Lessons learned from the design of ITER internal components

The traditional design sequence for a fusion device starts from desired plasma performance and a coil set that optimizes formation of the required plasma equilibrium. Designers then reserve space for internal components, vessel, and cryostat. The ITER process has taught us that this traditional sequence should be revised. The discovery of convective transport in the plasma scrape-off layer has greatly increased heat flux to the first wall (FW), implying that power is flowing along field lines. Shaping of the FW's plasma-facing-surface and divertor components is now a critical design consideration and constraint. Plasma duration has increased to the point that active cooling of internal components is required. Ever more complex plasma diagnostics require complex openings to view the plasma and complex routing of items like cables behind the blanket. As we move toward the next generation of fusion machines, there is a need for many engineering diagnostics to monitor the operating state of actively cooled components. Internal coils for ELM, resistive wall mode, and plasma rotation control further complicate the region between blanket modules and vessel. Adding new components between the vessel and blanket removes material that is either shielding external components or breeding tritium in a reactor. Traditionally, such additional internal components are added during later design phases when space has been fixed. Using design by analysis during the conceptual design phase allows the space required for internal components to be more accurately defined so balanced trade-offs among magnets, vessel, and internal components are made. The result is a concept that is easier to integrate and does not disproportionately constrain later design phases for any one system.