Computational studies on ECE spectrum for ITER, in the presence of a small fraction of non-thermals and radial resolution evolution for oblique view
1 ITER-India, Institute for Plasma Research, Gandhinagar, India
2 GNDU, Amritsar, Punjab, India
3 Kanchi Mamunivar Center for Post Graduate Studies, Poducherry, India
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Published online: 12 March 2015
In tokamaks, the temperature measurement using different techniques like Electron Cyclotron Emission (ECE), Thomson scattering etc. shows differences because of various phenomena. The physical reasons for this are not entirely understood. Thus to have comprehensive understanding of these difference, the contribution from each phenomenon needs to be individually understood. The phenomenon affecting radial temperature profile measurement includes harmonics overlap, relativistic down shifting, presence of non-thermals etc. For ITER like plasma, radial temperature profiles can be obtained from the first harmonics ordinary (O) mode or second harmonic extra-ordinary(X) mode of ECE spectrum. It is possible that, higher harmonics produced from the non-thermals can be relativistically downshifted to second harmonics and results a deviation in the measured temperature profile. We performed a parametric study on the effect of non-thermal electrons on measured ECE temperature for ITER scenario-2. All the numerical calculations reported in this paper are performed using NOTEC computer code which is capable of handling non-thermal populations. After proper validation of numerical methods using normal electron population (without non-thermals) a parametric study with non-thermals is performed. In the parametric study radial locations of non-thermals, energy of non-thermals and fraction of non-thermals are considered. This study is initially performed for normal view and later extended in to oblique views. The range of deviation of temperature over the examined parametric regime as well as the possible physical reasons will be presented. The effect of parallel component of non-thermal energy is also examined. Finally results of one set of study for oblique view (where the detector is not exactly normal to the magnetic field) with non-thermal electrons are also presented. In ITER apart from an Electron Cyclotron Emission (ECE) detector placed normal to magnetic field an oblique view detector is planned to grab information about non-thermal electrons. Usefulness of such an additional detector for a better radial resolution is examined. The differences in the ECE spectrum from a tokamak plasma between a direct LOS (normal to toroidal magnetic field) and a slightly oblique LOS have been modelled. A typical ITER tokamak scenario has been chosen in this study. The intensities of radiation, as observable from the low-field side, covering the first harmonic O-mode spectral frequencies 105-230 GHz have been compared. The physical reasons for the code-predicted results, regarding the differences between the direct and oblique spectra, are elucidated. Finally, signatures of the presence of non-thermals from a comparison of normal view and oblique view are also examined.
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