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A pellet injection system with controllability of the injection angle has been developed and installed in the JIPP T-IIU tokamak. A long helical ablation cloud (‘tail’) has been observed when an ice pellet is injected into the tokamak plasma using the controllable injection angle system. The ‘tail’ does not flow along the direction of the total magnetic field lines of the torus. In order to understand the details of the ice pellet ablation structure in the ‘tail mode’, the local parameters of electron density and temperature within the ablation cloud are obtained by a spectroscopic method. The electron density determined from Stark broadening of the Balmer β line is in the range 1.0 × 10 16 −1.0 × 10 17 cm −3 while the temperature obtained from the line-to-continuum intensity ratios is in the range 1–4 eV. It has also been observed by a heavy ion beam probe (HIBP) measurement that the potential at the plasma centre becomes more negative and the potential profile becomes deeper negatively after pellet injection.

Thin lithium layers are deposited on metal or graphite walls by evaporation in vacuum. Clean lithium surfaces have wide chemical activities on such gases as O 2 , CO and CH 4 . In particular, a strong gettering effect on oxygen is observed; the minimum number of 0 atoms gettered by the lithium layer is about one half the total number of Li atoms deposited on the wall, thus suggesting the formation of Li 2 O. On the other hand, H 2 gas hardly reacts with the lithium surface which, however, displays a large pumping effect in a hydrogen glow discharge. The maximum number of H atoms pumped by the lithium layer below 180°C is approximately equal to the number of Li atoms in the vessel, probably due to the formation of LiH. Helium glow conditioning of the used lithium layer allows the partial recovery of lithium from losses to hydrogen pumping or oxygen gettering. This lithium evaporation method has been applied to JIPP T-IIU. 100-250 mg of lithium was deposited onto a limited vessel area of ∼ 1 m 2 . The lithium coating leads to 20-50% reduction in oxygen and carbon impurities with less hydrogen recycling in ohmic and NBI discharges

By using the JIPP T-IIU device the wall surface carbonization experiment was carried out with ECR discharge (2.45 GHz, 1.6 kW) in a gas mixture (CH4/H2 = 5050, 1.5 × 10−2 Pa). The film is free from oxygen and metal. The mean deposition rate (on the first wall except special plane like limiter) can be predicted by simple measurement of CH4 partial pressure. Wall heating (up to 300°C or more) and ion bombardment (E > 100 eV) are effective to decrease the hydrogen concentration in the film.

In situ carbon coating was carried out with a glow discharge in methane-hydrogen mixtures on the wall of the JIPP T-IIU tokamak at room temperature. The concentration of hydrogen and deuterium in C-films was analyzed by thermal desorption spectroscopy (TDS) and elastic recoil detection analysis (ERDA). Good agreement was obtained between results deduced from these two methods. Hydrogen concentration is 40–55 at. % in the C-films. Replacement of H in the C-layer was tried using a pure D2 glow discharge. In order to achieve complete replacement, a fluence of 2 × 1017 cm−2 was needed at room temperature. The replacement process was monitored by a quadrupole mass analyzer and results are compared with Hα/Dα observation of tokamak discharges and the results of the surface analysis.

The effectiveness of the in-situ carbon coating (carbonization) has been demonstrated to reduce the radiation loss by iron impurities during ICRF heating in the JIPP T-IIU tokamak. As a result of carbonization, the total radiation loss decreased down to one fifth of the RF power, which resulted in an increase in electrons and total stored energy compared with these conditions before carbonization. The thickness of the carbon layer was 300–900 A, and its toroidal uniformity was within a factor of 3, although only one anode and one gas-inlet were used. A thin carbide layer is formed between the C-film and the stainless steel substrate with carbonization at room temperature. The hydrogen concentration is 40–50 at.% in the carbon layer. Deposition of carbon was observed on window materials. The deposition rate was relatively less on electrical insulators compared to the deposition rate on metals.

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 that the limiters are the major impurity sources both in the ohmic and RF heating phases. Although titanium radiations increased with RF power injected by an antenna with a titanium Faraday shield, the maximum intensity was much smaller than in the experiment where the titanium-flashed stainless steel limiters were used. Thus it has been found that the Faraday shield is less important as an impurity source than limiters. Toroidal asymmetry observed for O II radiation suggest 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 Hα + Dα radiation decreases during the RF pulse around the limiter, which may be due to the change in hydrogen/ deuterium recycling at the limiter. The reduction of Hα + Dα is greater with graphite limiters than with stainless steel limiters. The relation between recycling and impurity release is briefly discussed.

Surfaces of two sets of limiters were analyzed by PIXE and RBS. One is made of stainless steel SUS 304 and the other made of graphite. The two limiters are the same shape and size and have been exposed to similar numbers of tokamak discharges. Molybdenum and titanium were observed on both limiters. Although the deposited metals were larger on the graphite limiters than the amount on the steel limiters, the plasma radiation intensity from Mo or Ti was smaller with the graphite limiter than with the steel limiter during tokamak discharges. The sputtering rate of the metals on the graphite limiter was investiaged with an argon-ion beam. The apparent sputtering yields of metals are small on carbon limiters, and this results in the lower metal contamination of tokamak plasmas with carbon limiters than with metal limiters.

MHD activity and turbulence near the plasma edge are studied on the JIPP T-IIU tokamak using a new high-speed visible-light image-converter video-camera system. Different from conventional cinefilm and photo-diode array systems, this system is convenient for the instantaneous display of the high-speed optical plasma images after plasma discharges. The effectiveness of this instrument for research on the plasma-wall interaction is demonstrated in this experiment. The observed characteristics of the edge-plasma behavior are as follows: (1) The helical mode structure of the luminous plasma boundary suggesting plasma-surface interaction is identified in the case of OH or ICRF-heated discharge. (2) In the LH-current drive case, no clear large-scale coherent modes are identified, however, on the initial stage a medium-scale turbulence ( λ ~ a few cm, f ~10 kHz ) is found. (3) Before current disruptions, an m = 2 or m = 3 helical mode is found and up-down asymmetric light emissions are often observed during disruptions.