Edge transport and mode structure of a QCM-like fluctuation driven by the Shoelace antenna

• Published in: Nuclear fusion 2018-03, Vol.58 (5), p.56018
• Main Authors: Golfinopoulos, T., LaBombard, B., Brunner, D., Terry, J.L., Baek, S.G., Ennever, P., Edlund, E., Han, W., Burke, W.M., Wolfe, S.M., Irby, J.H., Hughes, J.W., Fitzgerald, E.W., Granetz, R.S., Greenwald, M.J., Leccacorvi, R., Marmar, E.S., Pierson, S.Z., Porkolab, M., Vieira, R.F., Wukitch, S.J.
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Alcator C-Mod; edge fluctuations; edge transport; fusion; quasi-coherent mode; shoelace antenna; tokamak
• ISSN: 0029-5515; 1741-4326
• DOI: 10.1088/1741-4326/aab2c8

The Shoelace antenna was built to drive edge fluctuations in the Alcator C-Mod tokamak, matching the wavenumber (k⊥≈1.5 cm−1) and frequency (30≲f≲200 kHz) of the quasi-coherent mode (QCM), which is responsible for regulating transport across the plasma boundary in the steady-state, ELM-free Enhanced Dα (EDA) H-mode. Initial experiments in 2012 demonstrated that the antenna drove a resonant response in the edge plasma in steady-state EDA and transient, non-ELMy H-modes, but transport measurements were unavailable. In 2016, the Shoelace antenna was relocated to enable direct measurements of driven transport by a reciprocating Mirror Langmuir Probe, while also making available gas puff imaging and reflectometer data to provide additional radial localization of the driven fluctuation. This new data suggests a   4 mm-wide mode layer centered on or just outside the separatrix. Fluctuations coherent with the antenna produced a radial electron flux with Γe/ne∼4 m s−1 in EDA H-mode, smaller than but comparable to the QCM level. But in transient ELM-free H-mode, Γe/ne was an order of magnitude smaller, and driven fluctuations reduced by a factor of ≳3. The driven mode is quantitatively similar to the intrinsic QCM across measured spectral quantities, except that it is more coherent and weaker. This work informs the prospect of achieving control of edge transport by direct coupling to edge modes, as well as the use of such active coupling for diagnostic purposes.