Thermal Behaviour of Deposited Carbon Layers on the Neutraliser Plates in Tore Supra

Abstract

The temperature measured on the surface of particle neutralisers of Tore Supra, i.e. the target plates under the toroidal pump limiter that channel particles into the pumping ducts, is significantly higher than expected because of thick carbon deposits formed on those plates. With growing deposit thickness their temperature and cooling time constants increase. Lower hybrid current drive heating and gas puffing seem to enhance the layer growth. The deposits form at first isolated hillocks giving rise to an enhanced luminance in the near infrared range. Later on, they may form continuous films (with black-body like spectra) which may flake off again distorting the spectra. The thermal behaviour of thick continuous films can be described by a radiative model. The conducted power flux density to the neutralisers for which this model is in accordance with the measurements is of the order of 0.1 MW/m2 in ohmic discharges. This requires characteristic scrape-off layer decay lengths of 7 to 8 mm as far as 5 cm outside the last closed magnetic flux surface. The thermal diffusivities deduced with this model from observations in Tore Supra and from laboratory experiments show values 2 to 3 orders of magnitude below the values characteristic for carbon fibre composites.