Title:Role of magnetic shear and aspect ratio in ITG turbulence in tokamaks and quasi-axisymmetric stellaratorsAuthors: R. Xu, H. Zhu, and A. Bhattacharjee

Regulating ion-temperature-gradient (ITG) turbulence is key to maintaining favorable core temperatures in tokamaks and stellarators. A recent study [1] found that the quasi-axisymmetric (QA) stellarator exhibits significantly higher transport than ITER and other stellarator types (QH, QI), due to weak, oscillating zonal flows. However, the cause remains unclear.

In this study, we identify an “equivalent tokamak” for Landreman-Paul’s precise QA base case (QABC) [2], with matched parallel connection length, safety factor, and magnetic shear. It differs in finite Larmor radius (FLR) physics due to QABC’s 3D geometry. We perform gyrokinetic simulations using the GX code and show that nonlinear heat fluxes are similar in QABC and its equivalent tokamak, while zonal flow in QABC is even more coherent and long-lived. Parameter scans reveal that Dimits shift occur only in low-aspect-ratio (~3), high magnetic shear (>0.5) regimes, suggesting that the transport difference between the QA configuration and ITER likely stems from ITER’s lower aspect ratio and higher magnetic shear. In contrast, low or negative shear reduces linear growth rate and transport but does not produce a Dimits shift.

In conclusion, QABC configuration exhibits transport levels comparable to its equivalent tokamak, indicating that its turbulent transport performance is acceptable. Our “equivalent tokamak” approach and parameter scans provide insight to guide future QA stellarator optimization for improved turbulence regulation.