Optical Thickness Corrections to ECE Transient Electron Temperature Measurements in Tokamak and Stellarator Plasmas

The conditions are examined under which optical thickness (tau) corrections to electron cyclotron emission (ECE) measurements of electron temperature (T-e) can be neglected. By means of simple algebra it is demonstrated that for measurements of T-e transients the ECE radiation temperature (T-rad) can be directly interpreted as T-e to a 15% inaccuracy for tau > 0.7. This method is a solution to the problem identified by Janicki in 1993, namely that to estimate the variation of T-e relative to the time averaged T-e ((T) over tilde(e)/[T-e]) by (T) over tilde(rad)/[T-rad] with the same accuracy requires tau > 3. This result implies that optical thickness effects have considerably less impact on the interpretation of ECE transients than suggested by Janicki and that they do not pose a serious problem to perturbative transport studies in small toroidal devices.