Discerning Why Some High-Z UFOs in C-Mod Caused Immediate Disruptions while Others Did Not

About 25% of disruptions during the current flattop on Alcator C-Mod were attributed to UFO disruptions, a type of disruption triggered by a rapid injection of high-Z material. Overheated molybdenum tiles were typical sources of UFOs on C-Mod, causing disruptions on ~5 ms timescales, too fast for effective mitigation. Reinjected deposits of sputtered tungsten can also cause UFOs, as seen on WEST. While some tokamaks experience few UFO disruptions, SPARC is expected to have high heat fluxes, high lifetime integrated particle fluence, and tungsten first wall and divertor tiles, so unmitigable UFO disruptions are a concern. To better understand UFO disruptions, we assemble a database of high-Z impurity injections during current flattop on C-Mod using vacuum ultraviolet spectrometry, soft x-ray, and bolometry signals. We find that while some high-Z injections triggered immediate disruptions, many were associated with disruptions on ~20 ms timescales or no disruptions at all. To discern why, we identify correlations between the time from injection to thermal quench (if present) and parameters such as the plasma thermal energy, power to the divertor, and strikepoint location and velocity. These correlations should help inform UFO disruption prediction on SPARC in future work.