Abstract
Future tokamak devices are envisioned to utilize a high-Z metal divertor with tungsten as the
leading candidate. However, tokamak experiments with tungsten divertors have seen significant
detrimental effects on plasma performance. The DIII-D tokamak presently has carbon as the
plasma facing surface but to study the effect of tungsten on the plasma and its migration around
the vessel, two toroidal rows of carbon tiles in the divertor region were modified with high-Z
metal inserts, composed of a molybdenum alloy (TZM) coated with tungsten. A dedicated two
week experimental campaign was run with the high-Z metal inserts. One row was coated with
tungsten containing naturally occurring levels of isotopes. The second row was coated with
tungsten where the isotope 182W was enhanced from the natural level of 26% up to greater than
90%. The different isotopic concentrations enabled the experiment to differentiate between the
two different sources of metal migration from the divertor. Various coating methods were
explored for the deposition of the tungsten coating, including chemical vapor deposition,
electroplating, vacuum plasma spray, and electron beam physical vapor deposition. The coatings
were tested to see if they were robust enough to act as a divertor target for the experiment. Tests
included cyclic thermal heating using a high power laser and high-fluence deuterium plasma
bombardment. The issues associate with the design of the inserts (tile installation, thermal stress,
arcing, leading edges, surface preparation, etc.), are reviewed. The results of the tests used to
select the coating method and preliminary experimental observations are presented.
