Electron Bernstein wave heating experiments based on the X-mode to Bernstein mode mode-conversion scenario were performed on the Tokyo Spherical Tokamak -2 (TST-2). Up to 140 kW of microwave power at 8.2 GHz was injected perpendicularly from the low-field side. Evidence of electron heating was observed as increases of the stored energy (by about 15%) and soft X-ray (hν > 1 keV) emission.

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Vacuum circuit breakers (VCB) have been widely used for power distribution systems. Vacuum Interrupters, which are the current interruption unit, have been increased its interruption capability with the development of vacuum arc control technology by magnetic field. There are three major type electrodes: disk shaped electrodes, radial magnetic field electrodes, axial magnetic field (AMF) electrodes. In the disk shaped electrode, the vacuum arc between the electrodes is not controlled. In the AMF electrode, the vacuum arc is diffused and stabilized by an axial magnetic field, which is parallel to the arc current. In the last type of electrodes, the vacuum arc column is rotated by magnetic force generated by the current flowing in the electrodes. The interruption current and the voltage of one break VCB is increased to 100 kA, 144 kV respectively. This paper describes basic configurations and functions of VCB, vacuum arc control technology in vacuum interrupters, recent researches and applications of VCB.

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Present knowledge on the formation mechanism of helium bubbles, which will enhance void swelling and lead to production of surface roughening and blistering as well as the high temperature intergranular embrittlement of metals during irradiation, is reviewed. Helium is strongly bound to a vacancy or a vacancy cluster (void), and it can stabilize the cluster, thereby increasing cluster lifetime by dramatically reducing thermal vacancy emission and by promoting thermal self-interstitial atom emission from the cluster. The thermal stability of a helium bubble greatly depends on the helium-to-vacancy ratio, i.e., the helium density, of the bubble. The characteristic precipitation (agglomeration) behavior of helium into voids in metals during irradiation was compared with that of other solute atoms, such as hydrogen and copper.

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To investigate the fundamental effects of the helium irradiation on metallic materials, transmission electron microscope (TEM) observation, surface observation using scanning probe microscope (SPM), and hardness measurements of the damaged region were carried out. Effects of pre-irradiation of helium ions on trapping of injected deuterium have also been examined. Several important experiment findings are briefly explained. These results emphasized that the formation of helium bubbles is a critical factor in understanding the plasma-surface interaction in fusion reactors.

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Experimental studies on mesoscale erosion of tungsten surface due to low-energy and high-flux helium/deuterium exposure are systematically described. Following the historical description of the accidental finding of helium bubbles and holes, possible changes in tungsten properties like the A coefficient and the work function of the rmo electron emission, the emissivity and the energy reflection coefficient are discussed in terms of thermal bifurcation. Multi-dimensional parameter area for He bubble formation is identified in the space of tungsten temperature, ion fluence and incident ion energy. In particular, the threshold energy was surprisingly found to be as low as 6 eV, in association with which the formation mechanism is proposed. Finally, the effect of bubbles and holes formation on deuterium retention is quantitatively clarified.

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In fusion experimental devices and fusion reactors, helium particles bombard the plasma-facing surfaces and may result in various effects. This helium to plasma-surface interaction will be an important issue in developing plasma-facing materials and maintaining plasma control. In this article, studies on the helium plasma-surface interaction in TRIAM-1M and LHD, considered especially from the view point of material science, will be reviewed. It was clear from the TRIAM-1M experiments that metals facing the plasma were damaged by relatively high energy charge exchange neutrals of helium. Formation of fine dense He bubbles was remarkable. It is considered that the affect of the damage on the surface properties is dramatic. For LHD experiments, heavy damage by divertor plasma deserves particular mentions. In spite of the very low energy of helium ions, surface erosion by blistering occurred. This phenomenon is important in the evaluation of lifetime of the divertor plates and contamination of the main plasma. Helium glow discharge cleaning in LHD caused very heavy irradiation effects in the sub-surface region. Though establishment of a fundamental understanding of helium-material interaction is in progress by theoretical and by experimental studies using small scale ion injectors, the high temperature plasma confinement devices play a significant role in foreseeing the phenomena of plasma-surface interaction under the complex conditions of reactors.

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Helium effects on the mechanical properties and microstructure of three low-activation structural materials are summarized based on the current understanding of the experimental data they have yielded. Void swelling of these low activation materials is not seriously enhanced by the presence of helium in their expected operating temperature regions. Degradation of the mechanical properties caused by helium is not considered to be serious up to regions of several tens of displacement per atom (dpa). The issues of material data collection for DEMO reactor design are discussed.

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Helium irradiation effects on non-metallic materials from an atomic to a macroscopic scale are reviewed from the viewpoint of fundamental lattice defects. Following a rough description of radiation damage phenomena in covalent and ionic materials, general but characteristic property changes associated with helium ion irradiation are introduced, taking the localized character of valence electrons incorporated in the atomic bonding into account. Further, simple kinetic equations are derived to discuss the damaging processes of the near-surface region under ion irradiation. Finally, a new method to precisely estimate the mechanical properties of ion-irradiated materials and future direction of this field are presented.

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Power absorption profiles of electron cyclotron resonance heating (ECRH) are calculated for a helical-axis heliotron device, Heliotron J, by a ray tracing code, TRECE. The three-dimensional structure of flux surfaces and magnetic field is considered to evaluate the power absorption profile accurately. The calculation results show that the power absorption profile can be generally controlled under the experimental conditions of the 70 GHz ECRH system by setting the appropriate launching condition. The well localized single pass power absorption can be realized when the ECRH power is launched with a tokamak-like B contour. The single pass absorption efficiency is as high as 92 % of the injected power in the case of moderate toroidally oblique launch because of the long path along the resonance region. On the other hand, the power is fully absorbed when ECRH power is perpendicularly launched at the poloidal cross section with a saddle-type B contour, although the absorption profile is wider. This is because the magnetic field gradient is gentle at the resonance region. These indicate that the control of power absorption is possible with the appropriate launching condition in the spatially winding plasmas. The calculation is compared with the experimental results on the ECRH plasma on Heliotron J.

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