External Excitation of a Drift-Alfv\'{e}n Wave Response in the Alcator C-Mod Edge Plasma and its Relationship to the Quasi-Coherent Mode

Experiments indicate that short-wavelength, $k_{\perp} \rho_s \sim 0.1$, drift-Alfv\'{e}nic turbulence plays an important role in C-Mod edge plasma transport. A Quasi-Coherent Mode (QCM, $50 < f < 150$ kHz, $k_{\perp} \sim 1.5$ cm$^{-1}$) regulates particle and impurity transport in C-Mod's EDA H-modes. A Weakly Coherent Mode (WCM, $150 < f < 500$ kHz, $k_{\perp}\sim1.5$ cm$^{-1}$) plays a similar role in I-mode discharges, suppressing the formation of a density pedestal while maintaining a temperature pedestal. ELMs are not present in either confinement regime. With the idea of exciting, probing, and perhaps exploiting this transport behavior, we have developed a novel antenna system to excite drift-Alfv\'{e}n-like modes at the outer midplane. A winding with a ``shoelace'' geometry is placed $\sim3-5$ mm from the LCFS. The principal design parameters, $k_{\perp}=1.5\pm0.1$ cm$^{-1}$ and $45 < f < 300$ kHz, match the QCM and WCM properties, so that the antenna induces parallel currents in the boundary plasma that mimic those observed for the intrinsic modes. Phase-locking to intrinsic modes is also accomplished via a custom circuit. The antenna produces perturbations in density and field comparable to amplitudes of the intrinsic QCM. The plasma response exhibits a resonance near the natural QCM frequency, which generally satisfies the drift wave dispersion relation. While a driven $\tilde{B}_{\theta}$ fluctuation is visible throughout the discharge, the driven $\tilde{n}_e$ is only observed during H-mode, though it precedes the onset of the intrinsic QCM. Like the QCM, the driven mode propagates in the electron diamagnetic drift direction and is approximately field-aligned. Recent mirror probe measurements show the intrinsic QCM structure is predominantly drift-Alfv\'{e}nic, and we might expect the same of the driven mode. However, the induced perturbation is not global, but is localized to field lines which map to the antenna, suggesting a damped response, and direct measurements of the damping rate indicate $\gamma/\omega_0\sim5$\%. If the antenna response is, indeed, a linearly-stable drift wave, this may suggest that additional interchange physics and curvature drive are involved to make the QCM unstable.