Abstract Details
Abstracts
Author: Runlai Xu
Requested Type: Poster
Submitted: 2025-03-31 23:13:13
Co-authors: H.X. Zhu, A, Bhattacherjee
Contact Info:
Princeton University
311 Court Dr
Princeton, NJ 08540
USA
Abstract Text:
The Dimits shift is a phenomenon where turbulent transport vanishes even when the
system is above the linear-instability threshold. While Dimits shift is well-documented in
tokamak configurations such as the Cyclone Base Case (CBC), it has been found to
disappear in the precise Quasi-Axisymmetric (QA) stellarator (Landreman and Paul,
PRL 2022). This study aims to systematically investigate the role of magnetic shear in
saturated heat flux in the context of Ion Temperature Gradient (ITG) turbulence and
zonal flow dynamics. Recent work by Yang et al. (Nucl. Fusion 2024) has shown that
the Dimits shift vanishes in tokamak as magnetic shear transitions from positive to
negative values. To extend this analysis, we compare the QA and equivalent tokamak
with similar effective parameters, identifying magnetic shear as a key parameter
influencing the existence of the Dimits shift. Since the precise QA stellarator shares
similarities with tokamaks, understanding the magnetic shear dependence in this
context is crucial for stellarator transport studies.
We employ the GX gyrokinetic flux tube code to compute the time-averaged saturated
heat flux as a function of the temperature gradient. Our study confirms the absence of
the Dimits shift in both QA and equivalent near zero-shear tokamak configurations. We
analyzed results at varying magnetic shear values (-0.5, -0.25, 0.25, 0.5). Our findings
indicate that both the ITG linear growth rate and zonal flow strength decrease as
magnetic shear decreases.
Future work involves two directions: (i) understanding why the ITG linear
growth rate and zonal flow strength decreases with decreasing magnetic shear and (ii)
examining and understanding differences in transport between QA stellarators and
equivalent tokamaks. We will focus on the mode’s spectral characteristics and zonal-
flow stability at different shear values. We aim to deepen our understanding of ITG
turbulence regulation and transport in tokamaks and
Characterization: 1.0
Comments: