Electron temperature measurements from optically gray third harmonic ECE in DIII-D (abstract)

The electron cyclotron emission spectrum on DIII-D is measured using an absolutely calibrated Michelson interferometer. During high performance discharges, electron densities above 9x1013 cm-3 preclude determination of the electron temperature from the second harmonic ECE. Under these conditions the third harmonic X mode is generally optically gray (0.4<τ<3.0) and can be used to determine Te by correcting the emission intensity for optical thickness. Overlap of the central third harmonic frequencies with optically thick inside-edge second harmonic frequencies on DIII-D allows a particularly simple formalism for determining the correction to the emission. To check the accuracy of the method, the Te values derived from the third harmonic can be compared to those from the second for discharges where it is not cutoff. These comparisons are presented along with an evaluation of various sources of error associated with calculations of the optical depth and the radiation transport