Noboru Fujisawa

Parametric Heating by Radio-Frequency near the Lower Hybrid Frequency in the JFT-2 Tokamak

Ion heating efficiencies of the central plasma obtained under various experimental conditions indicated that a nonlinear effect played an important role in the ion heating when the high rf power was irradiated to the plasma. Nonlinearly excited electromagnetic modes as well as the parametric decay spectrum of the electrostatic modes, which were also observed simultaneously, sugegested a further excitation of nonresonant quasi-modes decay instability. It wins shown that the ion heating for \(\omega_{0}/\omega_{\text{1h}}(0){\gtrsim}1.13\) (where ω 0 is frequency of irradiated rf field and ω 1h (0) is the lower hybrid frequency at the center of the plasma column) were well explained by the theoretical model having the empirical nonlinear heating rates resulting from the excitation of nonresonant quasi-modes in one-dimensional WKB approximation.

Loss Mechanism of Energetic Ions Produced during the Lower Hybrid Heating of JFT-2 Tokamak

The energetic ion (\({\gtrsim}3\) keV) produced by the lower hybrid waves, decays in 200–400 µsec after RF turn-off. The loss mechanism responsible for this fast decay is indicated as scattering into the loss cone and the toroidal ripple loss cone. The fast ions are considered to be present in \(0.6{\lesssim}r/a{\lesssim}0.9\).

Parametric Absorption of Lower Hybrid Wave in the Boundary Plasma of JFT-2 Tokamak

Power absorption rate of lower hybrid wave by ions has been estimated by the convection, conduction loss and the charge exchange loss both in the main plasma region and in the boundary region of a circular tokamak. It is shown that when an effective ion heating is performed in the main plasma region the rf power is not dissipated in the boundary region through the parametric decay instabilities. The rate of the dissipated rf power in the boundary region saturate at relatively low power level and does not give the fatal effect on the ion heating in the main region if we select an appropriate plasma parameters.

Molybdenum, Carbon and Silicon Carbide Limiter Experiment in the JFT-2 Tokamak

Molybdenum, pyrolitic graphite (PG), and silicon carbide were used for the limiter of the JFT-2 tokamak. It was found that the macroscopic discharge characteristics with the PG and SiC limiters were equivalent to those with the Mo limiter, though CV line radiation with the PG and SiC limiters was 1.3 to 6 times that with the Mo limiter. For these conditions , the maximum surface temperature of the limiter during discharge was 550°C for Mo, 1900°C for PG, and 1000°C for SiC, as observed by an infrared camera. Residual gas analysis indicated that chemical reaction between the plasma and the limiter did not play a major role in carbon generation into the plasma. The estimated amount of carbon contamination of the plasma can be caused by physical sputtering, which has been separately investigated for PG.

Measurement of Surface Temperature of a Limiter and a Study on Plasma Confinement Characteristics

The surface temperature of a limiter during discharges in the JFT-2 tokamak was measured by an infra-red scanning camera with a time resolution of 0.625 msec. The maximum surface temperature during a discharge was around 600°C in a discharge with a mean electron density of 1.3×1013 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.

Discharge Cleaning in JFT–2

High efficiency discharge cleaning has been demonstrated for a tokamak-type apparatus. Though the repetition interval of discharge was unusually long–longer than the pumping-out time, the present method is effective in removing impurities from surfaces because of large current fed in each shot. Hydrocarbons were observed to be produced in the hydrogen gas in association with the discharge.

Toroidal Theta Pinch Collapse by the Snowplow Model

Accurate equations for the cross section of the toroidal theta pinched plasma column are derived using the snowplow model, and then are solved numerically to study the shift of the column position and the deformation of the cross section during the collapse. The column shifts towards the outer wall and the shape of the cross section changes from the initial circle into a heart as the collapse proceeds. The shift and the deformation, however, are not serious, if the aspect ratio is larger than 10.

Role of Boundary Plasma in Lower-Hybrid-Frequeney Heating of a Tokamak

Boundary plasma of a circular tokamak has been investigated by means of electrostatic probes during lower-hybrid heating. The reflection coefficient is affected by the density gradient in front of the Iauncher. An effective ion heating is performed in the main plasma region when the boundary electron temperature is relatively high enough to suppress the parametric decay instabilities. The simultaneous injection of neutral beams as well as the lower-hybrid wave brings the suppression of instabilities with increase of the electron temperature coming from the neutral beam heating.

On the Origin of Arcing in a Tokamak

Experimental study of arcing is made in JFT-2 tokamak. Arcing is accompanied with the X-ray emission from a limiter. It is the most probable cause of arcing that accelerated electrons supply the large negatively biased potential to the limiter and offer the sufficient sheath potential to produce arc spot. This model well explains the monopolar arcing phenomenon in the current rising phase and in the plasma disruption phase.

Experimental Study on Limit of the Safety Factor in JFT-2 Tokamak

In medium q a ( q a ≃3) discharges, enhanced radiation loss power due to increased plasma-wall interaction plays an important role in the growth of low frequency oscillations. The m =2 tearing mode seems to be the most plausible cause of the disruption. The reduction of the impurity is essential in realizing the stable discharges of medium q a . Attainable q a is reduced with decreasing radiation loss power. In low q a ( q a <2) discharges, no enhancement of the radiation loss power was observed until the disruption. The plasma disrupts at \(q_{a}{\lesssim}2\). The m =2 kink mode is the probable cause of the disruption. Attainable q a is reduced with decreasing the ratio of shell radius to plasma radius. The possibility of the shell stabilization against the m =2 kink mode is proposed,