Physics Considerations in the Design of NCSX

Abstract

Compact stellarators have the potential to make steady-state, disruption-free magnetic fusion systems with β ~ 5% and relatively low aspect ratio (R/〈a〉 < 4.5) compared to most drift-optimized stellarators. Magnetic quasi-symmetry can be used to reduce orbit losses. The National Compact Stellarator Experiment (NCSX) is designed to test compact stellarator physics in a high-beta quasi-axisymmetric configuration and to determine the conditions for high-beta disruption-free operation. It is designed around a reference plasma with low ripple, good magnetic surfaces, and stability to the important ideal instabilities at β ~ 4%. The device size, available heating power, and pulse lengths provide access to a high-beta target plasma state. The NCSX has magnetic flexibility to explore a wide range of equilibrium conditions and has operational flexibility to achieve a wide range of beta and collisionality values. The design provides space to accommodate plasma-facing components for divertor operation and ports for an extensive array of diagnostics.

Component Manufacturing Development for the National Compact Stellarator Experiment (NCSX)

“…For the modular coils, the process began with studies of nonoptimized conceptual designs to develop engineering constraints such as the conductor bend radii, coil to coil spacing, and coil to plasma spacing, coil twisting, current density, and neutral beam access requirements. An optimized modular coil design was then developed through an iterative physics / engineering optimization process using a specifically developed suite of computer codes [1], which optimized the coil geometry to achieve targeted plasma physics properties, while satisfying engineering constraints. The resulting modular coil set, consisting of six each of three coil types, is shown in Figure 4.…”

Magnetic structure at the edge of a compact stellarator (NCSX)

“…By choosing a magnetic field magnitude which has axisymmetry, the stellarator configuration can have good tokamak-like transport while still retaining a vector magnetic field that is 3D. Using this quasi-axisymmetry (QAS), a low aspect ratio configuration can be achieved which combines the best features of both the stellarator and tokamak, and is the basis for the National Compact Stellarator Experiment (NCSX) [1]. The 3D plasma geometry then allows access to degrees of freedom to achieve plasma properties that are not available in 2D configurations, such as the stabilization of instabilities without close fitting conducting walls or feedback.…”

A Compact Quasi-axisymmetric Stellarator Reactor

“…In particular, compact, quasi-axisymmetric devices, which combine the feature of good particle orbits of a tokamak and the potential of being able to operate with MHD stable plasmas that are resistant to disruption at high pressure afforded by the three-dimensional shaping, open a new window of opportunity for confining steady-state, high β plasmas in magnetic fusion. A low aspect ratio (A=4.5), proof-of-principle device, NCSX, the National Compact Stellarator Experiment, is being designed and operation is expected to commence in 2007 [1] [2]. In conjunction with the development of NCSX, a reactor studies project, ARIES-CS, has been initiated to examine the competitiveness and the critical issues of compact stellarators as power producing reactors.…”