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Observations of core modes during RF-generated internal transport barriers in Alcator C-Mod

In the Alcator C-Mod tokamak, a high-resolution heterodyne ECE radiometer has been used to measure the electron temperature in plasma discharges with internal transport barriers (ITBs). ITBs are formed by the application of off-axis (r/a ∼ 0.5) ICRF power. Strong density peaking indicates the formation of the ITB. When the ITB is formed, the ECE radiometer detects a small amplitude mode localized near the magnetic axis. Surprisingly, as this mode amplitude grows a dip in the temperature profile is clearly observed at the same location. If sawteeth are present, the mode amplitude appears to be suppressed by the sawtooth crash and no dip in the temperate profiles is observed. TORAY, a ray-tracing code, has been used to investigate the possible refractive effects of the steep density gradients in the ITB and its effects on the ECE observations. The results show that refractive effects can explain the observed local changes in temperature. Ray-tracing also indicates that the observed modes are density fluctuations. Observations of broadband density fluctuations during 4.5 T ITBs are also described.