An experimental investigation of chemical sputtering of carbon in a tokamak discharge

An experiment has been undertaken to see if chemical sputtering of carbon can be directly observed under tokamak operating conditions. The test material consists of a well instrumented graphite probe limiter which can be heated up to 900°C under steady state conditions and up to >1200° C with additional plasma heating during a discharge. The carbon released from the limiter is monitored spectroscopically by using a visible spectrometer and a CCD camera with interference filters. Measurements have been made from 20°C to 700°C in steady state conditions and up to 1200°C with plasma heating. Within experimental error, there is no change in the carbon production rate over the whole temperature range. The inferred chemical sputtering yields are low, in agreement with beam simulations that indicate reduced yields at high incident current densities, low energies and in the presence of metal surface contaminants.