Characterization of ion cyclotron emission in L- and H-mode plasmas in the DIII-D tokamak

Coherent ion cyclotron emission (ICE) has been observed on many fusion devices but requires more detailed experimental understanding before it can be leveraged as a fast ion diagnostic for future machines. A dedicated experiment on the DIII-D tokamak explored the dependence of ICE at the deuterium cyclotron frequency and its harmonics on plasma current, outer gap, and fast ion distribution in both L- and H-mode plasmas. The recently upgraded ICE diagnostic measures frequencies in the 1-100 MHz range and has expanded its measurement capabilities to include toroidal mode number and approximate polarization at the edge. Centrally-localized ICE sensitivity to fast ion distribution was explored by using seven different neutral beam injection configurations in L-mode plasmas. Changing the counter-current injecting beams from on- to off-axis, for example, drives significantly reduced levels of ICE at different harmonics. Detailed measurements in L-mode plasmas will be useful in future modeling efforts and comparison to the more commonly observed edge-localized ICE in H-mode plasmas. ICE in both L- and H-mode plasmas are sensitive to changes in the separation between the plasma and the outer wall.