Radial Resolution Evaluation for Oblique-View Electron Cyclotron Emission for ITER

The differences in the electron cyclotron emission spectrum from a tokamak plasma between a direct line of sight (LOS) (normal to the toroidal magnetic field) and a slightly oblique LOS have been modeled. A typical ITER tokamak scenario has been chosen in this study. The usefulness of such an additional detector for obtaining a better radial resolution is examined. The intensities of the radiation, as observable from the low-field side, covering the first harmonic ordinary mode spectral frequencies ∼120 to 230 GHz have been compared. We find that at certain frequencies the radiation observed along the oblique view seems to come from a narrower region. This affords the possibility of realizing better radial spatial resolution, compared to that possible by a direct view alone, for localizing any fluctuations, identifying abrupt changes in the temperature profile, etc. The physical reasons for the code-predicted differences between the direct and oblique spectra are elucidated. The translation of the radial resolution calculations into realistic phenomena is studied for two situations: neoclassical tearing modes and a damped sinusoidal perturbation. For both cases, the oblique view yields a better reproduction of the situation.