Characterization of UFOs on Alcator C-Mod and WEST to inform SPARC operation

High-Z metal walled tokamaks can be prone to UFOs— rapid injections of high-Z metal— that sometimes lead to disruptions. Overheated molybdenum divertor tiles were typical sources of UFOs on C-Mod, causing disruptions on ~5 ms timescales, too fast for effective mitigation. On WEST, thick deposits of sputtered tungsten have been seen to develop near the inner strike point [1]. These deposits can overheat and flake off into the plasma, causing frequent disruptions on ~100 ms time scales [2]. SPARC will have tungsten walls and a high power density in the scrape-off layer, necessitating UFO prevention and mitigation strategies, such as divertor detachment, strike point sweeping, and real-time disruption warning systems. To better understand conditions leading to UFO disruptions, we assembled an automatically labeled database of UFO injections on C-Mod and identified parameters correlating with UFO appearance and time-to disruption, utilizing DisruptionPy [3]. We found that higher thermal pressures were linked to more frequent UFOs and that a global radiative cooling time separated UFO classes defined by their time-to-disruption, which was verified on UFO data from the WEST 2023 high fluence campaign [2]. Results were reviewed in the context of UFOs on other metal walled devices and mapped to SPARC.

[1] Fedorczak et al, NME, 2024

[2] Gaspar et al, NME, 2024

[3] Trevisan et al, Zenodo, 2024, 10.5281/zenodo.13935223