Fusion Science and Technology / Volume 54 / Number 3 / October 2008 / Pages 771-786
Technical Paper / Aries-Cs Special Issue
The critical issue of divertor configuration for heat and particle flux control in a conceptual ARIES compact stellarator (CS) reactor is addressed. The goal is to determine a divertor location and geometry with a peak heat load of not more than 10 MW/m2 for a CS equilibrium based on the configuration to be used in the NCSX experiment, optimized for high beta (6.4%) and designed for low alpha-particle power loss fraction (5%). The surface heat flux on the target has three components: thermal particles, lost energetic alphas, and radiation from the core and the scrape-off layer. The first two components are dominant and their magnitudes can be comparable. To maintain a tritium-breeding ratio of 1.1, the total target area should not exceed 15% of the boundary plasma surface area. The divertor concept consists of two pairs of target plates per field period, one pair each at the top and bottom of the plasma. The heat flux profile is assessed by assuming that the parallel transport can be represented by field line mapping and that cross-field transport can be modeled with a prescribed field line diffusion scheme. In this manner, the poloidal and toroidal extents of the plates and their shape and distance to the plasma are designed to intercept all the heat flux and to minimize the peak thermal heat load. An approximate scheme, based on particle drift orbits in the core and field line tracing in the edge, is derived to estimate the alpha-particle heat load distribution over the plates and the first wall. The best plate configuration to date yields total peak heat loads (thermal + alpha) ranging from 5 to 18 MW/m2. Further optimization of the target plates is required to reach the design goal, which will be addressed in a future study.