Observation and modelling of ion cyclotron range of frequencies waves in the mode conversion region of Alcator C-Mod

The fastmagnetosonic wave, mode converted ion cyclotron wave (MC ICW) and mode converted 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(He-3) plasmas at B-0 similar to 5.4T (f(RF) = 50 MHz) and B-0 similar to 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 modelling 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, the double hump spatial structure of the MC waves was observed experimentally and reproduced by the synthetic PCI. Such a 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.