We report the results of operation of Alcator C-Mod with all high-Z molybdenum Plasma Facing Component (PFC) surfaces. Without boron-coated PFCs energy confinement was poor (HJTER,89 1) due to high core molybdenum (nMo/neSO. 1%) and radiation. After applying boron coatings, nMJn was reduced by a factor of 10-20 with H1TERJ9 approaching 2. Results of between-discharge boronization, localized at various major radii, point towards important molybdenum source regions being small, outside the divertor, and due to RF- sheath-rectification. Boronization also has a significant effect on the plasma startup phase lowering Zcff, radiation, and lowering the runaway electron damage. The requirement of low- Z coatings over at least a fraction of the Mo PFCs in C-Mod for best performance together with the larger than expected D retention in Mo, give impetus for further high-Z PFC investigations to better predict the performance of uncoated tungsten surfaces in ITER and beyond. ...
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