Localization of Ion Cyclotron Emission (ICE) on NSTX-U

The toroidal array of wide bandwidth magnetic sensors on NSTX-U have been used to study ion cyclotron emission (ICE) on NSTX-U. Previously the data from this sensor array has been used to measure the toroidal mode numbers of ICE emission, document three-wave coupling between the harmonics, and look at growth rates and in some cases chirping of the ICE. On NSTX-U it is probably more appropriate to think of the ICE as an eigenmode, rather than a propagating wave. The ICE typically has relatively low toroidal mode numbers (long toroidal wavelengths). Likewise, although the 'poloidal' high frequency array is limited to ≈ ± 32 cm on the outboard midplane, the inferred poloidal wavelengths are also long. Here we extend those studies to search for any evidence of spatial localization of the ICE as was seen for high frequency Global Alfvén eigenmodes (GAE). For the fundamental (1^st harmonic) ICE, the amplitude of the emission appears to be, within experimental uncertainties, fairly uniform toroidally. For the higher harmonic, shorter wavelength ICE there is some evidence of local variability in relative amplitude. This does not seem to be an instrumental issue, e.g., some frequency dependence in the sensor, as ICE at similar frequency in other shots shows a different behavior. The strong localization suggests that the ICE is interacting with a much shorter scale phenomena. Understanding the observations will probably require further experimental work, with at minimum, a full set of diagnostics and upgrades to the fast magnetic sensor array. Work is underway to find correlations of this behavior with machine conditions.