Tungsten and Disruptions in SPARC*

The divertor surfaces and first wall in SPARC will be bulk tungsten or tungsten heavy alloy. Based on empirical evidence from Alcator C-Mod, there is concern that this could result in a significant fraction of disruptions triggered by sudden injections of tiny amounts of tungsten from overheated tile corners/edges (UFO's). C-Mod's first wall and divertor was bulk molybdenum, a high-Z metal similar to tungsten. About 25% of disruptions on C-Mod occurred with no detectable change of any measured plasma parameters more than 2-3 ms before the thermal quench. This is not enough warning time for any existing mitigation system to be effective. Since SPARC, like C-Mod, will have high plasma thermal density, high current density, high strikepoint heat flux, and a high-Z metallic first wall, there is concern that it too could have a significant number of UFO-triggered, unmitigatable disruptions. The ITPA MHD group initiated a joint study of UFO-triggered disruptions in tokamaks with high-Z first walls, and one conclusion is that careful engineering design to eliminate gaps between tiles and leading edges, and to ensure mechanical alignment even after repeated disruptions, could greatly reduce the likelihood of UFO-triggered disruptions, even in machines with high strikepoint power flux.