Collisionality Scaling of ETG Transport in NSTX

T. Singh, T. Rafiq, E. Schuster

68th Division of Plasma Physics (DPP) Annual Meeting of the American Physical Society (APS)

Chicago, IL, USA, November 2-6, 2026

The collisionality scaling of electron-temperature-gradient (ETG)-driven electron heat transport is investigated in an NSTX discharge using the fully electromagnetic gyrokinetic code CGYRO [1], where long-time saturation is regulated by zonal flows. Although the linear ETG instability exhibits only a weak dependence on collisionality, the nonlinear electron heat transport reproduces the experimentally observed collisionality scaling through collisional damping of zonal flows. The influence of equilibrium flow shear is further investigated, revealing two distinct saturation regimes. At low flow shear, the recently identified flow-shear destabilization mechanism in multiscale ETG turbulence is recovered [2], where reduced zonal-flow energy enhances ETG transport while preserving the strong experimental collisionality scaling. At higher flow shear, conventional flow-shear stabilization suppresses turbulence, resulting in a substantially weaker dependence on collisionality. These results demonstrate that the collisionality scaling of ETG transport is governed by the dominant nonlinear saturation mechanism, with important implications for predictive modeling of electron heat transport in future spherical tokamaks such as NSTX-U.

[1] J. Candy et al., J. Comput. Phys. 324, 73 (2016).
[2] E. A. Belli et al., Plasma Phys. Control. Fusion 66, 045019 (2024).

* Supported by DOE Award DE-SC0021385; computations used NERSC Perlmutter.