Optimizing Stellarators for Turbulent Transport

Up to now, the term “transport-optimized” stellarators has meant optimized to minimize neoclassical transport, while the task of also mitigating turbulent transport, usually the dominant transport channel in such designs, has not been addressed, due to the complexity of plasma turbulence in stellarators. Here, we demonstrate that stellarators can also be designed to mitigate their turbulent transport, by making use of two powerful numerical tools not available until recently, namely, gyrokinetic codes valid for 3D nonlinear simulations and stellarator optimization codes. Two initial proof-of-principle configurations are obtained, reducing the level of ion temperature gradient turbulent transport from the National Compact Stellarator Experiment baseline design by a factor of 2–2.5.

Figure 1

Comparison of $Q_{\mathrm{prox}}$ (red solid line) with $Q_{\mathrm{GK}}$ (black dashed line) for one flux tube of each of the 4 toroidal configurations studied in Ref. 9.

Figure 2

Comparison of radial curvature ${\kappa }_1(\theta )$ for 1 poloidal transit for NCSX (heavy dashed black line), QA_35q [dashed gray) green line], and QA_40n (solid red line).

Figure 3

Comparison of boundary shapes of NCSX (dashed black line) and QA_40n (solid red line) at values of $N\zeta$ ($=\mathrm{\text{number of field periods}}\times \mathrm{\text{toroidal angle}}$) $=0,\pm \pi /2,\pi$.

Figure 4

Comparison of line-averaged heat flux $Q_{\mathrm{GK}}$ versus time for NCSX (heavy dashed black line), QA_35q [dashed gray (green) line], and QA_40n (solid red line) from nonlinear Gene runs. QA_35q and QA_40n achieve reductions in turbulent transport over that in NCSX by factors of about 2.5 and 2, respectively.