UC Irvine

Diagnosis of the fast ion population, which is necessary for heating but can drive dangerous instabilities, is crucial in achieving and maintaining a burning plasma regime. Present fast ion diagnostic methods tend to rely on delicate components unlikely to survive in a reactor environment, motivating exploration into alternative diagnostic methods. One candidate, the subject of this thesis, entails measurement of collective ion cyclotron emission (ICE) spectra via robust magnetic pickup loops integrated with the first wall, making it potentially compatible with ITER and other reactor-relevant devices [116, 69]. ICE, distinct from thermal electron cyclotron emission, manifests as narrowband peaks at harmonics of the ion cyclotron frequency $f_{ci}$ and is generally localized near the core or the edge of the plasma. These modes have been observed to be sensitive to the fast ion population, be they introduced by auxiliary heating systems or fusion products. However, the connection between observed ICE spectra and the fast ion population has not been sufficiently established, meriting diagnostic upgrades and dedicated experiments.

This work aims to characterize ICE mode structure and fine-scale phenomena in L- and H-mode plasmas on DIII-D using the ICE diagnostic, which was recently upgraded to include additional channels and consequently new measurement capabilities. In addition to basic frequency and amplitude information, measurements of mode polarization at the plasma edge, amplitude as measured at the centerpost and outer wall of the machine, and toroidal mode number are now possible and will be demonstrated through this work. Additionally, mode sensitivity to the vacuum region, plasma shape, and particularly the fast ion distribution is explored. The detailed mode structure measurements and phenomenology presented in this work can be used to test and validate theoretical models en route to developing a more complete physical understanding of ICE, so that it can be leveraged as a passive fast ion diagnostic in future burning plasmas.