Electron temperature measurement from the ECE diagnostics in tokamak plasmas under transient conditions

The conditions under which the effective optical depth τn can be neglected when interpreting the electron cyclotron emission (ECE) signals in order to infer the electron temperature in tokamaks are examined. It is argued that the usual 'rule of thumb', IECE is proportional to Te for τ ≥ 2, is generally inadequate under transient or modulated plasma conditions and can lead to false conclusions regarding the possible coupling between the perturbed signals of the electron temperature and density (especially for low reflectivity systems); this issue is critical to the understanding of the transport phenomena in tokamaks. A simple solution of this problem is proposed, namely to modify the above criterion for IECE is proportional to Te to τ ≥ 4.5 for studies of transient conditions in order to reduce the spurious 'coupling effect' to below the 5% confidence level in low reflectivity systems. (author). Letter-to-the-editor. 9 refs, 2 figs