We have measured the distribution of the electric field in the sheath formed between electronegative Ar⁄SF₆ plasmas and a biased electrode by laser-induced fluorescence-dip spectroscopy. It has been found for the first time that the electric field in the sheath of an electronegative plasma has a multistage structure, which may be due to the reflections of negative ions and electrons at different distances from the electrode surface.

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It is generally considered that use of hydrogen in energy system is attractive because it produces no matters to cause air pollution or water pollution. Following this recognition, research to develop a car with low temperature fuel cell or hydrogen engine is extensively performed. The future energy system where hydrogen is used as the key material is shown in the figure below. To realize this system we must develop 1) the reasonable way to produce electricity or heat by using hydrogen as fuel, 2) the infrastructure to supply hydrogen to whole society, 3) energy sources and methods to produce huge amount of cheap hydrogen with minimum impact on nature, 4) together with assurance of resources to construct the system and 5) establishment of understanding about the basic behavior of hydrogen in various materials.

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SiC⁄SiC composite is a promising structural material candidate for fusion power cores and has been considered internationally in several power plant studies. It offers safety advantages arising from it slow induced radioactivity and afterheat, and the possibility of high efficiency of energy conversion through high temperature operation. The latest SiC⁄SiC-based power core design studies are summarized, and the key SiC⁄SiC parameters affecting the performance of power core components are high lighted.

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The present status and future prospects of silicon carbide (SiC) continuous fiber-reinforced SiC-matrix ceramic composites (SiC⁄SiC composites) is reviewed from the viewpoint of material development for applications to blanket ⁄ first wall structures in fusion power devices and other advanced nuclear systems. Emphases are placed on the directions of neutron-tolerant composite development. A rationalization of thermal conductivity limits for SiC⁄SiC composites is provided. It is concluded that, although today’s state-of-the-art SiC⁄SiC can contribute to ‘attractive’ fusion power, the expected emergence of novel composites shall further justify taking the risk of employing untraditional materials.

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Irradiation effects of SiC fiber reinforced SiC matrix composites were summarized. Based on studies of irradiation behavior of SiC⁄SiC composites, SiC fibers which have stoichiometric composition and highly crystallized structure are expected to have high resistance to irradiation, and advanced SiC⁄SiC composites for fusion application were developed using these advanced SiC fibers. In this paper, the mechanism of irradiation resistance and recent irradiation studies on size stability, mechanical properties and thermal diffusivity were introduced.

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Evaluation techniques and mechanical properties of silicon carbide composites (SiC⁄SiC composites) reinforced with highly crystalline fibers are reviewed for fusion applications. The SiC⁄SiC composites used were fabricated by means of the CVI method. The evaluation includes in-plane tensile strength by in-plane tensile test, transthickness tensile strength by transthickness tensile test and diametral compression test and shear strength by compression test using double-notched specimen. All tests were successfully conducted using small specimens for neutron irradiation experiment. As application technique, the novel tungsten(W) coating technique on SiC is reviewed. The W powder melted by high power lamp in a few seconds and formed coating on SiC. No thick reaction layers of WC and W₅Si₃, which are formed by the other coating methods, were formed by this method.

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In this article, the present and future prospects of the research and development regarding continuous SiC fiber reinforced ceramic matrix composites (CMCs) for aerospace applications are reviewed. These activities in Japan are described in term of their major applications, i.e. turbo fan engine components for aircrafts, rocket propulsion components, thermal protection system for future re-entry vehicles, thruster for satellites. It is suggested that high performance, affordable processing cost, and excellent reliability will be important factors in the practical use of CMCs in the future.

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The collisional-radiative(CR) model is introduced as an improvement to the corona model. Its formulation indicates that the population of every excited level is divided into two independent components, i.e., the ionizing and recombining plasma components. For the pulsed discharge with helium gas in a Pyrex tube which exhibits intense line radiations twice, the first and second peaks are found to correspond to the ionizing and recombining plasmas, respectively, from the different population distributions over the n³D levels. The spectrum taken in the stationary phase of the main discharge in the Large Helical Device (LHD) with helium gas suggests an ionizing plasma, and for the other two spectra taken in the plasma terminating phase, the first and the second spectra indicate the recombining plasmas of ionized and neutral helium, respectively. In all these analyses, the electron temperature and density are the variable parameters and are determined as a result of fitting of calculation to the experimental data. The spectrum observed in the helium glow discharge cannot be reproduced by CR model calculations even if the quasi-steady-state approximation for meta stable states of neutral helium is removed. The opacity effect may be the origin of this difficulty.

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An electron cyclotron heating (ECH) system has been installed on the TRIAM-1M tokamak, which produces a 170 GHz microwave up to 260 kW at the window of the gyrotron for 5 sec. After the Matching Optical Unit (MOU) converts the microwave to a Gaussian beam, a microwave power of 200 kW is transmitted by a corrugated waveguide of 63.5 mm in diameter. Tests confirmed these specifications, and the fundamental ordinary electron cyclotron (EC) wave of 170 GHz up to 120 kW was injected into the vacuum vessel of the TRIAM-1M in order to ionize neutral hydrogen gas to make source plasma, which is expected to reduce flux consumption in the case of current start-up by means of Ohmic heating (OH) coils. Though the breakdown does not occur at the input RF power, P_RF = 40 kW with the neutral gas pressure, p = 1.3×10⁻³Pa, high-density plasma (more than 0.7×10¹⁹m⁻³) is produced at P_RF = 100 kW. In the range of P_RF = 40−100 kW, the electron density is proportional to P_RF.

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