INVESTIGATION OF THE FLUXES TO A SURFACE AT GRAZING ANGLES OF INCIDENCE IN THE TOKAMAK BOUNDARY

This paper describes an experimental investigation of the effects of grazing angles of incidence on the fluxes of power and particles to a surface inserted into the boundary of the DITE Tokamak. Related thermographic data from the TFTR Tokamak are also presented. A summary is given of the important issues relating to angle of incidence which are relevant to large Tokamaks such as JET, NET and ITER and which require experimental confirmation. An array of Langmuir probes has been constructed to carry out experiments as a function of angle on DITE. Results from this Tilting Probe Array (TPA) are described, which show how the ion saturation current, electron saturation current, floating potential and the fitted electron temperature vary with surface angle. It is shown that when magnetic field is at grazing angles of incidence to the surface a conventional interpretation of the probe characteristics fails completely. Power flux distributions from infra-red thermography of the TFTR moveable limiter are presented, which show a substantially higher heat flux at the tangency point than is calculated conventionally. This is shown to agree with the analysis of the TPA thermocouple data, thus demonstrating the relevance of the results to large Tokamaks. The cross-field flux measured experimentally is about an order of magnitude higher than that predicted by the theoretical models considered. Spectroscopic observations of the TPA in the visible region are described, which measure the angular dependence of the recycled fluxes and carbon impurity production rate.