Investigations on dynamic wall recycling from tungsten divertor material in DIII-D *

Several tungsten samples were simultaneously exposed in the DIII-D lower divertor to ELMy low power H-mode attached plasmas using the Divertor Material Evaluation System (DiMES). Atomic and molecular deuterium recycling fluxes from tungsten samples were monitored through the measurements of spatially and ELM-resolved atomic Dα and molecular Fulcher-band emission [1]. The net variations of the molecular and atomic deuterium recycling fluxes observed above the various tungsten samples and the surrounding graphite head are similar during ELMs. Those results indicate that the nature of the wall material has virtually no effect on dynamic fuel recycling in the low power plasma conditions encountered in those experiments. In particular, two tungsten samples were pre-exposed to helium particles with an incidence ion energy about 60 eV and a fluence about  resulting in the formation of helium nano-bubble layers, suggesting that micro-structural defects weakly impact dynamic wall recycling in the considered plasma conditions. Modeling of deuterium recycling during an ELM from pristine and pre-loaded tungsten materials is performed with the cluster dynamics code Xolotl and assessed against experimental observations. Finally, we briefly discuss the observed toroidal shift of the molecular deuterium emission clouds with respect to the location of the tungsten samples, which may provide insights into the toroidal transport of deuterium molecules. 

 

[1] Bykov, I., et al. Physica Scripta 2020.T171 (2020)