New Classes of Quasi-Axisymmetric Stellarator Configurations
Authors: L.P. Ku and P.R. Garabedian
Date of PPPL Report: October 2005
Published in: Fusion Science and Technology 50:2 (August 2006) 207–215
Presented at the Fifteenth International Stellarator Workshop, 3–7 October 2005, Madrid, Spain.
We have identified and developed new classes of quasi-axially symmetric configurations which have attractive properties from the standpoint of both near-term physics experiments and long-term power producing reactors. These new configurations were developed as a result of surveying the aspect ratio-rotational transform space to identify regions endowed with particularly interesting features. 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 externally supplied rotational transforms so that the overall rotational transform, when combined with the transform from bootstrap currents at finite plasma pressures, will yield 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 NCSX-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 designed coils as well to ensure that the new configurations are realizable and engineering-wise feasible. The coil designs typically have coil aspect ratios R/Δmin(C-P) ≤ 6 and coil separation ratios R/Δmin(C-C) ≤ 10, where R is the plasma major radius, Δmin(C-P) and Δmin(C-C) are the minimum coil to plasma and coil to coil separations, respectively. These coil properties allow power producing reactors be designed with major radii less than 9 meters for DT plasmas with a full breeding blanket. The good quasi-axisymmetry limits the energy loss of α particles to below 10%.