Observation and Modeling of Ion Cyclotron Range of Frequencies Waves in the Mode Conversion Region of Alcator C-Mod

The fast magnetosonic wave (FW), mode converted ion cyclotron wave (MC ICW) and ion Bernstein wave (MC IBW) have all been observed and unambiguously identified in the mode conversion region of Alcator C-Mod. The influences of the species mix, mode conversion location and B pol /B tot have been studied in D(3 He) plasmas at B 0 ∼ 5.4 T (f RF = 50 MHz) and B 0 ∼ 8 T (f RF = 78 MHz). The RF waves were measured by a phase contrast imaging (PCI) system. The experimental observation is compared with the result from a synthetic PCI diagnostic based upon the full wave code TORIC. Good agreement between the observation and modeling has been obtained on the spatial structure of the RF waves. When the mode conversion layer was off axis, both MC ICW and MC IBW were observed. In 5.4 T near axis mode conversion discharges, double hump spatial structure of the MC waves was observed experimentally and reproduced by the synthetic PCI. Such structure is an indication of the up-down asymmetry of the MC ICW. In 8 T near axis mode conversion discharges, we had the first definitive observation of IBW dominated MC in Alcator C-Mod.