Operation of Alcator C-Mod with High-Z Plasma Facing Components With and Without Boronization

High-Z Plasma Facing Components (PFCs) are considered necessary for reactors due to their low tritium (T) re tention, capability to handle high heat fluxes with low erosion, and robustness to nuclear damage and activation. Recent Alcator C-Mod experiments provide divertor tokamak operational experience util izing all-metallic solid high-Z (molybdenum) PFCs, comparing boronized and unboronized surfaces. To assess the influence of boron coatings in plasma performance, previously deposited boron was removed from all PFC surfaces before the 2005 campaign. Subsequently, ICRF-heated H-modes were readily achieved, but energy confinement enhancements over L-mode were small. Molybdenum radiation is implicated. After boronization the Mo density was reduced by a factor of more than 10, and energy confinement doubled. The positive effects of boronization wear off, correlated with ICRF input energy. On the basis of experiments which included between-shot boronization, as well as from visual inspection, it appears that the Mo affecting the core comes from a small fraction of PFC surfaces, where the boron coating erodes rapidly. The evidence points to sputtering from RF sheath enhancement as the most important erosion mechanism. Plasma Performance with all molybdenum plasma facing surfaces Alcator C-Mod is unique among diverted tokamaks in its exclusive use of solid high-Z (molybdenum) plasma facing components.1 It is likely that ITER will require the substantial use of high-Z PFCs (probably tungsten), and in fusion power plants (DEMO and commercial), low Z PFCs (particularly graphites) may be totally precluded because of tritium retention and material degradation due to radiation damage. 2 Other than on C-Mod, worldwide experience with high Z PFCs in diverted tokamaks is very sparse. 3 On C-Mod, prior to the 2005 campaign, all low Z secondary limiters (BN protection tiles on the ICRF antennas) were replaced with Mo, and boron buildup from years of operation with boronization was removed as thoroughly as possible. H-mode performance with high power