Implication of Absolute Calibration for Michelson Interferometer ECE Diagnostic at JET for ITER

In the microwave and near infrared spectral range a Michelson interferometer diagnostic is dedicated to probe the Electron Cyclotron Emission (ECE) spectrum of several harmonics emitted by high-temperature plasmas in fusion experiments with magnetic confinement. The standard analysis of the interferogram data reveals the electron temperature profile Te. Besides the standard application a sophisticated technique can extract further information like about the electron density ne, the equilibrium and wall properties. However, the essential cornerstone for any analysis is the challenging absolute calibration of the diagnostic. Recently at JET, the calibration was carried out using the hot/cold technique with source temperatures well below 1000K (~0.0001keV). The attempt demonstrates that the calibration factors can be determined with the relative uncertainty of ~1.5% in the spectral range 0.1-0.35THz. With respect to the plasma parameters at JET the spectral range of interest is covered by the calibration. The situation changes for the ITER tokamak. For the envisaged plasma parameters B = 5.3T, Te = 25keV and ne = 1020m-3 the spectral range of interest lies beyond 0.5THz. Indeed the calibration and thus the Te measurement are assured by probing the first harmonic in O-mode polarization, physics studies like for non-Maxwellian plasmas demand the calibration up to at least 1THz including spectral ranges for which the optical thickness is low. To accomplish the calibration on a reasonable time scale of some days the design of the ITER Michelson interferometer diagnostic is constraint by the required sensitivity, and design studies are on-going. Nevertheless, the studies can be supported by implications of a well-characterized diagnostic like the JET interferometer and modelling the interferogram data giving constraints for the data acquisition system.
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