High heat loads producing large size dust particles in Alcator C-Mod

Rounded dust particles such as spheres and splashes were collected in the Alcator C-Mod tokamak. They are the result of high heat loads applied on the leading edges of plasma facing components. Disruptions destabilize these regions and molten material droplets are emitted through the vacuum chamber. The formed dust particles are characterized by an average size larger than in any other metal tokamak. To reduce their production, a slight rotation of the lower outer-divertor sectors was done in order to shadow every leading edge by the upstream neighboring sector. As expected, less dust particles were produced. Introduction Plasma facing units in the ITER divertor will be formed with chains of tungsten monoblocs (MB). One identified key issue is linked to the MB misalignment. Under cycles of heat loads and transient high heat loads, the MB leading edges could be melted and induce emission of molten W splashes and droplets [1-4]. Resulting surface damage could compromise plasma operation by changing the W mechanical structure and by reducing the MB lifetime. One effect of the tile misalignment was evidenced in the full-metal tokamak, Alcator C-Mod [5]. During plasma operation, camera videos have shown an over-light emission of various leading edges of plasma facing components (PFCs). These over-heated regions were destabilized during disruptions and led to the emission of molten material droplets through the vacuum chamber. Resulting typical rounded dust particles such as splashes and spheres were collected after 2007 and 2015 plasma campaigns. Their characteristics are presented in this paper as well as the solution adopted to reduce their production. Dust collection in Alcator C-Mod and analyses Alcator C-Mod is a molybdenum (Mo) tokamak divided in 10 toroidal sectors (named AB, BC .... JK). A toroidal row of tungsten (W) tiles was inserted in 2007, in the strike point region 45 EPS Conference on Plasma Physics P4.1026