Design and R&D for the C-Mod Outer Divertor Upgrade

Operational requirements and research considerations make a high-temperature, toroidally continuous outer divertor an important upgrade to the Alcator C-Mod tokamak. Leading edge melting of tiles, non-uniform heat loads, large electromagnetic forces, and localized impurity sources limit the performance of bulk plasmas. These issues can be addressed by the installation of a well-aligned, toroidally continuous outer divertor. Additionally, future long pulse operation will cause the temperature of the outer divertor to reach bulk temperatures as high as 500 - 600 °C. This future operational requirement combined with the strong temperature dependence of plasma surface interactions (especially fuel retention), makes a controllable, high-temperature outer divertor desirable and necessary. The motivation, criteria, design, and R&D for the upgrade are discussed below.