Gyrokinetic simulations of edge turbulence during NBI-only and NBI+ECRH plasmas*

Turbulent transport is the dominant mechanism driving heat losses in high temperature fusion-grade plasmas. While electrostatic drift wave instabilities have been identified as the primary drive for turbulent modes, their growth rates and saturation physics in the edge regions are still an active field of investigation. This study employs the CGYRO [1] gyrokinetic code to study the dominant instability in the edge regions during NBI only and NBI+ECRH heating in the DIII-D tokamak [2]. Linear simulations show that trapped electron modes (TEMs) are the most unstable modes in the pedestal region for early NBI only heating, whereas both TEMs and ion temperature gradient modes are prevalent for late NBI and NBI+ECRH heating. Line-averaged density measured by CO₂ interferometry is  2.9e19 m^-3 for early NBI and increases monotonically for each consecutive stage. Sensitivity studies that vary the simulated temperature and density gradient, among other profile parameters, show that TEMs are driven by the electron temperature gradient and stabilized by the density gradient and the ion temperature gradient. Quasi-linear estimates show that heat flux is predominantly electron-driven. Heat and particle fluxes from non-linear simulations will be presented, allowing for detailed comparison with experimental data. This will help validate the gyrokinetic approach with an appropriate collision operator in the edge region of tokamak plasmas and therefore help provide extrapolations towards ITER and future devices.

Ref.

[1] J. Candy et al. J. Comput. Phys., 324:73, 2016. doi:10.1016/j.jcp.2016.07.039.

 

[2] X. Qin et al. Measurement of Turbulence-driven Reynolds Stress and its Contribution to Intrinsic Toroidal Rotation in the DIII-D Tokamak, to be submitted