The Shoelace Antenna : a device to induce short-wavelength fluctuations in the edge plasma of the Alcator C-Mod Tokamak

Abstract

The "Shoelace" antenna is a unique device built to induce short-wavelength fluctuations in the edge plasma of the Alcator C-Mod tokamak, at a wave number and in the frequency range associated with the Quasi-Coherent Mode (QCM). The QCM is a continuous, drift-mode-like fluctuation, restricted to the low-field side of the tokamak in a 3 mm region around the last closed flux surface, and spanning both open and closed field lines. The study presented here is motivated by the fact that the QCM plays a crucial role in regulating particle transport across the plasma boundary in the Enhanced D[alpha](EDA) H-mode. It is this transport channel which sustains the EDA H-mode, flushing impurities from the plasma without the appearance of bursting Edge Localized Modes (ELMs). Because of the damage they cause to first-wall components, large-amplitude ELMs do not extrapolate to a full-size, steady state fusion reactor, and so it is of critical importance for the worldwide fusion research endeavor to identify, understand, and exploit ELM-free mechanisms of impurity flushing. It is in this context that the antenna's mission is defined. The Shoelace antenna is wound with field-aligned rungs spaced to produce a perpendicular wave number, k = 1.5 ± 0.1 cm-1, that precisely matches the QCM spatial structure, while the power system, with custom matching network, provides up to 2 kW of radio-frequency source power at any frequency in the band, 45 < f < 300 kHz. Initial experiments show that when the antenna is energized into L-mode plasmas, it produces a steady response in poloidal magnetic field, only. However, after transition to H-mode, the antenna drives both field and electron density fluctuations that are aligned with, and guided by, the background equilibrium field, propagate in the electron diamagnetic drift direction in the laboratory frame, have amplitude comparable to that of the intrinsic QCM, and display a weakly-damped resonance ([gamma]/[omega] ~ 5-10%). In EDA H-mode, the resonance is centered on the QCM frequency, but in ELM-free H-mode, it persists in the same frequency range, even in the absence of a QCM. This result is significant, offering the possibility that externally-driven modes might be used to enhance particle transport. However, additional measurements are required before a definitive statement can be made regarding transport resulting from the antenna-driven mode, as well as the driven mode's relationship with the QCM. This work has been scheduled for the 2014 Alcator C-Mod experimental campaign as part of a broader exploration of the plasma response to the Shoelace antenna.