Physics studies on helical confinement configurations with l=2 continuous coil systems

Physics studies have been carried out to optimize l=2 heliotron/torsatron configurations having continuous coil systems for the Large Helical Device (LHD), focusing on the beta orbit-divertor compatibility requirement, neoclassical and anomalous transports, bootstrap and Ohkawa currents, and divertor layer analysis. The optimal m number is found to be ∼ 10 for the LHD system on the basis of three physics criteria: MHD stability (β > or approx. 5%), particle orbit confinement (loss cone free radius rL > or approx. 1/3ap (plasma minor radius)) and a clean divertor layer (divertor-wall clearance Δdw > or approx. 3 cm). The pitch parameter of the helical coil γc is determined mainly from the beta-divertor condition. For the detailed prediction of LHD plasma parameters, 2 1/2-D equilibrium-transport simulations including empirical or drift wave turbulence models are carried out, which reveal that the global confinement time is sensitive to the edge electron anomalous transport, although ripple loss through the ion channel is dominant in the plasma core. When ripple transport optimized configurations are adopted, the bootstrap current is increased; however, the Ohkawa current may be utilized to cancel this current. The divertor layer study clarifies the peculiar magnetic properties of thin curved divertor layers and suggests the effectiveness of helical divertors. From these physics considerations within the related engineering constraints, a final standard LHD configuration (m=10, γc=1.25, α*=0.1, where m, γc and α* are toroidal mode, pitch and pitch modulation parameters of the helical coil, respectively) is determined. (author). 14 refs, 4 figs