K. Odajima

HEAT FLUX TO THE MATERIAL SURFACES IN A TOKAMAK

Heat flux to the material surfaces in the scrape-off layer of a tokamak plasma is investigated experimentally. The time response of heat flux measurement has been improved to 0.2 ms by the recently developed thin-film thermometer. The result shows that the heat flux is calculated from a simple sheath model including secondary electron emission effects even if epithermal electrons are present.

Confinement and fueling studies during additional heating phase in the JFT-2M tokamak

Increments of peripheral hydrogen/deuterium neutral gas pressures (PH2/PD2) during the additional heating phase (neutral beam and RF heatings) have been observed in a D2 gas-puff fueled JFT-2M tokamak with H2-absorbed graphite limiters/divertor plates. In the beam heating phase, a large increment of PH2 raises the plasma density 2 times or more without the degrading energy confinement time. The D2 gas-puff valve is closed in this phase. This improvement is interpreted as being due to “wall fueling”, i.e., fueling by desorbed hydrogen from the graphite wall by energetic particles. The first observation of the H-mode in the INTOR-type stubby open divertor with a short divertor channel (1–8 cm) at the high density regime (4–7 × 1013 cm−3) enhanced by the wall fueling is presented. The improved energy confinement time is comparable to or higher than that of ohmically heated discharges.

Transition from the L-mode to the H-mode by electron cyclotron heating of a tokamak edge plasma

Transitions of L-mode plasmas to the H-mode have been induced by an electron cyclotron heating (ECH) pulse. The transitions occur when ECH is applied to plasmas preheated either by a neutral beam or by waves in the ion cyclotron range of frequency with power levels well below their own threshold power for the H-mode transition. The position of the electron cyclotron resonance layer has been scanned and it has been shown that edge heating rather than central heating is effective in inducing the transition to the H-mode.

A new mode of improved confinement in discharges with stationary density in JFT-2M

A new mode of improved energy confinement, for which the confinement time is not worse or is sometimes even better than that for the well known H-mode and the density of which is in a quasi-stationary state, has been obtained in neutral beam heating experiments on JFT-2M. The new mode is different from the H-mode in many respects. The central electron temperature is higher in the new mode than in the H-mode. Radiation loss and density are reduced in the peripheral region but not in the central region. Therefore, the density and radiation profiles are highly peaked in the new mode, in contrast to the broad profiles in the H-mode. Particle confinement in the peripheral region seems to be worse in the new mode than in the H-mode. The new mode can be obtained in both divertor configurations and limiter discharges in JFT-2M.

H-mode phenomena during ICRF heating on JFT-2M

Significant improvement of energy confinement has been observed on JFT-2M during ICRF heating. This improvement is preceded by a sudden drop in the Hα/Dα emission and a successive increase in stored plasma energy, electron density and radiation loss. This is believed to be the same phenomenon as the H-mode transition observed in ASDEX, and in PDX divertor experiments with neutral beam injection. However, in JFT-2M, this transition is observed both in limiter discharges and in open divertor configurations.

Impurity reduction during ICRF heating in JFT-2M tokamak

Reduction of impurity line emissions associated with the ion cyclotron range of frequency (ICRF) heating is achieved by the phase control of a loop antenna array. Reduction in metal impurity emissions and radiation loss is closely correlated with the amount of power radiated from the antennae with parallel wave number near k∥ = 0. The maximum density attainable without disruption is increased over that in the Ohmic heating phase, by reduction of radiation loss.

Carbon wall experiment in diva

Abstract The sputtering characteristics of various types of carbon limiter surfaces, e.g. pyrolitic graphite (PG), pulverized carbon film and carbon film produced by methane discharges, are tested. Arcing is only observed on the pulverized carbon surface in a stable discharge or the other surfaces in an unstable discharge. Ion sputtering is shown to be the dominant process of carbon efflux from the normal surfaces. Employing PG limiters and a carbon wall coated by rf-sputtering method, very low q plasmas with a good confinement characteristic are obtained. The coated carbon surface is pure and still not contaminated after 600 or more discharges.

Ion sputtering, evaporation and arcing in DIVA

Abstract An experimental study of the metal impurity origin in DIVA is described. Three processes for the release of the metal impurities, that is, ion sputtering, evaporation and arcing have been identified. Among these processes, ion sputtering is the dominant process in the quiet phase of the discharge, that is the phase characterized by no spikes in the loop voltage and no heat flux to a specific part of the first wall. Moreover it is demonstrated that a honeycomb structure can decrease the release of the metal impurity. This paper is being published separate [1].

Measurement of peripheral electron temperature by electron cyclotron emission during the H-mode transition in JFT-2M tokamak

Time evolution and profile of peripheral electron temperature during the H-mode like transition in a tokamak plasma is measured using the second and third harmonic of electron cyclotron emission (ECE). The so called “ H-mode ” state which has good particle/energy confinement is characterized by sudden decrease in the spectral line intensity of deuterium molecule. Such a sudden decrease in the line intensity of D α with good energy confinement is found not only in divertor discharges, but also in limiter dischargs in JFT-2M tokamak. It is found by the measurement of ECE that the peripheral electron temperature suddenly increases in both of such phases. The relalion between H-transition and the peripheral electron temperature or its profile is investigated.

Effects of Radial Electric Field on Drift Wave Instability in a Weakly Ionized Plasma

The frequency and growth rate of drift wave instability in a weakly ionized rotating plasma are investigated experimentally in both stable and unstable regions. These dependence on the magnetic field strength and the axial wave-number are qualitatively in agreement with the characteristics of a collisional drift wave with a finite ion inertia. The frequency of the instability almost linearly increases with the radial electric field, but its slope is much smaller than that of the plasma rotation frequency due to E × B drift. The result can be explained by taking into account a centrifugal force.