Operation of Alcator C-Mod with high-Z plasma facing components and implications

Studies of potential plasma facing component (PFC) materials for a magnetic fusion reactor generally conclude that tungsten is the best choice due to its low tritium (T) retention, capability to handle high heat fluxes with low erosion, and robustness to nuclear damage and activation. ITER [F. Perkins , Nucl. Fusion 39, 2137 (1999)] may operate with all tungsten PFCs to provide the necessary operational experience for a reactor. Alcator C-Mod [I. Hutchinson , Phys. Plasmas 1, 1511 (1994)] operates with molybdenum (Mo) high-Z PFCs, which have very similar properties to tungsten. The experiments described herein have provided a unique comparison of operation with or without in situ boron coatings applied to the molybdenum PFCs; the latter are likely most relevant to ITER and beyond. ICRF-heated H-modes were readily achieved without boron coatings although the resultant enhancement in energy confinement was typically small (H-ITER,H-89 similar to 1). Molybdenum concentrations, n(Mo)/n(e), rise rapidly after the H-mode transition up to 0.1%, cooling the plasma by line radiation, reducing energy confinement, and/or causing a back H/L transition. Surprisingly, the primarily molybdenum PFC surfaces retain 3.5-5.0x10(20) of injected D-2 molecules per discharge, corresponding to 50% of the injected gas. Plasma current disruptions, both randomly occurring over the course of a day, or planned, reduce the retained D long term. After applying boron coatings, n(Mo)/n(e) was reduced by a factor of 10-20 with H-ITER,H-89 approaching 2. A world-record volume-average plasma pressure of 1.8 atm at 5.4 T was achieved at the ITER normalized beta. The effects of each boronization are found to be limited in time, correlated to time-integrated input energy. Intra- and inter-discharge boronization techniques have been developed with the latter being the most successful. This initial study indicates that a low-Z coating over at least a fraction of the Mo PFCs in C-Mod is needed to reduce core molybdenum levels and achieve the best energy confinement. This, together with the larger than expected D retention, raises concerns for the performance of uncoated tungsten surfaces in ITER and beyond. (c) 2006 American Institute of Physics.