Physics considerations in the design of NCSX

The National Compact Stellarator Experiment (NCSX) is being designed to study the physics of high-beta, quasi-axisymmetric (QA) stellarators. Its goals are to develop physics solutions for disruption avoidance and steady-state operation in magnetic fusion systems, and to broaden understanding of 3D plasma physics. The NCSX is a hybrid which uses both helical coils and plasma current to provide rotational transform, 3D plasma shaping to stabilize limiting MHD modes at high beta without the need for a conducting wall or feedback, and quasi-symmetry (low effective helical ripple along the magnetic field) to achieve transport-related properties as good as those of tokamaks. The NCSX design parameters (size, magnetic field strength, heating power, and pulse length) and the geometry of the main components (coils and vacuum vessel) are determined by physics considerations. These include: MHD stability at 4% beta with bootstrap current, low helical ripple (0.1% on axis, 2.7% at the edge), good magnetic surfaces, access to high beta states, control of impurities and neutral recycling, flexibility to vary the plasma's physics properties, and access for diagnostics. (author)