Spatial mode structure and propagation of ion cyclotron emission in the DIII-D tokamak

Ion cyclotron emission (ICE) spatial mode structure in DIII-D has been characterized for the first time, with the toroidal magnetic field fluctuation amplitude found to be consistently larger than its poloidal counterpart (δB_tor/δB_pol ~ δB_‖/δB_⊥ ≥ 2), thus demonstrating compressional polarization near the probe location. The modes were found to be poloidally extended through comparison of signal amplitude measured by loops on the low- and high-field sides of the machine, corroborated recently by δn measurements made by the high-frequency Doppler backscattering diagnostic¹. Finally, toroidal mode numbers of n ~ [-10, 5] were determined via phase information collected from three toroidally displaced outer wall loops. Measurements of ICE represent a possible fast ion diagnostic technique compatible with high radiation environments in future burning plasma experiments, which are prohibitive to many current fast ion diagnostics. However, we must better our physics understanding to discern fast ion characteristics from observed ICE spectra. To this end, an array of new toroidal and poloidal in-vessel loops was installed on the DIII-D ICE diagnostic, and the resultant spatial mode structure measurements will be used as crucial constraints for ongoing modeling work. ICE dependence on scrape-off layer (SOL) width was also investigated to assess mode propagation to the ICE diagnostic loops. ICE spectra were found robust to changing the SOL width from ~4–12 cm, suggesting very weak attenuation between the plasma and loop location and meaning that future diagnostics might be moved farther from areas of high radiation without adversely impacting measurement capability.

[1] N.A. Crocker et al. NF (2022) 026023.