Abstract
High-Z impurities released from plasma-material interactions have been shown to limit the performance of fusion plasmas, and understanding these impurity transport mechanisms throughout the plasma scrape-off layer is a major challenge. Presented herein is a study of tungsten (W) erosion and transport by uniquely measuring absolute quantities of isotopic W in order to determine the source of natural and enriched 182W isotopes that have traveled throughout the tokamak discharges on the DIII-D National Fusion Facility at General Atomics. Two primary analysis methods have been implemented to characterize this W on graphite collector probes that were inserted into DIII-D’s outboard midplane. Results from experiments using Rutherford backscattering spectrometry (RBS) have measured W particle areal densities down the centerline of the probes as high as 6E14 atoms/cm2 with a detection limit of 1E12 atoms/cm2. Laser ablation inductively coupled plasma mass spectrometry (LAMS) has confirmed the elemental trends found with RBS and has provided additional insight into collector probe surface profiles. Two-dimensional elemental and isotopic maps from LAMS are used to reveal new collector probe features and further refine the source of collected W. Variations in isotopic profiles and total W content are coupled to (a) the face of the probe being analyzed, (b) the dimensions of the probe, and (c) the plasma pulse parameters that were used during probe exposure. These results provide one-of-a-kind empirical evidence that is now being utilized for validation of tokamak impurity transport through theoretical models and in codes such as 3D-LIM and OEDGE.
