N. Fujisawa

Electron cyclotron resonance discharge cleaning of JFT-2 Tokamak (Jaeri)

Abstract A discharge cleaning experiment was carried out in JFT-2 tokamak by using a hydrogen plasma produced by electron cyclotron resonance (ECR) discharge. By the use of a 2.45 GHz, 2 kW CW microwave, a plasma with electron temperature of 4eV and density of 1.2 × 10 10 cm −3 was produced. Under quasi-equilibrium conditions, a comparison between the ECR-DC and the Taylor discharge cleaning (TDC) was done by observing the partial pressures of CH4, H2O, and CO gases. The TDC was carried out with a plasma with electron temperature of 4.1 eV and density of 3.5 × 10 12 cm −3 , produced with a pulse width of 10 ms and a repetition frequency of 0.67 Hz. Both cleaning methods were found to have an approximately equal cleaning effect. This shows the applicability of the ECR-DC to advanced tokamaks.

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.

Self-consistent numerical analyses for scrape-off plasmas and neutral particles in a FER divertor chamber

Abstract The present report deals with a numerical analysis of characteristics of poloidal divertors in the Fusion Experimental Reactor (abbreviated FER) now under the design study in JAERI. Diverted scrape-off plasmas are formulated and analyzed based on a fluid model including the interaction of the plasma with neutral particles through ionization and charge exchange reactions. The neutral particle behavior is calculated using Monte Carlo methods. The possibility of high density operation of the FER divertor is examined numerically and the pumping requirement for the helium ash exhaust is discussed. It is also shown that the same numerical model gives the results qualitatively consistent with the DIII divertor experiments.

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.

Dynamic behaviour of intrinsic impurities in Doublet III discharges

Before a complete set of impurity control techniques were implemented in the Doublet III tokamak, two distinct discharge types were obtained, with nearly equal probability at high power densities with the same external control parameters. In the normal discharge (type S), sawtooth activity develops quickly and no impurity peaking on axis occurs thereafter. The other type of discharge (type O) initiates with more plasma/wall interaction, proceeds through the current-ramp phase with no central MHD activity, and impurity density profiles are found to be peaked on axis. Later in the current flat-top phase, type-O discharges experience the sudden onset of a large oscillation with m = 1, n = 1, and a concurrent decrease of central impurity densities. The (sinusoidal) oscillation eventually evolves into sawteeth, and type-O discharges attain a final state identical to type-S discharges. Measurements of absolute impurity densities show a direct correlation of impurity confinement with the presence or absence of tearing-mode activity, which suggests that the impurity transport is modified by the influence of the m = 1 magnetic island.

Control of plasma vertical position in tokamak reactors

Abstract The methods to estimate the effects of the eddy currents in the conductive components, such as the first wall, blanket, shield and coil vacuum chamber, on the vertical position control in tokamak reactors are presented. The classical PID feedback control is investigated and the control properties of the Fusion Experimental Reactor (FER) design of JAERI are discussed. The shell structures in the breeding blanket modules, shield structures and belljar type of coil vacuum chamber have important effects on the plasma vertical position controllability. In this FER design, the feedback control power with the coil inside the shield and inner belljar is smaller by a factor of − 10 than the control by the outside coil. The derivative action of the PID controller is essential to control the plasma vertical position stably.

Numerical analyses of plasma and neutral particle behavior and design criteria for poloidal divertor in fusion experimental reactor

Divertor performance is investigated using a numerical model for various incoming ion and heat fluxes and geometrical configurations. It is shown that the solution is double-valued over a part of the range of the input fluxes, and that helium exhaust and cold and dense plasma formation will be attained even in open geometry for the expected range of the incoming ion flux.

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.

Bifurcated confinement solutions for diverted tokamaks

Divertor effects on the particle confinement time in a tokamak plasma are investigated by modelling of particle, momentum and energy transports in the divertor plasma and particle transport in the main plasma, with a simplified model for neutral-particle transport. The divertor plasma has three equilibrium states in a limited range of the ion flux entering the divertor. It is found that the existence of the three equilibrium states might lead to mode conversion of the particle confinement characteristics during additional heating.

Mechanism of unipolar arcs in tokamaks

An experimental study of arcing is made, by making use of time-resolved optical and electrical measurements. Photomicrographs are made, and surface features of two parallel rail limiters are examined visually. Arc phenomena are accompanied by X-ray emission from the limiter. Many arc tracks are observed on the limiter surface which is exposed to both runaway electrons and the scrape-off plasma, but hardly any arc is observed on the limiter surface exposed to scrape-off plasma only. The most probable cause of unipolar arcing is the fact that runaway electrons supply a sheath potential sufficient to produce arc tracks on the electrically isolated target or limiter in the tokamak scrape-off layer. This model explains well the unipolar arcing observed in the non-equilibrium phases such as the current rise and end phases as well as during plasma disruptions.