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

An operational scenario has been demonstrated on the DIII-D tokamak where the 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 arising 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 regions, the rate of carbon deposition is ∼ 3 cm/burn-year, with a corresponding hydrogenic co-deposition rate greater than 1 kg/(m2 burn-year); rates which are problematic for steady state fusion reactors. The carbon net deposition rate in the divertor is consistent with carbon arriving from the core plasma region. Carbon ion influx from the main wall is measured to be relatively large in the high density detached regime and is of sufficient magnitude to account for the deposition rate in the divertor. Divertor redeposition is, therefore, determined by non-divertor erosion and transport. Despite the success in reducing divertor erosion on DIII-D with detachment, no significant reduction is found in the core plasma carbon density, illustrating the importance of non-divertor erosion and the complex coupling between erosion/re-deposition and impurity plasma transport. (author)