Graphite limiters use in TFR

Abstract

Graphite limiters have been used in TFR. They prove to be satisfactory for tokamak operation, when fixed as tightly bolted tiles. However, strong efforts must be undertaken to cope with the problems related to their large hydrogen content as well as their mediocre surface structure. This has led us to install a limiter bakeable to 600°C in TFR. Radiofrequency antennae protection had also to be changed from inconel or titanium to graphite to avoid metallic redeposition on the main limiter. Conditions of recycling differ from those met with inconel limiters. The impurity content of the plasma is now dominated by carbon and oxygen. Moreover, large desorption occurs during the ICRH pulse. Strong toroidal asymmetries were noticed as far as the radiation and charge exchange power losses are concerned, with a large enhancement in the vicinity of the limiter. A high neutral pressure (one order of magnitude higher than elsewhere) is created around the limiter. A global energy balance of the discharge was deduced from both bolometric, spectroscopic and limiter surface temperature measurements. The radiative loss is increased when carbon limiters are used and may exceed 70% of the total input. ICRH experiments led to a noticeable power deposition in the scrape-off layer, independent of the mechanism of wave absorption in the bulk.