Optical thickness corrections to transient ECE temperature measurements in tokamak and stellarator plasmas

The conditions are examined under which optical thickness (τ) corrections to electron cyclotron emission (ECE) measurements of electron temperature (Te) can be neglected. By means of simple algebra it is demonstrated that for measurements of Te transients the ECE radiation temperature (Trad) can be directly interpreted as Te to a 15% inaccuracy for τ > 0.7. This method is a solution to the problem identified by Janicki in 1993, namely that to estimate the variation of Te relative to the time averaged Te (T-tildee/) by T-tilderad/ with the same accuracy requires τ > 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. (author). Letter-to-the-editor. 5 refs, 1 fig