Failure analysis of EAST Lower divertor

Full text: Experimental Advanced Superconducting Tokamak (EAST) has equipped the lower divertor since 2008. Lower divertor comprises total 2952 graphite tiles, 96 heat sinks and 48 support frames. With the length of plasma pulse in H-mode reached 100s thermal power deposited on the divertor has become very larger. Lots of hot spots had been observed on the lower divertor while plasma discharge. During plasma discharge tens graphite tiles damaged and a lot of dust of carbon made the plasma quench at last. By the analysis there are three main failure modes for graphite tiles: crack, erosion and release. A few graphite tiles cracked into two or three pieces and most graphite tiles had been erosion on the corners, edges and front surfaces. Some graphite tiles have no any damage on them but their position changed. Here the damage process has been researched by simulation and experiments to find the failure mechanism. Most failures are because of overheating. When the high heat flux on anywhere of graphite tiles expired the limits of lower divertor the erosion occurred some failures are because of electromagnetic force. When electromagnetic force on the graphite tiles was larger than the strength of graphite the crack occurred. Other failures are because of both thermal power and electromagnetic force. When the graphite tiles were heating the preload to mount the tiles reduced largely. The electromagnetic force would pull or push tiles to make the position of graphite tiles changed again and again and then they became released from heat sink. The failure analysis here helped to realize the performance limits of EAST lower divertor and helped to optimize or upgrade it in future. (author)