Modeling of helicon antenna in DIII-D using the VSim code

The helicon antenna recently installed in the DIII-D tokamak can become a valuable current drive actuator in future experiments. According to previous modeling results [R. Prater et al. NF {\bf 54} (2014) 083024], the propagation of helicon waves in the plasma core is sensitive to different parameters. In this study, we report the results of the helicon waves propagation computed with VSim [C. Nieter, J.R. Cary, JCP {\bf 196} (2004) 448] both for the prototype and final helicon antenna designs. Analytical fits to the plasma profiles from the DIII-D shot 165908 are used in these simulations. Several scans with the pedestal density gradient and outer gap for a simplified antenna geometry are conducted. It is demonstrated that the penetration of fast wave in the plasma core is reduced when the outer gap is increased. In a scan with the poloidal field, an offset between the magnetic field and the antenna polarization is introduced which is equivalent to the introduction of a poloidal field in the SOL to cause the polarization mismatch. A small level of mode competition with the slow wave propagating in SOL before encountering a lower-hybrid resonance near the pedestal base is observed. Development of a surface wave that carries energy along the plasma-wall interface is demonstrated.