Internal-transport-barrier correlating sub-ion cyclotron frequency fluctuations in DIII-D high-β_p plasma

Sub-ion cyclotron frequency fluctuations driven by fast ions have been linked to transport in internal-transport barriers (ITB) in spherical tokamaks [1], but not in conventional aspect ratio tokamaks, to our knowledge. We report new observations of such fluctuations in DIII-D experiments in which ITBs are routinely created. These experiments have ~10 MW neutral beam (NB) power, poloidal beta of ~3, and normalized beta of ~2.5. Line-integrated density fluctuations in the 6 to 8 MHz range are robustly detected in these experiments using a Faraday-effect radial interferometer-polarimeter (RIP). RIP provides novel measurements of the wave vector of fluctuations: the toroidal mode number is measured from -33 to -41 in the counter-current direction, and the vertical wave number is approximately 0.1/cm. By assuming the radial wave number is negligible and comparing with the plasma dispersion relation [2], these measurements unambiguously indicate that the fluctuations originate from global Alfvén Eigenmodes at the low-field-side mid-radius (normalized radius 0.4-0.6), which coincide with the ITB location. Notably, the fluctuation power does not solely depend on NB power but correlates strongly with the density gradient of the ITB. For example, the normalized density fluctuation power exponentially decreases by two orders of magnitude from to in a discharge, when the mid-radius density gradient is halved while the NB power is constant. The drive and transport effect of these fluctuations will be discussed.



Work supported by US DoE under Awards DE-SC0019004 and DE-FC0204ER54698