Fusion Science and Technology / Volume 50 / Number 2 / August 2006 / Pages 207-215
Technical Paper / Stellarators
We have identified and developed new classes of quasi-axially symmetric configurations that have attractive properties from the standpoint of both near-term physics experiments and long-term power-producing reactors. These include configurations with very small aspect ratios (~2.5) having superior quasi-symmetry and energetic particle confinement characteristics, and configurations with strongly negative global magnetic shear from the shaping fields so that the overall rotational transform, when combined with the transform from bootstrap currents at finite plasma pressures, will have a small but positive shear, making the avoidance of low-order rational surfaces at a given operating beta possible. Additionally, we have found configurations with National Compact Stellarator Experiment-like characteristics but with the biased components in the magnetic spectrum that allow us to improve the confinement of energetic particles. For each new class of configurations, we have also designed coils to ensure that the new configurations are realizable and engineering-wise feasible. The coil designs typically have the properties of R/min(C-P) 6 and R/min(C-C) 10, where R is the plasma major radius and min(C-P) and min(C-C) are the minimum coil-to-plasma and coil-to-coil separations, respectively. These coil properties allow power-producing reactors to be designed with R < 9 m for deuterium-tritium plasmas with a full breeding blanket. The good quasi-axisymmetry limits the energy loss of alpha particles to below 10%.