[en] Purpose: To remove impurities such as carbon, oxygen or the like adhered to a wall (first) surface face to plasma of a plasma confinement apparatus, by reduction of hydrogen or oxidation of oxygen or both. Constitution: Material forming a wall (first wall) face to plasma of plasma confinement apparatus is subjected to preliminary cleaning process, and then the thus preliminarily cleaned first wall forming material is heated to a high temperature. Next, it is solely or alternately exposed to hydrogen or oxygen atmosphere to remove the impurities adhered to the first wall surface to clean it. It should be noted that the first wall material comprises molybdenum, which is heated to a temperature of 500⁰C and alternately exposed to hydrogen 1 Torr and oxygen 0.1 Torr, and as a consequence, the percentage in atomic number between oxygen and carbon present in the surface may be reduced from more than 90% before processing to about 10 - 20%. (Kawakami, Y.)

[en] Purpose: To maintain the mechanical strength and facilitate the dismantling of blankets. Constitution: A blanket comprises a plurality of divisional pieces that can be divided in the torus direction, and key slots are provided to the sides of the divisional pieces of the blanket respectively in such a way that each paired slots are substantially in a symmetrical relation to each other with respect to the joining line between each of adjacent divisional pieces. Upon assembling the blanket, the divisional pieces are joined to each other and keys are fitted into the key slots respectively to form and maintain the torus configuration of the blanket. This enables to maintain the mechanical strength and facilitate the dismantling work of the blanket. (Furukawa, Y.)

[en] Purpose: To enable blanket exchange with slight dismantling thereby facilitate maintenance by moveably supporting the blanket rotatably in the torus direction and providing an access aperture for the blanket split pieces in the upper wall of the shieldings. Constitution: A blanket of a doughnut type and having ring-shaped cross section is formed with a plurality of split pieces connected in the torus direction, and the circumferential width of the split piece is made narrower than the width between the toroidal coils. While on the other hand, rollers are provided between the inner bottom face of the shieldings covering the blanket and the outer bottom face on the blanket so as to move the blanket rotationally in the torus direction. An aperture for allowing to pass the split piece therethrough is provided between the toroidal coils and at the upper wall of the shieldings, and the aperture is normally closed by the shielding block which can be removed with ease by lifting. (Yoshino, Y.)

[en] In the recycling process of impurities in a plasma, only the sputtering of first wall materials is usually considered. In the present study, not only sputtering of metal but desorption of sorbed layers by particles and reflection of incident impurity ions at the limiter are also introduced. The behaviour of impurities in the discharges of recent Tokamaks is interpreted quantitatively and the result is applied to a future large Tokamak. The nonmetallic impurity concentrations may attain plateaus in the early stage of a discharge which are lower than the permissible levels, because the desorption yield by particles may probably be decreased by discharge cleaning. On the contrary the metallic impurity concentrations tend to increase beyond the permissible levels, since the self-sputtering yield of impurity ions will become larger than unity with the increase of plasma temperatures. In this respect the method to reduce the metallic impurities is important. (auth.)

[en] The compositional change of the surface and near surface layer of TiC coating has been studied under 1keV hydrogen ion bombardment at various target temperatures. Depth profiles near the surface were observed by Auger electron spectrometer combined with Ar ion etching. The carbon composition decreases by a factor of 1.3 at the TiC surface bombarded at room temperature, indicating preferential sputtering. The carbon composition increases exponentially to the bulk value with increasing depth from the surface. The thickness of the altered layer is about 5nm, which is almost a half of the projected range. No carbon depletion, however, was observed at the target temperature of 700⁰C. The diffusion coefficient of carbon atoms in the altered layer, calculated from the steady-state depth profile at room temperature, is about 8 x 10^-18cm²/s which indicates enhanced diffusion in this layer. (author)

[en] A concept of tokamak fusion reactor maintenance is presented. Reactor structures and maintenance machines are arranged so that the component inside a shielding structure can be replaced through the hatches located on the upper side of the torus shielding structure. The plasma vacuum boundary is constituted by the inside wall of the shielding structure. The magnet cacuum chamber contains two toroidal magnets in a single room, so that strong support structures can be placed between these toroidal magnets. A merit of this reactor is that the inboard reactor structures are accessible with keeping the magnet cryogenic condition and without disassembling any major reactor components. The practicability of this method will depend on the time required to move the blanket segments in the toroidal direction and to weld pipes by remote handling. A number of ideas for reducing this time are presented. (author)

[en] Thermal testing of coating materials for tokamak limiters and walls was carried out by means of an electron beam equipment. The materials tested are TiC coated onto POCO graphite and TiC and TiN onto Mo by chemical vapor deposition. All the coating layers survived without exfoliation under 80% of the beam power which eroded the substrate material. This shows good thermal and mechanical contact between coatings and substrates. Allowable heat loads for upset conditions such as plasma disruption were estimated to be about 3.5 kW/cm₂ in uniform heating during 1 s for the three coating materials. The heat load limits in usual tokamak operation were also estimated in view of plasma contamination by limiter material evaporation. The limits in uniform heating for 1 s are about 3 kW/cm₂ for TiC on Mo, 2.5 kW/cm₂ for TiN on Mo, and 2 kW/cm₂ for TiC on POCO graphite. The TiC coating on Mo thus appears to be the most interesting choice within the framework of this study. (author)

[en] Noticeable correlations were observed between the changes of discharge characteristics, wall conditions and typical mass peaks with discharge cleaning in the JFT-2 tokamak. Atomic composition of the vacuum wall surface observed by AES becomes constant with continuing discharge cleaning in the level except hydrogen and helium: 30-50% C, 20-30% Mo, 15-30% stainless steel elements and 10-15% O. The stable reproducible plasma with Z sub(eff)=4.5 was obtained in this wall condition. The limiter and vacuum wall materials (Mo and 304 stainless steel, respectively), carbon and oxygen were observed depositing on the wall in the thickness of about 300 A at the minimum inner radius of the vacuum chamber and 40 A nearly at the maximum after 2900 cleaning pulses. The mechanism determining the wall condition is also discussed. (auth.)

[en] The surface temperature of a limiter during discharges in the JFT-2 tokamak was measured by an infrared scanning camera with a time resolution of 0.625 msec. The maximum surface temperature during a discharge was around 600⁰C in a discharge with a mean electron density of 1.3 x 10¹³ cm^-3, and it decreased with increasing electron density. The results indicate that evaporation of the limiter is not a mechanism of metal impurity production from the limiter. The heat flux to the limiter was estimated on the basis of one-dimensional heat conduction to the bulk. The heat flux to the limiter increased abruptly during the abrupt growth of low frequency oscillations just before a negative voltage spike: the enhanced heat flux was 13 times as large as the one before the abrupt increase. Relations between plasma characteristics and the heat flux to the limiter are discussed. (author)

[en] For tokamak wall conditioning, surface cleaning of molybdenum by chemical reaction with oxygen and hydrogen gases has been studied experimentally by Auger electron spectroscopy (AES) and Auger/Sputter techniques. Means of pretreatments, buffering with water containing Al₂O₃ powder, glow discharge cleaning, and electropolishing show similar results. By exposing molybdenum sample to 0.1 Torr oxygen and 1 Torr hydrogen successively at 500⁰C, the impurity content on the surface reduces to 1/3 -- 1/4 of the initial value. When the content is reduced below the detectable limit of AES by heating up to 1,800⁰C in vacuum, it becomes 1/6 -- 1/7. By heating at 500⁰C, the impurities in the surface penetrate inward, probably because of the increase of impurity diffusion at this temperature. (auth.)