Constructing stellarators with quasisymmetry to high order
Abstract
A method is given to rapidly compute quasisymmetric stellarator magnetic fields for plasma confinement, without the need to call a threedimensional magnetohydrodynamic equilibrium code inside an optimization iteration. The method is based on direct solution of the equations of magnetohydrodynamic equilibrium and quasisymmetry using Garren & Boozer's expansion about the magnetic axis (Phys Fluids B, vol. 3, 1991, p. 2805), and it is several orders of magnitude faster than the conventional optimization approach. The work here extends the method of Landreman et al. (J. Plasma Phys., vol. 85, 2019, 905850103), which was limited to flux surfaces with elliptical crosssection, to higher order in the aspectratio expansion. As a result, configurations can be generated with strong shaping that achieve quasisymmetry to high accuracy. Using this construction, we give the first numerical demonstrations of Garren and Boozer's ideal scaling of quasisymmetry breaking with the cube of the inverse aspect ratio. We also demonstrate a strongly nonaxisymmetric configuration (vacuum rotational transform ℓ > 0.4) in which symmetrybreaking mode amplitudes throughout a finite volume are < 2 × 10^{7}, the smallest ever reported. To generate boundary shapes of finiteminorradius configurations, a careful analysis is given of the effect of substituting a finite minor radius into the nearaxis expansion. The approach here can provide analytic insight into the space of possible quasisymmetric stellarator configurations, and it can be used to generate good initial conditions for conventional stellarator optimization.
 Publication:

Journal of Plasma Physics
 Pub Date:
 December 2019
 DOI:
 10.1017/S0022377819000783
 arXiv:
 arXiv:1908.10253
 Bibcode:
 2019JPlPh..85f9008L
 Keywords:

 fusion plasma;
 plasma confinement;
 Physics  Plasma Physics
 EPrint:
 J. Plasma Phys. 85 (2019) 815850601