Optical-Thickness Corrections to Transient Ece 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.