Electron cyclotron emission measurements of coherent and broadband density fluctuations in the Alcator C-Mod tokamak
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A novel technique for measuring density fluctuations using a heterodyne ECE radiometer has been developed. Although ECE radiometry is typically used for electron temperature measurements, the unique viewing geometry of this system’s quasi-optical antenna has been found to make the detected emission extremely sensitive to refractive effects under certain conditions. This sensitivity gives the diagnostic the ability to measure very low levels of density fluctuations in the core of the Alcator C-Mod tokamak. The refractive effects have been modeled using ray-tracing methods, allowing estimates of the density fluctuation magnitude and spatial localization to be obtained. This new diagnostic technique has been used to observe density fluctuations in a variety of plasmas. The bulk of the observations have been during internal transport barriers (ITBs) generated by off-axis ICRF heating at 5.4 T and 4.5 T toroidal fields. Density fluctuations during pellet injection into L-mode plasmas have been detected as well. A coherent mode with both temperature and density oscillations was observed in the 5.4 T ITB plasmas. Estimates of the magnitude of the temperature and density oscillations, their radial structure, and some information on their relative phases is discussed. Broadband fluctuations have been observed in ITBs generated at 4.5 T. Estimates of their magnitude and localization is presented. Directions for future work is also discussed.