H. Ohtsuka

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.

Graphite surface erosion by 100 keV helium and hydrogen bombardment

Abstract Surface erosion in pyrolytic graphite by 100 keV 4 He + and 200 keV H + 2 ion bombardment has been observed by scanning electron microscopy. The particle fluence ranged from 1 × 10 17 to 5 × 10 18 particles/cm 2 . Although the surface is eroded at 1 × 10 17 particles/cm 2 in helium bombardment, it is not eroded so heavily even at 5 × 10 17 particles/cm 2 in hydrogen bombardment. In helium bombardment flaking is significantly observed at 1 × 10 18 particles/cm 2 , and a cone structure appears at 5 × 10 18 particles/cm 2 , which is produced after the first cover flakes off completely. In hydrogen bombardment at 1 × 10 18 particles/cm 2 , many circular blisters are formed which are sputtered off at 5 × 10 18 particles/cm 2 . The surface roughness of the target also affects the erosion.

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.

Experimental results on boundary plasmas, resulting surface interactions and extrapolation to large fusion devices

Abstract Experimental results on boundary plasmas, impurity origins, and impurity transport in the boundary plasmas are summarized in relation to impurity control and wall erosion. Important parameters in the boundary plasma are surveyed in present-day tokamaks and the relation between the boundary plasma and the main plasma is induced. It is shown that the important parameters of the boundary plasma in a large device can be calculated from the assumed main plasma parameters. Impurity release from the first wall is governed mainly by the boundary plasma. Ion sputtering is shown to be the dominant process in the stable phase of a discharge. In addition to these processes, arcing, sputtering by charge-exchange neutrals etc., which may become serious in a large device, are considered. All released impurities do not flow into the main plasma, even without a divertor. Impurity contamination in the main plasma is governed by the impurity transport and atomic processes in the boundary plasma as well as by the transport in the main plasma and by the impurity production. Therefore, the study of impurity transport in the boundary layer is important and is summarized. It is shown that the impurity transport in the boundary layer is well simulated by Monte-Carlo calculation. Combining these results, impurity problems in a large device are discussed.

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].

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.

Probe measurements in the scrape-off layer of a Tokamak

A Langmuir probe is used to study the plasma behaviour in the scrape-off layer of JFT-2a, (a Tokamak with an axisymmetric divertor). The measurements reveal particle fluxes to the divertor and the wall as well as cross-field profiles of density and potential in the scrape-off layer. On the basis of these measurements, the cross-field diffusion coefficient in the scrape-off layer and the divertor efficiency are estimated. The existence of the magnetic limiter is also verified by the method.