The effect of detachment on carbon divertor erosion/redeposit ion in the DIII-D tokamak

An operational scenario has been demonstrated on the DIII-D tokamak where t he graphite covered divertor is free of net erosion. Reduction of divertor carbon erosion is accomplished using a low temperature (detached) divertor plasma that eliminates physical sputtering. Likewise. the carbon influx ari sing from chemical erosion is found to be very low in the detached divertor , although Uncertainties exist concerning chemical erosion yield due to the unknown effect of detachment on hydrocarbon transport. Near strike point r egions, the rate of carbon deposition is approximate to3 cm/burn-year, with a corresponding hydrogenic co-deposition rate greater than 1 kg/(m(2) burn -year): rates which are problematic for steady state fusion reactors. The c arbon net deposition rate in the divertor is consistent with carbon arrivin g from the core plasma region. Carbon ion influx from the main wall is meas ured to be relatively large in the high density detached regime and is of s ufficient magnitude to account for the deposition rate in the divertor. Div ertor redeposition is, therefore, determined by non-divertor erosion and tr ansport. Despite the success in reducing divertor erosion on DIII-D with de tachment, no significant reduction is found in the core plasma carbon densi ty, illustrating the importance of non-divertor erosion and the complex cou pling between erosion/re-deposition and impurity plasma transport.