Exploitation of a Diamagnetic Loop for Modulated ECH Power Absorption Measurements in TCV

摘要

Modulated ECH has been used to determine the ECH X2 and X3 power absorption from the measurement of the diamagnetic flux variations using the DML. Since only the modulation contribution is relevant to the analysis, the method does not require a perfect compensation of the diamagnetic flux measurement, although a good compensation of the vessel poloidal image current is crucial for ensuring a sufficiently large bandwidth to allow the use of high frequency modulation. The analysis of the behaviour of the amplitude and phase response in the modulation frequency scan has demonstrated the validity of the power absorption calculations with respect to the measured energy confinement time and the pitch angle scattering time. The method has been applied to the study of the absorption of the X3 ECH under different X2 pre-heat scenarios. The variations of amplitude and phase as a function of frequency, compared with different models, confirms the validity of the measurements. It has been demonstrated that single-pass absorption of the X3 is measured for central X3 injection with a X2 pre-heating in CO-ECCD, where only 55% absorption is predicted by the ray tracing code TORAY. The discrepancy is explained by the presence of an energetic electron tail generated by the X2 pre-heating. The presence of this tail is confirmed by hard x-ray photon spectra and by a high filed side ECE radiometer. The next step will consist of performing experiments at higher densities using the vertical X3 launcher of TCV.