Yoshihiro Okuno

Bio: Yoshihiro Okuno is an academic researcher from Tokyo Institute of Technology. The author has contributed to research in topics: Magnetohydrodynamic generator & Plasma. The author has an hindex of 16, co-authored 261 publications receiving 1212 citations.

Feasibility study on frozen inert gas plasma MHD generator

Abstract: The feasibility of a magnetohydrodynamics (MHD) generator with frozen inert gas plasma (FIP) is examined numerically. The FIP is produced by pre-ionizing helium of inert gas without alkali metal seed at the generator inlet. Because the three-body recombination coefficient of helium ions is low at the electron temperature more than 5000 K, the ionization degree of inert gas is kept almost constant in the entire region of the generator channel and the relation between the electron temperature and the electron number density deviates from the Saha equilibrium. In a small-scale disk-shaped generator, the enthalpy extraction ratio of 22.7% and the isentropic efficiency of 54.8% are obtainable for the load resistance of 3.0 /spl Omega/ and magnetic flux density of 4 T under the pre-ionization degree of 4.79/spl times/10/sup -5/. The auxiliary power to sustain the pre-ionized plasma corresponds to about 2% of the thermal input. For high-magnetic flux density, the generator performance can be improved as well as a conventional-seeded plasma MHD generator.

Observation of radio‐frequency discharges at various driving frequencies

Abstract: Radio‐frequency discharges at various driving frequencies f in the range of 1.9–28 MHz are observed, especially from the viewpoint of the sheath structure. The results reveal that for f≥10 MHz, the sheath thickness Zs measured by an emissive probe varies directly as f−1, while for f≤10 MHz, Zs∝f−1/2. The electron density estimated from the ion saturation current of a needle probe is almost proportional to f2 in the former case. High‐energy electrons accelerated by the expanding rf sheath can contribute to sustaining rf plasmas in the high‐frequency region, whereas in the low‐frequency region, secondary electrons emitted from an electrode can contribute to it in a similar manner to dc and ac discharges.

Numerical studies on the nonequilibrium inductively coupled plasma with metal vapor ionization

Abstract: Nonequilibrium inductively coupled plasma (ICP) where cesium metal atoms contained in argon gas as a seed material are dominantly ionized is investigated with two-dimensional (2-D) numerical calculations which are based on a fully time-dependent (FTD) model and sinusoidal approximation (SA) model. Calculation results with the FTD model indicate that the amplitude of electron temperature oscillation over one radio frequency cycle is below about 120 K for the mean value of about 5600 K, and that of the electron number density is negligible due to its long relaxation time. Results with the SA model coincide with that with the FTD model, and it is valid to predict the plasma properties with the SA model. Under the suitable operating conditions, the region where electron number density is kept constant is formed and extended in the ICP, since the electron temperature ranging from 4000 to 6000 K is realized, and cesium atoms are fully ionized while the ionization of argon atoms is not significant. Since no current is induced at the center axis of ICP, the density profile around the axis is almost determined by the diffusion of electrons from the region of full seed ionization.

Powering MEMS portable devices—a review of non-regenerative and regenerative power supply systems with special emphasis on piezoelectric energy harvesting systems

Abstract: Power consumption is forecast by the International Technology Roadmap of Semiconductors (ITRS) to pose long-term technical challenges for the semiconductor industry. The purpose of this paper is threefold: (1) to provide an overview of strategies for powering MEMS via non-regenerative and regenerative power supplies; (2) to review the fundamentals of piezoelectric energy harvesting, along with recent advancements, and (3) to discuss future trends and applications for piezoelectric energy harvesting technology. The paper concludes with a discussion of research needs that are critical for the enhancement of piezoelectric energy harvesting devices.

A Flux Splitting Scheme with High-Resolution and Robustness for Discontinuities(Proceedings of the 12th NAL Symposium on Aircraft Computational Aerodynamics)

Abstract: A flux splitting scheme is proposed for the general nonequilibrium flow equations with an aim at removing numerical dissipation of Van-Leer-type flux-vector splittings on a contact discontinuity. The scheme obtained is also recognized as an improved Advection Upwind Splitting Method (AUSM) where a slight numerical overshoot immediately behind the shock is eliminated. The proposed scheme has favorable properties: high-resolution for contact discontinuities; conservation of enthalpy for steady flows; numerical efficiency; applicability to chemically reacting flows. In fact, for a single contact discontinuity, even if it is moving, this scheme gives the numerical flux of the exact solution of the Riemann problem. Various numerical experiments including that of a thermo-chemical nonequilibrium flow were performed, which indicate no oscillation and robustness of the scheme for shock/expansion waves. A cure for carbuncle phenomenon is discussed as well.

Electric probes for plasmas: The link between theory and instrument

Abstract: Electric probe methods for diagnostics of plasmas are reviewed with emphasis on the link between the appropriate probe theories and the instrumental design. The starting point is an elementary discussion of the working principles and a discussion of the physical quantities that can be measured by the probe method. This is followed by a systematic classification of the various regimes of probe operation and a summary of theories and methods for measurements of charged particle distributions. Application of a single probe and probe clusters for measurements of fluid observables is discussed. Probe clusters permit both instantaneous and time-averaged measurements without sweeping the probe voltage. Two classes of applications are presented as illustrations of the methods reviewed. These are measurements of cross sections and collision frequencies (plasma electron spectroscopy), and measurements of fluctuations and anomalous transport in magnetized plasma.