ICRF Antenna – Plasma Edge Interaction in Alcator C-Mod: Comparison Between Experimental Results and 3D Full Wave Simulations*

We show that an optimal ratio of the power coupled by the center and outer straps of the Alcator C-Mod four-strap field-aligned antenna can facilitate H-Mode access and minimize unwanted near-field and far-field RF-enhanced heat fluxes, potentials, and impurities. The interaction between ICRF-induced ExB flows and turbulent filaments in the scrape-off layer (SOL) is studied, and we show that the sheared flows can slow down and destroy filaments, eventually suppressing convective transport in the far SOL. This interaction is eliminated by reducing RF-induced ExB flows via power ratio modulation. A 3D finite element cold plasma model of the C-Mod ICRF system is implemented in Petra-M using realistic antenna and vacuum vessel geometries. The simulations are compared with experimental results involving power ratio modulation. 3D Fourier analysis is used to study the excitation and propagation of different toroidal modes into the plasma. ICRF coupling simulations for field-aligned and toroidally aligned antennas during ELMs and confinement transitions are performed and compared with experimental trends. Preliminary far-field sheath simulations with realistic 3D limiter geometry will also be presented.