Modeling of convective cells, turbulence, and transport induced by a radio-frequency antenna in tokamak boundary plasmas

The BOUT++ based edge turbulence model Hermes [1] is set up for plasma boundary simulations with a radio-frequency (RF) antenna, using parameters characteristic of a tokamak edge. Cartesian slab geometry is used with thin plate limiters representing the antenna. Ad-hoc DC electric biasing of the limiters, motivated by calculations with VSim [2], represents an induced RF sheath in the plasma turbulence model. Flux-driven turbulence simulations demonstrate a realistic distribution of plasma profiles and fluctuations. There is a clear effect of the antenna sheath voltage leading to formation of convective cells; bias-induced convective transport flattens the SOL density profile and fluctuations penetrate into the shadow region of the limiters as the bias voltage increases. Turbulent transport for impurity ions is inferred by following ions trajectories in the simulated plasma turbulence fields, showing Bohm-like effective diffusion rates. All in all, the model elucidates physical phenomena governing the effects of an RF antenna on tokamak boundary plasma. [1] Dudson et al. PPCF 59 05401 (2017); [2] Nieter et al., J. Comp. Phys. 196, 448 (2004)