Comparison of high-Z triggered disruptions in metal-walled tokamaks

More than 25% of disruptions on Alcator C-Mod were triggered by injections from molybdenum tiles in the divertor. These so-called 'UFO' events triggered thermal quenches in just 2-5 ms, which precluded the possibility of successfully initiating any avoidance or mitigation actions. An ongoing ITPA joint study of other tokamaks with high-Z plasma-facing surfaces seems to show that C-Mod was unique in this regard. Some high-Z tokamaks do not experience UFO injections in significant numbers, while others have high-Z injections, followed by MHD leading to disruptions, but the disruptions occur hundreds of milliseconds after the injection, and therefore mitigation actions are potentially possible. Possible reasons for the observed differences include the precision of pre-campaign tile alignment, the ability to maintain tile alignment throughout campaigns, the prevalence of open diagnostic ports, and perhaps differences between molybdenum and tungsten, but a definitive explanation has not been determined yet. The source of high-Z injections is usually overheated tiles, but recent long-pulse operation on WEST has shown that build-up of loosely bound layers of redeposited tungsten can also be a significant source. These observations clearly have important implications for ITER, SPARC, and future tokamaks with high-Z plasma-facing components and high power densities.