-injected discharges in LHD. The peaking factor, which is the ratio of the central to volume-averaged densities, increased from around 0.8 in the gas puff fuelled phase, up to more than 2.0 after multiple pellet injection. The core density reached to several times 1020m-3. Temporal changes of density profiles were measured using a CO2 laser imaging interferometer. The change of the particle flux was estimated from temporal variations of measured density profiles. The diffusion coefficient (D) and the convection velocity (V) were then obtained from temporal traces of the relationship between the normalized density gradient and the normalized particle flux. Thus-obtained diffusion coefficients increased monotonically toward the plasma edge, while the convection velocity was inwardly directed. These observations are clearly different from those in the low collision regime, where D was spatially almost constant and V was outwardly directed. Three different regimes of particle transport were identified after the pellet injection. The first phase is characterized by a constant density peaking just after the pellet injection. In this phase, negative slopes between the normalized particle flux and the normalized density gradient were found, implying that D and V could not be determined. The second phase is at an additional density peaking, where the normalized particle flux increased with an increase in the normalized density gradient. In this phase, the core diffusion coefficient (ρ < 0.6) was reduced compared with that before the pellet injection, and an inwardly directed pinch was observed. The third phase is with the density broadening, where the normalized particle flux was reduced with a decrease in the normalized density gradient. Here, the core diffusion coefficients were further reduced, and the core pinch velocity (ρ < 0.8) became close to zero. The transition from the first (the constant peaking phase) to the second phase (the additional peaking phase) occurred almost simultaneously in the entire region of the plasma, while the transition from the second to third phase (the broadening phase) occurred from the outer region to the inner region of the plasma. The collisionality dependence of D and V at ρ = 0.4~0.7 was studied in the wide range of collisionality (νb* = 0.3~30) from density modulation experiments and the SDC analysis. A minimum value of D was likely to exist in the plateau regime (νb* = 1~5) together with almost zero convection velocity. This may be favourable for future high density reactor operations of the heliotron reactor.

Multiline FIR lasers by two wavelength cw CO 2 laser pumping

Simultaneously oscillated multi-line FIR lasers from a single FIR laser cavity pumped by two wavelength CO₂ lasers have been developed for plasma diagnostics and the other applications. We have obtained simultaneous oscillations as following : (i) 60.5- and 118.8 mum-CH₃OH lasers from pumping by 9R(18) CO₂ laser and 9P(36) CO₂ laser, (ii) 70.5- and 118.8-mum CH₃OH lasers from pumping by 9P(34) CO₂ laser and 9P(36) CO₂ laser, and (iii) 118.8-mum CH₃OH and 253.7-mum CD₃OH lasers from pumping by 9P(36) CO₂ laser and 10R(36) CO₂ laser. An output power of a 118.8-mum CH₃OH laser has been enhanced by two same CO₂ laser lines pumping.