[en] A feedback control system aided by a personal computer is developed to maintain plasma position on the required position in the JIPP T-IIU tokamak. The personal computer enables to adjust various control parameters easily. In this control system, a control demand for driving the power supply of feedback controlled vertical field coils is composed to be proportional to a total plasma current. This system has been successfully employed throughout the discharge where the plasma current substantially changes from zero to hundreds of kiloamperes, because the feedback control can be done, being independent of the plasma current. The analysis of this feedback control system taken into account of digital sampling agrees well with the experimental results. (author)

[en] Suprathermal electrons which carry a seed current are generated by non-resonant parametric decay instability during initial phase of lower hybrid current startup in the JIPP T-IIU tokamak. From the numerical analysis, it is found that parametrically excited lower hybrid waves at lower side band can bridge the spectral gap between the thermal velocity and the low velocity end in the pump power spectrum. (author)

[en] Precursor oscillations to the H-mode transition are identified in magnetic fluctuations of the ASDEX H-mode discharges initiated without a sawtooth. This precursor is m=4/n=1 mode, rotating with f ≅ 10 kHz in the opposite direction to co-injected neutral beams. Time behaviour of the amplitude suggests that the H-mode transition is caused, not by the edge electron temperature, but by the edge current density. (orig.)

[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] A plasma current is initiated and raised up to a quasi-stationary level of about 20 kA by injecting the lower hybrid wave into a cold and low density plasma produced by electron cyclotron resonance. The plasma current rises more slowly than the experimentally obtained Lsub(p)/Rsub(p) magnetic diffusion time of the bulk plasma. The current rise time decreases with the increase of the bulk electron density, and agrees well with the collision time of the current-carrying high energy electrons with the bulk plasma. (author)

[en] The gas-feed system controlling the mixing ratio and total pressure of two species of fuel gases has been constructed and tested in the JIPP T-IIU tokamak. The each of two species of pure gases is independently fed through piezo-electric valves respectively. The voltages applied to those valves are feedback-controlled by the each mass signals of which are measured by a quadrupole type of mass spectrometer. This system is successfully used during ICRF heating experiments. (author)

[en] In limiter H-modes of JIPP T-IIU, a new type of L-H transition preceded by an ELM is observed. The preceding ELM (pre-ELM) appears just prior to the L-H transition. This type of transition is usually observed in H-modes of JIPP T-IIU. The L-H transition without the pre-ELM is triggered only in the case when a sufficiently large rapid current ramp down is emploied. In H-modes with constant q(a)∼3.5-4.5, coherent magnetic oscillations with m=3/n=1 destabilized during L-phase are further enhanced at the pre-ELM, and suppressed suddenly at the transition. This mode is situated in the region of the transport barrier. Propagation frequency of the m=3/n=1 mode, which may be affected by plasma mass rotation, rises appreciably (by ∼ 10 %) during H-phase with frequent ELMs, but remains unchanged for at least 200 μs after the transition. Behaviours of the m=3/n=1 and m=2/n=1 modes are well explained by quasi-linear resistive tearing mode analysis for modelled toroidal current density profiles slightly detached from the limiter. These experimental results suggest that the transition is controlled by the change of a magnetic field structure relating to the modification of a toroidal current density profile near the edge. The possibility for the development of edge radial electric field as a consequence of the transition is discussed. (author)

[en] Multichannel polarization measurements of the Balmer α line of beam emission are performed using a 15-channel four-polarizer optical system on the JIPP T-IIU tokamak. The pitch angle of the internal magnetic field is obtained using a newly developed method where only σ-components of the motional stark spectrum are analyzed to avoid difficulty due to the π-component asymmetry caused by appreciable beam divergence. Radial profiles of internal poloidal magnetic field and safety factor are successfully obtained in a current ramp-up discharge. The safety factor at the magnetic axis q(0) is about 1.7 just before the current ramp-up and gradually decreases close to unity during the current ramping phase till the appearance of sawtooth oscillations. In the sawtoothing phase, q(0) is further decreased to be obviously below unity, i.e., 0.7-0.8, which suggests that no complete reconnection of the poloidal magnetic flux takes place at the sawtooth crash. (author)

[en] The H-mode, an improved confinement regime, has been observed recently in JIPP T-IIU high power heating experiments. It was obtained in a limiter configuration without any shaping of the plasma cross-section. This H-mode is not common because it is obtained with ICRF heating. The H-mode is also obtained in a combined heating with NBI and ICRF and it is found that the threshold power level is similar to that of averaged ICRF heating. The power threshold of the H-mode is then studied for its dependence on various plasma parameters: It increases with plasma current, insensitive to plasma density, and optimized against toroidal field intensity. The power deposition profile of ICRF heating is analyzed using a ray-tracing code, and used to explain the observed B_T dependence. This paper also deals with a class of strange shots observed in the same series of experiments. They appeared at a power level close to the H-mode threshold with a remarkable improvement of confinement. However, they are different from the H-mode in the time evolution of the profiles. (author)

[en] Plasma toroidal rotations not driven by the momentum input but by the electric field due to a loss of ions have been observed for the plasmas heated with perpendicular neutral beam injection. They are found to be increased in the counter direction to the plasma current as plasma ions are heated with ion cyclotron resonance frequency waves. The electric field derived from rotations are quantitatively consistent with those inferred from the ambipolarity of particle fluxes. (author)