Noble gas impurity balance and exhaust model for DIII-D and JET

Experiments to study the exhaust of noble gases (helium and neon) with cryo pumping in DIII-D (Advanced Divertor Program (ADP) configuration) and in JE T (Mk1 configuration) found significant differences in the global exhaust r ate of helium, while efficient neon exhaust was observed in both machines, An attempt to better understand the basic processes governing the exhaust o f noble gases in ELMy H-mode with cryopumping has been undertaken. Since di vertor geometries investigated in the DIII-D and in the JET cases have sign ificant differences, a comparative modeling study has been undertaken using the MIST core impurity transport code and the b2.5 time-independent divert or transport code. Photodiode measurements are used to determine ELM freque ncy, and charge-exchange recombination (CER) measurements are compared with the MIST ELM model to evaluate transport coefficients in the core plasma. A significant reduction in the anomalous diffusivities is found for the non -ELM component of radial transport without the need for a pinch velocity, a nd the model provides a more coherent description than the conventional ELM -averaged approach. Sensitivity to boundary conditions has been studied thr ough establishment of a database of divertor enrichment cases using b2.5. ( C) 1999 Elsevier Science Inc. All rights reserved.