The Fine-scale Structure of the Radial Electric Field in the Scrape-Off-Layer during ICRF Heating in Alcator C-Mod

By observing the radial structure in the poloidal dynamics of the SOL turbulence, we find a fine-scale radial structure in the broadband turbulence poloidal phase velocities (V$_{\theta })$ with the application of ICRF power (P$_{RF}>$0.6 MW). The radial profiles are very different from typical profiles in Ohmic plasmas. Since V$_{\theta }$(r) in the SOL is dominated by V$_{ExB}$, this structure implies a fine-scale E$_{r}$ profile is formed in the presence of the ICRF. The existence of such spatial structure is not predicted to be a result of the ICRF since the radial size-scale is much smaller than the fast-wave perpendicular wavelength ($\sim $10 cm). Such an E$_{r}$ profile may help to explain the increased impurity content observed with ICRF heating and will be important for SOL flows and transport. Using 2D Gas-Puff-Imaging we find that, in the $\sim $3 cm region outside the separatrix, the dominant propagation direction for V$_{\theta }$ reverses up to 3 times. This is found beyond the region where the antenna near-fields are present; the fast wave is propagating there. While the mechanism is still unknown, it must be consistent with the following: the structure is dependent on which antenna is energized; it changes with q$_{95}$ and is present when the RF frequency is 78 or 50 MHz.