The Role of a Higher-Order Coaxial Mode in the Parasitic Coupling of High-Harmonic Fast Waves to the Scrape-off Layer in NSTX-U

Understanding and controlling the parasitic coupling of the High-Harmonic Fast Wave (HHFW) power to the Scrape-off layer (SOL) on NSTX-U is crucial for developing a non-inductive plasma scenario and accessing the electron heating regime in a spherical tokamak. Past NSTX experiments [Hosea et al, PoP 15, 056104 (2008)] have shown that the HHFW power can couple to the SOL, evidenced by the RF-enhanced spiral heat flux pattern on the divertor plate. Experimental and modeling analyses indicate that the SOL propagation corresponds to satisfying the FW density cut-off condition, implying that low-density operation is beneficial, in contrast to the conventional picture of low-harmonic FW coupling. We show that a higher-order coaxial mode can be a coupling mechanism. Due to a relatively short perpendicular wavelength in the HHFW regime, the half-wavelength condition across the transverse (radial) direction can be satisfied between the wall boundary and the steep density gradient region at the last closed flux surface. Parametric dependences, including the SOL gap and field, are examined using a 2D cylinder model in Petra-M. The impact of the density gradient is studied using a realistic NSTX-U geometry. This study helps clarify the coupling picture in going from low- to high-harmonic FW regimes. A higher field on NSTX-U (1T) will result in a longer perpendicular wavelength, and is expected to help prevent such a geometric coupling, even when the SOL density remains high for a nominal gap width.