Identification of waves in the lower-hybrid frequency range in the scrape-off layer plasma of Alcator C-Mod

Polarization resolved measurements of the parallel refractive index N∥≡ck∥/ω of the driven RF waves in the lower hybrid (LH) range of frequencies are performed using arrays of RF magnetic probes in the scrape-off layer plasma of Alcator C-Mod. The measured N∥ of the RF magnetic field component parallel to the background magnetic field is about −1.6, which corresponds to the peak of the launched LH N∥ spectrum. Based on the wave dispersion relationship, this wave is identified as the LH slow wave. On the other hand, the RF magnetic field component perpendicular to the magnetic field is found to have a lower N∥ of −1.2, and is detected only near the last closed flux surface. This wave is identified as the LH fast wave generated by slow-fast wave mode conversion.