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.

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A pulse of methane gas is injected into the DIVA tokamak. The major portion of the injected impurity is shielded by the outer edge plasma, i.e. only 20% of the injected carbon are introduced into the discharge. After radial penetration of the injected carbon has been accomplished, accumulation doubles and a new steady state is sustained over the period of proton confinement. The transient radial diffusion process of the carbon is compared with calculations using an impurity transport code. The behaviour of the carbon impurity in the whole minor radius is well described by assuming perfect re-cycling at the gold-plated shell surface and using neoclassical diffusion superposed by anomalous diffusion of protons. It is shown that spreading caused by anomalous diffusion is important. The calculated result indicates that the carbon ions are lost dominantly in a He-like state.

Results from the JFT−2a (DIVA) experiments made with a separatrix magnetic surface are described. The main conclusions are: (1) A separatrix magnetic surface is stably located inside the material surface. (2) A plasma enclosed in a separatrix magnetic surface is similar to that of a conventional tokamak as far as the magnetohydrodynamic properties are concerned. (3) Measured parameters of the main plasma column are consistent with those expected from a conventional tokamak. (4) The electron density and temperature in the scrape-off layer are about ten times less than the values at the center of the main plasma column. (5) Heat and particle fluxes to the divertor region are axisymmetric and several times less than those of the total loss fluxes from the main plasma column. (6) Runaway electrons are well guided to the divertor region.

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