Profile Initialization from O-Mode Reflectometry

Abstract

Reflectometry will be used on ITER to study plasma fluctuations and to measure the density profile in the main and divertor plasmas. The basic information for profile evaluation is the group delay τ _g due to the propagation and reflection in the plasma of ordinary (O) or extraordinary (X) waves, with variable frequencies F. With O-mode the profile inversion is simpler because the cutoff depends only on the plasma density. However, as frequencies below F ₁ (∼15 GHz) cannot be used, plasma densities n _e ≤ 0.28 10¹⁹ m^-3 are not probed in reflection and the distance information about the lowest density profile cannot be obtained. X mode waves can be reflected from zero density but the cutoff depends both on n _e and on the total magnetic field B _tot Accurate information about B _tot necessary to invert the profile, may be difficult to obtain namely in long plasma ITER pulses (exceeding 3000s). It is therefore important to study the reconstruction of profiles from O-mode only and to assess the errors due to the initialization model used to complement the data below the first probing frequency.