High-fidelity simulations of helicon wave coupling in DIII-D H-mode plasmas

Helicon injection is being tested in DIII-D and KSTAR as a current drive source for steady-state tokamaks. While evidence of coupling and heat deposition was reported, a demonstration of effective core heating and current drive is still missing. The latter strongly depends on the ability to minimize and/or suppress the coupling with the slow mode, which might result in significant power losses in the scrape-off layer (SOL). We used Petra-M simulations in DIII-D conditions to quantify coupling effects with the slow mode. Here, we focused on two effects: (a) the misalignment between the antenna and background magnetic field and (b) the role of turbulent edge density on the helicon and slow waves. Parametric scans indicate that the misalignment of the Faraday screen is the most effective knob to reduce the slow mode and that a misalignment angle < 5 degrees minimizes the slow wave excitation. The predicted threshold for the misalignment angle closely matches the typical misalignment angle of 4-5 degrees in DIII-D. Therefore, the simulation findings suggested that even though the antenna can directly produce slow modes, the SOL power losses resulting from the slow mode might be insignificant. To study the effect of edge turbulence, we also carried out a full-wave simulation on a realistic background plasma obtained from the XGC code, including spatial density fluctuations in the edge and SOL, focusing on a DIII-D scenario with prominent edge turbulence (wide pedestal QH-mode). The results show that, in this scenario, edge density fluctuations strongly affect coupling by the effect of wave scattering in the core and wave trapping in the pedestal. The insights from the simulations discussed here inform the upcoming DIII-D experiments of helicon antenna coupling for long pulse scenarios.