Figures of merit for stellarators near the magnetic axis
Abstract
A new paradigm for rapid stellarator configuration design has been recently demonstrated, in which the shapes of quasisymmetric or omnigenous flux surfaces are computed directly using an expansion in small distance from the magnetic axis. To further develop this approach, here we derive several other quantities of interest that can be rapidly computed from this nearaxis expansion. First, the $\boldsymbol {\nabla }\boldsymbol {B}$ and $\boldsymbol {\nabla }\boldsymbol {\nabla }\boldsymbol {B}$ tensors are computed, which can be used for direct derivativebased optimization of electromagnetic coil shapes to achieve the desired magnetic configuration. Moreover, if the norm of these tensors is large compared with the field strength for a given magnetic field, the field must have a short length scale, suggesting it may be hard to produce with coils that are suitably far away. Second, we evaluate the minor radius at which the flux surface shapes would become singular, providing a lower bound on the achievable aspect ratio. This bound is also shown to be related to an equilibrium beta limit. Finally, for configurations that are constructed to achieve a desired magnetic field strength to first order in the expansion, we compute the error field that arises due to secondorder terms.
 Publication:

Journal of Plasma Physics
 Pub Date:
 February 2021
 DOI:
 10.1017/S0022377820001658
 arXiv:
 arXiv:2012.00865
 Bibcode:
 2021JPlPh..87a9012L
 Keywords:

 fusion plasma;
 plasma confinement;
 Physics  Plasma Physics
 EPrint:
 J. Plasma Phys. 87 (2021) 905870112