ITER physics: Design and operations

Physics considerations are central to the ITER Engineering Design Activity and impact it in several ways. First, the ITER size, shape, and magnetic field strength are chosen via a judicious balance of cost, technological constraints, and extrapolation of confinement data from present tokamak experiments by a process mostly empirical in nature but constrained to obey fundamental principles of plasma physics. Present experiments indicate that resistive modes and collisionality govern the beta-limit for discharges of the ITER shape and collisionality. The resulting machine parameters yield a design that, by intent, has modest margins with respect to obtaining the desired goal of steady, controllable thermonuclear burn. Operational responses, especially driven-burn operations, exist that will likely enable ITER to fulfill its mission of 1-1.5 GW of sustained fusion power if shortfalls in projected performance occur. Second, the scale of ITER presents especially difficult challenges in the areas of divertor and disruption effects, which are generic to tokamak fusion reactors. It is the role of ITER to meet these challenges and demonstrate reliable, continuing operations of a tokamak at the reactor scale. Third, the MR design strives to accomodate a variety of advanced tokamak operations. Plasma shaping capabilities are comparable to those of ASDEX-U, the principal constraint being that of a single-null divertor. A segmented central solenoid is under consideration as a potential design improvement that could augment plasma shaping capabilities and reduce cost. Site power requirements are set by a criterion that calls for the ability to apply almost the full auxiliary heating power (70 MW out of 100 MW) while ramping up the plasma current. NBI is planned to provide plasma rotation appreciably in excess of the natural diamagnetic rotation velocity