M. Maeno

Impurity reduction and remote radiative cooling with single-null poloidal divertor in Doublet-III

The successful operation of a single-null poloidal divertor in Doublet-III has demonstrated several new advantages of a diverted tokamak in addition to the suppression of impurity influx as demonstrated in DIVA: 1) The impurity contamination and radiation loss of the main plasma has been reduced by an open divertor geometry, i.e. without a divertor chamber; 2) The radiative cooling and formation of a dense and cold (ne≥5 × l013 cm−3, Te≤7 eV, torr) divertor plasma have been observed. – Up to 50% of the Ohmic input power is radiated in the divertor region, thus cooling the plasma in front of the divertor plate down to several eV. This remote radiative cooling greatly reduces the heat load on the divertor plate without cooling the main plasma. – The feasibility of remote radiative cooling in INTOR was studied by use of a volume integration technique of the radiation power along the field line.

Gross particle confinement characteristics by the boundary plasma in the JFT-2 Tokamak

The boundary plasma of a Tokamak device is investigated experimentally by using an electrostatic probe. It is shown that the density of the boundary plasma decays exponentially in the radial direction and perpendicular diffusion in the boundary plasma behaves like the Bohm type. The gross particle confinement time is evaluated from the particle flux to the limiters and the wall, which can be estimated from the density profile and the electron temperature of the boundary plasma.

Effects of ergodization on plasma confinement in JFT-2M

A steady-state H-mode plasma with frequent ELMs (edge localized modes) was obtained by applying magnetic fields with high poloidal mode numbers resonant in the plasma edge. The induced ELMs limit the density and impurity accumulation normally observed during ELM-free H-mode. An EML coil with numerous resonant modes ( m 10) in the plasma edge appears the most effective. This method of control is extended to higher auxiliary heating power by increasing the magnitude of the magnetic perturbations.

Improvement of plasma parameters by titanium gettering in the JFT-2 tokamak

Abstract Oxygen impurity has been reduced to about 1–2% of the electron density by titanium gettering onto the torus wall (~ 1 2 ) and the limiter. Radiation loss and effective ionic charge were decreased by a factor of ~2. As a result of reduced impurity influx, broader electron temperature profiles have been obtained. The energy confinement time and the scaling factor of the maximum electron density ( n e R/B t ) were improved by a factor of 1.6 with the titanium gettering. Limits on the density increase were investigated in connection with the radiation power. Mechanism limiting the density maximum is discussed.

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.

Impurity behaviour during ICRF heating in JFT-2M

Abstract The impurity behaviour during the ICRF heating phase was studied in the JFT-2M tokamak. In this experiment, impurity behaviour during the ICRF heating phase was studied with special attention to the boundary electron temperature. From these experimental results, it was concluded that carbon (limiter material) impurity was released by an ion sputtering accelerated by the sheath potential. Metal impurities such as iron and titanium released from the antenna and/or near the antenna region are caused by some additional mechanism by applying the RF during the ICRF heating phase.

High density, single-null poloidal divertor results in doublet III☆

Abstract Experimental results of divertor characteristics on impurity suppression, helium ash exhaust and remote radiative cooling are reviewed. In a throatless (open) divertor such as the single-null poloidal divertor in Doublet III, a dense and thick scrape-off layer provides strong particle shielding in high density divertor discharges, which results in a high neutral pressure and radiative cooling of the divertor even in an impurity-free condition. The feasibility of employing an open divertor in a reactor-grade device like INTOR is discussed. A preliminary result of divertor experiments with a 1.6 MW neutral-beam injection is presented.

Magnetohydrodynamic activity in the JFT-2 tokamak with high-power neutral-beam-injection heating

Neutral-beam power of up to 1.2 MW injected into the plasma has produced a volume-averaged β of up to 2.6% and a central beta β0 of up to 7%, due to the thermal components in the JFT-2 tokamak. In these beam-dominated discharges, the magnetohydrodynamic behaviour was studied. Four types of internal oscillations were observed: i) enhanced sawtooth oscillations with long repetition time and large sawtooth amplitude; ii) round sawtooth oscillations and/or reduced sawtooth oscillations with short repetition time and small sawtooth amplitude; iii) high-frequency oscillations without sawtooth oscillations, and iv) high-frequency oscillations with sawtooth oscillations. The measured beta values are compared with the critical ones as found from high-n ballooning-mode analysis, and the relationship between MHD behaviour and beta values is also investigated.

Particle and energy fluxes observed in the scrape-off layer of JFT-2 tokamak

Particle and energy fluxes in the scrape-off layer produced by poloidal limiters are studied in the JFT-2 tokamak with mean electron densities of 1.3 ? 1013 and 2.8 ? 1013 cm?3. The measurements were made by a combined Langmuir/thermoprobe and an infrared camera. The electron density in the scrape-off layer decays exponentially with increasing minor radius, implying that the cross-field diffusion is of the order of the Bohm diffusion. It is shown that heat transmission to the limiter more than 1 cm behind the edge radius is determined by a sheath of Maxwellian particles. Near the edge radius, accelerated electrons play a major role, which is indicated by toroidal anisotropy of the heat flux in the lower-density case.

Plasma parameters related to arc initiation on the tokamak first wall

The mechanism responsible for the initiation of unipolar arcs on the tokamak first wall is studied by measuring plasma parameters in the scrape-off layer. It is shown that a large electric field in the sheath region produced by runaway electrons is the dominant cause of the arc initiation. The difference in arc frequency in the toroidal direction on the limiter surface is explained by an asymmetry of the ion flux due to ion drift motion.