Characterization of RF-enhanced potentials with varying antenna power ratio on Alcator C-Mod

Impurity contamination caused by RF-enhanced potentials on antenna structures has long been detrimental to high-power operation of ICRF systems in tokamaks with metallic PFCs. With the C-Mod 4-strap field-aligned antenna, impurity generation was minimized with an appropriate power ratio between the inner and outer straps. In this work, we present a detailed characterization of RF-enhanced potentials during the antenna power ratio modulation experiments on C-Mod. With 1 MW of RF power and appropriate power ratio, the Gas Puff Imaging diagnostic, magnetically connected to the lower corner of the antenna, measures no change in poloidal velocity compared to the case with no RF. At the same time, the floating potential of a swept Langmuir probe, magnetically connected to the upper corner of the antenna, goes back to its background value. This suggests, for the first time, that near-field RF rectification effects can be cancelled with appropriate antenna power ratio. When deviating from the optimal power ratio, both the amplitude and width of enhanced potential structures are seen to increase. Moreover, with 1.5 MW of RF power, RF-enhanced potentials are minimized but not cancelled, suggesting the existence of a threshold residual induced current for rectified potentials to appear.