Physics issues in the design of high-beta, low-aspect-ratio stellarator experiments

High-beta, low-aspect-ratio ("compact") stellarators are promising solution s to the problem of developing a magnetic plasma configuration for magnetic fusion power plants that can be sustained in steady state without disrupti ng. These concepts combine features of stellarators and advanced tokamaks a nd have aspect ratios similar to those of tokamaks (2-4). They are based on computed plasma configurations that are shaped in three dimensions to prov ide desired stability and transport properties. Experiments are planned as part of a program to develop this concept. A beta = 4% quasi-axisymmetric p lasma configuration has been evaluated for the National Compact Stellarator Experiment (NCSX). It has a substantial bootstrap current and is shaped to stabilize ballooning, external kink, vertical, and neoclassical tearing mo des without feedback or close-fitting conductors. Quasi-omnigeneous plasma configurations stable to ballooning modes at beta = 4% have been evaluated for the Quasi-Omnigeneous Stellarator (QOS) experiment. These equilibria ha ve relatively low bootstrap currents and are insensitive to changes in beta . Coil configurations have been calculated that reconstruct these plasma co nfigurations, preserving their important physics properties. Theory- and ex periment-based confinement analyses are used to evaluate the technical capa bilities needed to reach target plasma conditions. The physics basis for th ese complementary experiments is described. (C) 2000 American Institute of Physics. [S1070-664X(00)94905-X].