[en] An experimental device named SUT (SUrface modification Teststand) was constructed for a boronization study. An ultra high vacuum (UHV) condition, a changeable high temperature liner and in-situ AES are three distinctive feature of the SUT device. Saturation density of oxygen atoms was as large as 1.2x10¹⁷/cm² on a boronized surface, whereas 1.5x10¹⁶/cm² on a bare stainless steel surface. It is found by AES analysis that the oxygen-contained layer was as thick as 50 nm from the top surface of the boron film. From such a large oxygen-saturation density, we expect that the oxygen-gettering ability of the boronized surface is likely to be maintained during one-day experiment of LHD. The oxygen-saturation behavior was quite similar between the boronized surfaces obtained with decaborane and diborane, which indicates that, as a working gas of the boronization, the decaborane works well compared with diborane, as far as oxygen gettering is concerned. (author)

[en] The Large Helical device (LHD), now under construction is a Heliotron / torsatron device with a closed divertor system. The edge LHD magnetic structure has been studied in detail. A peculiar feature of the configuration is existence of edge surface layers, a complicated three dimensional magnetic structure. However it does not seem to hamper the expected divertor functions. As a confinement improvement scheme in LHD, we have proposed a high temperature divertor plasma operation in which a divertor plasma with temperature of a few kev, generated by efficient pumping, leads to the confinement improvement. Conceptual designs of the LHD divertor components are under way. (author)

[en] A conceptual design has been completed for the divertor plates of the Large Helical Device (LHD, R = 3.9 m, a_p = 50 ∼ 60 cm, B_h = 3 ∼ 4T/ superconducting coils of NbTi) and the detailed technical design is now in progress. The design concept and the status of research and development (R and D) programs are described. (author)

[en] A hydrogen beam of 16 kV, 4 A was co-injected into hydrogen plasmas and coupled to an rf of 800 MHz, up to 150 kW. It has been observed with the measurement of parallel charge-exchange neutrals that the fast ions above 16 keV are enhanced by the interaction with the lower hybrid waves. The enhancement is critically reduced when the line-averaged electron density is higher than 1.6 x 10₁₃ cm^-₃ both in the toroidal fields of 22 kG and 16 kG. For the 22 kG case, the cause of the reduction may be related to the location of the turning point of the incident waves; that is, the turning point, in higher densities, locates outside the region where the fast ions produced by NBI are relatively abundant. For the 16 kG case, however, this interpretation is valid only when a part of the incident wave energy is transferred into waves of shorter wavelengthes. The ion-temperature increment in the simultaneous heating agrees with the sum of those in the individual heatings even when the beam-rf interaction is considerably strong. (author)

[en] Replacing stainless steel limiters by graphite limiters, we found that radiations from iron and titanium ions were significantly reduced. Total radiation and loop voltage also decreased. This indicates the limiters are the major impurity sources both in Ohmic and rf heating phases. Although titanium radiations increased with rf power injected by an antenna with titanium Faraday shield, the maximum intensity was much smaller than those in experiment where the titanium-flashed stainless steel limiters were used. Thus it has been found that Faraday shield is less important as an impurity source than limiters. Toroidal asymmetry observed for OII radiations suggests that the energetic charge-exchange neutrals play a role in releasing oxygen from the wall and that those energetic particles are relatively abundant in the toroidal sections near the antenna. The radiation H sub(α) + D sub(α) decreases during rf pulse around the limiter, which may be due to the change in hydrogen/deuterium recycling at the limiter. The reduction of H sub(α) + D sub(α) is greater with graphite limiters than with stainless steel limiters. The relation between recycling and impurity release is briefly discussed. (author)

[en] Reducing plasma heat flux on the material limiter and stabilizing fluctuations at the plasma boundary by the use of an RF ponderomotive potential barrier on the limiter (RF limiter) have been demonstrated in Compact Helical System (CHS) for ECH heated plasma. The RF current on a graphite limiter surface along magnetic field lines produces a ponderomotive potential barrier, which is large at the limiter edges. The high heat load on the limiter edges, which is measured using a infrared TV camera, has been shown to be reduced by an RF power of 1.4 kW at the frequency 10 MHz (ω_ci<ω<<ω_ce). Another remarkable effect of the ponderomotive potential barrier on the limiter that has been observed is the suppression of density fluctuations (<100 kHz) at the plasma boundary. (author)

[en] During discharge cleaning, it has been observed that decay time of water-vapor pressure changes when the pressure reaches a certain level. A long decay time observed in the later phase can be interpreted as a result of a slow deoxidization rate of chromium oxide, which may dominate the cleaning process in this phase. Optimization of plasma density for the cleaning is discussed comparing the experimental results on density dependence of water-vapor pressure with a result based on a zero-dimentional calculation for particle balance. One of the essential points for effective cleaning is found to be raising electron density of the plasma high enough that the dissociation loss rate of H₂O is as large as the sticking loss rate. A density as high as 10¹¹ cm^-3 is required for a clean surface condition where sticking probability is presumed to be around 0.5. (author)

[en] Particle transport and related phenomena are investigated in currentless electron cyclotron resonance heated plasmas. It is observed that during the initial phase the density stops rising and then starts to decay, in spite of continuous gas puffing. The delay time between the breakdown time and the time at which the density starts to decay depends inversely on rf power. The density profile changes drastically, becoming flat or hollow at this time. This density profile change becomes more significant as the rf power is increased. In order to sustain the density at some level the external particle source should be increased with increasing rf power. Here it is found that the particle confinement time decreases with increasing power. The substantial increase in the particle outflux from the main plasma causes the scrape-off plasma to have a steep gradient. As a result, the amplitudes of density and magnetic fluctuations are enhanced with increasing rf power. When the cyclotron resonance layer is shifted outside the half radius, it is found that the density profile changes from hollow to centrally peaked. This result shows that the density profile is a function of the power deposition profile. It is also shown that the behavior of impurity ions (injected by the laser blow-off technique) is similar to that of the bulk particles. (author)

[en] Optimization of discharge cleaning was studied by using 60 Hz AC and cw ECR discharges. Reduction rate of oxygen was investigated for variations of the wall temperature. It has been confirmed that, below 300₀C, the reduction rate increases as the wall temperature is raised. A simple model is proposed for the cleaning process and H²O pressure is calculated as a function of plasma density on the basis of this model. By comparing the calculated results to an experimental observation, it has been found that it is essential to raise the electron density of the discharge-cleaning plasma high enough that the dissociation-loss rate of H²O is as large as the sticking-loss rate in order to optimize the cleaning. (author)

[en] All the aperture limiters in JIPP T-II were changed to be TiC coated graphite. The maximum density in this condition is 7.6 x 10₁₃ cm^-₃ with the safety factor of 4.5, which is 2.7 times as high as the value obtained by the Murakami scaling. The z sub(eff) values with this density and with a medium density of 2.4 x 10₁₃ cm^-₃ are both close to unity after discharge cleaning for only 3 days. The behavior of limiter material release was found to be steady within a factor of two during the discharge, suggesting that the release mechanism is ion sputtering. Residual gas analysis also suggested that this sputtering is not chemical, which may be partly because the limiter surface temperature during the discharge is below 150₀C. (author)