ECE electron temperature mapping errors in high-performance DIII-D discharges

The proper mapping of diagnostic profiles to equilibrium flux surfaces is an important step in the analysis of tokamak data. On DIII-D, the mapping of electron temperature profiles obtained from second- and third-harmonic electron cyclotron emission (ECE) occasionally conflicts with that of Te profiles obtained by Thomson scattering, especially in high-performance negative central shear (NCS) discharges. Particularly, the ECE diagnostics report higher electron temperatures than the Thomson scattering measurements on the low-field side, although the profiles coincide within the measurement uncertainties on the high-field side. It is believed that this is the result of errors in the mapping of ∣B∣ to ρ, the normalized minor radius. This problem can be characterized for a particular equilibrium by calculating the percent reduction in ∣B∣ required to force agreement between the ECE and Thomson profiles. Calculations taking relativistic line broadening into account show that the required reduction in ∣B∣ in the range 0.2<ρ<0.8 is less than 5% in almost all cases, and is typically closer to 2%. Since the vacuum B-field is known to within a fraction of a percent, a discrepancy in total B must represent errors involving finite beta or other plasma parameters not related to vacuum magnetic field. The required reduction in ∣B∣ versus various parameters is investigated for a database of shots. It is found that in many cases the overlap error is due to the presence of large magnetic islands associated with locked modes.